Woody plants composition, structure and regeneration status of Muger Zala natural forest, Central Ethiopia

Abstract

Natural forests in Ethiopia are significant biodiversity repositories and climate change regulators, but they are under growing pressure from anthropogenic activities. The purpose of this study was to evaluate the composition, structure, and regeneration status of the Mugere Zala natural forest in Asagrit District, Central Ethiopia. A systematic sampling strategy was utilised to collect samples from 20 m × 20 m sample plots for trees, 10 m × 10 m for shrubs, and 5 m × 5 m subplots for saplings and seedlings. The sample plots were placed 400 m apart along transects laid at 200 m intervals. The composition and population structure data for woody species (diameter at breast height (DBH) > 2.5 cm and height > 2 m) were documented. To assess plant diversity, quantitative species diversity, richness, and evenness were calculated. A total of 62 species were identified in 47 genera and 43 families. With 5 (11.6%) species, the Rosaceae family had the highest species richness. In total, 557 mature trees, 747 saplings, and 1036 seedlings ha-¹ were observed in the sampled plots. Clutia abyssinica, Olea europaea subsp. cuspidata, Maesa lanceolata, Allophylus abyssinicus, Carissa spinarum, Phytolacca dodecandra, and Jasminum abyssinicum were associated with about 49.98% of the importance values. Compared to other similar forests in Ethiopia, the vegetation structure is similar and distinct. The forest is dominated by small plant species and largest native tree species despite its poor regeneration status. This is because highland forests are particularly difficult to manage due to population pressure, which is associated with grazing effects.

Introduction

Africa has 635,412,000 hectares of forest cover and 21% of its land area¹. Africa’s vast biodiversity is found primarily in its tropical rainforests, a species-rich ecology². There are more than 8000 plant species in Africa’s tropical rainforests, with 80% of them endemic³. They maintain the number of species, control erosion, watersheds, and influence global climate⁴. Dry Afromontane forests, the most threatened ecosystems due to fragmentation and deforestation⁵. Ethiopia is recognised globally as a centre of biological diversity and is one of the seven origins of Vavilov crops^6,7. The country’s highlands cover, 65% of its cattle, and 88% of its population⁸. There are estimated 6,500–7,000 species of plants in Ethiopia⁹. However, the forest ecosystem is affected by habitat loss and climate change¹⁰. Deforestation resulting in the loss of plant groups, associated fauna, and ecosystem services¹¹. Tropical forests, which have become severely fragmented, are the most exploited ecosystems in the world¹². Increasing human pressure impedes the natural regrowth of forests, putting native species at risk^13,14. According to studies, at the turn of the century, Ethiopia’s land area was covered by 35–40% highland forests^15,16. Studies on the composition, structure and regeneration status of woody plants are helpful in developing suitable conservation strategies^17–19.

The Mugere Zala Natural Forest is a rare Afromontane forest in Ethiopia’s Asagirt District, north of Shewa Administrative Zone. Field surveys indicate that the Mugere Zala forest is vital for preserving watersheds, stopping erosion, and supporting wildlife. Human activities such as timber production, firewood collection, overgrazing, traditional medicine, road construction, and erosion have degraded the forest despite its surroundings²⁰. Understanding the state of the forest is crucial for conservation planning and creating sustainable use of biological resources¹⁹. Hence, it is vital to document the characteristics of woody species before they vanish. Additionally, relevant organisations may find the study findings useful in developing appropriate managerial solutions. Thus, the objectives of this study were to collect data on: (a) the species composition of woody plants in the structural patterns of forest; (b) the woody species’ structural patterns; and (c) the evaluation of the regeneration status of the Mugere Zala natural forest. Consequently, the purpose of this study is to provide detailed information on the condition of the Mugere Zala natural forest and to suggest potential solutions for effective forest management.

Materials and methods

Description of the study area

The Mugere-Zala Natural Forest is in the Asgrit district, 99 km northeast of Addis Ababa and 96 km south of Debre Berhan in Ethiopia (Fig. 1). The forest spans 722 ha, ranging from 2,400 to 2,920 ma sl. and located between 39 ° 34′30′′ to 39 ° 35′30′′ E and 9° 18′0′′ to 90° 19′30′′ N. Ruining terrain with steep slopes and streams influences soil, hydrology, and vegetation distribution²⁰. Mugere-Zala historically connects the Menagesha Suba Forest and the Wof-Washa Forest in a highland forest corridor. Menagesha Suba, established by Emperor Zera Yacon, is Ethiopia’s oldest protected forest. Wof-Washa, formalised by Emperor Menelik II, is a key remnant of central highland dry afromontane forests. Mugere-Zala connects forests, helps species move and genes flow, and supports communities with vital services. Oral accounts show that the Mugere-Zala forest was once part of a continuous forest belt that covered much of the north Shewa highlands. Extensive deforestation in the 1970s and 1980s caused severe fragmentation, requiring community-based and government-supported interventions since the late 1990s for forest rehabilitation and protection. Despite these efforts, the forest remains under pressure from agricultural expansion, fuel wood collection, and settlement encroachment. The main threats to forests, such as cutting, grazing, cultivation near the edges, fires, have sped up soil erosion, emphasising the need for sustainable land management²⁰.

Fig. 1.

Map of the study area. The map was created using ArcGIS 10.8 (Esri, https://www.esri.com/en-us/arcgis/about-arcgis/overview).

Climate and topography

The study area has a cool and humid highland climate with two rainy seasons: a short one in March-April (Belg) and the main one from June-September, with the peak in July at 351.88 mm (Fig. 2). The mean monthly temperature is 12.7° C, with a maximum of 22.55 °C and a minimum of 2.83 ° C²¹. These climatic conditions, combined with rugged topography, strongly influence the composition, structure, and productivity of vegetation. The soils in the forest are primarily derived from Tertiary volcanic rocks and include Nitisols, Cambisols, and Leptosols, which vary according to slope and elevatio. Nitisols found on gentle slopes are deep and fertile, while Leptosols dominate steep ridges²². Mugere-Zala Natural forest is characteristic of the Dry Afromontane high land forest, dominated by Juniperus procera, Prodocarpus falcatus, and Olea europaea subsp. cuspidata, Hagenia abyssinica, and Schefflera abyaainica. Anthropogenic influence is evident through Eucalyptus globules plantations and fragmented agricultural lands, creating a mosaic of cropland, grazing land, and forest patches. The socioeconomic context is dominated by subsistence mixed farming, which combines crop cultivation and livestock rearing. The main crops include barley, teff, wheat, sorghum, beans, and peas. Livestock, primarily cattle, small ruminants, equines, and poultry, serve multiple purposes, including traction, food, income, and transportation²⁰. Population pressure and limited fertiliser use have shortened fallow periods, decreased soil fertility, and increased land degradation²³. Despite anthropogenic pressures, the Mugere-Zala forest is crucial for biodiversity conservation, watershed protection, carbon sequestration, and climate regulation. The ties to Wof-Washa and Menagesh highlight its importance to preserve forest continuity and diversity in central Ethiopia²⁴.

Fig. 2.

Average monthly temperature data and rainfall of the study area (January 2012–December 2021) (Debre Berhan meterological agency).

Land use and farming system

There are 3860 people living in the village, with 796 households; 2077 of them are men and 1783 are women²⁰. Rain-fed agriculture with a subsistence mixed farming system is practised in the study area. Oxes and “Chirosh” (Doma digging) are most commonly used for cultivation in mountainous areas²³. However, given the extreme scarcity of grazing land, this might be the result of land scarcity²⁰. The crops grown most commonly in the study area are barley, maize, wheat, sorghum, beans, peas and Teff. Both anthropogenic and natural factors contribute to the acceleration of land erosion²⁰.

Methods of data collection

In February 2021, a field survey was conducted to plan sample strategies for the Mugere Zala forest study. Data on woody vegetation were gathered using systematic sampling. Six-line transects were used to create a total of 55 quadrants²⁵. The selection of 55 quadrats per forest patch was based on preliminary field evaluations. These preliminary assessments looked at vegetation heterogeneity, stand structural variability, and spatial regeneration patterns throughout the Mugere Zala Natural Forest. During these assessments, we steadily increased the number of quadrats until additional sampling no longer provided substantial new information on the composition and structural traits. At 400 m apart, each line transect was laid, and the quadrates were systematically established at 200-meter intervals²⁶. Following, Braun-Blanguet technique, the plot sizes ranged from 20 m × 20 m for trees and shrubs to 5 m × 5 m for seedlings²⁷. To minimise the impact of disturbances, the first and last transect lines were placed 100 m from the forest’s edges²⁸. Vegetation data were collected in May, August, and September 2021 to address the blooming stage of the vegetation. Subsequently, all woody vegetation categories were identified, counted and recorded in each plot using their local and scientific names. The heights of mature plants (trees and shrubs) (height > 2 m), saplings between (1 and 2 m height) and seedlings (height < 1 m) were measured according to²⁹. Diameter tape was used to record DBH for trees/shrubs (DBH > 2.5 cm) at 1.3 m above ground³⁰. A geographical positioning system (GPS Garmen 12) was used to record the altitude of each quadrant. Voucher samples of each species in the quadrat were collected, pressed, and transported to Addis Ababa University’s National Herbarium for identification and recognized by Professor Sebsebe Demissew. All scientific names were verified using The World Flora Online (https://wfoplantlist.org/) and the Flora of Ethiopia and Eritrea books³¹. the International Code of Nomenclature for plants were confirmed following(https://www.iapt-taxon.org/nomen/main.php). Upon receiving permission, plant samples were stored in the Biology Laboratory at Debre Berhan University.

Sign of disturbance

The degree of disturbance in each quadrat was recorded via a scale of 1–3 as follows:

• 1 = no remaining indications of a stump cut.

• 2 = one cut in the stump and less frequent gathering.

• 3 = 2 to 3 stump removal, regular wood collection, and more than three stumps were removed and wood was frequently collected. Specifically, knowledgeable members of local communities were enlisted to identify dead woody plant stumps³². Furthermore, every indication of the grazing effect was observed in the plots.

Methods of data analysis

Structural analysis

Vegetation ecology takes into account the vegetation structure by taking into account the biomass structure, life form structure, stand structure. The following parameters were used in each graph to perform the structural analysis: density, relative density (RD), frequency, relative frequency (RF), diameter class distribution, height class distributions, basal area (BA), dominance (DO), relative dominance (RDO), breast height (DBH), and important value index (IVI)³³. The height of a woody species was measured and the findings were arranged into ten classes according to the height in metres: 1 = 3–6 m; 2 = 6.1–9 m; 3 = 9.1–12 m; 4 = 12.1 to 15 m; 5 = 15.1–18 m; 6 = 18.1–21 m; 7 = 21 to 24 m; 8 = 24.1–27 m; 9 = 27.1–30 m; 10 = > 30 m. Subsequently, the (DBH) class was divided into seven classes, each of which had a percentage: class1(< 2.5 cm); class 2(2.5–10 cm); class 3 (10–20 cm); class 4 (20–30 cm; ) class 5(30–40 cm); class 6 (40–50 cm> ) and class 7(> 50 cm) individual/h-1. The species were divided into five frequency classes based on their overall frequency expressed as a percentage. A = 81–100, B = 61–80, C = 41–60, D = 21–40, and E = 0–20%³⁴.

The following formula was used to calculate and summarise the vegetation data in a Microsoft Excel spreadsheet.

.

Basal area: It was calculated from the DBH records of trees and shrubs.

where π = 3.14.

Frequency: The number of times a species occurs at a specific number of sampling points is known as its frequency (F). It is frequently represented in percentages as follows and refers to both distributional and diverse patterns³⁴.

For woody species, the importance value index (IVI) integrates information from three parameters: relative frequency, relative density, and relative dominance. The following formula was used to determine the importance value index (IVI) of each woody plant species:

Importance value index (IVI): ³⁵.

Plant diversity data analysis

Shannon-Wiener diversity and Shannon evenness indices were calculated to describe the species diversity of the area³⁶. The Shannon-Wiener diversity index is thought to be the most widely used indicator of species diversity because it takes into account both the richness and evenness³⁷.

where H′ = Diversity of species

S = Number of species.

Pi = the proportion of an individual’s abundance of the i-th species.

ln = logarithmic base.

The evenness index (J) of each species was calculated via the Shannon evenness index via the following equation:

where = lnS; = Shannon diversity index.

lnS = the natural logarithm of the total number of species in each community.

S = number of species in each community.

The higher the values of Shannon evenness (J), the more even the species are in terms of their distribution. Similarly, the higher the value of the Shannon diversity index () is, the more diverse the community.

Species accumulation curve

Species accumulation curves can be used by scholars to analyse and compare population diversity and assess the benefits of increased sampling. The “library (vegan)” package in R was utilised to generate a species accumulation curve (SAC) to ensure sufficient plots for our investigation³².

Regeneration data analyses

Assessing the population sizes of seedlings, saplings, and mature trees and shrubs required knowledge of the state of forest regeneration. The number of individuals of each tree, sapling, and seedling species per hectare was calculated from the total number of individual species recorded from the sampled area. The regeneration status of the forest was evaluated using the following criteria: (1) ‘Good’ if the presence of seedlings > saplings > mature trees, (2) ‘Fair’ if the number of seedlings > saplings < mature trees, (3) ‘Poor’ if a species survives only in the sapling stage but not as seedlings, (4) ‘None’ if a species is absent both in the sapling and seedling stages, but present as mature; and (5) ‘New’ if a species has not matured but only in the sapling and/or seedling stages³⁸. Microsoft Excel software was used to analyse the data and compare the number of seedlings, saplings, and mature trees.

Results and discussion

Species composition and diversity

In this study, a total of 62 species belonging to 47 genera and 43 families were recorded in the Mugere Zala natural forest (Table 2). Rosaceae was the most species rich family, with 5 (11.6%) species, followed by Anacardiaceae. Unlike this Ruruki forest of Liban Jawi District, West Shewa Zone, Fabaceae were the most abundant family, followed by Oleaceae with five (7.14%)³⁸. Asteraceae, Myrisinaceae, Oleaceae, and Ranunculaceae, each of which contained three species (7% each), while Fabaceae, Moraceae, Sapindaceae and Scrophulariaceae each contained two species (4.6% each). Compared to other families, the preponderance of Rosaceae may benefit the local ecology, dispersal tactics, and superior adaptation to a range of ecological conditions. Floristically, these results recorded more number of woody plant species in Alka forest Beyeda District, North Gondar Zone, and Woody species diversity, richness and population structure of woody species in enclosed areas, North Gondar^39,40. On the contrary, the Wurg Forest in Southwest Ethiopia, the Woynwuha natural forest in North West Ethiopia and the Zijje Maryam Church Forest in Ethiopia comprise a higher number of woody species^41–43. The general Shannon Wiener diversity and evenness of the woody species in the Mugere Zala natural forest were 2.77 and 0.67, respectively (Table 1). The general Shannon-Wiener diversity and evenness of the current finding was more than the ealier scenario in Alka forest Beyeda district³⁹. This discrepancy can be caused by a variety of factors, including different topographical locations, microclimate conditions, and the awareness of the community of the need to protect forest resources.

Table 2.

Density of mature trees/shrubs, saplings and seedlings per hectare and regeneration status of Woody plants in the Mugere Zala natural forest during three growth stages.

List of species	Density of the three growth stages/ha			Regeneration status	Voucher ID
	Tree/shrub	Sapling	Seedling
Allophylus abyssinicus	26.4	0	15	Fair	MGK 01
Asparagus africanus	0	4	2	New	MGK 02
Berberis holstii	1.4	10	28	Good	MGK 03
Bersama abyssinica subsp. Abyssinica	8.6	14	50	Good	MGK04
Bridelia micrantha	0.5	0	0	None	MGK05
Buddleia polystachya	2.7	0	5	Fair	MGK06
Buddleia polystachya	0.5	0	0	None	MGK07
Buddleija polystachya	4.5	3	0	Poor	MGK08
Calpurnia aurea	5.5	16	12	Good	MGK9
Capparis tomentosa	0	2	0	New	MGK10
Carissa spinarum	26.8	17	1	Poor	MGK11
Cassipourea malosana	1	0	0	None	MGK12
Celtis africana	1	0	0	None	MGK13
Clematis hirsute	2.7	0	0	None	MGK14
Clerodendrum myricoides	0	19	3	New	MGK15
Climatis simensis	0	0	1	New	MGK16
Clutia abyssinica	2.7	8	15	Good	MGK17
Croton macrostachyus	3.6	2	2	Poor	MGK18
Discopodium penninervum	7.3	8	19	Good	MGK19
Dodonaea viscose	1	15	0	Fair	MGK20
Dombeya torrida	1.8	0	0	None	MGK21
Dovyalis abyssinica	16.4	30	17	Poor	MGK22
Ekebergia capensis	31.4	20	10	Poor	MGK23
Embelia schimperi	0	20	0	New	MGK24
Erica arborea	2.7	5	7	Good	MGK25
Euclea racemosa subsp. Schimperi	0	0	1	New	MGK26
Euphorbia abyssinica	2.7	0	0	None	MGK27
Ficus sur	6.4	3	2	Poor	MGK28
Ficus sycomorus	4.1	2	20	Fair	MGK29
Galiniera saxifrage	4.1	0	15	Fair	MGK30
Grewia villosa	2.7	0	0	None	MGK31
Hagenia abyssinica	6.8	10	0	None	MGK32
Haleria lucida	2.3	6	0	None	MGK33
Hypericum revolutum	0	25	0	New	MGK34
Inula confeptiflora	1	0	44	Fair	MGK35
Jasminum abyssinicum	0.5	0	0	None	MGK36
Juniperus procera	11	13	30	Good	MGK37
Lobelia rhynchopetalum	8.6	28	35	Good	MGK38
Maesa lanceolate	45	36	92	Good	MGK39
Maytenus arbutifolia	28.6	116	185	Good	MGK40
Myrica salicifolia	1.8	0	15	Fair	MGK41
Myrsine africana	2.7	162	142	Good	MGK42
Olea capensis	32.7	3	30	Fair	MGK43
Olea europaea subsp. Cuspidate	50	6	12	Fair	MGK44
Olinia rochetiana	37.3	11	7	Poor	MGK45
Osyris quadripartite	7.7	18	2	Poor	MGK46
Phytolacca dodecandra	0.5	20	0	None	MGK47
Pittasporum viridiflorum	10	2	4	Fair	MGK48
Podocarpus falcatus	83.2	73	70	Poor	MGK49
Prunus africana	5.5	0	5	Fair	MGK50
Pterollobium stellatum	0	0	1	New	MGK51
Ranunculus simensis	0	0	1	New	MGK52
Rhamnus staddo	1	0	0	None	MGK53
Rhus glutinosa	7.7	3	2	Poor	MGK54
Rhus retinorrhoea	1.4	0	0	None	MGK55
Rhus vulgaris	0	2	0	New	MGK56
Rosa abyssinica	0	0	2	New	MGK57
Rubus steudneri	1.4	0	1	Fair	MGK58
Schefflera abyssinica	1.8	0	6	Fair	MGK59
Tribulus cistoides	0.5	0	0	None	MGK60
Vernonia amygdalina	1.4	0	23	Fair	MGK61
Vernonia auriculifera	38.2	15	102	Fair	MGK62
Total	557	747	1036

Collector name M = Mengistu, G = Gebremicael, K = Kindye, 0–62 are the voucher ID number.

Table 1.

Shannon species diversity index in descending order.

Scientific name	Abundance	Pi	lanPi	PilanPi
Allophylus abyssinicus	58	0.05	− 3.05	− 0.15
Carissa spinarum	59	0.05	− 3.03	− 0.15
Calpurnia aurea	12	0.01	− 4.6	− 0.046
Dodonaea viscose	2	0.002	− 6.4	− 0.011
Olea europaea subsp. cuspidata	109	0.09	− 2.42	− 0.22
Euclea racemosa subsp. schimperi	0	0	0	0
Rhus vulgaris	0	0	0	0
Schefflera abyssinica	4	0.003	− 5.72	− 0.017
Bersama abyssinica subsp. abyssinica	19	0.0155	− 4.16	− 0.064
Haleria lucida	5	0.004	− 5.5	− 0.022
Ekebergia capensis	69	0.056	− 2.87	− 0.16
Ficus sur	14	0.011	− 4.47	− 0.05
Croton macrostachyus	8	0.006	− 5.03	− 0.03
Prunus africana	12	0.01	− 4.6	− 0.046
Juniperus procera	24	0.02	− 3.93	− 0.078
Myrica salicifolia	4	0.003	− 5.72	− 0.017
Vernonia auriculifera	84	0.07	− 2.68	− 0.187
Buddleia polystachya	6	0.005	− 5.31	0.026
Rubus steudneri	3	0.0025	− 6	− 0.015
Capparis tomentosa	0	0	0	0
Cassipourea malosana	2	0.002	− 6.4	− 0.011
Maesa lanceolata	99	0.081	− 2.5	− 0.2
Lobelia rhynchopetalum	19	0.0155	− 4.16	− 0.064
Rhus glutinosa	17	0.014	− 4.27	− 0.06
Celtis africana	2	0.002	− 6.4	− 0.011
Podocarpus falcatus	183	0.15	− 1.9	− 0.28
Olea capensis	72	0.059	− 2.83	− 0.17
Olinia rochetiana	82	0.067	− 2.7	− 0.18
Clerodendrum myricoides	0	0	0	0
Dombeya torrida	4	0.003	− 5.72	− 0.017
Maytenus arbutifolia	63	0.051	− 2.96	− 0.15
Buddleija polystachya	10	0.0082	− 4.8	− 0.04
Rhus retinorrhoea	3	0.0025	− 6	− 0.015
Dovyalis abyssinica	36	0.03	− 3.5	− 0.106
Myrsine africana	6	0.005	− 5.31	0.026
Osyris quadripartite	17	0.014	− 4.27	− 0.06
Pittasporum viridiflorum	20	0.016	− 4.11	− 0.066
Phytolacca dodecandra	1	0.0008	− 7.1	− 0.006
Embelia schimperi	0	0	0	0
Tribulus cistoides	1	0.0008	− 7.1	− 0.006
Asparagus africanus	0	0	0	0
Berberis holstii	3	0.0025	− 6	− 0.015
Clutia abyssinica	6	0.005	− 5.31	0.026
Clematis hirsute	6	0.005	− 5.31	0.026
Climatis simensis	0	0	0	0
Hagenia abyssinica	15	0.012	− 4.4	− 0.053
Grewia villosa	6	0.005	− 5.31	0.026
Jasminum abyssinicum	1	0.0008	− 7.1	− 0.006
Bridelia micrantha	1	0.0008	− 7.1	− 0.006
Rosa abyssinica	0	0	0	0
Inula confeptiflora	2	0.002	− 6.4	− 0.011
Hypericum revolutum	0	0	0	0
Discopodium penninervum	16	0.013	− 4.33	− 0.056
Rhamnus staddo	9	0.007	− 4.91	− 0.034
Vernonia amygdalina	3	0.0025	− 6	− 0.015
Galiniera saxifraga	9	0.007	− 4.91	− 0.034
Pterollobium stellatum	0	0	0	0
Ranunculus simensis	0	0	0	0
Ficus sycomorus	2	0.002	− 6.4	− 0.011
Buddleia polystachya	1	0.0008	− 7.1	− 0.006
Euphorbia abyssinica	6	0.005	− 5.31	0.026
Erica arborea	6	0.005	− 5.31	0.026
Total	1221			− 2.77
	Diversity (H) 2.77 Evenness (J) 0.67

Density

A total of 557 trees/shrubs, 747 saplings, and 1036 seedlings ha^{− 1} were recorded in the Mugere Zala natural forest. The Mugere Zala natural forest had relatively good number of saplings (747 ha^{− 1}) and seedlings (1036 ha^{− 1}). This sequence (seedling > sapling > trees) frequently indicates that the forest is in a good state of regeneration³⁸. Similar seedling counts were recorded in the Lephis forest in southeast Ethiopia (1036 ha^{− 1})⁴⁴. The tree/shrub species that showed the more density were P. falcatus (83.2 individuals ha^{− 1}), O. europaea (50 ha^{− 1}), M. lanceolata (45 ha^{− 1}), V. auriculifera (38.2 individuals ha^{− 1}), O. rochetiana (37.3 individuals ha^{− 1}), Olea capensis (32.7 individuals ha^{− 1}) and E. capensis (31.4 individuals ha^{− 1}), which represented approximately 57.1% of the total density in the forest. Almost 18% of the total woody plants (R. vulgaris, C. tomentosa, C. myricoides, E. schimperi, (A) africanus, C. simensis, R. abyssinica, H. revolutum, P. stellatum and R. simensis) were not represented in their mature stage. The woody plants in the Mugere Zala natural forest exhibited a concentrated regeneration pattern, as evidenced by the sapling densities that were dominated by a few species, including M. africana (162 ha^{− 1}), M. arbutifolia (116 ha^{− 1}), P. falcatus (73 ha^{− 1}), D. abyssinica (30 ha^{− 1}), M. lanceolata (36 ha^{− 1}), and C. aurea (16 ha^{− 1}) [Table 2]. The average tree density of the dry evergreen afro-montane forest in the southeast of Ethipia had less density⁴⁵. The average tree density of Dodola dry evergreen afro-montane Dodola forest in the southeast Ethipia had less density⁴⁵. This study revealed almost the same density of Wof washa natural forest, which had the density of mature trees / shrubs, saplings, and seedlings of 698.8, 1178 and 7618.7 individuals’ ha^{− 1} respectively, in the same agroecological zone and Menagesha forest (692 individual ha^{− 1})^46,47. The Mugere Zala Natural Forest, on the other hand, has a lower density than the Zegie Peninsula forest (3318 individual ha^{− 1}), Bale Mountain National Park (898 individual’s ha^{− 1}), and the Chilimo forest (888 individual’s ha^{− 1})^48–50. The density variations occur due to habitat preferences, topographic gradients, and human disturbances, the density of woody species varies greatly among forests⁵¹. In most cases, the density of large trees, such as J. precera, O. europaea, F. sur, C. africana and H. abyssinica, strongly decreased, whereas the density of shrubs and small trees, such as M. arbutifolia, M. africana, (B) abyssinica subsp. abyssinica, V. auriculifera, (C) abyssinica, M. lanceolata, and B. holstii, became dominant in the forest. Therefore, large trees face high deforestation, and the forest is dominated by shrubs and small trees, which challenges the sustainability of the forest.

Frequency

O. europaea subsp. cuspidata, P. falcatus, and E. capensis belong to frequency class C. A. abyssinicus, O. capensis, M. lanceolata, V. auriculifera, O. rochetiana, M. arbutifolia, D. abyssinica, D. penninervum, H. abyssinica and J. procera belong to class D, whereas the rest belong to class E. (Table 3). The frequency class of the present finding was inconsistent with the previous scenario⁴⁰. High values in the higher frequency class (Freqs. classes A and B) and low values in the lower frequency classes (Freqs. classes E and D) indicate constant⁵². High values in lower frequency classes and low values in higher frequency classes, on the other hand, indicate a greater degree of floristic heterogeneity. In the present study, high values were obtained in lower frequency classes, while low values were obtained in higher frequency classes (Table 3). Therefore, according to the above interpretation, it is possible to conclude that there is a high degree of floristic heterogeneity in the Mugere Zala natural forest. The frequency distribution of this forest was dominated by the lower frequency classes, which accounted for 95.2%.

Table 3.

Frequency classes of Woody plant species in the natural forest of Mugere Zala.

Scientific name	Frequency	Relative Freq.	% Freq.	Fr. Class
Olea europaea subsp. cuspidate	31	7.3	56.4	C
Podocarpus falcatus	29	6.9	52.7	C
Ekebergia capensis	28	6.6	50.9	C
Allophylus abyssinicus	22	5.2	40	D
Olea capensis	22	5.2	40	D
Maesa lanceolate	21	4.1	38.2	D
Vernonia auriculifera	20	4.7	36.4	D
Olinia rochetiana	19	4.5	34.5	D
Maytenus arbutifolia	18	4.3	32.7	D
Dovyalis abyssinica	17	4.0	30.9	D
Discopodium penninervum	16	3.9	29.1	D
Hagenia abyssinica	15	3.5	27.3	D
Juniperus procera	12	2.8	21.8	D
Pittasporum viridiflorum	10	2.4	18.2	E
Carissa spinarum	9	2.1	16.4	E
Osyris quadripartite	9	2.1	16.4	E
Rhamnus staddo	9	2.1	16.4	E
Galiniera saxifrage	9	2.1	16.4	E
Bersama abyssinica subsp. abyssinica	7	1.7	12.7	E
Rhus glutinosa	7	1.7	12.7	E
Ficus sur	6	1.4	10.9	E
Prunus africana	6	1.4	1.4	E
Buddleia polystachya	6	1.4	10.9	E
Grewia villosa	6	1.4	10.9	E
Euphorbia abyssinica	6	1.4	10.9	E
Erica arborea	6	1.4	10.9	E
Lobelia rhynchopetalum	5	1.2	9.1	E
Buddleija polystachya	4	1.0	7.2	E
Clutia abyssinica	4	1.0	0.5	E
Calpurnia aurea	3	0.7	5.5	E
Schefflera abyssinica	3	0.7	5.5	E
Croton macrostachyus	3	0.7	5.5	E
Myrica salicifolia	3	0.7	5.5	E
Rubus steudneri	3	0.7	5.5	E
Dombeya torrida	3	0.7	5.5	E
Myrsine africana	3	0.7	5.5	E
Vernonia amygdalina	3	0.7	5.5	E
Cassipourea malosana	2	0.5	3.6	E
Celtis africana	2	0.5	3.6	E
Rhus retinorrhoea	2	0.5	3.6	E
Clematis hirsute	2	0.5	3.6	E
Inula confeptiflora	2	0.5	3.6	E
Ficus sycomorus	2	0.5	3.6	E
Dodonaea viscose	1	0.2	1.8	E
Haleria lucida	1	0.2	1.8	E
Phytolacca dodecandra	1	0.2	1.8	E
Tribulus cistoides	1	0.2	1.8	E
Berberis holstii	1	0.2	1.8	E
Jasminum abyssinicum	1	0.2	1.8	E
Bridelia micrantha	1	0.2	1.8	E
Buddleia polystachya	1	0.2	1.8	E
	423	100	100

Keywords: A = 81–100%; B = 61–80%; C = 41–60%; D = 21–40%; and E = 0–20%.

Species accumulative curve

The accumulation of a woody species in Muger Zala forest was represented by a speciesaccumulative curve. The curve rise steeply within the first 15 plots, indicating that common and widespread plant species were quickly encountered. Between 15 and 35 plots, the curve would continue to rise but at gradually slowing rate, reflecting the addition of less common and habitat specific species (Fig. 3). By approximately 35–40 plots, the curve would show a clear tendency to flatten, approaching an asymptote, which signifies that most species present in the area have been recorded and additional plots yield few new species. The very gradual rise observed from 40 to 60 plots represent the long tail of the accumulation curve where only rare species. The species-area curve helps ecologists estimate species richness, assess sampling efficiency, compare habitats, comprehend spatial diversity patterns, and make conservation decisions. Without it, research may underestimate biodiversity and misread community structure, especially in complex systems like woody vegetation³⁸.

Fig. 3.

Species accumulation curve of the Muger Zala forest.

Basal area and importance value index

The total basal area in the Mugere Zala natural forest is approximately 20.67 m² ha^{− 1}. O. europaea subsp. cuspidata and P. falcatus were the dominant species in the forest, accounting for 59.26% and 5.5%, respectively, of the total basal area. The remaining woody species cover only 35.24% of the total basal area. The results of this study are much lower than the Wof Washa natural forest (64.32 m² ha^{− 1}), Chilimo forest (27.3 m² ha^{− 1}), Bale Mountain National Park (26 m² ha^{− 1})^46,49,50. This indicates that the Mugere zala natural forest is facing high deforestation, with the target of large trees. The relative ecological significance and/or dominance of tree species in a forest ecosystem could best be revealed from an analysis of IVI values⁴⁹. Approximately 49.98% of the importance values were attributed to C. abyssinica (22.6%) and O. europaea subsp. cuspidata (5%), M. lanceolata (4.7%), A. abyssinicus (4.5%), C. spinarum (4.5%), P. dodecandra (4.48%) and J. abyssinicum (4.2%). These species were abundant, frequent, and dominant in the forest. The remaining percentage (50.02%) was shared among the remaining species. In contrast to the above ones, the latter ones are less abundant, infrequent, and not dominant. A species that grows within its colour range and has the best regeneration and resistance is indicated by a high IVI price. Due to their high pollinator enrichment, which promotes seed dispersal under current environmental conditions in the area, these species are the least preferred by browsing animals and seed predators⁵³. In the current finding the top height was recoreded at 98 m. Consequently, the scheme classes are 33.3 m, > 16.7 m and < 33.3 m and 16.7 m for the upper, middle and lower story, respectively (Fig. 4). The emergent trees of the Mugere Zala natural forest cover 11% of the registered woody plants, including J. procera, P. falcatus, and O. europaea subsp. cuspidata and H. abyssinica. The middle layer of the forest occupies 20.3% of the total density, and some of the plant species included in this story are I. mitis, O. quadripartite, O. capensis, and A. abyssicus. The lower story covers the highest density of almost 68.7% of the total woody plants, which are largely dominated by shrubs and small trees such as M. lanceolata, M. arbutifolia, and L. rhynchopetalum (Fig. 4). The same results were reported in different forests in Ethiopia and these authors verified that most of the forest populations are dominated by small plant species^46,54,55.

Fig. 4.

Vertical structure of woody plant species in the Mugere Zala natural forest.

Diameter at breast height (DBH)

The DBH (Diameter at Breast Height) class distributions of all observed woody species of the Muger Zala forest was clearly shown (Fig. 5). As the size of the DBH class increased, the number of individuals decreased from 75.2% in the 1st and 2st classes to 2.2% in the 6th and 7th classes. This appears to be a regular distribution that resembles the inverted J-shaped distribution of individuals in the different classes of DBH, with slight depression in the first class (22.4%) compared to the 2nd class (52.8%). Selective cutting of certain DBH classes is continuous, the sustainability of the forest becomes in question⁵⁶. Approximately 91.2% of the number of individuals were contributed by DBH classes 1, 2 and 3, indicating the predominance of small individuals in the Mugere Zala natural forest, which contributed to the small base area (Fig. 5). This study aligns with the Wof Washa natural forest, which is located in the same ecological zone⁴⁶.

Fig. 5.

DBH classes versus the number of individuals in the Mugere Zala natural forest.

Population structure of some Woody plant species

The management, sustainable use, and conservation of trees and shrubs are significantly impacted by their population structure⁵⁷. Four representative patterns of population structure were displayed (Fig. 6a–d). The selection of representative species is based on economic importance and local community information about the prior abundance of these species and their size. Analysis of density distributions among the diameter classes of the woody species in the forest resulted in different patterns (Fig. 6a,c). The low densities in the small-diameter classes in this finding indicate a poor regeneration capacity of the woody plant species. An implication here is that the potential to replace such species will be very low once mature individuals have disappeared. This means that the species is endangered and needs conservation. The first population patterns represented by O. europaea subsp. cuspidata (Fig. 6a), indicated low abundance in the first class and an increase toward the last class. This pattern (J-shaped curve) indicates that seedlings and juveniles are not well represented and that there is high grazing and selective cutting for the purpose of construction, fuel wood, agricultural encroachment, and logging. Similar findings have been documented for J. Procera⁵⁸. In most cases, the demand for the highest DBH classes is very high because local people use timber as a source of income since this forest is the only potential forest in the area. The second population pattern was represented by P. falcatus (Fig. 6b). This pattern indicates the presence of medium density in the 1st class and the highest density in the 2nd class, with a gradual decrease in density toward the larger classes. It represents an almost inverted J-shaped curve, suggesting satisfactory reproduction but poor recruitment⁵⁵. P. falcatus is a large tree and, therefore, can have a large DBH, but its abundance is very low or almost non-existent in the last class. This species has a good regeneration status with a high sapling stage by overcoming the pressure of grazing at the seedling level, but as the removal of mature trees increases, the formation of seedlings and saplings becomes endangered since the only source of seeds are mature trees. As a result, a sustainable conservation system is needed. The third population pattern was represented by J. procera (Fig. 6c). This pattern shows a low abundance in the first class of DBH and a gradual increase in the middle class, followed by a decrease in the abundance of individuals towards the higher classes, which are mother plants. The same results for C. macrostachyus have been reported in previous findings⁵⁵. This pattern indicates poor reproduction and a decline in the number of large trees related to the death or selective cutting of large individuals by the local community near the natural forest for their local apparatus use; hence, conservation priority is needed. The fourth population pattern represented P. viridiflorum (Fig. 6d), which was an abnormal distribution pattern. P. viridiflorum is represented by a few individuals in the first class, with high abundances in the second and third classes, and is completely absent in the higher DBH classes.

Fig. 6.

Population structure of representative woody plant species.

Height

All woody plants taller than 3 m were considered mature trees / shrubs, while individuals shorter than 3 m were considered saplings and seedlings. The mature tree/shrub species in the study area were conveniently classified into 10 height classes (Fig. 7). The results revealed that the distribution of woody plants per ha decreased with increasing height class, except for the 10th class. The highest number of individuals represented in height classes 1, 2 and 3, which accounted for 53.6% of the total height classes. The woody plants in the remaining height classes accounted for 46.4% of the total height class. The species that contribute the most to the lower height classes (below 12 m) in the Mugere Zala natural forest were E. racemosa subsp. Schimperi, C. myricoides, M. salicifolia, M. lanceolata, D. abyssinica, H. revolutum, D. penninervum, and V. nobilis. These plants are shrubs or small trees in nature. The height distributions of the woody plants were highest in the lower class, followed by a considerable decrease, which promoted a normal inverted J-shaped distribution with little increase in the height distributions of the postreproductive plants (classes 8, 9 and 10) (Fig. 7). The species in the forest has a fair regeneration state, as indicated by the inverted J-shaped curve in the individual distribution⁵⁹. This pattern implies that forest vegetation has good reproduction potential and less recruitment potential. Similar findings have been reported in Ethiopian forests^50,54,58.

Fig. 7.

Height-class distribution of woody plants. 1 = 3–6 m; 2 = 6.1–9 m; 3 = 9.1–12 m; 4 = 12.1–15 m; 5 = 15.1–18 m; 6 = 18.1–21 m; 7 = 21.1–24 m; 8 = 24.1–27 m; 9 = 27.1–30 m; 10 = > 30 m.

Regeneration status of woody species

Investigating seedling and sapling data revealed that 37 species belonging to 29 families were represented in the seedling stage. Rosaceae and Asteraceae were represented by 4 and 3 species respectively (Table 2). The total seedling density was 1036 individuals ha^{− 1}, therefore, regeneration in Mugere Zala forest is greater than that of Wof Wahsa natural forest (32 species) and less than Menagesha forest (41 species)^46,60. The composition, distribution, and density of seedlings and saplings indicate the status of forest regeneration⁶⁰. The sustainability of natural forests depends on the regeneration capacity of each species in the forest⁶¹. Regeneration status of woody species are among the main factors that are useful for assessing their conservation status⁶². The sapling stage was made up of 36 species representing 29 families. Myrsinaceae was the family most represented, with 3 species (Table 2). This forest is much less dense than the Wof wahsa natural forest (33 species)⁴⁶, this may be the result of overgrazing. Deforestation followed by frequent cultivation, weeding, site preparation, stump splitting, and animal trampling exhaust native woody species propagating on arable and grazing lands in Ethiopia⁶⁰. M. africana (27%), M. arbutifolia (12%), P. falcatus (7.5%) and D. abyssinica (5%), all shrub by species, made up 51.5% of sapling regeneration. On the other hand, only 48.5% of the e remaining species. M. arbutifolia (14.3%), M. africana (11%), B. abyssinica subsp. abyssinica (9.2%) and V. auriculifera (7.5%) were the most dominant species in the regeneration stage of seedlings, accounting for approximately 42% of the total seedlings. The patterns of these life stage distributions show a higher number of individuals at the germination stage and a gradual decrease toward the life stages of seedlings, saplings, and mature trees. Similar results have been reported in the Menagesha forest⁶³. This distribution pattern is commonly referred to as an inverted ‘J’ shape, which has good regeneration potential⁶⁴. This shows that the future status of the forest will be covered by few dominant species, leading to less diversification of the forest. Some species with less valuable woody species for commercial purposes are resistant to grazing by livestock^{60, 65}. Therefore, M. africana, M. arbutifolia, B. abyssinica subsp. abyssinica and V. auriculifera may be in this category but need further investigation, whereas P. falcatus is due to the presence of many propagates. Among the 62 woody species in the Mugere Zala natural forest, 17.7% showed good regeneration, 24.2% fair, 16.1% poor, 24.2% lacked regeneration and 17.7% were new emerging plant species.

Regeneration status of some representative woody plant species

The selection of the representative woody plant species for this analysis was based on locally threatened, indigenous trees and representations of the different regeneration categories. On the basis of the above, the following representative species are selected and discussed in the following categories.

• i.The distribution of J. procera (Fig. 8a) was greater for seedlings and saplings than for mother plants, and the number of saplings was greater than that of mature trees. The pattern has many individuals in the seedling stage and a decreasing number of individuals successively at the sapling and adult stages. Thus, it exhibits typical inverted J-shaped curves⁶⁶.
• ii. P. falcatus (Fig. 8b): This regeneration pattern represents a species with few seedlings and saplings, whereas mature tree individuals are abundant. This pattern shows a typical J-shaped curve, which indicates a healthy or good regeneration status if sustainable protection from selective deforestation is achieved⁶⁶.
• iii. O. europaea subsp. cuspidata (Fig. 8c): This distribution pattern has some seedlings with very few saplings and few mature tree individuals, which indicates poor regeneration and needs intervention for conservation.
• iv. H. abyssinica (Fig. 8d): This distribution pattern has matured stages with few saplings where seedlings are absent. This is poor regeneration. Possible reasons for hampered regeneration include human disturbance, damage caused by free livestock grazing, particularly goats, and changes in environmental conditions⁶⁷.

Fig. 8.

Seedling, sapling, and tree/shrub distributions of some selected species in the MZNF.

Conclusions and recommendations

Conclusion

The Mugere Zala natural forest is one of the few remaining dry Afromontane forests in Ethiopia. A total of 62 species belonging to 47 genera and 43 families were identified and recorded. It has relatively high species density, accounting for 557, 747 and 1036 individuals ha^− 1 of mature trees/shrub, saplings and seedlings, respectively. Species such as P. falcatus, O. europaea, M. lanceolata, V. auriculifera, O. rochetiana, O. capensis, and E. capensis are the dominant tree/shrub species in the forest. The regeneration status of the large and indigenous tree species is poor, and the forest is dominated by small plant species. This could have resulted from the selective cutting of high-economic value plant species and/or from grazing effects. Highland forests face life-threatening challenges due to population pressure. The management intervention was insignificant, especially during the state transition. Currently, it is owned by the government, but free grazing and selective cutting for agricultural utilities and domestic activities are still evident. Menshion Fur Menshion, a non-governmental organisation, encouraged and called for public mobilisation and collaboration among stakeholders for its effectiveness.

Recommendations

Based on the results, possible recommendations are as follows.

• The urgent mobilisation of local communities and building of a sense of ownership and conservation interest through discussion and consultation with various stakeholders are crucial.

• To increase the germination capacity of the soil seed bank and ensure forest sustainability, the ‘cut and carry” method should be used instead of free grazing in the forest.

• A participatory forest management strategy should be implemented to ensure forest sustainability.

• Agroforestry activity should be practised in the area to ensure the biophysical and socioeconomic well-being of the society in the study area.

• Further studies on seed bank, ethnobiology and physiology of seeds that provide additional information on vegetation status could be useful for forest management and conservation efforts.

Limitation of the work

The soil seed bank and soil mineral analysis of the forest were not completed due to a lack of funds, resources, and a well-equipped laboratory. With this we recommend that further investigation regarding on these parameters is essential.

Author contributions

M. A., G.F., and K.B. W. wrote the main manuscript and prepared Figs. 1, 2, 3, 4, 5, 6, 7 and 8. All authors reviewed the manuscript.

Data availability

All data generated or analysed during this study are included in this published article.

Declarations

Ethics approval and consent to participate

The authors state that this publication does not contain any information concerning human experiments or the use of human tissue samples. Plant species data was collected from forests in collaboration with the Asagirt District Agriculture Office. The research followed the IUCN policy guidelines and restrictions.

Consent for publication

Not applicable: This manuscript does not contain personal data from the authors.

Competing interests

The authors declare no competing interests.