Graminification and Reversal of Mesophication in a Restored Oak Woodland

ABSTRACT

Modern fire exclusion in upland oak- and oak-pine-dominated forests has resulted in reduced flammability of surface fuels as a result of the replacement of flammable fuels of oaks and associated vegetation by less flammable fuels produced by mesophytic trees. Some consequences of such “mesophication” include reductions in herb diversity, C4 grasses, and oak regeneration. We tested two hypotheses of the effects of C4 grasses and tree leaf litter on fuel consumption in the field in restored and unrestored oak woodlands: 1) the presence of C4 grasses increases fuel consumption by a late growing-season prescribed fire (“graminification”), and 2) removing tree leaf fuels reduces fuel consumption by a late growing-season fire to a greater extent in areas lacking substantial, slow-to-dry mesophyte tree leaf litter (mesophication reversal). Consistent with graminification, removing C4 grasses in a restored oak woodland reduced fuel consumption, and fuel consumption was greater following tree leaf litter removal in restored woodland plots containing C4 grasses than in areas that had not been restored and that lacked such grasses. Consistent with mesophication reversal, removing tree leaf fuels reduced fuel consumption to a greater extent in areas where tree leaf litter was dominated by pyrophytic oaks than in an untreated area with a significant amount of mesophyte tree leaf litter. We conclude that mesophication can be reversed at early stages of restoration by thinning mesophytic trees and opening the canopy. Increases in C4 grasses (graminification) at late stages of restoration further increase flammability.