Chapter 2
The deep history of fire
Identifying past fire events
There are many features of fire systems in vegetation that cannot be assessed without recourse to fire history. We can view fire history on a number of different scales, from the length of a human lifetime to a geological scale of millions of years. Each scale will, however, require a different approach to the gathering of data. To establish the characteristics of fire regimes we need historical data, to learn about, for example, fire return intervals (FRI—the time between fires in a defined area, usually in a single vegetation type) that will aid current policies about wildfire. Today, we have the advantage of recorded information either by individuals or organizations including satellite information. While this is giving us a better global view of fire, we still need a much better perspective of fire through time, especially if we are able to assess the impact that humans have had on natural fire systems and to get to grips with the impact on wildfire of climate change. Apart from written or oral records for recent history, we can access data about fire stored in nature itself.
One approach is to use the growth rings within the wood of trees. We can date a tree from growth rings and obtain a considerable amount of climate data in addition. But growth rings can also tell us about fires of the past. As we have already seen, surface fires are common in many forested areas and can pass through without killing the trees. The trees may, however, be scorched or burnt in one area, giving rise to a fire scar that may be healed over. When the tree is felled, we can then establish not only that a fire has occurred in the past, but also the year in which the fire occurred. In trees with a long run of tree rings fires may have occurred many times and this pattern of scars provides information on the frequency of fire—the FRI.
If trees from a wide area can be studied then this may provide data on the size of a particular fire in the past. If it proves possible to obtain fire data from trees across a whole region then this can be coupled with information on climate and allow an interpretation of how fire frequency is affected by climate change, as well as seeing the effects of major fluctuations caused by the phenomena of El Niño and La Niña, which are the warm and cold phases respectively of a large-scale atmosphere and ocean coupling cycle called the El Niño Southern Oscillation (ENSO). During an El Niño phase, parts of the western Pacific become particularly dry. In some places, such as the Sequoia National Park in northern California, it has been possible to use tree rings to obtain a fire record that goes back nearly 3,000 years.
Obtaining tree ring/fire scar data is limited to forest settings, where trees can be sampled. Fires, when burning, emit smoke into the atmosphere that comprises not only water vapour but complex chemicals such as polycyclic aromatic hydrocarbons (PAHs), gases such as carbon dioxide, nitrous oxides, ammonia, recombined organic carbon in the form of soot, and small charred particles (charcoal, Box 1). While many of these compounds, such as soot, PAHs, and ammonia, have been used to interpret fire events, they have proved controversial, especially when interpreting the size of fires. Small charcoal fragments (termed micro- and meso-charcoal) have more frequently been used in the interpretation of fires that occurred over the past million years or so (Figure 6). Such studies have proved most effective in interpreting fires over the past 70,000 years, as rock sequences can be readily carbon dated and this data on fire return intervals and climate change can be analysed. Charcoal (generally less than 1 mm in size and often counted in categories greater and less than 100 µm) can readily be deposited in lake and peat sediments where there may be a long history of sedimentation and where age profiles of the sedimentary sequence may more easily be obtained. Researchers have been encouraged to pool their data to create a global charcoal database that can be used to study fire patterns on both a regional and global scale.
6. Types of fire seen through time.
Box 1. The formation of charcoal
When wood is burned there are distinct zones of combustion, charring, and pyrolysis (that is, heating in the absence of oxygen, which causes thermal alteration and decomposition) (Figure 7). This is important as it means that any branch or tree trunk, or indeed any piece of wood, burns in a similar manner. We are able to see this if we look at a piece of wood that has not been fully combusted. We have seen how a high-temperature pulse is needed to initiate the process (although in the rare case of self-ignition the temperature rise may be more gradual). At 20 to 110 °C the wood absorbs heat as it is dried giving off moisture vapour (steam). The temperature remains at or slightly above 100 °C until the wood is ‘bone dry’. At 100 to 270 °C the final traces of water are given off and the wood starts to decompose, giving off some carbon monoxide, carbon dioxide, acetic acid, and methanol. Heat is absorbed. At 270 to 290 °C, this is the point at which exothermic decomposition of the wood starts. At this point heat is produced and breakdown continues spontaneously, providing the wood is not cooled below this decomposition temperature. Mixed gases and vapours continue to be given off, together with some tar. Some of this tar may move further down into the charcoal structure and be precipitated there. This may result in the formation of glassy carbon that is often found by archaeologists, and that has been shown to be a result not of very high temperature but of the precipitation of compounds in the charcoal residue. At 290 to 400 °C, as breakdown of the wood structure continues, the vapours given off comprise the combustible gases carbon monoxide, hydrogen, and methane, together with carbon dioxide gas and the condensable vapours: water, acetic acid, methanol, acetone, etc., and tars which begin to predominate as the temperature rises. At 400 °C the transformation of the wood to charcoal is practically complete.
The charcoal at this temperature still contains appreciable amounts of tar, perhaps 30 per cent by weight trapped in the structure. This ‘soft burned charcoal’ needs further heating to drive off more tar and raise the fixed carbon content of the charcoal to about 75 per cent. To drive off this tar, the charcoal is subject to further heat inputs to raise its temperature to about 500 °C, thus completing the carbonization stage.
While this describes the formation of wood charcoal, a range of plant tissues follows a similar pattern. Despite this alteration of the cell walls, the anatomy of the plant is still preserved (Figure 7). However, mass is lost during this process so the plant drops in weight.
In addition, shrinkage of the various organs can occur. The carbon content of the plant tissues increases and the carbon domains within the cell wall become ordered. The consequence is to make the plant tissues more resistant to decay. This feature is the rationale behind the production of biochar as a mechanism for CO2 sequestration. The solid material that remains from this pyrolysis process is known as charcoal and itself may be consumed if the combustion process is not halted.
7. How charcoal forms. (a) Charred log showing cracking; (b) Section of charred wood showing charring zones and movement of volatiles; (c) Cut tree after wildfire showing only outer zone is charred.
The data collected by a number of different teams of scientists need to be standardized and data on charcoal flux may be calculated and departures from a background norm can be used to identify significant fire events. The record becomes more common over the past 2,000 years and has proved a powerful tool in the interpretation of fire regimes in relation to climate change and in relation to human activity, although not all changes are picked up using this method.
Charcoal records are obtained and interpreted rather differently in the Quaternary and Holocene (the last 2.5 million years) compared to those from earlier times. Small charcoal particles can easily be identified by their black colour and their often lath-like shape on the glass slides of organic residues obtained from rocks showing ancient pollen and spores, known as palynological slides, that have traditionally been used to identify vegetation and its changes in the rock record. This data has been used to interpret fire occurrence, and has led to the idea of constructing a global charcoal database. But while this approach works for recent deposits, more ancient records become more difficult to interpret. The first problem is related to the identification of small charcoal particles on the palynological slides. As organic material is buried, the colour of the organic particles changes because of increased heat experienced through the burial process (it gets hotter as you drill down through sedimentary layers) from yellow to brown to black—a process known as maturation or coalification. This means that distinguishing black charcoal particles from dark coalified particles becomes more difficult, although oxidative acids may play a useful role by preferentially changing the colour of the coalified particles from black or brown back to yellow. In addition, absolute dating becomes more difficult in rocks older than 70,000 years as carbon isotopic dating is not viable and other methods lack the precision to obtain yearly, decadal, century, or even millennial accuracy.
Another approach to interpreting fires in the past is to examine macroscopic charcoal (>180 µm but usually over 1 mm in length) that can be extracted from sediment or rock samples. Charcoal fragments in rocks may reach dimensions of centimetre cubes and even larger. These will predominantly have been buried near the fire site (such as in peats) or moved, usually by water, to a depositional site that could have been a river, lake, or even the sea. Experiments on charcoal settling rates and water transport have shown unexpectedly that larger charcoal particles can travel greater distances than smaller particles, as they take longer to sink in the water column.
The study of larger charcoal fragments has one distinct advantage (Figure 8). Charcoal may preserve the anatomical details of the plant being burned, so charcoals may provide evidence not only that there was a fire but also what vegetation was burned.
8. Modern and fossil charcoal. Light and scanning electron microscope (sem) images. (a) Fossil wood charcoal from the Jurassic rocks of Yorkshire (scale 1 cm); (b) sem of beech wood charcoal (scale 100 µm).
Surprisingly there have been few studies on macroscopic charcoal assemblages associated with microscopic charcoals from the more recent past. However, it is the data from macroscopic charcoal that have opened up the deep time history of fire although the interpretation of fire frequency can be much more difficult. For the use of charcoal to interpret fire history in older rock sequences it is necessary to be able to recognize such fragments in the rocks. Until now Pre-Quaternary charcoals have been studied in detail in a limited number of regions and time periods and our understanding of ancient fire systems is relatively recent, developing only over the past forty years or so.
Fire and the evolution of vegetation
It goes without saying that the history of wildfire is intimately tied up with the evolution of plant life on land. Vegetation provides the fuel to burn but the nature and quantity of that fuel has changed through the course of geological history since plants first appeared on land.
The earliest land plants evolved somewhere between 450 and 420 million years ago, during the Silurian Period of the Geological Timescale. However, although there may have been small patches of algae, mosses, and liverworts for a long time, we tend to think of the greening of the landscape as occurring from the evolution of vascular land plants (that is plants with specialized tissues for the transport of water and the products of photosynthesis). These plants had a number of characteristics that allowed them to live on land, including a cuticular covering with water/gas exchange pores (stomata). These green plants absorbed carbon dioxide from the atmosphere and water from the soil, and used energy from the Sun in the chemical process of photosynthesis to transform the water and gas into carbohydrate molecules such as sugars and other chemicals that allowed the plants to build their organic skeleton. In the process oxygen was released into the atmosphere as a by-product. The process of photosynthesis can be summarized in general terms in the following chemical equation:
As we shall see, this equation has great significance in our discussion of fire.
The second feature of these early vascular land plants is that they produced vascular tissue that comprises several cell types such as the water-conducting elements known as tracheids (xylem) that are made from cellulose impregnated with the strengthening chemical lignin, and phloem, which transports food for the plant.
Thirdly, the plants developed a reproductive strategy that involved producing spores. These spores were shed by the plant (known as the sporophyte) onto the damp soil surface. Here they developed into small predominantly underground gametophytes, which produced the male and female sex organs. Sperm from the male gametophyte swam in the damp soil to reach and fertilize the female gametophyte and a new generation of the plant, the sporophyte, was produced. This type of reproduction can be seen in ferns today.
The earliest plants from the late Silurian Period, around 420 million years ago, included the diminutive form known as Cooksonia. Indeed many of the earliest land plants were only a few centimetres tall, so they would not provide sufficient fuel for a large wildfire. In spite of that our earliest charcoalified plants that demonstrate the occurrence of fire on Earth came from this Period. Such fires would have been small and very localized. These early plants were restricted in their habitats to being near water and were patchy in their occurrence.
Through the subsequent period of geological time, the Devonian (419–358 million years ago), plants evolved a number of new strategies that allowed them to spread into new environments and increased potential fuel loads. Plants evolved a range of growth habits that helped in their spread. One of these was to reproduce by a mechanism known as clonal growth (where the plant reproduces vegetatively from a single individual). In such cases the plants may be connected underground. But all early land plants were restricted by their method of growth. They were only able to grow by primary growth via their growing tip (meristem), so growing tall was a significant problem. Some plant groups of the Devonian overcame this by developing a method of secondary growth: new cells are produced not at the growing tip alone but also around the circumference of a stem, in a layer between xylem and phloem known as the cambium. The cambial cells produce secondary xylem and phloem cells. These secondary xylem cells (wood) are chemically composed of around 70 per cent cellulose and 30 per cent lignin, so they provide significant strength. The ability for the plant to increase in girth allows the plant to become taller and develop an arborescent (or tree) habit. This also helps the plant live longer, and a larger potential fuel load is produced.
However, growing taller produces other effects too, not least the need to increase photosynthetic activity. Another structure evolved during the Devonian that solved this problem: the leaf. The increased area available via leaves to capture sunlight and photosynthesize not only helped some plants in their growth strategy but also created some shade on the soil surface, and this led to a further diversification of plants adapted to an ever-increasing range of habitats.
These early plants, even trees, still reproduced by spores so that the plants needed to have a ready source of water. It was the evolution of the seed habit in the late Devonian, around 370 million years ago, that finally allowed plants to spread into much drier habitats.
The spores that produce male and female gametophytes became differentiated, with those that evolve into female gametophytes growing bigger, into megaspores encased in ovules, and becoming retained on the plants and only released when they were fertilized to form a seed. The development of seeds allowed the plant to provide an increased food reserve for the new plant and to release it from the need to grow in damp soil. The seed habit allowed plants to grow and thrive in much drier habitats.
By the mid to late Devonian, as plants grew larger, providing more fuel, and spread into drier environments, larger and more frequent wildfires must have become possible. Yet this pattern is not seen in the record of fossil charcoal. As we have seen there are three parts of the fire triangle—fuel, heat, and oxygen. But in deep time, we need to consider a slightly different triangle (Figure 9).
9. Deep time fire triangle.
One side of the deep time triangle represents the evolution of plants or fuel; the second represents the evolution of the atmosphere, especially oxygen in the atmosphere; and the third represents climate. If we had a significant build-up of fuel by the mid to late Devonian and the plants were living in drier climates then why do we not see more indications of fire in the rock record? The answer almost certainly lies in the oxygen content of the atmosphere.
Fire and the atmosphere
For sustained fire we need a minimum of 15 per cent oxygen in the atmosphere, though recent research suggests a more realistic figure may be 17 per cent. Today the figure is 21 per cent. If we have a build-up of fuel on the landscape with dry conditions then the absence of fire may be the result of low atmospheric oxygen. The occurrence of fire in the late Silurian, 420 million years ago, suggests that oxygen levels were at least 17 per cent. The subsequent absence of fires for much of the period 400–360 million years ago could be a result of the atmospheric oxygen levels falling below this critical figure. The calculation of atmospheric oxygen has come traditionally from geochemical models that have used a range of data to predict oxygen through time. Some of these models suggest that there was a fall in oxygen levels through the Devonian and this may have been the reason for such limited or no fire activity. These models also suggest that oxygen levels rose again in the very latest Devonian and into the Carboniferous Period (359–340 million years ago) and we also see an increase in charcoal in rocks of this age, providing evidence of wildfire activity.
A number of geochemical models also indicate that from around 350–250 million years ago oxygen levels were significantly above the modern level of 21 per cent. Some models have suggested levels between 30 and 35 per cent oxygen during this period. The implications of this are significant. Experiments have shown that increasing oxygen content above 21 per cent significantly increases the possibility of fire. Increasing oxygen content allows wetter plants to burn. Some experiments have indicated that in an atmosphere of between 30 and 35 per cent oxygen even very wet plants could burn, and so fires would be difficult to extinguish. Clearly such levels of oxygen are unsustainable as the vegetation would always be burning. New models that have included fire feedbacks in their calculations still, however, indicate oxygen levels higher than that of the present throughout this time period.
There are no obvious proxies for atmospheric oxygen but it has been suggested that the charcoal fossil record might provide some clue as to changing levels. This idea is based on the amount of charcoal that is found in ancient peats that, through time, have been transformed into coal. We know that peats were formed in wet conditions not prone to fire, so that in modern peats there is less than 5 per cent charcoal present globally. For oxygen levels below 21 per cent there would be even less charcoal present, and none at all at 17 per cent oxygen. As oxygen levels rise up to 30 per cent the very wet plants may burn and charcoal levels can be expected to increase. Data from charcoal content in coal indicates that in the late Paleozoic Era—the Carboniferous and Permian Periods—many coals have over 30 per cent charcoal and some have over 70 per cent. Such data can be used to calculate atmospheric oxygen levels and the charcoal record shows, along with the traditional geochemical models, that the late Paleozoic was a period of high oxygen and also high wildfire activity.
Both approaches also show that oxygen levels varied considerably over the following Mesozoic Era (encompassing the Triassic, Jurassic, and Cretaceous Periods, 250–66 million years ago) but the mid to late Cretaceous (120–66 million years ago) was also a time when oxygen levels were higher than today. All data further suggest that the modern levels of oxygen were stabilized at around 21 per cent around 40 million years ago.
Fire and climate
In the world today climate plays a fundamental role not only in the occurrence of different vegetation types but also in the occurrence of wildfire. There are several aspects of climate that need to be considered. The first is temperature. As temperatures rise, fuels may dry and make fires more likely. However, rainfall is also important: if it is wet, then in spite of temperatures being high fire will not take hold, even if started by lightning strike. More subtly the length of dry and wet periods may play an important role. If there are wet winters and springs yet dry summers this may lead to fire activity but if it is wet all year long then fire would be unlikely (rainfall limited). In areas of prolonged dryness plant growth is much reduced, and as a consequence, fuel build-up would be less as fire that could more easily start because of the dry condition of the fuels would not develop because of the lack of fuel to burn (fuel limited).
The differences in both rainfall and temperature not only play an important role in the types of vegetation that may be found but also in the occurrence of fire in these different environments. We might not expect much natural wildfire in tropical rainforests as a whole as while there is a significant amount of potential fuel the rainfall does not allow it to dry naturally and hence be available for fire. The intervention of humans may change this. This is one reason for the outrage generated by the deliberate starting of fires across the Amazon rainforest in 2019. Mediterranean-type climates with long hot summers make for very fiery regimes but even in cooler areas such as in boreal forests periods of dryness may lead to extensive fires. It is a surprise to many that Alaska experiences large numbers of wildfires.
The distribution of fire on Earth has only been fully understood with the development of satellite monitoring. It has allowed us to understand more about natural flammable vegetation biomes as opposed to those in which fire is unnatural but may potentially be modified by human activity.
Low and high fire worlds
The realization that atmospheric oxygen levels play an important role in the occurrence and amount of fire through time has led to the idea of low and high fire worlds. We may consider that the Devonian Period was a low fire world, initially because of limited fuel but later because of reduced atmospheric oxygen concentration. In contrast, the Late Paleozoic was a high fire world even though climate changed considerably through that time, and it was predominantly an icehouse world with polar ice. But while the history of fire through these early times provides many interesting insights into how fire and the biosphere evolved, it is the history of fire through the subsequent periods that has most relevance to our current understanding of wildfire.
The vegetation of the Mesozoic, in particular the Triassic and Jurassic (250–145 million years ago), was dominated by seed plants and the oxygen levels varied significantly over the time period but did not fall below the 17 per cent level. By far the most important changes took place on Earth, however, during the Cretaceous Period (140–66 million years ago).
One of the most important changes in vegetation occurred during the Cretaceous, with the evolution of flowering plants (angiosperms). These plants developed new ways of reproduction via flowers and pollination that were successful both in wind but also most especially through insect pollination. During the early part of the Cretaceous (140–100 million years ago) land floras were dominated by a range of seed-bearing plants including conifers, cycads, and cycad-like plants known as the Bennettitales. Ground cover was dominated by ferns and horsetails, both spore-bearing plants. The atmospheric oxygen levels rose through the Cretaceous and by around 120 million years ago many types of vegetation burned, both conifer and fern-dominated types, extensively during this high fire world.
This, however, was when the flowering plants first evolved. The changes in fuel structure may have significantly altered fire regimes. Many early flowers were weedy plants that thrived in disturbed environments. So frequent fires would have aided the spread of these early flowers and many flower fossils found from this period are preserved as charcoal (Figure 10).
10. Scanning electron micrograph of fossil charcoalified Cretaceous flower.
Our understanding of this high fire world and its impact on the evolution of life has been considerably enhanced by molecular studies looking at the DNA of modern plants. For example, both pines (conifers) and Proteaceous plants have been predicted using molecular clock techniques to have evolved their ability to survive fire during the Cretaceous, and recent fossil finds have tended to support this hypothesis.
Fire is likely to have played a significant role in landscapes that were dominated by the dinosaurs. The extinction of the dinosaurs and many other animals and plants in a mass extinction some 66 million years ago has been linked to a large asteroid impact. The impact is quite well established, and left the massive Chicxulub crater in Mexico, and many accept that it played a significant role in the mass extinction, although large-scale volcanism and climate changes are also likely to have been major causes. The suggestion of a global wildfire following the impact has, however, received much less support. Research suggests that there is insufficient evidence for a global wildfire and also experiments and a general understanding of wildfire dynamics suggest that such a global wildfire was unlikely, even in the high fire world of the Cretaceous.
After the Cretaceous, changes in climate, with increased rainfall together with the evolution of tropical rainforests around 40 million years ago, suppressed fire activity. The move from a relatively high global temperature during the Eocene Epoch (46–34 million years ago), to a much cooler world (going from a greenhouse to an icehouse world) from 20 million years ago until the present, shaped the development of modern vegetation and fire regimes. However, a significant change took place around 7 million years ago, during the Miocene Epoch, with the spread of savanna grasslands, especially in Africa.
Grasses first evolved during the early part (between 66 and 30 million years ago) of the Cenozoic Era (66–2.5 million years ago) but these had a traditional biogeochemical pathway for photosynthesis known as the C3 pathway. However, during drier intervals some grasses developed an efficient new pathway known as C4. This helped grasses thrive and spread in drier climates and soils. The rapid growth of grasses in drier habitats also provided a significant surface fuel load, giving rise to a grass-fire cycle, which itself aided the spread of such grasses and resulted in the development of savannas such as are seen in Africa today. While fire may burn off dead dry plants the roots of the grasses are not killed. If fire occurs regularly (i.e. every ten years or so) then competing larger plants that may grow into shrubs or trees are killed. Fire, then, is important for the maintenance of large tracts of savanna in many parts of the world. We may then consider that the modern fire world began around this time, 7 million years ago.
Evolution of fire traits
Plants have evolved a number of traits that help them live in a variety of conditions or habitats. It is probable that a trait that evolved in response to some other environmental pressure may be found to be useful in a fiery landscape. For example, having a thick bark may provide a tree with a number of advantages as it protects the outer cambial growth layer of a tree or shrub. There is no doubt, however, that a thick bark layer is particularly advantageous to some conifers, such as pines, which experience regular fire. Such a trait may be thought of as a fire trait. Other traits that plants have developed that have been associated with fire include re-sprouting, serotiny (an ecological adaptation by which seed release is predominantly in response to being heated by fire), and germination in response to heat or smoke. However, it has been suggested that these traits are not simply an adaptation to fire but rather to a fire regime. A fire regime includes characters such as fire frequency and fire intensity, as well as patterns of fuel consumption (Figure 11).
11. The fire regime triangle.
Recent molecular analysis of a range of plant groups including pines and Proteas have suggested that these traits, important to plants living in a fiery landscape, evolved in the high-fire world of the Cretaceous Period, around 100 million years ago. So the traits may be considered adaptive in fire-prone environments and convey a resilience to specific fire regimes (Table 2).
Table 2. Fire traits in plants
| Fire-related plant traits | Description | Comment |
|---|---|---|
| Thick bark | Fire resistant tissues and self-pruning of branches from tree—adaptation to frequent surface fires, e.g. pines. | Bark thickness strongly influences stem survival. Trees and shrubs with buds above ground are most vulnerable to fire damage. |
| Post-fire re-sprouting, woody species | Often below-ground re-sprouting from meristomatic tissues followed by post-fire re-sprouting. Epicormous growth such as in Eucalyptus. | Eucalypts have buds deeply embedded in the bark and are able to re-sprout even after severe fires. Basally sprouting woody plants can regenerate their whole canopy. Clonal spread is often stimulated after removal of above-ground stems. |
| Fire-stimulated germination | Often found in fire-prone systems such as chaparral. | Heat shock germination is common in a wide range of flowering plant species especially from Mediterrannean-type climates. |
| Smoke-stimulated | Smoke-stimulated flowering such as in some Proteas. | Found in a large number of plants including many angiosperms. |
| Serotiny | Canopy seed storage and fire-stimulated seed release. Often associated with crown fire regimes. | Found in many conifers including Pinus but also in some southern hemisphere taxa of both conifers and angiosperms. |
| Pyrophytic annuals | Regular germination following fire. | |
| Fire-stimulated flowering | Effective in synchronizing recruitment to the post-burn environment. | Found in many monocot angiosperms including grasses and orchids. |
We have already noted that some pines that first evolved in the Cretaceous developed a thick bark that helped them survive frequent fires. However, some taxa also self-prune their lower branches, opening up a gap between litter on the forest floor and the tree canopy by removing ladder fuel. Other pines, especially those living in higher altitudes such as jack pine and lodgepole pines, do not possess thick bark but have evolved a cone that opens only after a high-intensity, stand-replacing fire and the seeds are then released.
Another adaptation that some trees have evolved is re-sprouting (Figure 12). Such is the case with eucalypts in Australia, which may experience high-intensity crown fires but have developed a technique of re-sprouting from different parts of the plant.
12. Fire adaptive plant traits. (a) Re-sprouting; (b) serotiny.
Some plants, such as Proteas, many of which thrive in the Cape area of South Africa, have developed a sensitivity to smoke whereby they sense a coming fire and produce seeds that are released only after the fire has passed. Dormant seeds in the soil seed bank may also sprout following a fire.
While ascribing a particular plant trait to fire may be problematic there is no doubt that fire has been a major pressure in some ecosystems throughout geological history and adaptations to one environmental factor may also have been useful and selected for because of fire. A good example is clonal growth. This is a habit that first evolved in the earliest land plants but is a particularly useful trait in disturbed environments, even for example in volcanic terrains. Horsetails, a group of plants that became so important through the Carboniferous and after, developed this strategy and exhibit it today, much to the despair of gardeners trying to eradicate them from their garden. Yet clonal growth is useful in fiery regimes. One of the largest clonal plants is the quaking aspen (Populus tremuloides; it is in fact the world’s largest plant in terms of biomass and also one of the oldest), which often thrives following fires, and is especially associated with pine forest fires.
An increasing number of studies now recognize the importance of fire in the ecology of a number of different vegetation types. We may think of different communities of plants, some of which may be totally destroyed by fire but others of which may not only survive fire but in some cases need fire. Fire cannot and should not always be excluded from the landscape. The integration of fire studies with traditional plant ecological studies has provided significant debate. We now have a better understanding of the role of fire on Earth, and that has proved fundamental to conservation policy. It is now appreciated that many of the grasslands of Africa are ancient and are maintained by fire rather than simply be degraded forest. Excluding fire for some ecosystems may have a devastating consequence for some habitats. Fire, then, may be thought of as an ecosystem process and fire regime may be considered as the complex interaction of primary productivity, seasonality, ignition source, and fuel structure. One element that is key is the fire return interval. It is possible for one type of vegetation to be completely destroyed by a fire where there are rare or absent fires, such as in tropical rainforest. In contrast, some grasslands thrive on fire return intervals of 1–25 years. A change in fire frequency even in vegetation that is well adapted to fire may have catastrophic results.
Fire and animals
While we often consider the relationship between fire and plants and the increasing acceptance that fire has shaped many plant adaptive traits, the relationship between fire and animals has been explored far less. Juli Pausas of the University of Valencia, Spain, has documented a wide range of interactions of fire and animals, though this type of study is in its infancy (Table 3). He suggests that fire is an important evolutionary driver for animal diversity. This is not an aspect that has been explored in the fossil record.
Table 3. Possible benefits to animals of fire and fire-altered habitat
| Benefit | Category | Fauna |
|---|---|---|
| Fresh grasses, and leaves | Food resource | Herbivores, e.g. large mammalian grazers, insect herbivores, arboreal marsupials |
| Fire-released seed, and more exposed seeds in the soil | Food resource | Granivores including rodents, seed-removing ants |
| Animals fleeing or dying | Food resource | Predators, scavengers (e.g. birds, kites, owls, ants) |
| Weakened and dead trees | Food resource | Bark beetles, cavity-dependent (hollow-nesting) animals like woodpeckers, other birds, lizards, possums |
| Dead wood | Food resource | Saproxylic insects |
| Flowers, post-fire blossom | Food resource | Insect pollinators, hummingbirds |
| Meeting point | Mating cue | Saproxylic insects, smoke flies, mole crickets |
| Synchronization of the emergence | Mating cue | Insects (some beetles) |
| Reduced habitat complexity: increased visibility | Habitat alteration | Birds of prey; large herbivores, primates (easier to move and detect their predators) |
| Reduced habitat complexity: movement through the environment | Habitat alteration | Grouse (gaps for mating); seed-dispersing ants (move further with fire) |
| Microclimate change | Habitat alteration | Ectotherms—e.g. thermophilous reptiles, insects (warmer post-fire environment) |
| Reduction of parasites | Biotic interactions | Vertebrates |
| Reduction of predators | Biotic interactions | Insects (e.g. reduction of insectivore vertebrates) |
Modified from Table 2 by permission from Springer, Evolutionary Ecology, ‘Towards an understanding of the evolutionary role of fire in animals’, Pausas, J. G. and Parr, C. L, COPYRIGHT 2018, 32:113–125 https://doi.org/10.1007/s10682-018-9927-6 with permission of author.
Pausas’s hypothesis is based on the observation that many animals present today in fire-prone landscapes have characteristics that contributed to their adaptation to open environments and also that in some cases animals inhabiting fire-prone ecosystems may show specific fire adaptations (Figure 13).
13. Some fire adaptive traits in animals. (a) An emu that blends into a burned grassland. (b) An owl hunting in advance of a fire front.
In 2017 one such example hit the newspaper headlines, when it was shown in Australia that some birds of prey may even have started fires. Intentional fire spreading by several raptors such as the Black Kite, Whistling Kite, and Brown Falcon has been reported in Australian savanna grasslands. The birds have been observed to grasp burning twigs in their talons or beaks, apparently to encourage the spread of a fire so they may have easier access to their prey. Many animals, such as insects, can also thrive following a fire.
Pyrogeography and pyrodiversity
The increasing use of satellite data has transformed our understanding of fire and shows just how widespread and common fire is on Earth. This revolution has led to a broader recognition of the special and temporal patterns of fire, which has led to the new science of pyrogeography that has been championed by David Bowman of the University of Tasmania. It has been demonstrated that there is a significant relationship between net primary productivity (which is controlled by climate) and area burnt. Clearly in those areas that are very dry, such as deserts, the area burned is constrained by the available fuel, which is low because of low primary productivity.
But in areas of high primary productivity such as rainforests the burnt area is also controlled by climate, with rain excluding fire. In areas which have intermediate levels of primary productivity and a high frequency of dry periods, sustainable fire may be more widespread (Figure 14). Such environments include tropical savannas, which are the most flammable widespread environments on Earth. These systems have both abundant fuels and suitable weather conditions to sustain fire. Savannas have hot wet seasons that promote growth and hence fuel, and this is followed by a dry season that helps dry this abundant fuel, and weather that promotes fire (Box 2).
14. Plant productivity, fire, and switches. (a) Increasing fire with plant productivity; (b) Productivity and fire frequency; (c) Fire switches and time.
Box 2. Savanna grasslands and wildfire
Savanna grasslands are widespread across different continents but all are part of a diverse grass-fire cycle. Savannas are found in central and southern parts of Africa, in Australia, especially in the north tropical regions, parts of India, and in many regions of South America, Brazil in particular. Within Brazil there are a range of savanna types, each with a different incidence of fire. The best known of these is called Cerrado. As is the case with all savannas, the frequency of fire controls the growth of shrubs and trees in the ecosystem. In some cases if there are small trees that lose their leaves at drier times of the year the grass-rich ground cover becomes more evident. Such regions have diverse animal communities and fire also produces a patchwork of varied ecosystems. In the north of Brazil, on the borders of Venezuela and Guyana, indigenous peoples have successfully used fire to maintain their local agricultural practices, but the range of pressures from other competing activities such as from logging or from rainforest protection (such areas border rainforest) means that local fire practices are under threat even where fire is being used in a controllable and ecologically sensitive manner. Fire use by indigenous communities involves a process not just of land clearance and use but also of protection against even larger fires. Is it the duty of a central government to make fire policy or can local communities with their fire knowledge make such decisions?
Cerrado, which originally covered around 25 per cent of Brazilian territory, is the largest savanna area in South America. Unlike African savanna it is characterized by a layer of grasses but also contains small palms, trees, and shrubs. Like many other fire-prone regions, it has a wet season promoting high biomass (fuel) production and a long dry season. Also in common with other such areas the grasses are highly resistant to fire and rapidly re-sprout after fire. The fires that occur here, whether natural or human set, are surface fires.
A major problem in such regions has become the significant replacement of the native vegetation by a combination of introduced exotic grasses and mechanized agricultural practices that have altered the natural fire regime to more frequent fires, which have a significant impact on the ecosystem.
It has been claimed that there may be alternative stable states of vegetation that exist within a region depending on whether fire is present or absent.
An understanding of pyrogeography is proving vital to our approach to conservation as the distinction between ‘natural’ and ‘human-started’ fire becomes ever more blurred. As we shall see, humans are altering vegetation, and that has a dramatic impact upon fire regimes. A key element in our understanding of fire is the boundaries between flammable (pyrophylic) and less flammable (pyrophobic) vegetation.
To the recent concept of pyrogeography we can add the emerging idea of pyrodiversity, again championed by David Bowman, and defined as the coupling of biodiversity and fire regimes in food webs. The importance of feedbacks relating to fire regimes, biodiversity, and ecological processes has been emphasized in this approach. In this context humans can both alter and shape pyrodiversity through the manipulation of fire, in terms not only of frequency of burn and severity of burn, but also of the timing and extent of burning. In addition, fire systems can be altered by the introduction of exotic or invasive species. Clearly any discussion of conservation of biodiversity needs to consider pyrodiversity and an appreciation of the role that fire has played and still plays on Earth.