Invasive grass fuel loads suppress native species by increasing fire intensity and soil heating.
Abstract
Non-native invasive grasses are driving intense fires across the globe but the impacts of native versus invader-fuelled fires on community assemblages are poorly understood. By increasing fire intensity, grass invasions might increase below-ground mortality of heat-sensitive seeds and buds, thereby shifting community composition. We compared fuel loads in native and non-native invasive (cogongrass, Imperata cylindrica) plant-dominated areas of pine savannas in Florida. Then, we conducted a field experiment to examine how fuel loads and native and invasive fuel types affected soil heating and seedling emergence or resprouting of native and invasive plant species. Average fuel loads in invaded communities were 152% greater than that in native communities. Soil heating, including heating duration >60°C, maximum temperature and heat flux >60°C, increased, and seedling emergence and resprouting decreased with greater fuel loads; these relationships were similar across the overlapping range of native and invasive fuel loads. However, longer durations of soil heating at the higher average fuel loads of invaded communities resulted in 23% lower predicted probability of seedling emergence compared to average fuel loads of native communities. Invasive cogongrass resprouting was not affected by fuel loads, indicating that cogongrass tolerates the intense fires it generates. In contrast, seedling emergence and resprouting of most other species was reduced by greater fuel loads. Synthesis and applications. By increasing fuel loading and soil heating, grass invasions may alter post-fire community assemblages and facilitate invasive grass dominance at the expense of native species via an invasion-fire cycle. Fuel loads can be used to predict soil heating duration and depth, and these data, combined with information on species tolerances to heating, can be used to forecast the impacts of invasions on post-fire community composition. To maintain fire regimes that promote native communities and resist invader dominance, it is critical to manage invasive species that increase fuel loads.