Keywords to note: wildfire, wildland fire, bushfire, prescribed fire, land cover, climate change
Soil is the ‘living, breathing skin of the Earth’ . 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. They also represent the largest terrestrial organic carbon stock, and act as stores and filters for water. 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.
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 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, and indirectly through the changes to its vegetation cover and enhanced redistribution of soil through accelerated post-fire erosion. 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. 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. 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. 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. 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. 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. 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.
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). …
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 …
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, 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. The effects of more modern uses of fire on soils, such as fuel reduction or ecological burns, are examined in 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.
Fire Effects On Soils
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 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. 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, although physical properties such as soil water repellency and aggregate stability can also be altered. 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. Even transformations of soil minerals can occur when high temperatures (above 350°C) are reached, for example, under logs or slash piles. The combination of all these changes typically results in a more friable and erodible soil.
Importantly, soil temperature during burning does not normally exceed 100°C until the soil water is evaporated. Furthermore, soils are poor conductors of heat. 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. 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. In addition to this, soil temperature reached and duration of heating can vary substantially even over small scales, 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. 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. 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. 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. 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. 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.
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. While some ash is derived directly from charred topsoil, much of it typically originates from the burnt living or dead above-ground biomass . 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, and hence enhances soil fertility, which is one of the motivations for burning of crops and pastures. 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. 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. 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.
Fire can substantially alter soil characteristics both directly during burning and indirectly during the post-fire recovery period.
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.
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 oxygen availability and the characteristics of the soil (e.g. organic matter content, moisture content mineral composition and thermal properties. 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, although physical properties such as soil water repellency and aggregate stability can also be altered. 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. Even transformations of soil minerals can occur when high temperatures (above 350°C) are reached, for example, under logs or slash piles. The combination of all these changes typically results in a more friable and erodible soil.
Importantly, soil temperature during burning does not normally exceed 100°C until the soil water is evaporated. Furthermore, soils are poor conductors of
heat. 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. 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. In addition to this, soil temperature reached and duration of heating can vary substantially even over small scales, 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 . The loss of protective vegetation and litter, combined with a loss in soil structure and, in some cases, enhancement of water
Fire effects on soils: the human dimension
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.