Seasonality of Fire Weather Strongly Influences Fire Regimes in South Florida Savanna-Grassland Landscapes

Abstract

Fire seasonality, an important characteristic of fire regimes, commonly is delineated using seasons based on single weather variables (rainfall or temperature). We used nonparametric cluster analyses of a 17-year (1993–2009) data set of weather variables that influence likelihoods and spread of fires (relative humidity, air temperature, solar radiation, wind speed, soil moisture) to explore seasonality of fire in pine savanna-grassland landscapes at the Avon Park Air Force Range in southern Florida. A four-variable, three-season model explained more variation within fire weather variables than models with more seasons. The three-season model also delineated intra-annual timing of fire more accurately than a conventional rainfall-based two-season model. Two seasons coincided roughly with dry and wet seasons based on rainfall. The third season, which we labeled the fire season, occurred between dry and wet seasons and was characterized by fire-promoting conditions present annually: drought, intense solar radiation, low humidity, and warm air temperatures. Fine fuels consisting of variable combinations of pyrogenic pine needles, abundant C4 grasses, and flammable shrubs, coupled with low soil moisture, and lightning ignitions early in the fire season facilitate natural landscape-scale wildfires that burn uplands and across wetlands. We related our three season model to fires with different ignition sources (lightning, military missions, and prescribed fires) over a 13-year period with fire records (1997–2009). Largest wildfires originate from lightning and military ignitions that occur within the early fire season substantially prior to the peak of lightning strikes in the wet season. Prescribed ignitions, in contrast, largely occur outside the fire season. Our delineation of a pronounced fire season provides insight into the extent to which different human-derived fire regimes mimic lightning fire regimes. Delineation of a fire season associated with timing of natural lightning ignitions should be useful as a basis for ecological fire management of humid savanna-grassland landscapes worldwide.

Pyrogenic fuels produced by savanna trees can engineer humid savannas

“…, Platt et al. ). We reduced variation in external variables by conducting fires under similar weather and management conditions; we indicate external components that we managed or excluded from our conceptual model at the bottom of Fig.…”

“…These fire seasons follow dry seasons during which moisture content of fine fuels is reduced, and they occur before moisture increases substantially in ensuing wet seasons (Platt et al. ). Post‐fire regrowth of C 4 grasses during these wet seasons results in widespread abundant fine fuel that is loosely packed (sensu Simpson et al.…”

“…2010 a , Platt et al. ). Ignition and spread of fires occurs almost every year in some humid warm temperate and subtropical savannas (e.g., Platt , Slocum et al.…”

Defining Extreme Wildfire Events: Difficulties, Challenges, and Impacts

“…The period of time when fires typically occur or when they are likely to be of high intensity or burn large expanses [174].…”

Clarifying the confusion: old-growth savannahs and tropical ecosystem degradation

“…Most savannahs are dependent on frequent fires and/or megafaunal herbivory, forces that are part of the internal ecological dynamics of savannahs, rather than externally imposed disturbances [9,50]. Although in pre-human history, savannah fires were ignited by lightning strikes, in most of the world's savannahs, humans are now the primary sources of ignitions [2,51,52]. Similarly, most savannahs support populations of native and/or domestic herbivores, often under human management [11,53].…”