Invasive grass fuel loads suppress native species by increasing fire intensity and soil heating

Abstract

1. 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 belowground mortality of heat-sensitive seeds and buds, thereby shifting community composition.2. 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.3. 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. 4. 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.

Perennial pasture grass invasion changes fire behaviour and recruitment potential of a native forb in a temperate Australian grassland

“…The highest fuel mass and residence times in grasslands observed in this study were those that had been invaded by an exotic pasture grass. While most native species in temperate grasslands regenerate from fire by resprouting (Lunt and Morgan 2002;Morgan 1999a;Morgan and Salmon 2020), elevated soil temperatures may reduce recruitment opportunities for new cohorts (Morgan 2001;Tomat-Kelly et al 2021) or potentially affect resprouting from underground storage organs or bud banks (Gagnon et al 2015;Pausas and Paula 2020). The residence time of fires may therefore influence the local distribution of species (Bond et al 2005;Keely et al 2011), as would an invasiondriven increase in fire residence time.…”

“…Given that residence time and soil heating can influence plant responses to fire, including seed germination and patterns of resprouting (Auld and O'Connell 1991;Auld 1995, Gagnon et al 2015;Neary et al, 1999;Tomat-Kelly et al 2021), it is surprising that it is measured in relatively few studies that investigate fire impacts on plants.…”

“…Invasion-driven shifts in fire behaviour may lead to changes in the intensity (the amount of heat released) and residence time of fires (how long they burn at a given point). Socially, increased fuel loads and fire intensity pose risks to communities and infrastructure (Adams et al 2020); environmentally, changes in fire behaviour and increased soil heating may influence native plant species recruitment and persistence, with corresponding decreases in species diversity and regeneration postfire (Gagnon et al 2010;2015;Tomat-Kelly et al 2021). Recent studies in the region have assessed the variation in fire behaviour between native grassland types (Morgan and Salmon 2020), but no studies have investigated how exotic grass invasion into native grasslands may alter fire behaviour.…”

Interacting global change drivers suppress a foundation tree species

“…To test the effect of a third stressor, fire, on longleaf pine, we evaluated fine fuels, fire characteristics and post‐fire tree survival in relation to plot treatments six years into the experiment. Given the very high biomass of the invader (Dillon et al, 2021; Tomat‐Kelly et al, 2021), we hypothesised that invasion would result in greater fuel loads, fires with higher temperatures and taller flames, and lower tree survival compared to plots dominated by native species. We also expected that the treatments would additively or synergistically interact such that trees that performed relatively poorly (i.e.…”

Prefire Vegetation Structure of High Severity Wildfires in Nonherbaceous‐Dominated Rangelands in the Western United States

“…Fire is a significant component of many rangeland ecosystems and is essential for the sustainability and productivity of these rangelands (Mitchell, 2000; Twidwell et al., 2021). Large high severity rangeland wildfires, however, can have enormous negative impacts on human safety and infrastructure (Calkin et al., 2014), as well as short‐ to long‐term effects on ecosystem structure, function, and services especially in nonherbaceous‐dominated rangeland ecosystems (Dillon et al., 2011; Fernández‐Guisuraga et al., 2021; Lentile et al., 2007; Tomat‐Kelly et al., 2021). Similar to the trends for all wildfires in the western United States (Dennison et al., 2014; Westerling et al., 2006), areas burned in large rangeland wildfires with high severity, as well as those with moderate and low severities, had increasing trends over the recent decades (Li et al., 2021).…”