Ecological quality as a coffee quality enhancer. A review

Abstract

As both coffee quality and sustainability become increasingly important, there is growing interest in understanding how ecological quality affects coffee quality. Here we analyze, for the first time, the state of evidence that ecological quality, in terms of biodiversity and ecosystem functions, impacts the quality of Coffea arabica and C. canephora, based on 78 studies. The following ecosystem functions were included: pollination; weed, disease, and pest control; water and soil fertility regulation. Biodiversity was described by the presence, percentage, and diversity of shade trees. Coffee quality was described by the green bean physical characteristics, biochemical compounds, and organoleptic characteristics. The presence and diversity of shade trees positively impacted bean size and weight and reduced the percentage of rejected beans, but these observations were not consistent over different altitudes. In fact, little is known about the diversity of shade trees and their influence on biochemical compounds. All biochemical compounds varied with the presence of shade, percentage of shade, and elevation. Coffee beans from more diverse tree shade plantations obtained higher scores for final total organoleptic quality than simplified tree shade and unshaded plantations. Decreasing ecological quality diminished ecosystem functions such as pollination, which in turn negatively affected bean quality. Shade affected pests and diseases in different ways, but weeds were reduced. High soil quality positively affected coffee quality. Shade improved the water use efficiency, such that coffee plants were not water stressed and coffee quality was improved. While knowledge on the influence of shade trees on overall coffee quality remains scarce, there is evidence that agroecosystem simplification is negatively correlated with coffee quality. Given global concerns about biodiversity and habitat loss, we recommend that the overall definition of coffee quality include measures of ecological quality, although these aspects are not always detectable in certain coffee quality characteristics or the final cup.

Contents

• 1. Introduction

• 2. Definition of ecological quality

• 3. Definition of coffee quality

• 3.1 Physical coffee quality

• 3.2. Biochemical composition quality

• 3.3. Organoleptic quality

• 4. Methods

• 4.1. Search criteria for database query

• 4.2. Procedure for selecting relevant publications, data extraction and handling

• 5. Global distribution of publications, and their focus on coffee species and varieties

• 6. Effects of ecological quality on coffee quality

• 6.1. Biodiversity

• 6.1.1. Effects of biodiversity on physical quality and interacting factors

• 6.1.2. Effects of biodiversity on coffee bean biochemical compounds and interacting factors

• 6.1.3. Effects of biodiversity on organoleptic quality and interacting factors

• 6.2. Effects of ecosystem functions on coffee quality

• 6.2.1. Pollination

• 6.2.2. Pest, disease and weed control

• 6.2.3. Soil fertility regulation

• 6.2.4. Water regulation

• 6.2.5. Interacting effects of ecosystem functions on coffee quality

• 7. Conclusions

• Acknowledgements

• References

Introduction

Coffee is among the most important agricultural commodities grown in tropical and subtropical regions, often replacing biodiversity-rich ecosystems (Somarriba and Lopez-Sampson 2018). FAO (2021) reports that coffee production covers 11.1 million hectares worldwide. Global coffee production in 2019–2020 was estimated at 10.2 million tons (International Coffee Organization, ICO 2021). Initially, global coffee consumption was expected to decrease to 10.06 million tons due to the COVID-19 pandemic; instead, 2020 ended with an unexpected coffee consumption increase of 92,400 t compared to 2019 (ICO 2021).

Growing global demand for coffee has put rising pressure on tropical and subtropical biodiversity hotspots (Myers et al. 2000), resulting in their conversion to coffee crop systems, while diversified shaded coffee plantations being converted to unshaded coffee plantations (Donald 2004). At the same time, low international coffee prices during the last decade further fueled such conversions, as coffee growers need to produce even greater quantities in unshaded coffee plantations in order to achieve a living income (ICO 2020). While coffee farmers are often incentivized to maintain diverse coffee agroforestry systems through certifications and better prices (Perfecto et al. 2005), farmers often opt instead to produce higher short-term yields in unshaded coffee plantations rather than seeking to obtain improved quality in shaded coffee plantations. Indeed, in shaded coffee plantations, yields are often lower compared to shaded coffee plantations, though this is not always the case. For instance, Sarmiento-Soler et al. (2020) studied the effect of shade on yields in Uganda and found that coffee yields could be increased by maintaining 30% shade cover in coffee plantations.

Coffee is traditionally grown in shaded plantations under structurally and floristically diverse tree canopies. These complex habitats not only support the livelihoods of coffee producers, but also provide habitat for a high diversity of associated flora and fauna (Perfecto et al. 2005). A meta-analysis on biodiversity in cacao and coffee agroforestry systems reported that when comparing total species richness between forests and agroforests, the latter had, on average, only 11% less forest species than the former (De Beenhouwer et al. 2013). Other studies also reported a high biodiversity of ants (Perfecto and Vandermeer 2002; De la Mora et al. 2013; Arenas-Clavijo and Armbrecht 2018), bees (Jha and Dick 2008; Jha and Vandermeer 2010; Cepeda-Valencia et al. 2014), birds (Calvo and Blake 1998; Sánchez-Clavijo et al. 2009; Leyequien et al. 2010; Bakermans et al. 2012; McDermott et al. 2015; Narango et al. 2019; Estrada-Carmona et al. 2019; Schooler et al. 2020), butterflies (Mahata et al. 2019), hoppers (Rojas et al. 2001), frogs (Murrieta-Galindo et al. 2013, b), mammals (Gallina et al. 1996; Bali et al. 2007; Caudill et al. 2014; Guzmán et al. 2016), epiphytes (Moorhead et al. 2010; De Beenhouwer et al. 2015), and soil biota (Cardoso et al. 2003; Arias et al. 2012; Naeem Shahid et al. 2012; Arias and Abarca 2014; De Beenhouwer et al. 2015; Prates Júnior et al. 2019; Teixeira et al. 2021), specifically in shaded coffee plantations.

Coffee is also grown in simplified shaded plantations, where shade trees can be sparse. The meta-analysis by De Beenhouwer et al. (2013) also compared the total species richness between forests and simplified shaded coffee and cacao plantations and found the latter to have, on average, 46% less forest species than the former.

Further, agroforestry systems are often regarded as compatible with conserving biological integrity and healthy ecosystems, which are two characteristics of ecological quality (Somarriba and Lopez-Sampson 2018). Agroforestry systems enhance soil quality, carbon sequestration, and water holding capacity; control weed, diseases, and pests; and exhibit increased pollination activity compared to unshaded coffee plantations (Tscharntke et al. 2011; Kremen and Miles 2012). Coffee agroforestry systems could thus be considered as having high ecological quality, since they harbor and conserve a wide range of biodiversity and ecosystem functions (Fig. 1). However, the ecological quality will vary depending on the complexity of the agroforestry system (De Beenhouwer et al. 2013; Mokondoko et al. 2022).

Fig. 1.

Coffee agroforestry systems with diverse tree shade. Photo credit: Vania Torrez.

Moreover, it is broadly recognized that high biodiversity plays an important role in the provision of ecosystem functions for agriculture, i.e., pollination, pest, and disease control (Paetzold et al. 2010; Power 2010). A decline in biodiversity jeopardizes the provision of ecosystem functions (Hooper et al. 2012). For instance, diverse bee communities enhance pollination of crops (Ricketts 2004), diverse bird communities are effective in predation of insect herbivores (Bael et al. 2008), and diverse tree communities produce biomass more efficiently than species-poor assemblages (Zhang et al. 2012). In their meta-analysis, De Beenhouwer et al. (2013) compared the provision of ecosystem functions in cacao and coffee agroforestry systems, unshaded coffee and cacao plantations, and forests in a meta-analysis. They reported that the provision of ecosystem functions decreased 37% in agroforestry systems compared to forests and decreased 27% in unshaded plantations compared to agroforestry systems.

Meanwhile, consumers are increasingly interested in coffee quality, such that demand for high quality and specialty coffee is growing (Upadhyay and Rao 2013; Ufer et al. 2019; Giacalone et al. 2019). Coffee quality arises from interactions between the environment, management, and plant genetics (Läderach et al. 2011; Sunarharum et al. 2014). However, the definition of specialty coffee not only includes intrinsic attributes—e.g. cupping score, physical appearance, size, roast color, and descriptive profile—but also broader characteristics such as origin, certification, and name of the farm, as well as aspect of ecological, economic, and social sustainability (Specialty Coffee Association, SCA 2021). Both quality and specialty coffee are potentially of critical importance for the coffee industry and conservation, due to their increasing demand and their potential importance for biodiversity, as high-quality coffee could be grown under conditions of high ecological quality conditions.

Moreover, the increased market for quality coffee has further raised interest in research on environmental factors and local production systems that affect coffee quality (Avelino et al. 2005). Additionally, the impact of coffee production on the environment has also spurred research interest on how to reconcile ecological quality and coffee production.

Coffee quality encompasses physical appearance, biochemical composition, and organoleptic characteristics (Fridell 2014). The most common physical characteristics analyzed are the length, width, weight, shape, and color of green coffee beans (Eskes and Leroy 2004), as well as damaged beans (SCAA 2013). The most important organoleptic characteristics are fragrance/aroma, flavor, sweetness, acidity, aftertaste, bitterness, body, and balance (SCA 2015). The most common biochemical characteristics are sugar composition, trigonelline, caffeine, chlorogenic acids, and lipids (De Maria et al. 1996).

Numerous factors and their interactions affect coffee quality characteristics, including shade (Muschler 2001; Vaast et al. 2006a; Lara-Estrada and Vaast 2007; Geromel et al. 2008), elevation (Avelino et al. 2005; Bertrand et al. 2006; Villarreal et al. 2009), temperature patterns (Bertrand et al. 2012), amount and distribution of rainfall (Bermúdez-Florez et al. 2018), physical and chemical properties of the soil (Woldesenbet et al. 2008; Bosselmann et al. 2009), fertilizers (Lara-Estrada and Vaast 2007), genotypes or varieties (Avelino et al. 2005; Leroy et al. 2006; Bertrand et al. 2006; Oberthür et al. 2011; Belete et al. 2014; Cheng et al. 2016), pre-harvest management practices (Vaast et al. 2006a; Bertrand et al. 2006; Läderach et al. 2011), and post-harvest practices (Yadessa et al. 2020). The variation of all these factors from place to place contributes to the quality variation of coffees around the world.

Despite trends towards intensification of coffee plots to increase production, coffee grown in biodiverse rich conditions—such as coffee agroforestry systems—is highly valued by roasters, baristas, and consumers not only because of its environmental friendliness, but because of its quality (Hernandez-Aguilera et al. 2018). To date, however, there has been no comprehensive review assessing the effects of ecological quality on coffee quality. There is a need for evidence on the value of diversified shaded coffee plantations, not only for conservation, but also to improve coffee quality.

In this way, the present review addresses an important knowledge gap regarding whether coffee quality aligns with maintaining ecological quality, such that land use change for coffee production does not come at the expense of ecological quality. To this end, we reviewed the current state of scientific evidence about the impacts of ecological quality on coffee quality. We sought to identify whether and how ecological quality—in terms of biodiversity and ecosystem functions—enhances coffee quality.

Definition of ecological quality

Various definitions of ecological quality have been proposed. They broadly encompass concepts of biological integrity (Karr 1999) and ecosystem health (Rapport et al. 1998; Burkhard and Müller 2008). A healthy ecosystem has been defined as “being stable and sustainable, maintaining its organization and autonomy over time and its resilience to stress” (Rapport et al. 1998). Biological integrity is typically defined by the closeness of diversity, species composition, and functional organization to natural habitats (Angermeier and Karr 1994). In this way, indicators of ecological quality rely on “naturalness” as a criterion (Paetzold et al. 2010); however, in agricultural landscapes, there are marked practical difficulties due to land use change. Paetzold et al. (2010) have proposed a useful framework for assessing ecological quality in human disturbed landscapes, such as agricultural systems, based on ecosystem functions.

Ecological quality in coffee agroforestry systems includes several indicators of biodiversity and ecosystem functions, such as pollination, disease and pest control, water quality, soil quality, and climate change mitigation and adaptation (Paetzold et al. 2010; Nair 2011; Nair et al. 2021). For the purpose of this review, the definition of ecological quality is limited to the characteristics of biodiversity and ecosystem functions in coffee plantations (Fig. 2).

Fig. 2.

Definition of ecological quality for this review.

Our review is restricted in terms of indicators of biodiversity. This is because existing published research examining both agroecosystems and coffee quality almost exclusively focuses on the quality-related effects of the presence of shade trees, the percentage of shade, and/or shade tree diversity. It became clear from our literature search that research on the quality-related effects of other types of biodiversity (e.g., insect presence and diversity) remains scarce, with very few exceptions (Moreaux et al. 2022; Martínez-Salinas et al. 2022). However, it is widely known that shaded coffee plantations provide habitat niches for a high diversity of flora and fauna (Muschler 2004; Perfecto et al. 2005) and that levels of biodiversity in these agricultural landscapes are often comparable to natural forests (De Beenhouwer et al. 2013). The diversity of flora and fauna harbored by coffee agroforestry systems depends on the complexity of the system (De Beenhouwer et al. 2013).

Against this background, our review assumes that diversified shade can be taken as proxy for biodiversity. Relevant tree species include leguminous trees that contribute nitrogen to the soil, as well as tree species that are valued for their timber or for their fruits (Descroix and Wintgens 2004). In any case, the presence of shade trees creates habitat for biodiversity—to varying degrees. In this review, we therefore consider the presence of shade trees, the percentage of shade cover, shade tree diversity, and shade tree density as descriptors of biodiversity.

The ecosystem functions studied in the selected literature that have reported effects on coffee quality are pollination, pest control, disease control, weed control, water regulation and provision, and soil fertility regulation. As a result, only these ecosystem functions were included in our review.

Definition of coffee quality

Coffee quality results from the interaction of several intrinsic and extrinsic factors such as elevation, soil characteristics, coffee variety, production methods, geographical origin, seasonal conditions, post-harvest processing methods (washed, pulped or natural processing, fermentation, and drying methods/conditions), storage, and transport, as well as roasting and brewing methods (Ribeiro et al. 2011; Tolessa et al. 2016; Toci and Boldrin 2018; Craig et al. 2018; Monteiro et al. 2018; Belchior et al. 2019).

Coffee quality definitions can be very subjective and vary according to the research question, regulatory organization, country, or coffee sector (Tolessa et al. 2016; Baqueta et al. 2020). In addition, various methods are used to classify coffee quality at different stages of production (Ribeiro et al. 2009). However, the Specialty Coffee Association has standardized methods and is training Q-graders for consistency in the sensory analyses of quality coffee (Pereira et al. 2017).

The coffee quality parameters reported in the selected publications are (1) physical quality characteristics, (2) biochemical composition, and (3) organoleptic characteristics. However, these vary depending on the species and varieties. For instance, Coffea arabica and Coffea canephora have distinct physical, biochemical, and organoleptic characteristics. Beverages from C. arabica have a lower caffeine content and tend to have fruity notes and more intense flavor, as well as wider variations in the body and acidity (Dias et al. 2018). On the other hand, beverages from C. canephora tend to have higher caffeine content and taste more bitter (Davis et al. 2011; Craig et al. 2018).

Physical coffee quality

Physical quality is assessed in green coffee beans. The descriptors of coffee bean physical quality are bean size, bean weight, and rejected bean percentage—including as elephant beans, triage beans, and pea berries (Eskes and Leroy 2004).

To determine bean size, coffee beans pass through different numbered screens until they no longer fall through the holes. For instance, if a bean falls through the hole of a 16/64-inch screen (6.5 mm), but is not able to pass through a 14/64-inch screen (5.5 mm), it is graded as a size 16. Larger beans are the best quality in terms of bean size, as required by the coffee industry, and thus are more expensive (Wintgens 2004a). Depending on the region or country, bean size is classified differently (Table 1).

Table 1.

Grading bean size classification per region or country

Screens 1/64 inch	mm	Classification	Central America and Mexico	Colombia	Brazil	Africa and India
20.0	8.00	Very large	Superior	Supremo	Santos NY 2/3	AA
19.5	7.75
19.0	7.50
18.5	7.25	Large
18.0	7.00					A
17.0	6.75			Excelso	Santos NY 4/5
16.0	6.50	Medium	Segundas			B
15.0	6.00
14.0	5.50	Small	Terceras			C
13.0	5.25	Shells	Caracol			PB
12.0	5.00
11.0	4.50		Caracolli
10.0	4.00
9.0	3.50		Cracolillo
8.0	3.00

Bean weight is measured by weighing 100 coffee beans. The higher the weight of the 100 beans, the higher the deemed quality.

Further, coffee beans are classified as either normal or abnormal beans (Wormer 1964). Abnormal beans consist of peaberries, empty beans, elephant beans, triangular beans, and shell beans (Wintgens 2004b). These abnormal beans are often rejected for coffee quality measurement purposes. Other beans that are often rejected from physical quality measurements are immature beans, withered beans, floater beans, full or partial black beans, full or partial sour beans, fungi damaged beans, insect damaged beans, and broken or chipped beans (SCAA 2013). The lower the percentage of rejected beans, the higher the deemed physical quality of the green coffee beans.

The publications selected for review use bean size and bean weight as indicators of physical quality.

Biochemical composition quality

Various biochemical characteristics of beans interact and react among themselves during the roasting process, resulting in various compounds that later determine the perceived beverage quality (Ribeiro et al. 2011). The roasting process involves hundreds of simultaneous chemical reactions, which include the Maillard and Strecker reactions, degradation of proteins, polysaccharides, trigonelline, and chlorogenic acids (De Maria et al. 1996).

Sucrose acts as an aroma precursor that generates substances during roasting—including furans, aldehydes, and carboxylic acids—which influence the eventual flavor and aroma of the coffee beverage (Farah and Donangelo 2006). Trigonelline is a pyridine derivative that is known to indirectly affect the formation of desirable aromas (Ky et al. 2001). Caffeine is an alkaloid that affects coffee bitterness, which is important for coffee flavor (Trugo 1984). Chlorogenic acids are phenolic compounds that are known to be responsible for coffee pigmentation, aroma, bitterness, and astringency (Trugo 1984; Clifford 1985; De Maria et al. 1995).

The publications selected for this review consider the levels of caffeine, trigonelline, lipids, sucrose, and chlorogenic acids contained as indicators of biochemical composition quality.

Organoleptic quality

To evaluate organoleptic quality, the cupping method is usually applied. This methodology is based on the sensory perception of professional, calibrated coffee tasters that evaluate and score each coffee attribute. The cupping consists of a group of tasters that assess the same coffee sample. After roasting and grounding following standardized protocols proposed by the SCA and the Specialty Coffee Association of America (SCAA), the fragrance of the dry coffee is scored. Next, the coffee is brewed according to standardized protocols proposed by the SCA and SCAA; these consider the water/coffee ratio, water temperature and water quality, time of extraction, and brewing method (SCAA 2015; SCA 2018).

Finally, the tasters must rate the following attributes (SCAA 2015; SCA 2018):

• Fragrance: smell of the ground coffee when still dry.

• Aroma: smell of the coffee when infused with hot water.

• Flavor: combined impression of all gustatory sensations and retro-nasal aromas that go from the mouth to nose.

• Aftertaste: length of positive taste and aroma qualities emanating from the back of the palate and remaining after the coffee is expectorated or swallowed.

• Acidity: described as “brightness” when favorable or “sour” when unfavorable. Acidity contributes to the liveliness, sweetness, and fresh-fruit character of the coffee beverage.

• Body: tactile feeling of the liquid in the mouth, perceived between the tongue and the palate.

• Balance: how flavor, aftertaste, acidity, and body of the coffee beverage work together and complement or contrast with each other.

• Sweetness: refers to a pleasing fullness of flavor as well as sweetness. The opposite of sweetness is astringency, sour, or “green flavors.”

• Clean cup: refers to a lack of interfering negative impressions from the first sip to the final aftertaste.

• Uniformity: consistency of flavor of the different cups of the coffee sample tasted.

• Overall score: reflects the holistic integrated rating of the sample perceived by the individual taster.

• Defects: negative or poor flavor that detract from the quality of the cup.

A final score for the organoleptic quality is obtained by summing up all scores of all evaluated coffee attributes and subtracting the defect score. The final score is rated from 0 to 100. The Specialty Coffee Association (Table 2) considers coffee beverages with scores of 80 and above of high quality.

Table 2.

Scores, grading, and classifications of coffee beverages

Score	Grading	Classification of specialty	Brazil
90.00−100.00	Outstanding	Specialty	Strictly soft
85.00−89.99	Excellent
80.00−84.99	Very good		Soft
75.00−79.99	Bellow specialty quality	Not specialty	Just soft
68.00−74.99			Hard
<67.00			Riado

The selected publications use flavor, aroma/fragrance, aftertaste, acidity, body sweetness, and total score as indicators of organoleptic quality. Additionally, bitterness and astringency are evaluated in a few publications, but these are not suggested by the SCA.

Methods

Search criteria for database query

We conducted a literature search in Web of Science (all databases) in June 2022 by searching for the following combinations of keywords: (“quality coffee” OR “coffee quality” OR “specialty coffee”). All retrieved publications in the Web of Science were in English.

Moreover, we conducted a literature search on key national-level coffee organizations and national research centers in Latin America. Thus, we decided to work with the virtual library of CATIE (http://repositorio.bibliotecaorton.catie.ac.cr) and CENICAFE (https://www.cenicafe.org/es/index.php/nuestras_publicaciones/busqueda) for publications in Spanish, and SBICafé (http://www.sbicafe.ufv.br) and Incaper (https://biblioteca.incaper.es.gov.br/digital/) for publications in Portuguese. We opted to use diverse databases because we wanted to include gray literature in our analysis and did not want to limit our analysis strictly to studies published in English. In those virtual libraries, the keyword combinations that we used were (“calidad de café” OR “café de calidad” OR “café de especialidad”) for virtual libraries in Spanish; and (“qualidade do café” OR “cafés especiais”) in Portuguese. All searches were done between February 2021 and June 2022.

In addition, publications of interest were obtained by scrutinizing the reference lists of the publications retained from the searches. We also complemented the literature with our own literature collections. We considered articles written in English, Spanish, French, and Portuguese.

Procedure for selecting relevant publications, data extraction, and handling

Our initial search resulted in 460 publications. Firstly, to identify relevant publications for this review, we screened the titles and abstracts. Our main criterion for inclusion was a primary focus on the effects of ecological quality on coffee quality. Secondly, publications that passed the first screening were screened a second time based on the full text and excluded if/when they had unclear results or no obvious connection with ecological quality or coffee quality. The second screening narrowed our sample to 78 publications. These specific screening steps have been done to extract all publications dealing with the effects of ecological quality on coffee quality based on experimental and observational studies.

We manually extracted all qualitative and quantitative data from graphs, tables, and text – including significant single factors and significant interactions between factors—to determine the type of effect on coffee quality.

Global distribution of publications and their focus on coffee species and varieties

From the 78 selected publications, Costa Rica (n=16) and Brazil (n=15) were the countries featured in the most studies (Fig. 3), followed by Ethiopia (n=8), India (n=7), Mexico (n=6), and Nicaragua (n=6). It is interesting to note that several countries base much of their economy on coffee production, and among are the largest producers of coffee, published 1 to 3 studies about the effects of ecological quality on coffee quality—for example, Vietnam, Colombia, Indonesia, Uganda, and Honduras. Other lesser-important coffee producing countries were featured in the same number of publications each—such as Tanzania, Guatemala, Kenya, Puerto Rico, Thailand, Peru, Jamaica, and Togo. In addition, it is interesting that Latin America is significantly more represented in our literature sample; only Ethiopia, Kenya, Tanzania, and Togo were featured for Africa and India and Thailand for Asia.

Fig. 3.

Geographical distribution of publications regarding the effects of ecological quality on coffee quality. The country is understood as the location of the field site (n=78).

Regarding the coffee species and varieties, most of the studies reported results from Coffea arabica (n=68); the reported varieties were Caturra, Catimor, Pacas, Parainema, Obatã, Costa Rica 95 (or CR95), and Typica. A few publications reported on Coffea canephora (n=10); the reported variety was Robusta.

Effects of ecological quality on coffee quality

Shade coffee plantations are highly valued because they support diverse flora, fauna, and related ecosystem functions (Perfecto et al. 1996; Vandermeer et al. 2010; Cunningham et al. 2013). The presence of shade in coffee systems is generally associated with a favorable microclimate, such as lower air temperature fluctuations, increased relative air humidity, lower wind speed, and decreased frost (De Giusti et al. 2019). Forest-like conditions of shaded coffee systems allow for various ecosystem functions, resulting in a positive impact on soil fertility, total organic matter, nutrient cycling, reduced soil evaporation, and soil erosion (Beer et al. 1997; Padovan et al. 2018; Sarmiento-Soler et al. 2019; Jácome et al. 2020; de Carvalho et al. 2021), mitigation of microclimate extremes (Lin 2007; de Carvalho et al. 2021), carbon sequestration (Guillemot et al. 2018; Caramori et al. 2020; Zaro et al. 2020), natural pest control (Vandermeer et al. 2010; Karp et al. 2013), natural weed control (Staver et al. 2001), and improved pollination of coffee (Boreux et al. 2013b). These ecosystem functions can have diverse effects on coffee quality.

Biodiversity

Effects of biodiversity on physical quality and interacting factors

Reviewed studies reported that the presence of shade and different percentages of shade impacted positively on physical quality (Table 3). For instance, Tolessa et al. (2016) reported that coffee beans of C. arabica were 10% heavier from medium (50–55%) and dense (65–85%) shade coffee plantations than those grown unshaded in Ethiopia. Similarly, Muschler (2001) found that beans of C. arabica var. Caturra were 20% heavier and beans of C. arabica var. Catimor were 29% heavier when grown under dense shade (>80% of shade) compared to those grown unshaded. Somporn et al. (2012) found that 65% of shade from Litchi chinensis positively affected beans weight of C. arabica var. Catimor; they were 23% heavier than those grown unshaded.

Table 3.

List of studies reporting either positive or negative impacts of shade on the physical quality of coffee beans. Abbreviation: m a.s.l. meters above sea level, USH unshaded, SH shade

Reference	Coffee species	Type of shade	Impact
Guyot et al. (1996)	C. arabica	Inga spp.	69% of beans of >6.5 mm in the yield of shaded coffee plantations at 1100 and 1400 m a.s.l. compared to 65% recorded in USH coffee plantations at same elevation
Muschler (1998)	C. arabica	Erythrina poeppigiana Treatments: USH, pollarded, open (40–60% of SH), dense (>80% of SH), and shade cloth (55–60% of SH)	>1% of deformed fruits on open and dense SH, but on USH and pollard 10% and 7% of deformed fruits
Salazar et al. (2000)	C. arabica	Erythrina poeppigiana 40–60% of SH, 60–80% of SH and USH	Percentage of beans of >6.7 mm of diameter was 60% of the total yield on USH coffee plantation, and at 40–60% of SH and 60–80% of SH had 75% of larger beans of the total yield
Muschler (2001)	C. arabica vars, Caturra and Catimor	Erythrina poeppigiana Treatments: USH, pollarded, open (40–60% of SH), dense (>80% of SH), and shade cloth (55–60% of SH)	C. arabica var., Caturra 20% heavier beans in dense SH C. arabica var., Catimor 29% heavier beans in dense SH
Muschler (2004)	C. arabica vars, Caturra and Catimor	SH vs. USH	Higher proportion of larger beans
Wintgens (2004a, b)	C. arabica and C. canephora	SH vs. USH	Larger beans in shaded coffee than in USH
Yadessa et al. (2008)	Wild C. arabica	SH of Acacia abyssinica, Cordia africana, Albizia schimperiana, and Albizia gummifera	Proportion coffee beans of >6 mm was higher under Acacia abyssinica (92.73%) and Cordia africana (91.79%) than under Albizia schimperiana (89.48%) and Albizia gummifera (88.42%)
Bote and Struik (2011)	C. arabica	Ethiopian forest (557 lux) vs USH (1193 lux)	Larger and heavier beans
Somporn et al. (2012)	C. arabica var. Catimor	Litchi chinensis (lychee trees)	Approx. 23% heavier beans
Boreux et al. (2013, b)	C. canephora	Grevillea robusta vs native diverse SH tree canopy	Lower physical quality under Grevillea robusta shade than under diverse SH
Tolessa et al. (2016)	C. arabica	Ethiopian forest, medium (50–55%), dense (65–85%) and USH	10% heavier beans in medium and dense SH
Boreux et al. (2016)	C. canephora	Grevillea robusta vs native diverse SH tree canopy	Decreased fruit weight under Grevillea robusta 5.6% increase in fruit set and 6.25% increase in the proportion of lager beans in coffee shaded by native diverse SH trees
Nesper et al. (2017a, b)	C. canephora	Grevillea robusta vs native multi-species tree SH	Higher proportion of grade A beans (>6.65–7 mm) and reduced proportion of pea-beans on multi-species tree shade compared to G. robusta dominated shade
		Levels of SH cover (20–80%)	Increasing shade cover decreased percentage of grade AAA, AA and A beans (>6.65–>7.5 mm)
Worku et al. (2018)	C. arabica	SH vs USH	Physical quality scores were lower in coffee beans grown under SH

Coffee bean weight and size of C. arabica were compared between coffee plantations grown shaded and unshaded by Bote and Struik (2011) in Ethiopia. They found larger and heavier coffee beans in coffee trees grown under shade compared to unshaded coffee trees.

Bean size is widely considered an indicator of quality. However, larger beans do not necessarily taste better than smaller ones (Wintgens 2004b). It is more important to have a uniform bean size for a better heat transfer during roasting and to roast the beans homogeneously.

Shade trees buffer air temperature, minimum night temperatures were reported to be 0.5−2 °C higher in shaded coffee plantations than unshaded, and maximum daytime temperatures 4−5 °C were lower in shaded coffee compared to coffee grown unshaded (Lin 2007; Siles et al. 2010; Souza et al. 2012; Rigal et al. 2020; Merle et al. 2022). Additionally, shade reduces the daily variation of the mean air temperature (de Carvalho et al. 2021) and reduces frost events (De Giusti et al. 2019).

As a result of air temperature reduction, fruit maturation in shaded coffee is slower than in unshaded coffee (Muschler 2004; Ricci et al. 2011). As flower initiation is light dependent, a reduced number of flowers are developed under lower solar radiance resulting in reduced fruit set, but these factors enable bigger fruits due to a longer assimilation into fewer fruits (Muschler 2001; Vaast et al. 2006a; Clifford 2012). In other words, shade enhances bean physical quality because it promotes fewer flowers and a smaller number of larger-sized fruits that have more space and energy from the plant to develop.

The percentage of deformed fruits of C. arabica was reduced in the presence of Erythrina poeppigiana trees (Muschler 1998). Less than 1% of deformed fruits were reported under dense (77–90%) and open (46–78%) shade; 10% and 7% of rejected fruits—whether diseased, mummified, sunburnt, or dried—were reported in unshaded and under heavily pruned shade trees (<50% of shade), respectively (Muschler 1998). The highest percentage of rejected fruits was found in unshaded and heavily pruned shade trees, possibly related to intense heat and higher incidence of diseases combined with reduced vigor of heat-stressed plants (Muschler 2004).

Studies on the effect of shade tree diversity on coffee physical quality indicated that coffee quality might increase with shade tree diversity. Boreux et al. (2016) reported lower coffee physical quality of C. canephora under the shade of only Grevillea robusta in comparison with more diverse native shade tree canopy in Kodagu, India. The proportion of larger coffee beans declined in agroforestry systems with low structural shade tree diversity (Boreux et al. 2016). Further, the researchers found that the fruit set increased by 5.6% and the proportion of larger beans increased by 6.25% in coffee agroforestry systems featuring diverse native shade trees when compared to coffee agroforestry systems with only one shade tree species.

Nesper et al. (2017a) compared the effects of diverse multispecies coffee agroforestry systems on bean size when compared to Grevillea robusta dominated shade in Kodagu, India. They found that reduced tree shade diversity negatively affected the bean size of C. canephora. High shade tree diversity increased bean quality in terms of higher proportion of grade A (>6.65–7 mm) bean size. They also found that diverse tree shade decreased the proportion of pea beans. Ineffective pollination has been suggested to be the cause of the occurrence of pea beans (Wintgens 2004a, b). Diverse agroecosystems are likely to harbor a higher diversity of pollinators and benefit from its services, though not always. Boreux et al. (2013a) reported that an increase in the abundance of G. robusta increased the abundance of pollinators in coffee plantations in Kodagu, India. However, another reason for the increased pollinator abundance was that they also had an irrigation system that allowed coffee farmers to control for coffee flowering, such that not all coffee plantations were flowering at the same time (Boreux et al. 2013b).

In addition, Nesper et al. (2017a) found that as shade cover increased, the percentage of larger beans decreased, irrespective of shade tree diversity. In contrast, Wintgens (2004a, b) reported fewer but larger beans in shaded coffee than in unshaded coffee plantations.

Boreux et al. (2013b) found that the shade of only Grevillea robusta or the low density of this species decreased fruit dry weight of C. canephora compared to diversified shade coffee systems in Kodagu, India. A low density of shade trees is unlikely to be as effective in providing a favorable microclimate buffer for weather fluctuations.

When comparing the effect of different tree shade species on coffee bean size of C. arabica (Ethiopian wild arabica coffees), Yadessa et al. (2008) found that the proportion of coffee beans measuring >6 mm was higher under Acacia abyssinica (92.73%) and Cordia africana (91.79%) than under Albizia schimperiana (89.48%) and Albizia gummifera (88.42%). In addition, they found that the proportion of small coffee beans was higher under both Albizia species than under C. africana and A. abyssinica.

When including interacting factors in the analysis, physical quality varied depending on the percentage of shade and elevation. For instance, at lower elevations (700 m a.s.l.), Salazar et al. (2000) reported that the percentage of larger beans of C. arabica (>6.7 mm of diameter) was 60% of the total yield on unshaded coffee plantations. At lower elevations, they found that intermediate shade (40–60% of shade) and dense shade (60–80% of shade) coffee plantations featured 75% larger beans of the total yield. At higher elevations (1650 m a.s.l.), they reported similar patterns, but differences between shaded and unshaded coffee plantations were less evident. Guyot et al. (1996) reported an average of 69% of larger beans of C. arabica (>6.5 mm) in the yield of shaded coffee plantations at higher elevations (1100 and 1400 m a.s.l.), which was higher compared to 65% recorded in unshaded coffee plantations at 1100 m a.s.l. in Guatemala. Physical quality is improved by shading and higher elevation because both delay fruit ripening (Guyot et al. 1996). In particular, shade seems beneficial at low elevations or in suboptimal environmental conditions (Muschler 2001).

Vaast and Raghuramulu (2012) studied the effects of tree shade diversity and rainfall conditions on coffee bean size of C. canephora in Kodagu, India. Under low rainfall conditions, they found a higher proportion of larger beans (>7 mm) in a coffee plantation under the shade of Artocarpus heterophyllus, Dalbergia latifolia, and Lagerstroemia microcarpa compared to those grown under only Grevillea robusta. However, under high rainfall conditions, they found lower proportion of larger beans under the shade of A. heterophyllus or G. robusta.

One study also analyzed the interacting effects of post-harvest processing methods (dry and wet processing) and shade on coffee bean physical quality score of C. arabica (primary defects + secondary defects + odor; score over 40 points) in Ethiopia (Worku et al. 2018). These authors found that coffee beans grown unshaded had higher physical quality scores than coffee beans grown shaded. Moreover, they reported that coffee beans grown unshaded that were dry processed obtained higher scores on physical quality compared to wet processed coffee beans. However, bean physical quality scores of coffee beans grown under shade did not differ significantly between dry and wet processing. Worku et al. (2018) obtained unexpected results, since they hypothesized that shade and wet processing produce good quality coffee beans. They suggested further research using larger and repeated experiments to arrive at reliable conclusions regarding these interactions.

Indicators of biodiversity had positive effects on coffee bean physical quality of C. arabica and C. canephora (15 of 17 studies). In addition, highly diverse agroforestry systems were found to positively affect bean size and weight of C. canephora. Tree shade species have different effects on bean size of C. arabica. Indicators of biodiversity interact with elevation, rainfall conditions, and post-harvest methods and have differential effects on coffee bean physical quality of C. arabica and C. canephora.

Effects of biodiversity on coffee bean biochemical compounds and interacting factors

The chemistry of coffee quality involves a wide range of biochemical compounds that change during fruit development, and these compounds can be degraded during roasting or can stay stable and act as flavor, aroma, or bitterness attributes in the final cup (Wintgens 2004a). Biochemical compounds such as caffeine, trigonelline, lipids, sugars, and chlorogenic acids significantly influence coffee quality. For instance, high trigonelline and 3,4-dicaffeoylquinic acid levels in coffee beans are correlated with a high organoleptic quality (Farah et al. 2006). However, high levels of caffeoylquinic acids, feruloylquinic acids and their oxidation products are associated with a low organoleptic quality and flavor (Farah et al. 2006; Farah 2012). In this way, the effect of biochemical compounds on coffee organoleptic quality is variable.

Biochemical composition and organoleptic attributes of coffee beans can vary with environmental conditions (Bertrand et al. 2012), agricultural practices (Vaast et al. 2006a), varieties (Leroy et al. 2006; Dessalegn et al. 2007), and post-harvest processing techniques (Duarte et al. 2010; Taveira 2014).

Caffeine

Caffeine is an alkaloid that affects coffee bitterness and thus contributes to coffee quality. It is formed in immature coffee fruits and gradually accumulates during seed development (Koshiro et al. 2006). Caffeine is found in green coffee beans, in the range of 0.9–2.5 g/100 g (Farah 2012).

Caffeine accumulation and final concentration has been reported to be shade dependent (Table 4): when shade cover increased 0–80% caffeine concentration increased consistently in coffee beans of C. arabica (Odeny et al. 2014). Additionally, Guyot et al. (1996) found an increase of 4% in caffeine content in beans of C. arabica that were grown under shade compared to unshaded coffee, and similar patterns were found by Delaroza et al. (2017) and Tuccio et al. (2019) for beans of C. arabica.

Table 4.

List of studies reporting significant impacts of the biodiversity on the content of biochemical compounds in the dry coffee beans. Abbreviation: USH unshaded, SH shaded. Meaning of the symbols: +, increased under shade; −, decreased under shade; =, no differences between shaded and unshaded. *Albizia gummifera, Albizia grandibracteata, Albizia schimperiana, Acacia abyssinica, Blighia unijugata, Celtis africana, Cordia africana, Croton macrostachyus, Fagaropsis angolensis, Macaranga capensis, Millettia ferruginea, Olea welwitschii, Pouteria adolfi-friedericii, Sapium ellipticum, Trichilia dregeana, and Trilepisium madagascariense. ** Banana (Musa spp.), peach (Prunus persica), nance (Byrsonima spp.), orange (Citrus X sinensis), lemon (Citrus X limon), tangerine (Citrus tangerina), pink cedar (Acrocarpus fraxinifolius), guamo (Inga edulis)

References	Coffee species	Shade	Fructose	Glucose	Sucrose	Caffeine	Trigonelline	Chlorogenic acids	Lipids
Guyot et al. (1996)	C. arabica	Inga spp. vs USH			+ 3%	+ 4%	−10%	+ 10%	=
Alpizar et al. (2005)	C. arabica var. CR95	SH vs USH							+
Somporn et al. (2012)	C. arabica	70% SH Litchi chinensis (lychee trees) vs USH	−16%	Only detected under SH	Only detected under SH			+
Odeny et al. (2014)	C. arabica	Cordia africana 80%, 70%, 50%, 30% SH and USH				+	−at 0–50% SH +at 70% SH −at 80% SH	−	+
Delaroza et al. (2017)	C. arabica	SH vs USH				+			−
Tolessa et al. (2017)	C. arabica	Ethiopian forest Medium (50–55%), dense (65–85%), and USH				+at 65–85% SH −40–55% SH		+
Alves et al. (2018)	C. canephora	SH vs USH						−	−
Worku et al. (2018)	C. arabica	Forest* (30–40%) vs USH			−	=		=
Tuccio et al. (2019)	C. arabica var. Parainema	Banana trees (Musa spp.) vs USH				+	−	=
		Agroforestry system ** vs USH					=	=
de Oliveira et al. (2021)	C. arabica	Hevea brasiliensis vs USH							−

In addition, Tuccio et al. (2019) compared the effects of diverse multispecies coffee agroforestry system to Musa spp. (banana) dominated shade on caffeine content in beans of C. arabica var. Parainema and found that caffeine content was higher in diverse coffee agroforestry systems.

Analyzing the interaction effect of elevation, shade, and harvesting period on caffeine content in beans of C. arabica, Tolessa et al. (2016) found the highest caffeine content (17.9 g kg⁻¹) in early harvested beans (during the first week of the first month of the harvesting period) from a dense shade (65–85% of shade) at 1550–1750 m a.s.l., with the lowest caffeine content (14.5 g kg⁻¹) found in beans that were harvested in the middle of the harvesting season (last week of the first month of the harvesting period) from medium shade plantations (40–55%) at > 1750 m a.s.l.

A study on the interaction effect of elevation and shade on caffeine content in beans of C. arabica found no differences in caffeine content between coffee beans grown shaded and unshaded (Worku et al. 2018). Instead, they found that caffeine content decreased 0.12 g kg⁻¹ per 100 m altitude increase.

Most of the reviewed studies (5 of 6) reported that shade positively affects caffeine content. However, a few reported that shade interacts with elevation and harvesting period variously affecting caffeine content.

Trigonelline

Trigonelline is the second-most abundant alkaloid in coffee beans, which is an aroma precursor that contributes to desirable flavor attributes formed during roasting (De Maria et al. 1996). It follows the same biosynthetic pattern as caffeine: it accumulates rapidly in young coffee fruits and decreases before the maturity stage (Koshiro et al. 2006). Trigonelline concentration is reported to decrease in beans grown under shade: Guyot et al. (1996) found that trigonelline concentration was on average 0.92% in beans of C. arabica grown shaded and 1% in beans grown unshaded.

Regarding the effects of monospecific, diversified shaded and unshaded coffee plantations on trigonelline concentration in beans of C. arabica, Tuccio et al. (2019) found no differences on trigonelline concentration between beans grown unshaded and under diverse shade, but trigonelline concentration was lower on beans grown only under Musa spp. shade compared to beans grown unshaded.

However, trigonelline concentration content is also reported to be inconsistent with increasing shade levels (Table 4). For instance, Odeny et al. (2014) found that trigonelline concentration in beans of C. arabica decreased when shade level increased 0–30% or 50% and then a rise of trigonelline concentration at 70% and a reduction at 80% of shade.

In this way, the effects of shade on trigonelline concentration in beans of C. arabica are inconclusive.

Phenolic compounds and chlorogenic acids

Phenolic compounds are secondary metabolites of plants that are involved in the protection from ultraviolet radiation or aggression by pathogens (Farah and Donangelo 2006). Most of these compounds have received attention as they are considered potential factors against human chronic degenerative diseases, cancer, and cardiovascular diseases (Scalbert et al. 2005).

Phenolic acids in coffee are chlorogenic acid, vanillic acid, caffeic acid, protocatechuic acid, sinapic acid, p-Hydroxybenzoic acid, syringic acid, p-coumaric acid, ferulic acid, and gallic acid (Farah and Donangelo 2006; Somporn et al. 2011). The effect of phenolic compound content on coffee quality is variable because it depends on the initial content in green coffee beans, on the roasting temperature, and the final chemical compounds that are formed (Fuller and Rao 2017). For instance, a light roasting process converts chlorogenic acid into chlorogenic acid lactones, and at this point, the bitterness is low and considered pleasant (Frank et al. 2007). However, further roasting converts chlorogenic acid lactones into phenylindanes, which increase bitterness (Frank et al. 2007), and that is not considered acceptable in terms of cup quality (Fischer et al. 2001; Fuller and Rao 2017).

Phenolic compounds directly determine coffee cup quality and play an important role in coffee flavor (Farah et al. 2006). However, the correlation between final cup quality scores and total phenolic content before and after roasting is variable (Laukaleja and Kruma 2019).

Regarding biodiversity and total phenolic content (Table 4), Somporn et al. (2012) reported that total phenolic content in beans of C. arabica grown under a 70% canopy of lychee trees (161.9 g kg⁻¹) was higher than in unshaded coffee beans (155.3 g kg⁻¹).

The most studied phenolic compounds are the chlorogenic acids. These are present in high concentrations in green coffee beans (41–113 g kg⁻¹) and are determinants of cup quality (Carrelli et al. 1974; Clifford and Wight 1976; Trugo and Macrae 1984; Variyar et al. 2003; Farah et al. 2006; Farah 2012). Chlorogenic acids are degraded during roasting and contribute to increasing certain organoleptic attributes, such as bitterness (Trugo 1984), astringency (Carrelli et al. 1974; Variyar et al. 2003), acidity (Trugo and Macrae 1984), and aroma (Upadhyay and Rao 2013).

Results of the effect of the percentage of shade on chlorogenic acid content in coffee beans are inconsistent in the literature (Table 4). For instance, Tolessa et al. (2016) found that total chlorogenic acid content in beans of C. arabica increased from unshaded to 85% of shade cover. Similarly, Guyot et al. (1996) and Somporn et al. (2012) found increased chlorogenic acid content in beans of C. arabica that were grown under shade compared to those grown unshaded. Somporn et al. (2012) reported that beans of C. arabica grown under 70% of lychee shade had 125 g kg⁻¹ of chlorogenic acid content and that coffee beans grown unshaded had 62.1 g kg⁻¹ of chlorogenic acid content. Almost 100% more chlorogenic acid content was found in shade-grown coffee. Guyot et al. (1996) found 10% more of chlorogenic acid content in beans of C. arabica grown under shade compared to beans grown unshaded. In contrast, Alves et al. (2018) found higher chlorogenic acid content in beans grown unshaded. However, Tuccio et al. (2019) did not find significant differences in chlorogenic acid content in beans of C. arabica grown under Musa spp. shade, in an agroforestry system comprising eight tree shade species and in unshaded plantations.

When including elevation in the analysis along with shade, Tolessa et al. (2016) found significant interactions: total chlorogenic acid contents in beans of C. arabica decreased from medium altitudes (1,600–1680 m a.s.l.) to high altitudes (1950–2100 m a.s.l.) and from 65–85% of shade cover to unshaded at high altitudes. Worku et al. (2018) also analyzed the same interaction and found that the chlorogenic acid content in beans of C. arabica was not significantly different between coffee beans grown shaded or unshaded, but chlorogenic acid content decreased with increasing elevation.

Overall, the effects of shade on phenolic compounds and chlorogenic acid contents are diverse and inconsistent in the reviewed literature.

Sucrose

Sucrose plays a crucial role in coffee organoleptic quality, since its breakdown during roasting releases several aroma and flavor precursors (Somporn et al. 2012). The higher the sucrose content in green beans, the more intense the coffee cup flavor and aroma (Ky et al. 2001), partly explaining the better coffee cup quality. Sucrose degrades during roasting and forms volatile and non-volatile compounds (Grosch 2001). Sucrose is also important for coffee color, aroma, and flavor based on the Maillard reaction and caramelization during roasting and brew acidity after roasting (Farah 2012).

Sucrose remains constant in the early growth stages of the perisperm–endosperm transition of the fruit, then it increases during endosperm development and drastically increases during the middle stage of fruit development (green to yellow fruit), and slows down until maturity (Guyot et al. 1996; Privat et al. 2008; Joët et al. 2009).

The effect of shade on sucrose content remains inconclusive (Table 4). Guyot et al. (1996) reported that sucrose content in coffee beans of C. arabica grown under shade was 3% higher than those grown unshaded.

Regarding the interaction effects of shade and elevation, sucrose content increased with elevation in beans grown under shade and unshaded (Worku et al. 2018). However, researchers found that sucrose content was higher in beans of C. arabica grown unshaded: it increased by 2.11 g kg⁻¹ per 100 m altitude increase, compared to 0.93 g kg⁻¹ in coffee beans grown under shade.

Regarding other sugars, Somporn et al. (2012) compared the sugar content in beans of C. arabica grown with 70% lychee shade versus unshaded and found a higher concentration of fructose in coffee beans grown unshaded. They also reported the presence of glucose and sucrose in coffee beans grown under 70% lychee shade (0.051 g kg⁻¹ and 0.013 g kg⁻¹, respectively). However, these sugars were not detected in coffee beans grown in unshaded coffee plantations. Hence, diverse types of sugars can be found in shaded coffee beans, which might have an effect on coffee quality. According to Vaast et al. (2006a) and Wintgens (2004a), coffee beans grown under cooler environments, such as shaded coffee plantations and higher elevations, accumulate more sugars.

Given that high sugar levels can be found in both shaded and unshaded coffee plantations, it appears that other additional factors and factor constellations play a role in determining whether shaded or unshaded coffee has more or less sugar.

Lipids

Coffee lipids contribute to the texture and mouth feel of the beverage as they carry flavors and fat-soluble vitamins (Oestreich-Janzen 2010). Roasting does not change most coffee lipids, but they are difficult to retain in the final beverage, depending on the extraction method.

Lipid content was found to increase with shade levels (Odeny et al. 2014), because more lipids are accumulated in cooler environments (Lara-Estrada and Vaast 2007). Further, a slower maturation of coffee beans promoted by shade allows a higher lipid accumulation (Muschler 2001; Lara-Estrada and Vaast 2007).

Alpizar et al. (2005) compared the lipid content of beans of C. arabica var. CR95 grown under shade (45%) with beans grown unshaded, and they found that the lipid content was higher in beans grown under shade. Additionally, Alpizar et al. (2005) tested the cup quality and found that it increased with the lipids content. However, Alves et al. (2018) found a higher lipid content in beans of C. canephora grown unshaded versus shaded. Guyot et al. (1996) reported that lipid concentration did not differ in beans of C. arabica grown unshaded or under Inga spp. shade.

In addition, de Oliveira et al. (2021) studied the gene expression related to two isoprenoid diterpenes, cafestol and kahweol, which are relevant lipids for coffee cup quality. They used beans of C. arabica grown unshaded and under Hevea brasiliensis shade. They reported that the expression of genes related to these lipids was higher in coffee beans grown unshaded. In addition, they studied the total lipid content in coffee beans and reported higher lipid content in coffee beans grown unshaded (about 11% more) than in those grown under shade. Delaroza et al. (2017) found a similar pattern; they found that lipid content in beans of C. arabica grown unshaded was higher compared to those grown shaded, 7.85 ± 6.69 μg mL⁻¹ versus 7.66 ± 5.73 μg mL⁻¹ respectively.

According to these findings, the effect of shade on lipid content varies, and other factors might be affecting lipid content as well.

In summary, the effects of biodiversity on coffee bean biochemical compounds remain inconclusive. Nevertheless, it appears that elevation and other factors or factor constellations play a role in determining coffee bean biochemical compound content.

Effects of biodiversity on organoleptic quality and interacting factors

Biochemical compounds are correlated with the organoleptic qualities of coffee beverages. It is well known that sucrose, caffeine, and trigonelline are essential flavor precursors during roasting (Grosch 2001; Homma 2001). Caffeine is correlated with body and bitterness of the coffee beverage and trigonelline is correlated with high cup quality (Farah et al. 2006; Oestreich-Janzen 2010). Hence, biochemical compound contents, which are differently affected by shade, have an effect on organoleptic quality.

Fragrance/aroma

The aromatic aspects of coffee cupping include fragrance and aroma. Fragrance is defined as the smell of recently grounded coffee beans when still dry. Aroma is defined as the smell of the coffee when infused with hot water. Coffee is smelled at three steps during the cupping process: (1) when coffee grounds are placed into cups before pouring water, (2) when the aromas are released while breaking the crust, (3) when the aromas are released as the coffee steeps (SCAA 2015).

Regarding the fragrance and aroma of the coffee beverage, Bosselmann et al. (2009) reported that diversified shade composed of Inga spp., Erythrina spp., Musa spp., Ficus spp., Cordia spp., and citrus trees negatively affected the score of C. arabica var. Caturra (Table 5) compared to unshaded coffee plantations. However, Tuccio et al. (2019) found that shade enhanced the coffee fragrance of C. arabica var. Parainema beverages. Tuccio et al. (2019) reported that diverse tree shade resulted in a higher coffee fragrance score than coffee grown in unshaded plantations. Conversely, when only Musa spp. was shading coffee, the fragrance score was similar to those from unshaded plantations. Similarly, Odeny et al. (2015) reported that C. arabica shaded with Cordia alliodora had a similar beverage fragrance score as coffee grown unshaded.

Table 5.

List of studies reporting effects of biodiversity on the organoleptic quality of coffee. Abbreviations: F flavor, A acidity, FR fragrance/aroma, B body, AF aftertaste, BI bitterness, AS astringency, SW sweetness, BA balance, T total final score, USH unshaded, SH shade. Meaning of symbols: +, increased under shade; −, decreased under shade; =, no differences between shaded and USH. *Treatments: (1) Inga spp. (I, 50–80% SH), (2) Musa spp. (M, 25– 43% SH), (3) I + M (50–74% SH), (4) I + M + Cordia alliodora (50–79% SH), (5) M + other species (28–60% SH), (6) rustic (63–83% SH), (7) forest remnant (47–76% SH), (8) multistrata polyculture (Persea americana, Citrus spp., Musa spp., 33–76% SH), (9) USH. **Albizia gummifera, Albizia grandibracteata, Albizia schimperiana, Acacia abyssinica, Blighia unijugata, Celtis africana, Cordia africana, Croton macrostachyus, Fagaropsis angolensis, Macaranga capensis, Millettia ferruginea, Olea welwitschii, Pouteria adolfi-friedericii, Sapium ellipticum, Trichilia dregeana and Trilepisium madagascariense. ***Banana (Musa spp.), peach (Prunus persica), nance (Byrsonima spp.), orange (Citrus X sinensis), lemon (Citrus X limon), tangerine (Citrus tangerina), pink cedar (Acrocarpus fraxinifolius), guamo (Inga edulis)

Reference	Coffee species	Shade	F	A	FR	B	AF	BI	AS	SW	BA	T
Guyot et al. (1996)	C. arabica	Inga spp. vs USH		=	=	=		−18%	=
Salazar et al. (2000)	C. arabica	Erythrina poeppigiana		+	+until 1300 m	+
Muschler (2001)	C. arabica	>80% SH of Erythrina poeppigiana		+	−	+
Lara-Estrada (2005)	C. arabica var. Caturra	0–25%, 30–55%, and 60–85% of SH in different treatments*		=	=	=	=	+16% at 60–85% SH
(Vaast et al. 2006a, b)	C. arabica	Eucalyptus deglupta or Terminalia ivorensis vs USH		+				−	−
Yadessa et al. (2008)	C. arabica	Acacia abyssinica	+	+	=	=	=					+
		Cordia africana	+	+	=	=	=					+
		Albizia schimperiana	−	−	=	=	=					−
		Albizia gummifera	−	−	=	=	=					−
Bosselmann et al. (2009)	C. arabica var. Caturra	Inga spp., Erythrina spp., Musa spp., Ficus spp., and Cordia spp., citrus trees vs USH		−	−	−	=			−
(Vaast et al. 2011)	C. canephora	Grevillea robusta vs USH			−	−
Odeny et al. (2015)	C. arabica	Cordia africana 80%, 70%, 50%, 30% SH, and USH	=	+	=	+	=				=	=
Tolessa et al. (2017)	C. arabica	Ethiopian forest Medium (50–55%), dense (65–85%) SH and USH at 1950–2100 m	=	−	=	+in USH and medium −at dense
Rocha et al. (2017)	C. canephora	25% shade of Bactris gasipaes, 60% shade of Gliricidia sepium, 70% shade of Musa spp., 70% SH of Inga spp. and USH										+at 25% SH
Worku et al. (2018)	C. arabica	Forest ** 30–40% of SH		+
Silva Neto et al. (2018)	C. arabica var. Otabã	Anadenanthera falcata, Albizia polycephala, and Cassia grandis (1–6m distant to tree SH)										+closer to tree SH
Sánchez-Hernández et al. (2018)	C. arabica	Acrocarpus fraxinifolius vs diverse tree SH										=
Souza et al. (2019)	C. canephora	25% SH of Bactris gasipaes, 60% SH of Gliricidia sepium, 70% SH of Musa spp., 70% SH of Inga spp., and USH		=	+under Bactris, Gliricidia −under Musa, Inga
Tuccio et al. (2019)	C. arabica	Musa spp. vs. USH	=	=	=	=	=				−	=
		Agroforestry system *** vs USH	+	=	+	+	+				+	+

Muschler (2001), Guyot et al. (1996), and Vaast et al. (2011) compared the effects of a simplified shade and unshaded on coffee fragrance and aroma. Muschler (2001) used Erythrina poeppigiana as a shade tree for C. arabica and found that this type of shade negatively affected the coffee fragrance. A similar pattern was found by Vaast et al. (2011) with Grevillea robusta as a shade tree for C. canephora. Guyot et al. (1996) used Inga spp. as shade for C. arabica and found no significant effects of unshaded and Inga spp. shade on coffee aroma.

In a study where C. canephora was grown under the shade of different shade tree species and in unshaded plantations, Souza et al. (2019) reported that coffee beans grown under 25% shade of Bactris gasipaes and under 60% shade of Gliricidia sepium obtained similar scores for coffee fragrance (7.3 over 10) and that these were higher scores than those from unshaded plantations. However, coffee beans grown under 70% shade of Musa spp., under 70% shade of Inga spp., and unshaded had similar scores (6.5 over 10).

Tolessa et al. (2016) compared the aroma of beans of C. arabica grown unshaded, under 40–55% of shade cover, and under 65–85% of shade cover and found no differences. Lara-Estrada (2005) also analyzed the effects of different percentages of shade and unshaded growing conditions on the aroma of C. arabica var. Caturra beverages and found no differences in coffee aroma between the different percentages of shade.

Yadessa et al. (2008) compared the effects of different shade tree species on the coffee fragrance of C. arabica beverages. These authors reported that Acacia abyssinica, Cordia africana, Albizia schimperiana, and Albizia gummifera shade did not affect coffee fragrance.

Studying the interaction effect of shade and elevation on coffee fragrance, Salazar et al. (2000) reported that the coffee fragrance of C. arabica beverages improved under Erythrina poeppigiana shade but only until 1300 m a.s.l. Above this altitude, they no longer found this positive effect. Similarly, Bosselmann et al. (2009) reported that higher altitudes from 1590 to 1730 m a.s.l. and 33–55% of shade positively affected the coffee fragrance of C. arabica var. Caturra beverages.

Regarding coffee aroma, Tolessa et al. (2017) found higher aroma scores at higher elevations (1950 to 2100 m a.s.l.) for C. arabica, but shaded and unshaded treatments did not influence the aroma at these elevations.

Overall, the shade effect on coffee aroma and fragrance appears inconsistent. Also, shade tree species and the percentage of shade cover differently affect coffee aroma and fragrance. The presence and percentage of shade and elevation interact and variously influence the coffee aroma and fragrance.

Flavor

Flavor is a combined impression of all taste sensations and retro-nasal aromas stretching from mouth to nose, including the first impressions when smelling the coffee recently grounded to its final aftertaste (SCAA 2015).

Coffee beverages from unshaded and shaded beans of C. arabica received similar scores (Odeny et al. 2015). Tolessa et al. (2016) compared the coffee flavor of beans of C. arabica grown unshaded, under 40–55% shade cover and under 65–85% shade cover, but found no effects of the different percentages and presence of shade on coffee flavor.

When comparing the effects of different shade tree species, Yadessa et al. (2008) reported that C. arabica under Acacia abyssinica and Cordia africana shade produced a coffee beverage with higher flavor scores than those under Albizia schimperiana and Albizia gummifera shade (Table 5).

Tuccio et al. (2019) compared the effect of diverse tree shade, only banana shade, and unshaded plantations on the coffee flavor of C. arabica beverages. They found that diverse tree shade improved coffee flavor compared to unshaded plantations; however, growing exclusively under the banana shade, on the one hand, and unshaded plantations, on the other, did not have significantly influence on coffee flavor.

When studying the interaction effect of the percentage of shade and elevation, the coffee flavor score of C. arabica beverages varied. For instance, at higher elevations (1950–2100 m a.s.l.), Tolessa et al. (2016) found higher coffee flavor scores from coffee beans grown unshaded or at 40–50% of shade cover. At this range of elevation and at denser shade cover (65–85%), the coffee flavor score decreased. At lower elevations (1600–1680 m a.s.l.), lower scores of coffee flavor were found irrespective of the presence or percentage of shade.

Overall, the shade effect on coffee flavor appears inconsistent; other factors might be affecting the coffee flavor in addition to shade. However, when comparing diversified shade to monospecific-dominated shade, diversified shade appears to improve coffee flavor, based on the results of one study (Tuccio et al. 2019). In addition, tree species shade differently affects coffee flavor. In this sense, the tree shade species identity appears important—at least as observed in one particular study (Yadessa et al. 2008).

Acidity

Acidity when favorable is described as “brightness” and when unfavorable as “sour” (SCAA 2015). Excessive acidity may be unpleasant and may not be appropriate for the profile of the coffee sample (SCAA 2015).

Diverse tree shade as well as monospecific tree shade were found to increase coffee acidity (Salazar et al. 2000; Muschler 2001; Vaast et al. 2006b; Yadessa et al. 2008; Odeny et al. 2015; Worku et al. 2018) (Table 5). For instance, the coffee beverage acidity of beans of C. arabica grown under the shade of Eucalyptus deglupta or Terminalia ivorensis was 8–35% more acidic than those grown unshaded (Vaast et al. 2006b). Other studies reported no significant difference in coffee acidity between coffee beans of C. arabica grown shaded versus unshaded (Guyot et al. 1996; Tuccio et al. 2019). Bosselmann et al. (2009) found that the beans of C. arabica var. Caturra grown under diverse tree shade have lower coffee acidity scores compared to beans grown unshaded.

Yadessa et al. (2008) compared the effects of different shade tree species on the beverage acidity of C. arabica. The authors report that coffee beans grown under Acacia abyssinica or Cordia africana shade produced coffee with higher scores of acidity than those grown under Albizia schimperiana or Albizia gummifera shade.

In the same study where C. canephora was grown under the shade of different shade tree species and different percentages of shade, Souza et al. (2019) reported that coffee beans grown under 25% shade of Bactris gasipaes had higher coffee acidity scores than those grown under 60% shade of Gliricidia sepium, 70% shade of Musa spp., 70% shade of Inga spp., and unshaded. Vaast et al. (2006b) also found that 45% of shade cover enhanced the coffee acidity of C. arabica. In contrast, Lara-Estrada (2005) reported that different percentages of shade did not have an effect on the coffee acidity of beans of C. arabica var. Caturra.

Regarding the combined effect of shade and elevation on coffee acidity, Worku et al. (2018) found that the acidity of beans of C. arabica grown under shade increased 0.22 points for each 100 m (over 10 points), whereas the coffee acidity of beans grown in unshaded plantations was not significantly different across the elevation (0.01 point every 100 m). Tolessa et al. (2017) found higher scores of coffee acidity from beans of C. arabica at higher elevations (1950–2100 m a.s.l.) but from coffee beans grown unshaded or at 40–50% of shade cover. At denser shade cover (65–85%), coffee acidity decreased at these elevations. At lower elevations (1600–1680 m a.s.l.), lower scores of coffee acidity were found irrespective of the presence or percentage of shade. However, Bosselmann et al. (2009) reported that coffee acidity increased at higher altitudes (1590–1730 m a.s.l.) but only in beans grown in unshaded plantations of C. arabica var. Caturra.

Overall, scores of coffee acidity vary in both shaded and unshaded coffee plantations; it appears that other factors, including altitude, play a role in determining coffee acidity.

Body

Body testing is based on the feeling of the coffee beverage in the mouth, especially perceived between the tongue and the palate (SCAA 2015). Coffee beverages may have heavy or light body and both may receive high scores in terms of quality; however, coffee tasters know the standards each type of coffee should have. For instance, Sumatra coffees are expected to be high in body and Mexican coffees are expected to be low in body, and both can receive high scores in body equally (SCAA 2015).

The increase in the percentage of shade cover decreased the coffee beverage body of C. arabica; Tolessa et al. (2017) found that coffee body was higher at 50–55% of shade cover and unshaded, and it decreased at 65–85% of shade cover.

Coffee body was differently affected by the presence of shade and diversity (Table 5). Guyot et al. (1996) and Yadessa et al. (2008) reported no differences in coffee body between beans of C. arabica grown unshaded and shaded. However, Salazar et al. (2000) and Muschler (2001) found that beans of C. arabica grown under Erythrina poeppigiana shade received higher coffee body scores compared to beans grown unshaded. Similar results were obtained by Odeny et al. (2015) but for beans of C. arabica grown under Cordia africana shade.

When comparing the effects of shade of different tree species on coffee body (Table 5), Yadessa et al. (2008) reported that no differences could be detected in coffee body from the beans of C. arabica grown under the shade of Cordia africana, Acacia abyssinica, Albizia schimperiana, and Albizia gummifera.

Tuccio et al. (2019) demonstrated that diverse tree shade (seven tree species and banana) positively affected the body of the coffee beverage compared to a single species (banana) as shade, with the latter scoring the same score as unshaded plantations. On the other hand, Bosselmann et al. (2009) reported a lower coffee body from beans of C. arabica var. Caturra grown under diverse tree shade, composed of Inga spp., Erythrina spp., Musa spp., Ficus spp., and Cordia spp., compared to unshaded coffee beans.

Regarding the effect of shade percentage on coffee body, Lara-Estrada (2005) reported that different percentages of shade did not have an effect on the coffee body of C. arabica var. Caturra.

When the interaction effect of elevation and shade on the coffee body was analyzed, Bosselmann et al. (2009) found at higher elevations (1950–2100 m a.s.l.) and denser shade (65–85%) that the coffee body of C. arabica var. Caturra was negatively affected. At lower elevations (1355–1523 m a.s.l.), coffee beverages from shaded and unshaded plantations had similar coffee body scores. Tolessa et al. (2017) found higher coffee body scores from beans harvested at higher elevations (1950–2100 m a.s.l.), but lower coffee body scores from beans of C. arabica grown unshaded or at 40–50% of shade cover. With denser shade cover (65–85%), the coffee body scores decreased at these elevations. At lower elevations (1600–1680 m a.s.l.), lower coffee body scores were found irrespective of the presence or percentage of shade.

Overall, coffee body varied among both shaded and unshaded coffee plantations. Additional factors may play a role in determining it.

Aftertaste, balance, and sweetness

Coffee aftertaste is defined as the length of positive flavor emanating from the back of the palate and remaining after the coffee is swallowed. If the aftertaste is short and unpleasant, a lower score is given (SCAA 2015). Coffee balance is how the flavor, aftertaste, acidity, and body of a coffee sample complement or contrast with each other (SCAA 2015). Coffee sweetness refers to a pleasant fullness of flavor, as well as, any perceived sweetness that results from the presence of certain carbohydrates (SCAA 2015).

Odeny et al. (2015) found that the coffee beverage aftertaste of C. arabica beans grown under shade and unshaded conditions was not significantly different. Bosselmann et al. (2009) found aftertaste higher scores for coffee from beans of C. arabica var. Caturra grown in unshaded plantations in comparison to those grown under shade.

Regarding the effect of the percentage of cover on coffee aftertaste, Lara-Estrada (2005) reported that the different percentages of shade did not have an effect on the coffee aftertaste of C. arabica var. Caturra. In one study of the effect of different tree shade species, Yadessa et al. (2008) reported that beans of C. arabica grown under Acacia abyssinica, Cordia africana, Albizia schimperiana, and Albizia gummifera shade did not have an effect on the coffee aftertaste (Table 5).

When comparing the effect of diverse tree shades and banana shade to unshaded plantations on the coffee aftertaste, Tuccio et al. (2019) found that diverse tree shades improved the aftertaste of C. arabica compared to unshaded coffee. However, coffee beans aftertaste values were not significantly different from banana shade and unshaded coffee.

The coffee balance of beans of C. arabica grown under shade and unshaded plantations was not significantly different (Odeny et al. 2015). Tuccio et al. (2019) compared the effect of diverse tree shade, Musa spp. shade, and unshaded plantations on the coffee balance of C. arabica var. Parainema. They found that diversified tree shade improved the coffee balance compared to unshaded plantations; however, Musa spp. shade decreased coffee balance compared to unshaded plantations.

The sweetness of C. arabica was decreased by shade compared to unshaded plantations at higher elevations (1950–2100 m a.s.l.). However, at lower elevations (1355–1523 m a.s.l.), coffee sweetness had similar scores from unshaded and shaded coffee plantations (Bosselmann et al. 2009).

Bitterness and astringency

Bitter and astringent tastes are universally unpleasant. The caffeine and chlorogenic acid content, the quality of green beans, the roasting process, and the brewing process can all impact the final bitterness in the cup (Yu 2018). Tannins and chlorogenic acids cause astringency (Rao 2020). Astringency in coffee beverages creates a dryness sensation in the mouth and mutes flavors (Gagné 2019).

Coffee beverages from unshaded C. arabica was 8–16% more bitter and 19–32% more astringent than that grown under the shade of Eucalyptus deglupta or Terminalia ivorensis (Vaast et al. 2006b). Guyot et al. (1996) found that coffee bitterness decreased by 18% in beans of C. arabica grown under shade. However, Lara-Estrada (2005) reported that coffee bitterness increased with increasing shade. It increased by 16% in beans of C. arabica var. Caturra at high percentages of shade (60–85%) compared to beans grown in unshaded plantations. In contrast, Bosselmann et al. (2009) found that coffee bitterness of C. arabica var. Caturra was not dependent on shade but rather was affected by the degree of roasting.

The astringency scores of coffee beverages of shaded C. arabica and unshaded were not different (Guyot et al. 1996). However, Vaast et al. (2006b) found that astringency increased in beans of unshaded C. arabica compared to shaded coffee.

Astringency and bitterness were found to be mainly related to the degree of roasting (Decazy et al. 2003; Farah and Donangelo 2006).

Final total organoleptic quality score

Total organoleptic quality score is calculated by summing all individual attributes, and subtracting the defects (SCAA 2015). Defects are negative scores that denote unpleasant flavor sensations (SCAA 2015). Scores above 80 points are considered of high quality and specialty coffees.

In a study of the sensorial quality of coffee grown unshaded or under the shade of different species, Rocha et al. (2017) reported that beans of C. canephora grown under 25% shade of Bactris gasipaes had the highest score of total organoleptic quality and obtained on average 80 points over 100. Coffee beans grown under 60% shade of Gliricidia sepium obtained on average 77 points over 100, coffee beans grown under 70% shade of Musa spp. obtained on average 65.5 points over 100, coffee beans grown under 70% shade of Inga spp. obtained on average 68 points over 100, and beans from coffee grown unshaded obtained on average 72 points over 100 (Table 5). These results demonstrate that a low percentage of shade, to whatever extent present, positively affects the total organoleptic quality score.

Regarding the effects of the spatial distribution of shade trees on coffee fruits, Silva Neto et al. (2018) reported that the best cup quality was obtained with beans of C. arabica var. Obatã coming from coffee trees closer to shade trees.

Sánchez-Hernández et al. (2018) compared the effect of diverse Mexican traditional shade (Inga spp. as a dominant species, and other species valued for their timber and fruits) and shade of only Acrocarpus fraxinifolius. They reported that both types of shade produced high quality of C. arabica that did not significantly differ from each other. Coffee beans grown under diverse traditional shade obtained on average 83 points, and coffee beans grown under Acrocarpus fraxinifolius shade obtained on average 84 points.

In this sense, the coffee beverage quality was affected by the percentage of shade and by the shade species according to Rocha et al. (2017), but not by different tree shade species or diversity, with high coffee beverages quality being maintained in either case (Sánchez-Hernández et al. 2018).

When comparing the organoleptic quality of coffee under diversified shade and single species shade to unshaded coffee, the C. arabica grown under diversified shade scored higher regarding total score than coffee grown unshaded (Tuccio et al. 2019). C. arabica growing under a single species shade scored similarly to unshaded coffee (Odeny et al. 2015; Tuccio et al. 2019).

On the other hand, the beverage of Coffea canephora grown under shade obtained 10 fewer points on the final total organoleptic quality score than the same species grown unshaded (Alves et al. 2018).

All of the studies in our sample reported that shade improved the final total organoleptic quality score of C. arabica, with the exception of C. canephora.

Overall, the effects of biodiversity on organoleptic quality remain inconclusive. However, biodiversity and elevation clearly interact and affect organoleptic quality.

Effects of ecosystem functions on coffee quality

Pollination

Coffea species have different dependencies on pollinators (Prado et al. 2019). Coffea arabica is self-compatible and can benefit from outcrossing with insect pollination by developing a higher fruit set, heavier fruits, and fewer abnormal beans (Roubik 2002; Ricketts 2004). Coffea canephora is self-incompatible and relies on wind and insect pollinators for cross-pollination and effective fruit production (Willmer and Stone 1989; Prado et al. 2019). Despite their differences on pollinator dependencies, both benefit from pollination.

For instance, Prado et al. (2018) demonstrated that pollination promoted 1.5 times higher fruit set of C. arabica in shaded coffee plantations than in unshaded plantations. In addition, they found higher coffee bean weight in shaded compared to unshaded plantations. Moreover, the authors found that coffee beans from shaded plantations had a higher average score of coffee beverage quality than unshaded plantations.

The fruit set of C. arabica increased 10–30% in coffee plantations with a high richness of bee species (Klein et al. 2003; Saturni et al. 2016; Hipólito et al. 2018). Moreaux et al. (2022) developed a meta-analysis with eleven case studies and evaluated the biotic value of pollinators on the fruit set of C. arabica by comparing open and exclosure pollination treatments. They reported that the fruit set increased by 18% on average in the open pollination treatment compared to the pollination exclosure treatment.

In a full-factorial pollinator and vertebrate exclosure experiment in coffee production systems at Mount Kilimanjaro, Classen et al. (2014) found that the fruit weight of C. arabica decreased on average by 7.4% when pollinating insects were excluded, but not when excluding vertebrates. Roubik (2002) also reported a 7% average reduction in the weight of C. arabica berries in pollinator exclosure compared to open pollination treatment. Philpott et al. (2006) reported similar results but a higher reduction in the fruit weight of C. arabica. They compared fruit weight in full-exclosure, ant-exclosure, and no-exclosure and identified a decrease in fruit weight of 41% on average in full-exclosure and ant-exclosure compared to no-exclosure.

Philpott et al. (2006) proposed that the fruit weight of C. arabica (self-compatible coffee) increased due to increased pollen loads and higher genetic pollen diversity as a consequence of cross-pollination; however, detailed information about the specific mechanisms are lacking.

When comparing the composition of pollinators along a land use gradient at Mount Kilimanjaro, Classen et al. (2014) found that unshaded plantations of C. arabica attracted very few pollinators besides Apis mellifera, while shaded coffee plantations attracted and provided habitat for various wild pollinators (native bees, syrphid flies, and butterflies). Thus, management forms of cultivating coffee under diversified shade trees that support wild pollinators may produce better coffee quality and be less sensitive than those relying on a single pollinator species, such as honeybees.

Pollination and presence of shade are interrelated. Nesper et al. (2017a) investigated the effect of the shade tree on C. canephora and showed that pollination is diminished when shade tree diversity is low and dominated by one tree species. For instance, they reported that the proportion of pea beans was higher in agroforests dominated by Grevillea robusta as a shade tree compared to coffee systems with diversified shade. A major cause of pea beans is inadequate pollination (Wintgens 2004a). According to Boreux et al. (2013b), the dominance of one shade tree species affects the microclimate by increasing temperatures and humidity fluctuations, which may lead to erratic pollinator visits (Nesper et al. 2017a). For instance, the shade of Grevillea robusta is relatively open and is not effective in buffering weather fluctuations (Boreux et al. 2013b). More diversified shade may better buffer weather fluctuations because each tree species has a different type of shade and phenology.

On the other hand, Boreux et al. (2013a) reported that increasing density of G. robusta, which simplifies shade tree diversity, increased the abundance of pollinators. In addition, Boreux et al. (2013a) studied the effects of the distance and size of the nearest forest fragment on pollinator abundance and final fruit set in irrigated and rain-fed C. canephora agroforestry systems in Kodagu, India. The coffee was shaded by either native trees or a mixture of native and exotic trees (the latter consisting mainly of Grevillea robusta). In rain-fed coffee agroforests, the researchers found that shade tree species richness negatively affected pollinator abundance. An increase in the abundance of G. robusta increased the abundance of pollinators. In irrigated agroforests, they found that none of the variables affected the abundance of pollinators except for relative humidity. Distance and size to the nearest fragment forest did not affect final fruit set in either rain-fed or irrigated agroforests. The coffee fruit set increased with an increase in pollinators and flowers in rain-fed agroforests even though the abundance of pollinators was low in rain-fed agroforests compared to irrigated ones. On the other hand, shade positively affects pollination, especially when there is diversified tree shade. Even though pollination dependencies of Coffea species are different, both benefit from pollination, and pollination improves coffee quality.

Pest, disease, and weed control

The multistrata—i.e., the diversity of trees present in agroforestry systems and associated biodiversity—support biological regulations to reduce diseases and pests (Soto-Pinto et al. 2002; Ratnadass et al. 2012; Durand-Bessart et al. 2020). Regulation effects that reduced weeds, diseases, and pests include the dilution of host density, reduction of soil diseases favoring beneficial micro-fauna, allelopathic effects, reservoir of natural enemies, and creation of unfavorable microclimates for diseases (Ratnadass et al. 2012). Coffee plants themselves naturally attract a large range of natural enemies to pests and diseases, such as birds, lizards, ants, mites, wasps, and microorganisms like entomopathogenic fungi (Venzon 2021; Perfecto et al. 2021). However, unshaded coffee plantations do not provide a suitable environment to maintain them. Based on a meta-analysis of the effects of agroforestry on pest, disease, and weed control, Pumariño et al. (2015) found a reduction of pest abundance and plant damage in perennial crops, such as coffee.

Specific traits of shade trees such as the canopy openness and leaf area have been found to have differential effects on pests and diseases that affect coffee plants (Avelino et al. 2020; Gagliardi et al. 2021) because these traits affect microclimate conditions (Merle et al. 2022). For instance, taller canopies, double strata, and more vertical tree leaf angles were found to have higher levels of coffee leaf rust (Hemileia vastatrix, CLR) incidence on C. arabica (Gagliardi et al. 2021). However, canopies with a single stratum had lower CLR incidence. Additionally, canopies with greater openness and larger leaves increased CLR incidence. The researchers found that shade tree leaf functional and canopy traits reduced wind speed and thus the spore dispersal and deposition of CLR.

Other studies report that lower canopy openness increased CLR incidence on C. arabica due to increased rain-induced dispersion (Boudrot et al. 2016), diminished wash-off of spores from leaves (Avelino et al. 2020), and improved microclimatic conditions for specific pests and diseases (López-Bravo et al. 2012; Allinne et al. 2016). In Kenya, a study reported that shade trees improved coffee health without reducing coffee yield, with increasing tree shade cover decreasing CLR intensity and coffee berry disease caused by Colletotrichum kahawae (Barkaoui et al. 2019).

A common shade tree species is Inga spp., generally chosen by farmers due to its potential benefits for improving soil quality. Rezende et al. (2014) found that this species also enhances natural pest control for the coffee plant in proximity to its canopy due to its extrafloral nectaries. Shade tree diversity in coffee agroforestry systems may increase natural enemy populations because they can provide alternative food to natural enemies.

In contrast, the shade tree Chloroleucon eurycyclum was reported to promote CLR by reducing spore wash-off by rain (Avelino et al. 2020). They found that the spores of CLR were produced and preserved 2.2 times more often in shaded C. arabica than in unshaded coffee plantations and that the wash-off of these spores was 1.62 times lower in shaded coffee than in unshaded coffee. According to Avelino et al. (2020), raindrop kinetic energy is reduced by the canopy of shade trees, which then reduces the capacity of coffee leaves to intercept raindrops.

There is growing evidence that the loss of biodiversity in agroforestry systems negatively impacts multitrophic interactions, resulting in reduced pest control services, which in turn, negatively affects crop quality (Vandermeer et al. 2010; Liere et al. 2012; Gras et al. 2016; Nesper et al. 2017a, b). For instance, decreasing shade tree diversity increased the infestation rates of coffee berry borer beetle (Hypothenemus hampei, CBB) in C. canephora (Nesper et al. 2017a), which might be attributable to the reduced diversity of predators, such as ants and birds (Infante et al. 2009; Vidya 2011; Gonthier et al. 2013). Karp et al. (2013) found that shade enhanced bird abundance, which reduced infestation rates of CBB by 50% in C. arabica, in turn enhancing coffee quality.

In contrast, Bosselmann et al. (2009) reported higher occurrence of CBB in moderately shaded coffee plantations (29.2±19.1% of shade) compared to unshaded plantations from 1270 to 1630 m a.s.l. The researchers studied C. arabica var. Caturra plantations with moderate shade and suggest that the higher occurrence of CBB might be due to the often-lower quantity and diversity of natural pest controllers in moderate shade coffee compared to dense shade (Beer et al. 1997). Staver et al. (2001) and Soto-Pinto et al. (2002) determined the relationships between different ecological characteristics of shade and the incidence of CBB, CLR, and weed cover in shaded C. arabica plantations. They found that tree shade diversity and structural complexity in shaded coffee plantations maintained a healthy system in terms of providing ecosystem functions. The incidence of CBB and CLR were low, 1.5% and 10.1%, respectively. Weed cover was 34%. The number of strata of shade trees was negatively correlated with CLR.

In Eastern Uganda, Jonsson et al. (2015) studied how shade cover levels (dense >50 trees per 0.4 ha, moderate 21–50 trees per 0.4 ha, and low 0–20 trees per 0.4 ha) and elevation (high 1717–1840 m a.s.l. and low 1511–1605 m a.s.l.) influenced the abundance of CBB and the white stem borer (Monochamus leuconotus, WSB) in C. arabica plantations. They found that the effect of shade cover differed between the two pest species. The CBB was more common in unshaded plantations, whereas the WSB was more common in shaded plantations. Further, the effect of shade cover on the WSB depended on elevation, with the effect of shade cover most pronounced in plantations at low elevations.

Bagny Beilhe et al. (2020) assessed the potential effects of the percentage of shade cover, tree area surface, C. arabica density, and farm management (i.e., conventional, integrated, and organic) on ant predatory groups; the abundance and related damage of CBB; and their interactions. The researchers found that crop management practices (i.e., shade management, plant diversity, preventive phytosanitary measures) and abundance of predatory ants helped to maintain low levels of CBB infestation and damage. They reported that the best ways to regulate CBB are to conserve a high level of tree diversity within coffee plantations to maintain good predator diversity, to regulate shade intensity, and to practice sanitary harvest at the end of the harvesting season.

Several mechanisms for negative effects of shade trees on CBB have been suggested. First, natural enemies of CBB, such as birds and parasitoid wasps, may be more effective on shaded coffee plantations (Karp et al. 2013). Second, shade may reduce CBB development rates (Infante et al. 2009). Third, shade may modify the biochemical composition and emission of chemical compounds from coffee berries, which may make it more difficult for CBB females to locate coffee berries for oviposition (Jaramillo et al. 2013).

CBB impact on cup quality depends on the number of holes per bean. For instance, one hole per bean is associated with reductions in aroma, flavor, and acidity. The presence of more than one hole increases bitterness and exhibits off-flavors (Ribeyre and Avelino 2012).

Bukomeko et al. (2018) found that bigger shade trees and a high density of sap-exuding trees are effective in reducing infestation of Xylosandrus compactus on a C. canephora plantation, a beetle that is commonly called the black coffee twig borer. They found that Albizia chinensis increased and Carica papaya decreased the proportion of X. compactus-infested coffee plants.

Regarding foliar diseases, Durand-Bessart et al. (2020) report that the presence of shade in C. arabica plantations increased the incidence of foliar diseases; however, shade trees also increased soil fertility which in turn decreased disease prevalence. These authors recommend considering the role of shade trees in a holistic approach, making sure to assess the trade-offs between shade management, soil quality, disease regulation, and coffee quality.

Beer et al. (1997) reported that Cercospora coffeicola, a defoliating fungus, had a greater incidence in unshaded C. arabica plantations compared to shaded ones and concluded that shade trees favored the natural enemies of C. coffeicola.

Coffee leaf miner (Leucoptera coffeella) damaging activity can be influenced by shade trees: Righi et al. (2013) experimented with Hevea spp. as a shade tree of C. arabica var. Obatã. These authors found that during cold seasons, shade trees act as shelters for coffee leaf miners. Coffee plants closer to Hevea spp. displayed greater damage from coffee leaf miners.

Within a full-factorial experiment to determine the effects of agricultural management (unshaded and shaded) and bird pest reduction services (absent and present) on the proportion of CBB infestation in C. arabica plantations, Johnson et al. (2010) found that CBB infestation rates ranged between 11 and 21% when birds were present in both coffee systems. This range was similar for shaded and unshaded coffee. However, when birds were excluded from the systems, infestation rates increased by 40% in unshaded plantations and 21% in shaded plantations. The lower infestation rate in shaded plantations can be explained by complex trophic interactions among arthropods and birds reported in coffee systems (Philpott et al. 2009). Insect predators such as spiders and ants, which are more abundant and diverse in shaded coffee systems, serve to suppress insect pests (Borkhataria et al. 2006).

Bird-related ecosystem functions on coffee farms can increase coffee quality by reducing damaging pests. Farmers can adopt farming practices that are attractive to insectivorous birds, such as agroforestry systems, can maintain non-coffee habitats like forest patches within and adjacent to farms (Greenberg et al. 1997; Perfecto et al. 2003; Karp et al. 2013; Medeiros et al. 2019), and can preserve arboreal epiphytes (Cruz-Angón and Greenberg 2005).

Cerda et al. (2020) compared C. arabica var. Caturra losses due to pests and diseases in a wide range of coffee management conditions in Costa Rica, including unshaded, low diversified shade, and high-diversified shade. The researchers found lower losses in shaded coffee plantations and less use of fungicide, reducing costs for farmers.

Regarding weeds, a shaded environment is less favorable to weed development (Eccardi and Sandalj 2002; Aguilar et al. 2003; Ricci et al. 2008; Staver et al. 2020), irrespective of the tree species (Staver et al. 2020). Muschler (1998) reported a decline from 3.6 Mg weeds ha⁻¹ in unshaded C. arabica to less than 0.1 Mg ha⁻¹ under 50% of shade cover or more. Additionally, Staver et al. (2020) found that weed suppression from shade trees did not have an effect on coffee quality, but it did have an effect on labor and herbicide use.

The effect of shade on pests and diseases depends on several factors, including the identity of the pest or disease, as well as the microclimate, elevation, microclimate preferences of the pest or disease, and microclimate preferences of natural enemies. Specific site management of shade may reduce the effects of pests and diseases on coffee quality.

Soil fertility regulation

One major contribution of shade trees is the production of easily recyclable nutrient-rich biomass. Dossa et al. (2008) studied below and aboveground biomass in C. canephora var. Robusta plantations under Albizia adianthifolia shade and unshaded plantations. They found that Albizia trees contributed 87% of total aboveground biomass and 55% of total root biomass on shaded coffee plantations. However, biomass of coffee bushes was higher in unshaded coffee plantations than in shaded plantations. The effect of the higher biomass in shaded coffee plantations was that the total carbon stock was 3.5 times higher than in unshaded coffee plantations. In addition, nutrients such as N, K, Ca, P, and Mg were found in higher concentrations on shaded coffee plantations. By contrast, Souza et al. (2012) found no significant differences in soil characteristics between unshaded C. arabica plantations and shaded ones. In addition, they compared soil characteristics of both coffee systems to reference forests and found a positive trend in the soil quality of shaded coffee plantations, more closely resembling the soil conditions of reference forests. This suggests higher soil quality in shaded coffee plantations than in unshaded coffee plantations.

Shade trees not only directly affect coffee quality by improving the microclimate of the area but also indirectly by influencing soil mineralization and soil nutrient availability. Babbar and Zak (1994) and Beer et al. (1997) found that a reduced canopy cover and understory vegetation density indirectly impact coffee trees through altered soil characteristics. Babbar and Zak (1994) found that N availability was greater in unshaded C. arabica var. Caturra plantations compared to shaded ones; however, N losses by leaching were higher in unshaded plantations. Hence, N is more easily recycled in shaded coffee plantations than in unshaded ones.

Geeraert et al. (2019) studied the effects of the following soil characteristics on organoleptic beverage quality of C. arabica: soil acidity, electrical conductivity, available B, available P, available K, cation exchange capacity, exchangeable Ca, exchangeable Mg, exchangeable Na, exchangeable K, organic C percentage, total N percentage, and micro-nutrients such as Mn, Fe, Cu, and Zn. They found that the variation in soil characteristics negatively affected beverage acidity and the flavor of the final cup. However, as they reduced their dataset by using principal component analysis on soil characteristics, it was difficult to determine which soil characteristics had a specific effect.

In addition, Márquez et al. (2020) studied the correlation between beverage quality of C. arabica and soil characteristics. They found a positive correlation between Al concentration and balance and between final total organoleptic quality score and cation exchange capacity. A similar study by Suárez et al. (2015) identified the highest final total organoleptic quality score in C. arabica grown in soil with high acidity, high Al content, and low levels of Ca and Mg. The lowest cup quality was from coffee grown in moderately acidic soil, with high levels of Fe, Cu, Zn, and S, and low levels of Ca and Mg.

Tassew et al. (2021) studied the relationships between soil quality variables and forest coffee quality in Ethiopia (C. arabica). The soil quality variables they evaluated were total N, available P, K, Ca, Mg, S, Fe, Mn, Cu, Zn, B, Mo, Na, Co, Si, organic C, and C/N ratio. Most of these variables had positive relationships with beans greater than 5 mm, specifically soil S and Zn content. Soil content of K, S, B, Co, organic C, and C/N ratio were negatively correlated with coffee beverages acidity. Significant and negative correlations were obtained between most soil quality variable and coffee beverages body. Similarly, coffee beverages flavor and final total organoleptic quality score were negatively and significantly correlated with C/N ratio, B, Co, and organic C contents. In terms of soil texture, a positive correlation was found between soil clay percentage and beverage acidity, flavor, and total scores. In summary, Tassew et al. (2021) found that the best forest quality coffee was produced in soils with high Mo content, a high percentage of clay, and reduced content of most soil nutrients, pH, and silt percentages. Yadessa et al. (2009) reported for wild Ethiopian coffees (C. arabica) positive correlations between available P, K, Mg, Mn, and Zn with high final total organoleptic quality scores for coffee beverages.

Several studies have compared the effects of diverse tree shade versus monospecific tree shade on nutrient cycling (Hairiah et al. 2006; Hertel et al. 2009; Gama-Rodrigues et al. 2010; Zaia et al. 2012; Nesper et al. 2017b). However, few reported whether these affected coffee quality (Nesper et al. 2017a). Often, leguminous trees are dominant in monospecific agroforests, but, in some cases, timber species are incorporated as dominant species in coffee agroforestry systems in order to diversify farmer incomes (Garcia et al. 2010).

Nesper et al. (2017b) demonstrated that the simplification of shade tree diversity in C. canephora plantations with the monospecific shade of Grevillea robusta reduced the inputs and cycling of several micro- and macronutrients. Moreover, this simplification of shade tree diversity negatively affected coffee quality in terms of bean size (Nesper et al. 2017b).

On the other hand, a comparison between diverse shade C. arabica plantations and Inga latibracteata shade coffee plantations showed no differences in soil nutrient availability or organic matter and no effects on coffee bean size (Romero-Alvarado et al. 2002).

In the long term, diverse shade coffee plantations may more effectively sustain more effectively micro- and macronutrients in the soil and may reduce risks of nutrient loss (Nesper et al. 2017b). Simplification of agroforestry systems based on a single shade tree species dominating the system may also impact the soil structure and nutrient retention (Hairiah et al. 2006).

In a study evaluating the direct and indirect effects of agricultural management practices on plant diversity, soil quality, and crop quality in C. arabica plantations, Teixeira et al. (2021) found that intensive management, such as use of external inputs and weeding, in large-scale and unshaded coffee plantations did not result in increased soil quality or coffee quality compared to diverse shaded coffee plantations. By contrast, agroecological practices on shaded coffee plantations increased plant diversity, positively affecting soil microbial biomass carbon and soil quality (Teixeira et al. 2021). Regarding coffee quality in terms of physical quality, the researchers did not find differences in coffee quality between shaded and unshaded coffee plantations. However, agroecological practices were found to be efficient in maintaining satisfactory physical coffee quality and soil fertility without the need for external inputs and weeding (Teixeira et al. 2021).

Shade trees have deeper roots than coffee trees, allowing them to absorb nutrients that have leached below the rooting zone of coffee. These nutrients, in turn, are later recycled via leaf litter and made available for coffee trees (Buresh et al. 2004). Hence, nutrient-use efficiency in shaded coffee plantations is higher than in unshaded ones.

Overall, the soil-related beneficial effects of ecosystem functions of shaded coffee plantations found on coffee quality can be attributed to various factors, especially enhancement of soil quality via nutrient-recycling litter. The soil characteristics that affect coffee quality are diverse and site specific; however, higher soil quality generally improves coffee quality.

Water regulation

Water is key to maintaining healthy ecosystem functioning. Its relative presence or absence determines supporting and regulating functions (Coates et al. 2013). Water underpins many ecosystem benefits, such as food production. It is thus a key factor to be managed in agriculture, whether in rainfed or in irrigated farming systems. Due to their locations, most coffee plantations mainly rely on rainfall, with management practices aiming to maximize soil infiltration and water-holding capacity.

It is well known that deforestation can decrease rainfall due to loss of cloud-forming evapotranspiration from the forest (Xu et al. 2022). Thus, an agroforestry system may be a good agricultural practice to secure water retention in the agroecosystem and mitigate against droughts. Drought increases water stress on coffee plants and prevents ovules from reaching full size, resulting in smaller coffee beans (Cannel 1974). It also triggers flowering, while excessive rain and cool conditions during the quiescent growth phase can repress flowering, compromising coffee quality and yields (DaMatta and Ramalho 2006).

Coffee is native to Ethiopian forests, which are characterized by abundantly distributed rainfall and atmospheric humidity frequently approaching saturation (Pinheiro et al. 2005). Coffee might have evolved as a “water spender” species (DaMatta and Rena 2001). Therefore, coffee is a highly environmentally dependent crop, and changes in temperature and drought periods can substantially affect its quality. However, it is also well known that coffee plants have a wide plasticity response to avoid and endure heat and drought stresses (DaMatta and Ramalho 2006; Matos et al. 2009; Worku and Astatkie 2010).

The amount and timing of rainfall can impact coffee bean quality. Too little rainfall during the growing season stresses plants, causing branch death and defoliation, reducing resources for fruiting, and leading to small and damaged coffee beans (DaMatta et al. 2018; Kath et al. 2020). Too much rainfall can dislodge flowers and fruits. If heavy rain occurs during harvest, increased moisture can enable conditions for fungus growth (Poltronieri and Rossi 2016), diseases, and fruit fermentation, increasing coffee bean defects (Taniwaki et al. 2014).

Specific and seasonally adapted agricultural management practices, such as use of shade trees, are critical to reduce the drivers of coffee bean defects and small bean sizes (Vaast et al. 2006b). Shade not only reduces heat stress, but can also attenuate the effects of drought by protecting the photosynthetic apparatus against excess energy use as well as enabling higher photosynthetic rates in water-stressed coffee plants (DaMatta 2004). Adequate shade cover can improve the water status of both soil and coffee plants after a prolonged drought (DaMatta and Ramalho 2006).

When comparing the efficiency of water use of C. arabica plants during the warmest hour (1 p.m.) on unshaded plantations and plantations under different shade tree species in Brazil, Christo et al. (2021) found that coffee grown under shade was more efficient in water use compared to those grown unshaded. They intercropped coffee with banana (Musa spp. var. Nanicon), cassava (Manihot esculenta), and palm (Euterpe edulis) and found that the most favorable shade, in terms of efficiency of water use, came from the banana.

Competition for water is higher in shaded coffee systems, as coffee plants and shade trees compete for water uptake, which can be then detrimental to coffee quality (Govindarajan et al. 1996; McIntyre et al. 1997; Rao et al. 1997). Importantly, however, this depends on the root system and distribution of each shade tree species. For instance, in Nicaragua Padovan et al. (2015) compared the root distribution of C. arabica var. Pacas plants and two shade trees (Tabebuia rosea and Simarouba glauca). They found that coffee roots reached 150–170 cm depths in shaded and unshaded plantations, but that 56.9% and 50.6% of the coffee fine root system was concentrated in the upper 30 cm on shaded and on unshaded coffee plantations, respectively. Shade tree roots proliferated below 100 cm in soil depth (Padovan et al. 2015). This points to clear root niche differentiation, including hydrological niche segregation between coffee and shade trees.

Regarding evapotranspiration, C. arabica alone transpired 20–50% more in unshaded plantations than in shaded ones; however, considered as a whole, shaded coffee plantations in total (coffee + shade trees) transpired 10–30% more than unshaded coffee plantations (Cannavo et al. 2011) due to the contribution of shade trees. van Kanten and Vaast (2006) and Lin (2010) reported the same pattern; however, Lin (2010) additionally measured the effects of high (60–80%) and low (10–30%) shade cover and found that a high tree shade cover reduced coffee transpiration by 32% compared to low shade cover.

Soil moisture was similar in shaded and unshaded C. arabica plantations between 0 and 120 cm (Cannavo et al. 2011), but below 120 cm, it decreased more in shaded than in unshaded coffee plantations. Below 150 cm in depth, in both coffee systems, soil moisture increased. In contrast, de Carvalho et al. (2021) and Christo et al. (2021) reported that soil moisture of shaded C. arabica plantations was lower at all depths above 100 cm than unshaded ones, but water loss was almost 100% higher in unshaded plantations (338 L ha⁻¹) compared to shaded ones (150 L ha⁻¹).

According to Sarmiento-Soler et al. (2019), soil water uptake occurs mostly in the top 40–70 cm of the soil profile. This is echoing van Kanten and Vaast (2006) and Padovan et al. (2015), who observed that coffee fine root density was greater than that of shade trees up to the depths of 20–30 cm. On coffee plantations shaded by Lonchocarpus guatemalensis, Inga vera, and Trema micrantha in Veracruz-Mexico, Muñoz-Villers et al. (2020) found that shade trees relied on water sources from >15 to 120 cm in depth, while C. arabica var. Typica relied on water sources above 15 cm in soil depth, with both patterns being registered during normal to more drier seasons. Hence, coffee trees and shade trees must not necessarily compete and can instead use water in a complementary manner when there is no water stress in the agroecosystem.

Water runoff was 4% higher in unshaded C. arabica plantations than in shaded coffee with Inga densiflora (Cannavo et al. 2011). This was mainly due to lower interception and more water reaching soil surface in unshaded coffee plantations.

A study in Uganda assessed the effect of three C. arabica plantations (unshaded, shaded with Musa spp., and shaded Cordia africana) on soil moisture, water use of coffee, Musa spp., and C. africana and water competition or complementary use between coffee and shade trees (Sarmiento-Soler et al. 2019). The researchers found that soil water content in shaded plantations was reduced by 59% when shaded with C. africana and by 6% when shaded with Musa spp., in comparison with unshaded plantations. Daily water use of coffee trees was 1.2 ± 0.6 l day⁻¹; hence, it did not differ between the three types of plantations. Water use of Musa spp. was lower than water use of C. africana, namely 3.1 ± 1.8 l day⁻¹ and 42 ± 20 l day⁻¹, respectively. The researchers did not find competition for water use between coffee, Musa spp., and C. africana, since coffee water use was similar across the plantations.

Shade trees in coffee plantations thus improve water use efficiency. These prevent water losses and do not compete with coffee trees for water, due to both of them having different hydrological root niches.

Interacting effects of ecosystem functions on coffee quality

Typically, ecosystem functions (EF) are most of the time evaluated independently. However, in reality, they frequently interact (Dainese et al. 2019). EF can have positive interactions, understood as synergies, in which the increasing provision of one EF can increase the provision of another EF. Negative interactions are trade-offs, in which the increased provision of one EF reduces the provision of another EF. Finally, EF may have no interactions that are additive or complementary, such that their effects are, indeed, independent of each other EF (Lundin et al. 2013; Garibaldi et al. 2018).

Martínez-Salinas et al. (2022) assessed the interacting effects of pollination and pest control on physical coffee quality of C. arabica in a full-factorial design, in which they had open and closed treatments for birds and bees on coffee branches. They found synergetic interactions between these EF; the combined effect of birds and bees increased the fruit set by 24%, fruit weight by 6.6%, fruit weight uniformity, and decreased coffee berry borer infestation. Classen et al. (2014) found that pollination and pest control have complementary effects on coffee bean quality. They reported that the fruit weight of C. arabica decreased on average by 7.4% when pollinators were excluded, and the fruit set was reduced on average by 9% under vertebrate exclusion. Two recent studies confirm the importance of pollinator diversity and pest control by birds for coffee yields—in terms of both quality and quantity (Moreaux et al. 2022)—and also show that the combined effect of birds and bees on coffee farms is stronger than their individual effects (Martínez-Salinas et al. 2022).

Conclusions

We found that ecological quality, expressed in terms of biodiversity and ecosystem functions, impacts coffee quality in different ways:

Shade trees were found to influence physical quality. There is evidence that the presence of shade positively influenced bean size, weight, and reduced the percentage of defects. However, when the percentage of shade and elevation interacted, coffee quality varied greatly.

High shade tree diversity improved bean size and reduced the percentage of defects. In addition, there is evidence that each shade tree species has a different effect on coffee bean quality. In this sense, it is important to conduct more research on the effects of shade tree species selected to be part of coffee agroforestry systems, also including local farmers’ knowledge.

All biochemical compounds were found to vary with the presence of shade, percentage of shade, elevation, and harvesting period. However, tree shade diversity was not correlated with biochemical compounds content. These compounds are largely affected by the roasting process, which can later affect organoleptic quality. As these biochemical compounds are transformed during roasting, it is still not well understood how their quantity in green beans eventually impacts the organoleptic coffee quality after roasting.

The organoleptic quality varies according to type of the tree shade species, percentage of shade, and elevation. There is evidence that coffee beans from diversified shaded coffee systems produced coffee beverages that obtained higher scores for final total organoleptic quality when compared with simplified tree shade and unshaded coffee plantations.

Shade positively impacts pollinator diversity, especially when shade tree diversity is high. Hence, pollination is improved, in turn improving coffee quality due to the reduced percentage of defects.

Shade differently affects pests and diseases. Its effect depends on several factors, including the identity of the pest or disease, microclimate, elevation, the microclimate preferences of the pest or disease, and microclimate preferences of natural enemies. Specific site management of shade may reduce the effects of pests and diseases on coffee quality. Regarding weeds, shade reduces their presence, which reduces herbicide use and labor.

Soil characteristics that affect coffee quality are site-specific and may interact with other factors; however, high soil quality positively affects coffee quality. The simplification of shade tree diversity negatively affected nutrient content and nutrient cycling, which negatively impacts coffee quality in terms of bean size.

The simplification of shade tree diversity negatively affected pollinator diversity and natural enemy’s diversity, jeopardizing pollination and pest control functions. Decreasing shade tree diversity was found to decrease the fruit set and increase infestation rates of the coffee berry borer beetle, Hypothenemus hampei.

In this way, reduction of tree shade diversity negatively impacts ecosystem functions, compromising coffee quality.

Shade trees on coffee plantations also improve water use efficiency. Shade trees prevent water losses and generally do not compete with coffee trees for water as long as they have different root distribution and hydrological root niches.

The findings of this review highlight that ecological quality enhancement can have multiple benefits for coffee quality, which can be translated directly into monetary benefits for coffee farmers and at the same time preserve biodiversity and healthy ecosystems.

Currently, the definition of “coffee quality” merely depends on the demand and flavor. Considering the effects of ecosystem functions on coffee quality, the concept of coffee quality should be expanded to include other qualities such as ecological quality, even though it may not necessarily be detectable in the final cup. Markets should be designed around appropriate economic and social policies that foster ecological quality. Currently, there is a need to harmonize ecological quality conservation with agricultural activities. Further, considering the overall positive effects that ecological quality has on coffee quality—and acknowledging certain negative effects—maintaining ecological quality on coffee farms needs to be considered in certification schemes for coffee quality.

As the importance of sustainability for emerging specialty coffee markets continues to grow, more empirical studies are needed that specifically focus on the effects of ecological quality—and interactions—on coffee quality in order to obtain clearer conclusions regarding the contextual relationships. In addition, studies on the individual and combined effects of shade tree species on coffee quality are needed, as there is still a lack of data on specific effects.

Authors’ contributions

Conceptualization, V.T., J.J., C.B.; literature search and data analyses, V.T.; writing—original draft, V.T.; writing—review and editing, J.J., C.B., C.I.S.; supervision, J.J; C.I.S.

Funding

This review was conducted in the frame of the research project “Exploring the interlinkages between specialty coffee farms’ biodiversity and farmers practices in Bolivia,” Seed Money Grant 1912, with the financial support of the Leading House for the Latin American Region, Latin-American-Swiss Center (CLS-HSG), University of St. Gallen, and the State Secretariat for Education, Research and Innovation (SERI), Switzerland.

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The authors declare no competing interests.