Invasive plants can alter natural fire regimes, including the intensity, duration, and extent of fires, by changing the abundance and structure of fuels. Fire-invasion interactions can significantly affect ecosystem properties, such as community composition, nutrient cycling, landscape connectivity, and habitat use, and may feed back to promote invader dominance. Still, relatively little is understood about the mechanisms underlying fire-invasion interactions or how specific invasive plants affect fire intensity. Imperata cylindrica (cogongrass) is one of the most widespread and problematic invasive plants in longleaf pine forests and other habitats in the southeastern U.S., and putatively affects fuel loads and fire intensity, but remains understudied. We sought to determine if cogongrass invaded habitats had greater fine fuel loads compared to uninvaded habitats, and how variation in invader fuel mass, structure, and moisture (a proxy for season of burn) affected fire intensity. First, we measured fuels in paired invaded and uninvaded habitats across nine sites in north-central Florida. We then experimentally manipulated fuel type (mixed native herbaceous fuel or cogongrass), invader biomass, fuel structure (standing vs. litter), and fuel greenness (i.e., moisture), and measured maximum and sustained fire temperatures at ground level, 12 cm, and 50 cm, and flame height.
Our results show that habitats invaded by cogongrass have significantly greater fine fuel loads, including over 250% more aboveground biomass and more than 130% greater litter biomass compared to native plant dominated habitats. Peak fire temperatures for the highest density of standing cogongrass fuel (~3000g/m2) with low proportion of greenness was over 900 ºC at ground level and nearly 1000 ºC at 12 cm and 50 cm above ground level, suggesting intense fires in invaded areas may cause significant damage to native vegetation and could promote invader dominance. In addition, invader fueled fires sustained high temperatures (> 300 ºC) for more than 25 seconds and flame heights exceeded 2.5 m. High temperature, long-duration fires fueled by cogongrass, or other invasive species that occur in high densities, may damage or kill native plants such as longleaf pine and associated understory species that are otherwise well-adapted to relatively high fire temperatures. Moreover, the rapid regrowth or colonization of cogongrass after disturbances including severe fire indicates the potential for positive fire-invasion feedbacks that can further enhance invader dominance unless targeted management activities are implemented.