The interaction of fire and mankind

We are becoming increasingly familiar with the potential influence of fire on modern short-term timescales, however the major feedbacks that operate on long timescales are inherently different. Changes in the abundance of atmospheric oxygen (O₂) are the main long-term forcing on fire-activity. Laboratory experiments have explored the sensitivity of fire to changes in O₂ and report that ~16% O₂ is required to enable self-sustaining combustion. The record of charcoals over the past 420Ma indicates that O₂ has never fallen below 16%. However, the amount by which it has risen to super-ambient levels remains debated because it involves consideration of a complex interplay between vegetation and fire. We have used the Cretaceous period to explore aspects of this interplay because it is a time that was likely characterised by high O₂ and major evolutionary innovations in land plants. We coupled laboratory experiments with mathematical models of fire spread to create the first predictions of palaeofire behaviour. These were based on changes to fuel driven by the invasion of early angiosperms. We explored the implications of these changes on long-term carbon and oxygen cycles, by imposing an increase in the flammability of ecosystems in the COPSE biogeochemical model between 135-100Ma. This coupled O₂-angiosperm flammability increase strengthens negative feedbacks that regulate O₂ revealing that by 80Ma atmospheric O₂ was lowered from ~28.5% to ~25%. We conclude that past evolutionary innovations in plants not only had major consequences for fire regimes, but that these altered the strength of Earth system feedbacks that affect the habitability of our planet. 

Numbers of animal species react to the natural phenomenon of fire, but only humans have learnt to control it and to make it at will. Natural fires caused overwhelmingly by lightning are highly evident on many landscapes. Birds such as hawks, and some other predators, are alert to opportunities to catch animals including invertebrates disturbed by such fires and similar benefits are likely to underlie the first human involvements with fires. Early hominins would undoubtedly have been aware of such fires, as are savannah chimpanzees in the present. Rather than as an event, the discovery of fire use may be seen as a set of processes happening over the long term. Eventually fire became embedded in human behaviour, so that it is involved in almost all advanced technologies. Fire has also influenced human biology, assisting in providing the high quality diet which has fuelled the increase in brain size through the Pleistocene. Direct evidence of early fire in archaeology remains rare, but from 1.5 million years ago onward surprising numbers of sites preserve some evidence of burnt material. By the Middle Pleistocene recognisable hearths demonstrate a social and economic focus on many sites. The evidence of archaeological sites has to be evaluated against postulates of biological models such as the ‘cooking hypothesis’ or the ‘social brain’, and questions of social co-operation and the origins of language. Although much remains to be worked out, it is plain that fire control has had a major impact in the course of human evolution.

Fire history reconstructions from lowland Amazonia provide insights into millennial-to-centennial scale trends and drivers of Neotropical fire activity since the Ice Ages.  Sedimentary proxies are used to reconstruct the development of fire and vegetation within lowland tropical ecosystems, including the seasonally-deciduous tropical forest, Cerrado, tropical rainforest, and flood-adapted forests and savanna systems in lowland Amazonia.  Sedimentary charcoal archives were compared to regional paleoclimate reconstructions and evidence for human land-use in the lowland Neotropics to understand drivers of past fire regimes. A network of Neotropical charcoal records captures long-term trends in fire activity, suggesting regional heterogeneity that is likely linked to natural climate variability.  In recent decades, and on long-time-scales, fire activity has been more prevalent in tropical ecosystems with greatest seasonality, which also corresponds to regions with elevated human land-use. Results of this research suggest fire frequency has been decreasing in the tropical lowland tropics during the last several thousand years, reaching a nadir in the past millennia, particularly following the demographic collapse after the Columbian Encounter around ~AD 1500. The long-term history of fire in the Neotropics is linked to natural climate variability, and potentially, may have been amplified (or dampened) at times by changing human demographics. The present view of Amazonia as a landscape without fire and containing pristine forests needs to be reevaluated. 

Since ancient times people have recognized fire as fundamental yet different.  For centuries Westerners tried to imagine it as a substance or an informing principle; but however conceived, it was integral and central, as it was in daily life.  This changed with the Enlightenment.  Fire disintegrated into subcategories of chemistry, physics, and engineering.  Interestingly, this transformation occurred at the same time that fire as a source of general power shifted from burning living biomass to fossil biomass.  With industrialization working fire has increasingly disappeared from the quotidian world, as it did from the intellectual world. Today, in developed societies it flourishes primarily in nature preserves and amid sites of social unrest.

Yet it may be reasserting a claim to significance.  It is the source of power behind the many global changes wrought by humanity and summed in the expression Anthropocene.  It has also become fundamental to the management of nature preserves, where its removal has unhinged ecosystems.  An increasing number of phenomena connect to humanity, as the keystone species for fire on Earth, shifted its fire practices.   Perhaps the time has likewise come to reassert a claim for fire's intellectual presence.  The other ancient elements have academic disciplines devoted to their study.  The only fire department on a university is the one that sends emergency vehicles when an alarm sounds.

Recent studies have suggested that the first arrival of humans in the Americas during the end of the last ice age is associated with non-trivial anthropogenic influences on landscape, in particular with the use of fire which would have given even small populations the ability to have broad impacts on the landscape. Understanding the impact of these early people is complicated by the dramatic changes in climate occurring with the shift from full glacial to full interglacial conditions. Despite these difficulties here we attempt to test the extent of anthropogenic influence using the Californian Channel Islands as a smaller, landscape scale test bed.

In 1954 Phil C. Orr discovered two human femora on Santa Rosa Island, California Channel Islands deposited in water lain sediments at the mouth of Arlington Springs Canyon which have become known as ’Arlington Springs Man’ direct and indirect radiocarbon suggest an age no younger than c.13,000 yrs BP. The probable stratigraphic positioning of the human remains has also been recently ascertained. This presentation places these existing investigations into a wider stratigraphic framework of the California Channel Islands. Over 20 field sections from Arlington Canyon coupled with >30 new radiocarbon dates (dating from c.20-11,000 yrs BP) has allowed a detailed chronostratigraphy to be developed for the Last Glacial-Interglacial Transition. Landscape changes and development both before and after ’Arlington Springs Man’ are discussed including possible changes in fire activity, vegetation, soil development and fluvial processes.

Forests and human communities in western North America are now extremely vulnerable to large, severe wildfires as a consequence of fire exclusion and other land use practices and increasingly warm and persistent droughts. Combined paleoecological and archaeological studies of climate, fire and human histories from the same landscapes can help reveal the relative roles of people and climate in driving past and modern trends in fire regimes. In this paper we evaluate human and climate controls of wildfire using (1) a mountain range-scale network of more than 1,000 fire scarred trees from the Jemez Mountains, New Mexico (~2,000 km²), and (2) a continental-scale network of fire history reconstructions from more than 800 sites in western North America (~3,000,000 km²), including 2,000 year-long fire histories from giant sequoia groves in the Sierra Nevada of California. The Jemez Mountains are an instructive case of the influences of long-time human occupation on fire-prone, forested landscapes. Here, in ponderosa pine and mixed conifer forests, we find that humans eliminated widespread surface fires during peak populations prior to 1650 CE, whereas many small fires still occurred at locations distant from villages and agricultural fields.  After depopulation of forested areas circa 1680 CE, widespread surface fires became much more regular and these extensive fires were highly correlated with wet/dry switching of moisture regimes. Continental-scale fire climate patterns emphasize the consistently important roles of inter-annual to decadal-scale climate variations on spatial and temporal fire activity, regardless of human influences at finer spatial scales during recent millennia.

Australia is an island continent with ecosystems renowned for their flammability, high endemism of plants and animals, the keystone role of marsupial herbivores and carnivores, ancient Aboriginal traditions of landscape burning, and more recently, extraordinarily high mammalian extinctions and irruptions of mammalian herbivores and carnivores introduced by Europeans.

There much debate about the causes and consequences of the recent extinctions in uncleared Australia landscapes. Several prominent theories have stressed, to differing degrees, the importance of change fire regimes following the breakdown of Aboriginal land management and the introduction of non-native herbivores and carnivores.  Some of these theories have also highlighted the legacy effects of the initial impact of Aboriginal colonisation in the late Pleistocene, including the extinction of very large marsupial herbivores and carnivores, collectively known a ‘megafauna’. 

It is possible that specific spatiotemporal patterns of landscape burning (pyrodiversity) influences habitat quality and hence mammal diversity and abundance. Recursively, pyrodiversity is reinforced by mammalian food webs via a range of ecological processes (such as nutrient cycling, soil turnover, plant regeneration and growth, plant species diversity because mammals affect ecosystems process via digging, trampling, phytomass off-take, seed and spore dispersal, and concentrating nutrients) all of which directly and indirectly influence fire regimes. Critically, humans are both keystone predators that regulate herbivores and affect pyrodiversity by controlling the frequency, extent and seasonality of landscape fire.

This human controlled food web – pyrodiversity hypothesis has substantial management implications for restoring ecosystems in the Anthropocene globally. While manipulating fire regimes, humans must simultaneously manage mammalian food webs, possibly by introducing new species to compensate for the extinction of herbivorous and carnivorous vertebrates that have occurred recent and more distant past.

The African continent has the largest extent of savannas and associated grasslands in the world. The savannas support frequent fires so that Africa also accounts for the largest fraction of the world’s annual burnt area than anywhere else. Thus Africa is an interesting test case for the influence of fire on vegetation and of humans in shaping fire regimes and thereby shaping the vegetation.  For more than a century, Africa’s grasslands have been widely interpreted as the products of deforestation promoted by anthropogenic burning. The deforestation hypothesis has led to official policies of fire suppression in many African countries. It has motivated plans for large-scale ‘reforestation’ projects targeting, in particular, Africa’s humid savannas and grasslands. Here I review different lines of evidence for the age and origins of African savannas and the fires that maintain them. These point to ancient origins of the grassy biomes long before intensive human use of fire. Nevertheless, except for the last few decades, there is considerable uncertainty as to the influence of people in expanding the area of derived savannas at the expense of the forest they are supposed to have displaced.  A better understanding of the attributes of ancient versus secondary grasslands is needed to evaluate human impacts on the spread of savannas.