Fire-Induced Seedling Recruitment Responses to Fuel Type in a Restored Oak Woodland

Abstract

Canopy closure and altered tree species composition in oak woodlands caused by fire exclusion has decreased fuel bed flammability in the eastern USA, which can reduce the effectiveness of prescribed burning in promoting plant diversity during ecological restoration. Increased soil heating and more complete removal of surface litter can increase seedling recruitment and thus diversity of fire-adapted species. I tested the effects of C4 grass fuel removal and tree leaf litter source (mesophyte-rich litter from fire-excluded forests vs. mesophyte-poor litter from oak woodlands) on fire-stimulated seed germination in an open oak woodland using a field experiment. I counted post-fire seedling densities in paired burned and unburned plots and in response to pre-fire C4 grass removal, the source of tree leaf litter, and post-fire preemergent herbicide application. I included the preemergent herbicide treatment to help distinguish treatment effects on seedling survival from those on seedling emergence. Densities and richness of seedlings were significantly greater in burned than in unburned plots. Positive effects of fire on seedling emergence (especially of Chamaecrista fasciculata and Lespedeza repens) were strongly reduced by removing C4 grasses immediately before the fires due primarily to lower surface temperatures in these subplots during fire. Although the effect was not as strong, seedling densities increased more in response to burning mesophyte-poor litter than to burning mesophyte-rich litter. Decisions to restore oak woodlands should consider the promotion of flammable C4 grasses through the removal of mesophytic trees and the formation of canopy gaps in fire-excluded forests.