Pyrogeography

Origins

Before humans, climate determined the general fire patterns in accord with climate-soil-vegetation relationships. Precipitation produced fine grass fuels. Seasonal lightning propagated fire from grassy lowlands into forested uplands and vice versa. Fire recycled nutrients and thus promoted biodiversity. It was by this process that fire renewed the landscape (see surface fire photo).

Surface fire reduces ground fuels, recycles nutrients, and maintains open, healthy conditions.

Source: U.S. Forest Service.

Processes

Fire is a keystone ecological process, standing among the few natural phenomena humans can control. Pyrogeography seeks to clarify the evolutionary importance of fire and human culture. Before humans, fire was ignited primarily by lightning and occasionally by geological actions (e.g., volcanism and rock falls). Today, many wildfires are started by people. Combustion potential is controlled by vegetative fuel moistures: Live fuel moistures respond seasonally to broadscale interannual variations in precipitation, whereas dead fuel moistures can respond hourly to local meteorological conditions (e.g., relative humidity, wind).

(A) Bright areas represent high live-fuel moisture stress, thus an elevated fire potential. Southwestern North America shows particularly high stress in June. (B) The arrival of monsoonal rains in summer eases live-fuel moisture stress across southwestern North America by September.

Source: Yool, S. R. (2004). Wildfires. In J. Stoltman, J. Lidstone, & L. M. DeChano (Eds.), International perspectives on natural disasters (chap. 5, pp. 107–130). London: Kluwer.

Fuels are both a source and a sink for atmospheric carbon: Emissions from biomass burning are sources of particulates and carbon dioxide (CO2) that affect Earth's radiant energy budget and atmospheric chemistry, influencing climate on regional and global scales. Live fuels harvest atmospheric CO2 and thus complement Earth's oceans as terrestrial sinks for planetary carbon. Fire scars in tree rings and charcoal in sedimentary deposits record terrestrial carbon exchanges through deep time, showing the close coupling between climate, fire, and vegetation. Modern fire atlases from ground observations and satellites show increased fire activity in many regions. Complex combinations of climate warming and drying, land use change, and the spread of pyrogenic invasive plant species likely explain these recent changes.

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