No abstract

No 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.

Mesophication has reduced fuel-bed flammability in the Mid-Southern US, limiting the effectiveness of fire alone in promoting disturbance-adapted woody species. We applied combinations of thinning (none, 7, and 14 m 2 ha -1 residual basal area) and seasonal fire (none, October, and March) at three sites and monitored understory woody response from 2008 to 2016. In combination, thinning and burning had strong negative effects on some mesophytic species (Pinus strobus, Ostrya virginiana, and Fagus grandifolia) and positively affected many shade-intolerant and fire-tolerant species formerly suppressed under closed canopies. Such compositional shifts were greatest at our most xeric site, and were related to treatment effects on overstory and midstory density. Seedling density of Quercus spp. nearly doubled (+2,256 stems ha -1 ) from pre-to postmanagement. Sapling response was less dramatic; however, indicator and ordination analyses often associated mesophytic and disturbance-dependent saplings with unmanaged and managed treatments, respectively. Fire-season effects were subtle, but more species and greater understory densities were associated with March relative to October burning. Although some mesophytic species (Acer rubrum and Liriodendron tulipifera) responded positively to thinning and resprouted aggressively after fire, our results demonstrate how thinning and burning can initiate the reversal of mesophication's effects on understory woody vegetation.