Fire ecology of American savannas and grassland-forest mosaics
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abstract
For most of the past century, forestry has been dominated by the view that climate controls biome distributions; that forests are the climatic-climax vegetation in all but the driest of regions; and, that fire is a recent, human-caused phenomenon. These ideas are proving untenable in light of evidence that fire-maintained savannas have existed for millions of years and often co-occur with forests in mosaic landscapes, under the same climatic conditions. Indeed, across much of the globe, fire - as both an ancient ecological force and a tool of human land management--controls biome distributions and the carbon cycle. Because of land-use change, it is often difficult to identify where contemporary fire regimes (i.e., the frequency, intensity, and seasonality of fire) are consistent with, or deviate substantially from, historical ecosystem-fire relationships.Historical relationships between ecosystems and fire have profound implications for contemporary relationships between people and fire. For example, in the context of forest management for timber or biofuel, anthropogenic fires can threaten future harvests by killing tree species that did not evolve to tolerate fire. In stark contrast, fire in savannas serves to limit woody thickening and maintain species diverse herbaceous (grasses and forbs) plant communities that did evolve with frequent fire. Given the radically different role of fire in distinct ecological contexts (i.e., savannas versus forests), this research will demonstrate that knowledge of long-term fire history is critically important to our understanding of how people influence key ecosystem functions (e.g., flammability and carbon storage) and how people rely on fire-dependent ecosystems for their livelihoods (e.g., as habitat for game and livestock, and provisioning of ground water).Savannas and grassland-forest mosaics are the dominant native ecosystems of Texas and the North American Coastal Plain and cover vast regions of South America. Although these fire-dependent ecosystems are foundational to the region's livestock and timber production, a lack of science-based prescribed fire management threatens future productivity and has set the stage for catastrophic wildfires. In savannas, widespread fire exclusion results in woody encroachment and a loss of forage production. In fire-excluded forests, which often occur in mosaics with grasslands, high densities of highly flammable trees (e.g., pines) are prone to destruction in inevitable wildfires. A key challenge for ecologists is to determine when fire exclusion reduces ecosystem flammability, leading to the "demise of fire," or sets the stage for wildfires due to the accumulation of highly flammable biomass. This research will use savannas of Texas and ecologically similar grassland-forest mosaics of South America and the Southeastern USA to answer questions about fire-vegetation feedbacks and their effects on biome distributions, wildfire risk, and provisioning of ecosystem services.
