Litter to glitter: promoting herbaceous groundcover and diversity in mid-southern USA oak forests using canopy disturbance and fire

Abstract

Background: In oak-dominated communities throughout eastern North America, fire exclusion and subsequent woody encroachment has replaced the “glitter” of once robust and diverse wildflower and grass layers with leaf-litter dominance. Restoring the important herbaceous components of Eastern oak ecosystems could involve pairing heavy canopy disturbance with growing-season fire, but potential negative effects warrant research. Beginning with 20 ha replicates of closed-canopy forest at three sites across Tennessee and North Carolina, USA, we monitored groundcover response to combinations of thinning (none; light: 14 m2 ha−1 residual basal area; and heavy: 7 m2 ha−1) and seasonal fire (none; March: pre leaf expansion; and October: pre leaf abscission) from 2008 to 2016. Results: Before treatments, woody plants and leaf-litter-dominated groundcover and herbaceous plants were rare (<6% groundcover, 118 species). By 2016, herbaceous groundcover averaged 59% after heavy thinning and three biennial burns, and 359 herbaceous species were documented. Only 6% (23) of these species appeared negatively affected by applied disturbances. Across sites, thin-and-burn treatments increased graminoid groundcover 14-fold, forb groundcover 50-fold, herbaceous richness 9-fold, and herbaceous diversity 10-fold, relative to unmanaged stands. These increases were often greater where fire was repeatedly applied, and only after repeated fire was herbaceous response greater in heavily thinned stands relative to lightly thinned stands. Burn-only treatments rarely affected herbaceous metrics, and thin-and-burn treatments more than doubled woody groundcover. This suggests that canopy reduction, leaf-litter consumption, and pulses of bare ground were more related to positive herbaceous responses than to the control of woody competition in the understory. Fire season effects were not observed, but herbaceous response after less intense October fires was comparable to that following more intense March fires. Conclusions: Our results conflict with warnings concerning the potential negative effects of disturbance on herbaceous diversity east of the prairie–woodland transition zone. Canopy disturbance and repeated fire, regardless of season, widely restored herbaceous groundcover and diversity in Eastern oak ecosystems. Herbaceous components were resilient to extended periods of fire exclusion, but current conservation programs often prioritize existing, high-quality sites. Our results suggest that such policies may overlook the tremendous restoration potential present in otherwise inconspicuous understories of closed-canopy oak forests throughout eastern North America.