Emeritus Professor of Biological Sciences Louisiana State University Tallahassee, Florida, United States
Recent analysis of occurrences of fires and tropical cyclones worldwide has identified the southeastern North American coastal plain as a global hotspot for frequent fires and tropical cyclones. This region also is a biodiversity hotspot, with an overwhelming predominance of endemic taxa associated with what historically was the most widespread fiery upland ecosystem, pine savannas/woodlands. Both fires and tropical cyclones, historically and currently, occur every few years in these coastal plain terrestrial ecosystems. Each disturbance occurs in the context of the other disturbance; in concert, these disturbances generate distinct environments. Long-term study over >4 decades in one of the remaining old-growth pine savanna/woodland ecosystems has enabled a synthesis of data, resulting in a conceptual model used to predict how interactive effects of fires and tropical cyclones result in diverse ecosystems containing fire- and wind-adapted species across multiple trophic levels.
Wind disturbance of the overstory pines results in a range of effects from frequent landscape-level increases in fine fuels due to increased needlefall in high winds to periodic wind damage of trees that produces both firebreaks (tree boles) and intensely burning patches (tree crowns). Frequent fires, when fueled by cyclone increased needlefall, have longer durations of elevated temperatures at ground level, resulting in increased fuel consumption and elevated heat production at ground level and selecting for highly pyrophilic ground layer plants and microorganisms (fungi and bacteria involved in decomposition). This multitrophic ground-level component contains endemic taxa and modifies fine fuels in ways favoring very frequent, almost annual fires.
Periodic high winds of tropical cyclones select for shorter-stature, slower-growing, higher wood density overstory pines that have open canopies and branches with terminal needle tufts that reduce wind damage. In addition, boles of cyclone-fallen pines produce firebreaks and reduced ground level fire intensities that can shelter pine seedlings. In contrast, both fallen crowns and stumps of large pines burn intensely in fires, suppressing ground layer vegetation. Resulting reduced ground-level intensities in subsequent fires then facilitate recruitment of new pines and growth to life cycle stages resistant to fires in open areas away from surviving pines. In these ways, non-resprouting, long-lived tree species with spatially separated cohorts of different sized and aged trees persist as non-equilibrium populations of signature trees.
These complex interactions between fires and tropical cyclones thus are hypothesized to produce unique fiery, windblown southeastern pine savannas/woodlands rich with endemic species in coastal warm temperate regions of southeastern North America.
