By Dulcinea Groff
Picture the landscape of a tropical savanna, composed of grasses, shrubs and a sparse number of trees. The savanna biome is dominated by a wet season supplying thousands of herbivores with forage and a dry season accompanied by intense lightning and fire. These wildfires maintain the savanna as grassland by killing the saplings (suppressing tree growth) and the grasses quickly regenerate. Imagine early hominids in the fire-constructed savanna, they begin to use fire, control fire and even make it! Fire is a permanent link now between biome and human.
Fire was used by humans for presumably many reasons: communication, prepare food, drive and corral prey, warmth during cold periods, etc. Humans have long suppressed and ignited fires for various reasons. In fact, as a source of ignition, humans have been shaping landscapes since the earliest known hominids were thought to use fire one million years ago in South Africa (Berna et al. 2012).
Several things have to be *just right* in order to have a wildfire, and we know this from present day parameters for wildfire conditions. Fire ecologists recognize the conditions that must exist to have a wildfire on the landscape and the ecological value of regenerating specific plant taxa to larger ecosystem functions. The U. S. Forest service uses prescribed fires to reduce invasive species, reduce pest insects and disease, improve soil health (accelerating nutrient recycling), and maintain the existence of fire-dependent species.
We can then deduce that natural wildfires in the past were valuable for these same reasons, to improve ecosystem function. The temperature, humidity, wind, moisture of the vegetation, and fuel load are regarded as important variables in achieving a wildfire. Not only must the climatic conditions be fire-conducive (warm temperatures and low precipitation), but the right vegetative characteristics (e.g. xeric woodlands, scrub-steppe, grasslands, etc) must exist on the landscape, and a source of ignition is essential for a wildfire to occur. Several sources of ignition exist for paleofires: humans, lightning, and volcanoes (Fig. 1 and 2).
Paleoecologists study fire by using charcoal found in sediments as a proxy for past vegetation occurrences that can be correlated to past climatic conditions. Charcoal evidence of paleofires can tell a story of how climate potentially influenced a powerful ecological process on terrestrial landscapes. More specifically, this information may illustrate how fluctuations in precipitation, and higher spring and summer temperatures influence vegetation.
This provides clues to the types of vegetation that existed, periods of succession, and even insight into behavior of humans or herbivores. However, explaining the observed fire patterns from charcoal is not always straightforward. After ruling out volcanic activity as a source of ignition, a paleoecologist may find difficulty in detecting the difference between human and lightning sources of wildfire ignition (Markgrav 1993).
One approach to resolve the matter is to first decide whether the region lacks high convective airflow needed to produce lightning. If this is the case, it might be logical then to investigate the role of paleo-indian fires. If a charcoal investigation is omitted, then any attempt to reconstruct and interpret the patterns of the vegetation on the terrestrial landscape may be obscured. One may simply attribute these changes in vegetation to be driven by climate change. On the other hand, changes in vegetation at a regional scale may have been driven by human fire regimes versus a climate change explanation.
When and where did paleofires occur? Have they occurred on every continent? Currently, several groups devote their efforts to study paleofires. The international Global Paleofire Working Group (initiated by Dr. Jennifer Marlon) pulled together to resolve these questions. The product is a geographic distribution of paleofire sediment records from various depositional environments called the Global Charcoal Database. The distribution of known paleofires (Fig. 3) is nearly global (minus Antarctica). There also appears to be a high number of paleofire records in North America. Perhaps the conditions and vegetation were more conducive to wildfire in North America, or maybe there is a bias in sampling on part of the interests of a wealthy nation capable of investing in past climate and landscape reconstructions.
The Global Charcoal Database has been used in analyses to show how abrupt climate changes influence wildfires/biomass burning (Marlon et al. 2008; Marlon et al. 2009). For example, regions situated outside of the tropics in North America, Europe and South America had reduced fire regimes during deglaciation (21 to 11 kya BP). Then, fire activity in the Northern Hemisphere increased during the early and mid Holocene. The rapid climate change from 15 to 12.9 kya leading up to the Younger Dryas increased the average number of fires in North America. The cooler temperatures of the Younger Dryas cold period halted the increasing fire activity from 12.9 to11.7 kya. Then fire frequency increased again, after 11.7 kya, in response to an abrupt rise in temperature. Fire activity also responded to variations in temperature in the Northern Hemisphere during the Medieval Warm period by increasing and decreasing during the Little Ice Age period.
What is the value in knowing about paleo wildfire events? Learning about the vulnerability and resiliency of ecosystem processes to fire disturbance during past climate scenarios is valuable because it advances the forecasting of fire events. One international research group, the WildFIRE PIRE is working with scientists, land managers, and non-profit conservation organizations, and focusing on the “causes and consequences” of fire in the past and future relative to anthropology, biodiversity conservation, fire climatology, etc.
Fire is an important force in shaping the structure and function of terrestrial landscapes and it is closely tied to the climate system. Humans have long manipulated wildfire, but with the forecast for fire frequency set to increase, it may become more challenging to manipulate wildfire in the future human-biome link. It is likely new and more complex interactions between ecosystems and the climate system will arise in the future of our abruptly changing climate.
Berna, F., P. Goldberg, L. K. Horwitz, J. Brink, S. Holt, M. Bamford, and M. Chazan. 2012. Microstratigraphic evidence of in situ fire in the Acheulean strata of Wonderwerk Cave, Northern Cape province, South Africa. Proceedings of the National Academy of Sciences 109: 1215-1220.
Markgrav, V. 1993. Younger Dryas in South America. Quaternary Science Review 12: 351-355.
Marlon, J., P. Bartlein, C. Carcaillet, D. G. Gavin, S. P. Harrison, P. E. Higuera, F. Joos, M. J. Power, C. I. Prentice. 2008. Climate and human influences on global biomass burning over the past two millennia. Nature Geoscience 1: 697–701.
Marlon, J., P. Bartlein, M. K. Walsh, S. P. Harrison, K. J. Brown, M. E. Edwards, P. E. Higuera, M. J. Power, R. S. Anderson, C. E. Briles, A. Brunelle, C. Carcaillet, M. Daniels, F. S. Hu, M. Lavoie, C. J. Long, T. Minckley, P. J. H. Richard, A. C. Scott, D. S. Shafer, W. Tinner, C. E. Umbanhowar Jr, C. Whitlock. 2009. Wildfire responses to abrupt climate change in North America. Proceedings of the National Academy of Sciences 106: 2519–2524.
Power, M. J., J. R. Marlon, P. J. Bartlein, S. P. Harrison. 2010. Fire history and the Global Charcoal Database: A new tool for hypothesis testing and data exploration. Palaeogeography, Palaeoclimatology, Palaeoecology 291: 52-59.