Fire effects on soils: the human dimension

Abstract

Soils are among the most valuable non-renewable resources on the Earth. They support natural vegetation and human agro-ecosystems, represent the largest terrestrial organic carbon stock, and act as stores and filters for water. Mankind has impacted on soils from its early days in many different ways, with burning being the first human perturbation at landscape scales. Fire has long been used as a tool to fertilize soils and control plant growth, but it can also substantially change vegetation, enhance soil erosion and even cause desertification of previously productive areas. Indeed fire is now regarded by some as the seventh soil-forming factor. Here we explore the effects of fire on soils as influenced by human interference. Human-induced fires have shaped our landscape for thousands of years and they are currently the most common fires in many parts of the world. We first give an overview of fire effect on soils and then focus specifically on (i) how traditional land-use practices involving fire, such as slash-and-burn or vegetation clearing, have affected and still are affecting soils; (ii) the effects of more modern uses of fire, such as fuel reduction or ecological burns, on soils; and (iii) the ongoing and potential future effects on soils of the complex interactions between human-induced land cover changes, climate warming and fire dynamics.

This article is part of the themed issue ‘The interaction of fire and mankind’.

1. Introduction

Soil is the ‘living, breathing skin of the Earth’ [1]. It allows the growth of terrestrial vegetation and thus supports, directly or indirectly, most forms of life on the Earth's land surface, including our own kind. Soils have enabled the development of human agro-ecosystems and the associated acceleration of human population growth [2,3]. They also represent the largest terrestrial organic carbon stock, and act as stores and filters for water [4,5]. The time to form a fully developed soil can range from centuries to millions of years, and, therefore, soils are considered one of the most valuable non-renewable resources on planet Earth [2].

The occurrence of fire is closely linked to soils. Without soils, there would be very limited vegetation cover on the Earth and hence very little, if any, of the more than 400 million ha [6] that, on average, burn across its land surface every year would be affected by fire. The interactions between soils and fire, however, go much deeper, even in the literal sense. Fire can directly influence soil properties through heating and combustion processes (figure 1), and indirectly through the changes to its vegetation cover and enhanced redistribution of soil through accelerated post-fire erosion [25,26]. Indeed fire is currently regarded by some as the seventh soil-forming factor (in addition to time, organisms, parent material, climate, topography and man), having influenced soil development and properties since the advent of vegetation fires over 400 Ma [27]. The rise of human societies has exerted a strong influence on the fire–soil interaction. On the one hand, for several thousand years humans have used fire as a vegetation-management or land-clearing tool, introducing or increasing fire impacts in some ecosystems [3,5,8,28–34]. On the other hand, landscape fragmentation and conversion to agricultural and urban land has decreased forest, shrub and grassland covers and their associated fire occurrence from large parts of the Earth surface [35] (figure 2). Over the last century, advances in fire suppression and fuel management, but also afforestation, increased ignition opportunities, and invasion of alien plants has led to further changes in fire occurrence and behaviour in many parts of the world [38,39]. Indeed, of the fires that currently burn ca 4% of Earth's vegetated land surface every year, up to 95% are directly caused by humans in densely populated areas such as Europe or Southeast Asia [40,41]. Furthermore, human-induced climate change is already affecting fire occurrence and behaviour in some regions with much more pronounced changes to be expected in the future [42,43]. These direct and indirect human interferences with fire, vegetation (i.e. fuels) and climate affect the role fire has played and will play in the development and functioning of soils.

Figure 1.

Effects on the biological, chemical and physical properties of soil and associated temperature ranges reached near the mineral soil surface for different types of human-induced fires (slash-and-burn, underburning, pile burning and ecological burning). See box 1 for a definition of ‘human-induced’ fires. The temperature ranges provided are broad estimates. Specific temperatures and associated soil effects will depend on the characteristics of each fire and soil. Temperature scale is nonlinear. SOM: soil organic matter; PyOM: pyrogenic organic matter; SWR: Soil water repellency. Data derived from [13–24].

Figure 2.

Current (a) and potential natural (b) global land cover under present climatic conditions, showing the extent of forests and woodlands (dark green), shrublands and open woodlands (pale green), grass biomes (orange), croplands and urban areas (red), croplands and mixed vegetation (pink) and bare land or ice/snow (grey). Data source: Current land cover map derived from GLCNMO2008 [36]; Global potential natural vegetation map derived from Ramankutty & Foley [37].

Here we aim to explore the human dimension in the interactions between fire and soils. First, we summarize the main effects of fire on soils (§2), and then focus specifically on how traditional uses of fire, such as slash-and-burn or vegetation clearing, have affected and still are affecting soils (§3). The effects of more modern uses of fire on soils, such as fuel reduction or ecological burns, are examined in §4 followed by a discussion of the ongoing and potential future effects on soils of the complex interactions between human-induced land cover changes, climate warming and fire dynamics (§5).

2. Fire effects on soils: a brief review

Fire can substantially alter soil characteristics both directly during burning and indirectly during the post-fire recovery period. These effects have been covered extensively in a series of reviews [14,18,19,21,23,25,26,44–49] and are therefore only briefly summarized here. The heat transfer from combustion of biomass and necromass above the soil and combustion of live and dead organic matter in the soil itself leads to some of the most common direct changes to the soil. These are generally dependent on the temperature the soil reaches, as illustrated in figure 1, which provides broad estimates of temperature ranges and associated effects on the biological, chemical and physical properties of soil. It is, however, important to recognize that the specific changes, and their magnitude, will be driven not only by temperature, but also by other fire parameters, such as heating duration and oxygen availability [15,50], and the characteristics of the soil (e.g. organic matter content, moisture content, mineral composition and thermal properties [22,25,49,51]). The main changes at lower temperatures (below 200°C) affect mostly biological properties (e.g. reduction of microbial biomass and destruction of the seed bank and fine roots, figure 1), although physical properties such as soil water repellency and aggregate stability can also be altered (figure 1). At higher temperatures (above 200°C), chemical properties are affected through combustion of soil organic matter and production of pyrogenic organic compounds and increases in soil pH, and physical properties also change, with alterations in water repellency and aggregate stability (figure 1). Even transformations of soil minerals can occur when high temperatures (above 350°C) are reached, for example, under logs or slash piles (figure 1). The combination of all these changes typically results in a more friable and erodible soil [25,49].

Importantly, soil temperature during burning does not normally exceed 100°C until the soil water is evaporated [52] (figure 1). Furthermore, soils are poor conductors of heat [16]. Therefore, even a very intense-flaming fire consuming most of the available ground and above-ground fuel may only lead to limited heat penetration into the soil [51,53]. Thus, unless fires are very slow moving, or large amounts of ground fuel are consumed (e.g. pile burning), the direct alterations summarized above are typically confined to the top few millimetres or centimetres of the soil [16,18]. In addition to this, soil temperature reached and duration of heating can vary substantially even over small scales [50], so direct effects of fire on soils can be spatially very heterogeneous.

Some of the even more consequential changes to the soil are indirect and often occur gradually in the post-fire period. This fact is easily overlooked when studying burnt areas shortly after fire, or when examining impacts of heat or burning on soil material in the laboratory. The most studied post-fire effect is that of enhanced erosion and hence thinning of soils on hillslopes [25]. The loss of protective vegetation and litter, combined with a loss in soil structure and, in some cases, enhancement of water repellency, result in more of the rainfall impacting the soil surface directly and in enhanced surface runoff and erosion [25,26]. This can lead to strongly accelerated losses of surface soil after the fire, with published values of 0.1–41 Mg ha⁻¹ per year after moderate to severe fires compared with 0.003–0.1 Mg ha⁻¹ in long-unburnt terrain [25]. However, it is important to remember that these enhanced erosion rates are (i) often restricted to the first months to years following fire, and (ii) usually decrease at larger spatial scales due to redeposition within hillslopes or catchments [47,54,55]. Given that surface soil holds the greatest amount of soil organic matter, nutrients and microorganisms, this fire-triggered process could be viewed as ‘soil destruction’. However, it must not be forgotten that soil erosion is a natural process that acts on the land surface irrespective of fire [56]. The resultant redistribution of often organic- and nutrient-rich sediment leads to the accelerated generation of fertile soils at the base of slopes, in riparian zones and floodplains within and well beyond a given burnt area. Only material that is deposited in lacustrine or marine sediment can therefore be considered as being removed from the pedosphere in the longer term [56].

Perhaps less acknowledged are the inputs of new material to the soil that occur after fire and which go beyond the sediment redistribution discussed above. The most obvious among these is the deposition of wildfire ash, the particulate post-fire residue consisting of mineral materials and charred organic components [57]. While some ash is derived directly from charred topsoil, much of it typically originates from the burnt living or dead above-ground biomass [14]. Ash production values up to 150 Mg ha⁻¹ have been reported [14]. Some of the ash will be redistributed by wind or water erosion, but some will become incorporated into the soil via infiltrating water or bioturbation. Ash is typically rich in nutrients [14], and hence enhances soil fertility, which is one of the motivations for burning of crops and pastures [58,59]. Further inputs of organic materials also occur in the form of unburnt vegetation killed by the fire, and more importantly, also by incorporation of charcoal produced during the fire. Charcoal and other types of pyrogenic organic matter (e.g. fine charred materials contained in ash) have an enhanced resistance to degradation that allows them to survive in soils for centuries to millennia, and hence can act as long- or medium-term carbon sinks [60]. Recent estimates suggest that vegetation fires annually generate 56–385 Tg yr⁻¹ of pyrogenic carbon worldwide, which equates to approximately 0.5% of the annual terrestrial net primary production [48,61]. The potential for fire to increase the fertility of soils has been the focus of much research since the discovery of Terra preta in Amazonia (see box 2). Thus, by accelerating the breakdown of living and dead organic matter above the soil and its input (largely in charred forms) into the soil, fire can enhance soil formation [18,65].

Box 2. The role of fire in Terra preta soils and Nordic dark earths.

These anthropogenic soils (figure 3b) were formed centuries ago by addition of fire residues (charcoal and ash) and other waste materials (biomass waste, excrement, manure and fish bones) to nutrient-poor soils by indigenous communities in tropical (Terra preta de Indio, 500–2500 BP [62]) and temperate (Nordic Dark Earth, approximately 3000 BP [63]) regions. These are very fertile soils in comparison with the surrounding natural soils and, therefore, an ancient model of sustainable agriculture in poor soils [62]. They are also increasingly studied as models for long-term carbon sequestration in soils, given that they have accumulated up to 100 times more carbon than the adjacent natural soils, mostly in the form of charcoal [62]. Although it is unlikely that these soils were intentionally created to improve soil fertility at large spatial scales [62], their spatial extent now ranges from less than 1 hectare up to several square kilometres, and it has been estimated that they may cover up to 3% of the total Amazonian region [64].

A further and even less understood fire-related process which contributes to soil formation is that of increased breakdown of parent material (i.e. rock and regolith) into smaller fragments. The high-temperature gradient caused by fire often leads to spalling, i.e. small rock fragments (mm–cm thick) flaking off from exposed rock surfaces [66]. This weathering process has, for example, been estimated to be 10–100 times more effective over the longer term than frost for locations in southern France that are prone to freeze–thaw weathering [67]. Finally, particularly where fires cause high tree mortality (stand-replacing fires), the accelerated uprooting of trees brings regolith or deep soil to the surface and exposes it to accelerated weathering. For example, following winter storms in a subalpine forest in the southern Rocky Mountains, local soil distribution of 450–600 Mg ha⁻¹ has been reported within patches of uprooted trees [68].

Thus, while the soil degradational impacts of fire are often those that are at the forefront of the debate, it is also clear that fire can increase soil fertility, organic carbon content, weathering and, ultimately, soil formation, particularly in areas where limited topography or rapid vegetation cover limit post-fire erosion. In areas of greater topography and where vegetation recovery is slow, fire may lead to accelerated soil erosion, and localized soil degradation will be the medium-term outcome (figure 3a). This, however, has to be seen in conjunction with enhanced soil formation in other areas where the soil is redeposited. Thus, on balance, and excluding areas where the occurrence of extremely severe or frequent fires prevent medium-term recovery or de novo formation of soil, fire has probably been an important factor driving soil formation in many fire-affected regions around the globe.

Figure 3.

(a) Soil erosion caused by recurrent burning for pasture generation (Asturias, northwest Spain). (b) Terra preta soil, made by addition of charcoal and ash, versus a natural oxisol in Central Amazonia (see box 2; Photo courtesy of B. Glaser). (c) Underburning in a dry eucalypt forest with very limited effect on soil (September 2014, southeast Australia). (d) High-intensity ecological burn in Jasper National Park (May 2015, Alberta, Canada; Photo by Parks Canada).

3. Fire as a traditional land management tool

Fire is one of the most ancient human tools, and its mastership has contributed enormously to the development of humankind [7,69]. First evidence of the use of fire dates back 1 Myr [70], but records of a widespread control of burning are not evident until 400 000 years ago [71]. Traditional uses of fire are many and very diverse (cooking, illumination, warmth, religious and cultural rituals), but, very importantly, fire allowed humans to exert the first impacts on landscape scales through burning of the land for hunting, vegetation modification and, later on, agricultural practices. The traditional (i.e. pre-industrial) uses of fire as a land management tool can be classified into three main types, according to the type of human society that carries out the burning [35]: (i) Hunter–gatherers: fire to promote habitat diversity and grass for game, and also to keep landscapes open to facilitate mobility. These are early season low-intensity surface fires with short fire intervals of only a few years; (ii) Pastoralists: fire to kill unpalatable species and stop woody encroachment, to promote grass growth and to control parasites and animal movements. These are also low-intensity fires but with a longer return interval (approx. 20 years); (iii) Farmers: burning of crop residues after harvest and pastures for domesticated grazers and also to prepare new cropland areas. These have shorter fire intervals (up to several times per year), but are also low-intensity fires with part of the burning happening in slash piles, or even away from croplands (i.e. residues used as house fuels).

The origins of these traditional uses of fire range from ancient to more recent, but all of them still occur today. Burning of crop residues, for instance, remains a fundamental agricultural practice for millions of people around the world, affecting over 20 million ha of croplands every year [6]. Traditional uses of fires have thus been modifying our landscapes for thousands of years. But what are the consequences for soils? As highlighted in the previous section, not all fires impact soils directly through heat transfer and combustion. The traditional uses of fires described above are mainly low-intensity surface fires (i.e. burning of grass and shrubs for grassland regeneration or of ground vegetation for forest maintenance) and are, therefore, mostly conducted under conditions that result in relatively ‘cool’ burns (i.e. at high relative humidity and fuel moisture, and low wind speeds; see box 1). This usually translates into limited direct effects on soils (figure 1). Overall, the greatest impacts of the traditional uses of fire on soils are mainly indirect, via changes in the vegetation cover. Transformation of forests, shrublands or grasslands into agricultural lands brings major impacts in the long term as soils under these types of land covers will evolve differently. For instance, cultivated soils normally lose 25–50% of their ‘natural’ organic carbon stock, due to enhancement of soil organic matter losses through decomposition, leaching and erosion and, at the same time, a decrease in new inputs of organic materials [72].

Box 1. Human-induced fires.

In this article, we focus on ‘human-induced’ fires. With this term, we refer to fires ignited by humans for a specific purpose, mainly land management. These fires have distinct characteristics compared to ‘natural’ wildfires: (i) In seasonal climates, they can occur at almost any time, but are often ignited prior to, or early in, the fire season(s), whereas natural fire occurrence peaks in the dry (hot) periods [7]. (ii) Their frequency is often higher than that of natural fires [8,9]. (iii) The area burnt by a single fire is usually smaller than for wildfires [10,11]. (iv) Owing to points (i)–(iii), the intensity of human-induced fires is usually lower [10]. (v) They tend to occur in the proximity of human settlements and infrastructures [12]. However, it is important to recognize that in many regions of the world human ignitions, whether accidental or arson, can be the cause of wildfires, which mostly share the characteristics of wildfires following non-human ignitions such as lightning.

There are, however, some traditional uses that can have severe direct effects on soils, such as slash-and-burn practices for forest clearance purposes. Here, the forest is cut and subsequently burnt, which can result in high temperatures in the surface soil due to heavy fuel loads (figure 1). Even without pre-fire logging, deforestation fires can lead to dramatic impacts on soils. The arrival of the Māori in the South Island of New Zealand approximately 700–800 years ago serves as an extreme example. It led to losses of around 40% of the island's forest cover in only a few decades. Fire was the main tool for forest clearance and resulted in enhancement of slope instability and large-magnitude soil erosion events [73]. Furthermore, it also needs to be considered that human-induced low-intensity fires can occasionally escape, resulting in larger and more severe wildfires with much higher impact on soils. In addition to this, the unsustainable use of fire as a landscape management tool, however—for example, burning outside of the appropriate season or using a too short fire recurrence interval—may lead to more damaging impacts on soils (figure 3a).

4. Soil impacts from modern prescribed uses of fire

A prescribed fire in the modern sense is any supervised burn conducted to meet specific land management objectives [74,75]. Although their behaviour and outcome will not always differ from those of some of the ‘traditional uses' of fire described in §3, they are essentially a modern phenomenon that involves land management agencies. The objectives of prescribed burning are many and very diverse. Here we focus on two types that have become increasingly important in the last few decades: (i) fuel reduction burns and (ii) ecological burns. Other prescribed uses of fire include generation of pastures for cattle, control of weeds, training of firefighters and even wildfire suppression operations such as backfires and burn out [76].

(a). Fuel reduction burns

Also known as hazard-reduction burns, this term refers to ‘any planned application of fire [in the landscape] to reduce hazardous fuel quantities, undertaken in prescribed environmental conditions within defined boundaries’ [74]. During the last couple of centuries, the Central European approach to wildfire of 100% suppression has reduced, or even excluded, fire occurrence in many ecosystems around the world [77]. This has resulted in unnaturally excessive accumulation of dead and live biomass in some fire-prone environments, with an associated increase in fire hazard. This trend has been exacerbated in some regions by depopulation of rural areas and resulting abandonment of previously managed woodlands, shrublands and pastures and/or spreading of invasive fire-adapted species [38,78]. In these areas fuel reduction burns are, therefore, used to mitigate the risk of severe wildfires. In addition, fuel reduction burns are also used in fire-prone environments like, for example, eucalypt forests in southeast Australia, where the natural fuel loads are very high and the natural fire recurrence interval is relatively short (less than 50 years), posing a substantial risk for human communities [79]. Fuel reduction burns are gaining relevance in the global context as a more economic alternative to mechanical treatments. They are widely used in Australia since the 1950s [79], already the most common fuel reduction practice throughout the USA [16], and still limited but increasingly used in Europe [78].

Fuel reduction burns can basically be divided into: (i) underburning (surface fires in woodlands and grasslands where fire burns ground and understory fuels but does not affect the overstory where present; figure 3c); (ii) post-harvest slash-and-burn (coarse fuels are cut, sometimes masticated, left on the ground and burnt); and (iii) pile burning (coarse fuels are cut, piled up and burnt) [16]. These different types of fuel reduction burns have different effects on soils as illustrated in figure 1. These fires are generally carried out within a ‘prescription window’, i.e. weather and fuel conditions that allow burning successfully, but with a low risk of resulting in an uncontrolled fire. They are, therefore, usually conducted under weather conditions that are not particularly hot, dry or windy. Thus, unless the accumulation of ground fuel available for burning is high, the effects on soils are negligible or very limited (figure 1) [16,78]. For example, Meira-Castro et al. [80] examined the effects of underburning in a Portuguese pine plantation. The burn achieved its fuel reduction goal and no changes in soil properties were detected. A thin layer of unburnt litter remained, serving as an insulator during the fire and protecting the soil from post-fire erosion. A notable exception to this is pile burning, where high fuel loads burn for hours, exerting great impacts on the soil (figure 1) [16]. However, these soil impacts are localized and, unless the piles occupy large or sensitive areas, effects are considered as being irrelevant at the landscape scale [16,81].

When addressing the effects of fuel reduction burns on soils, the overall impacts need to be carefully evaluated, taking into account the characteristics and resilience of the specific soil and ecosystem, and the cumulative effects of the prescribed fire regime in the long term [47,82]. In addition to this, the potential effects of the absence of fire should also be considered. In fire-prone areas, wildfire will occur eventually, with fire intensity being typically higher than during fuel reduction burns, and, therefore, soils could be more strongly affected.

(b). Ecological burns

An ‘ecological burn’ is the burning of vegetation for the preservation or enhancement of ecological processes [74]. The concept of ecological burning is not new [83], but it has not been until recently that awareness of the importance of fire for the conservation of biodiversity and ecological processes has grown substantially [84]. Ecological burns typically aim to restore the natural role of fire in ecosystems where fire has long been suppressed. A good example is the series of large-scale prescribed fires that are currently carried out in several national parks in Canada (figure 3d; www.pc.gc.ca/eng/pn-np/mtn/feuveg-fireveg/dirige-prescribed/projet-projects.aspx).

Achieving conservation objectives, however, can be very challenging. They can conflict with other management goals, and, also, sometimes require fire conditions that are simply too risky from a safety perspective. Van Wilgen [85] illustrated the failure of ‘safe’ prescribed burning to meet conservation objectives through the example of the flowering shrub Mimetes stokoei. This South African fynbos species needs high soil temperatures during the fire to complete its life cycle. In 1971 and 1984, two prescribed burns were carried out in the only area where the species still remained but, due to safety limitations, fires could not be made intense enough to lead to germination of the soil seed bank. Luckily for the species, a more intense wildfire burnt the area in 1999 and saved it from extinction. Thus, impacts on soils of ecological burn can vary widely, depending on their objectives. When they successfully emulate natural fire conditions their effects on soils will be as variable as those from wildfires, ranging from negligible to severe (figure 1).

5. Human-induced changes to land cover and climate and their impacts on fire–soil interactions

Human-induced fires have modified many of the world's landscapes for thousands of years [69]. Burning for forest clearance and slash-and-burn practices has been carried out from the beginning of the Holocene; however, at what stage they became important at wide spatial scales is still the subject of debate [86]. Wildfires and human-induced fires are difficult to differentiate in the palaeorecord, and, in most regions of the world, the human role in past fire activity may have been overridden by the stronger and broader influence of climate [87]. However, during the last two centuries, our footprint on the planet has become much easier to trace. In this short period, humanity has changed the world as much as it was changed, for instance, over 200 million in the Mesozoic. Our enormous impact on the Earth may even lead to the ‘privilege’ of having our own geologic epoch: the Anthropocene [88]. Although this term is still under debate, this epoch is suggested to have begun with the industrial revolution, which was also a ‘fire revolution’, switching from vegetation burning to burning of fossil fuels. Here, we explore how current human-induced changes in land cover and climate are affecting fire dynamics worldwide and, with that, the impact of fire on soils.

Human activities have greatly altered global land cover, with agricultural and urban lands currently accounting for 39% of the Earth's total ice-free surface [89] (figure 2). The natural vegetation in most of these transformed areas would have been mainly forest, shrubs and grasslands (figure 2). For most of those natural ecosystems, fire occurrence would have been higher than it is in the current agricultural and urban systems. Also very importantly, another 37% of the Earth's total ice-free surface, although not directly used by humans, has also been modified to some extent [89]. Even in those areas where natural land covers have not been drastically changed, human activities may have led to landscape fragmentation, which can decrease the probability of fire to spread [90], and, thus, can reduce the occurrence of large and severe wildfires [32]. Partially due to these human-induced land-cover transformations, current global fire activity is considered to be lower than at any time in the last 2000 years [77,91]. Therefore, we can conclude that human intervention has decreased the effects of fire on soils for a considerable proportion of the global land surface.

Notwithstanding this trend, there is an opposing trend in some areas of the world where human-induced changes to land cover have led to an increase of fire effects on soils. For example, in many parts of Europe, large-scale afforestation or reforestation, often replacing native broadleaf forests with conifer and eucalypt plantations, combined with rural depopulation and abandonment of agricultural lands and pastures, have increased fire incidence in recent decades [39,78] and, therefore, the concomitant effects on soils. In other regions, tree planting programmes for carbon sequestration are threatening to profoundly alter fire regimes, for example, by introducing high-intensity forest fires and their associated impacts on soils into areas not prone to such fires under natural conditions [92,93]. Probably the best example of enhancement of fire effects on soils through anthropogenic landscape modifications are the mega-fires. These extremely severe and extensive wildfires are usually a combination of extreme fire weather and unnaturally high fuel accumulations over large areas as a result of fire exclusion policies and other land management decisions [94]. These mega-fires, which are at least partially human-induced, generally burn at greater intensities than normal wildfires, resulting in higher impacts on soils [95].

In addition to transformations of the land cover, climate change is the other main anthropogenic alteration to fire regimes at the global scale. It has been suggested that the reported increase of CO₂ in the atmosphere 8000 years ago was driven by conversion of forested areas to agricultural lands through human-induced fires [96]. The underlying hypothesis that this and other modifications of the past global climate were caused by human activities is still subject to scientific debate [87]. However, what is widely accepted is that the current climate is changing as a result of anthropogenic emissions of greenhouse gases [97]. The effects of ongoing global warming on fire regimes are very complex [76]: in some ecosystems increased drying will enhance fire (e.g. tropical forest and boreal and tropical peatlands, [98]) whereas in others fire will decrease due to reduced vegetation growth (e.g. drylands). The raised level of CO₂ in itself should, in principle, enhance vegetation growth and thus fuel loads [99]. The outcome, however, would not necessarily be more fire as, for example, in tropical savannas, where this could also translate into an expansion of forest cover to the detriment of grass, decreasing landscape flammability [92].

The effect of climate change on the probability of ignition is also hard to predict. Although there is a projected increase of lightning in some regions of the world, such as at the tropics, North America or northeast Asia [100,101], the relationship of this increase to fire ignitions is not straightforward, as these will also be controlled by other factors such as fuel availability [100] and human activities [77,102]. Moreover, other effects of climate change, such as insect outbreaks or spread of invasive flammable plant species, will also alter fire dynamics. In addition, changes in fire activity themselves will modify vegetation cover and fuel availability, which in turn will affect fire patterns [103].

As outlined in the previous paragraphs, it is difficult to generalize the effects of climate change on global fire dynamics. Notwithstanding this, most global models agree that the severity and length of the fire season will have increased substantially by the end of this century over most of the Earth [43,104]. These changes will probably lead to enhancement of fire activity [105]. Thus, assuming these predictions hold, more fire is to be expected in the warmer future world and, importantly, the fires will overall burn more intensely [43]. We could, therefore, conclude that the resulting effects of fires on soils will also be intensified in many areas. In others, however, clearing of vegetation and expansion of built-up areas will reduce fire in the landscape. The overall future outcome of these opposing trends is uncertain. What seems clear, however, is that the human dimension of fire effects on soils will become even more significant than in the present day. A thorough understanding of the complex interactions between human activity, fire, climate and soils is therefore essential if we aim to preserve our soils as one of the most valuable non-renewable resources on our planet.

6. Conclusion

The existence and fate of fire and soils are closely linked. The presence of soils is a principal prerequisite for the occurrence of fire, and fire can be both a forming and degrading agent for soils. Fire can alter the physical, chemical and biological characteristics of soils both during and after burning. These changes range from negligible to very severe, with their nature and direction depending on many factors and thresholds. In this complex fire–soil interaction, humans have exerted a key role since their early days. After all, fire has been the first human tool that allowed modification of entire landscapes.

Unlike some of the more severe naturally occurring wildfires, most traditional and modern human uses of fire do not usually have significant direct impacts on soils. They can, however, lead to substantial soil alteration through indirect effects including changes in vegetation cover. Human-induced fire has transformed landscapes, but also, human-induced land-use changes have altered fire regimes. Thus, the effects of fire on soils have been, on the one hand, enhanced by human interference in some areas (e.g. through human ignitions and generation of more fire-prone vegetation covers); but on the other hand, have been reduced in other areas (e.g. by replacing fire-prone vegetation with crops). Humans have long been, and will keep on influencing the complex interactions between fire and soils. The advent of climate change now emerges as yet another key anthropogenic factor influencing fire dynamics. The resulting effects on soils will inevitably vary, but if the global projections of more burning hold true, fire is likely to gain further importance as a factor influencing our global soil resources.

Authors' contributions

Both authors contributed to the ideas presented here and to drafting the article, and approved the final version.

Competing interests

We have no competing interests in relation to the article's content.

Funding

Both authors have been supported by a Leverhulme Trust grant (RPG-2014-095).