Grass invasion reduces the resilience of tree regeneration to fire in the Central Hardwoods Region

Non-native grass invasions are altering fuels and fire behavior in forests, with uncertain consequences for tree regeneration and forest dynamics. We examined whether invasion by Microstegium vimineum, the most widespread invasive grass in the eastern United States, interacts with prescribed fire to reduce tree regeneration and evaluated how such interactions might influence long-term regeneration dynamics in the Central Hardwoods Region. Using paired invaded and uninvaded plots subjected to fall or spring burning, we quantified differences in pre-fire juvenile tree regeneration, fire intensity, and survival and resprouting rates of naturally established juvenile trees of varying sizes and species. Field data were then used to parameterize the Forest Vegetation Simulator (FVS), a forest growth and yield simulation model. Prior to burning, seedling density was 43% lower in invaded than uninvaded plots and the seedling size-class distribution skewed toward larger individuals, suggesting grass invasion and fire previously acted as a filter on small tree regeneration or that invasion occurred in areas with larger and fewer trees. Burning resulted in similar rates of fuel consumption among treatments, with 1-h, 10-h, and 100-h fuels decreasing an average of 23%, 42%, and 18.5% respectively. However, invaded plots exhibited lower flame lengths, shorter fire residence times, and smaller burned areas than uninvaded plots, indicating lower fire intensity. Despite experiencing lower fire intensity, invaded plots had 46% higher mortality of small trees (<3 mm stem diameter) and nearly 54% lower post-fire seedling persistence overall compared to uninvaded plots. Resprouting rate was positively related to seedling size and decreased marginally with invasion. Both a structural equation model and FVS simulations demonstrated that grass invasion had large, negative effects on seedling resprouting, regardless of fire intensity effects. Post-fire sapling persistence did not differ with invasion status. These results suggest that, while grass invasion can have varying effects on fire intensity, it consistently alters forest dynamics by reducing the resilience of tree regeneration to fire. Lengthening the time between prescribed fire applications in grass-invaded forests may be necessary to allow juvenile trees to reach an adequate size to survive burning.