Following decades of fire control, many oak forests have developed dense subcanopy layers of shade-tolerant trees. To restore these forests to a more open-structured and sustainable condition, prescribed fire and mechanical thinning are being applied on public lands. Although both fire and thinning create more open-structured forests, the function of these disturbances may be different in terms of vegetation response. This study examines the response of herbaceous layer vegetation to burning and thinning alone and in combination. Three study sites, supporting mature oak-dominated forests, were located in the Allegheny Plateau of southern Ohio. Each site had four 20-ha treatment units: an untreated control (C), thin only (T), burn only (B), and thin+burn (TB). Thinning was conducted between the 2000 (year 0) and 2001 (year 1) growing seasons; a single prescribed fire was conducted on all B and TB units in the early spring prior to the year 1 growing season. Cover of all vascular plants was sampled in 12 1-m2 quadrats within 10 0.1 ha plots per unit (n = 120 plots), before treatments in year 0, and in post-treatment years 1 and 4.
Results/Conclusions
Fire had little effect on overstory structure. By year 4, stand basal area was reduced by only 2% on B units; by contrast, it was reduced 28% and 27% on T and TB units. Although thinning had a larger effect on structure, the cover of herbaceous plants (forbs and graminoids) increased significantly only after fire (B and TB). In year 4, herbaceous cover averaged 5.2% and 7.5% on the C and T plots, respectively, compared with 16.3% and 17.5% on B and TB plots. Perennial forbs accounted for most of this increase. In contrast, woody plant cover increased significantly after all treatments; much of this increase was due to sprouting of topkilled or cut shade-tolerant trees and establishment of several shrub and tree species from the seedbank. Modest increases in total species richness (per 1-m2) occurred on all three treatments, with TB plots exhibiting the largest increase, from 8.2 in year 0 to 11.1 species/m2 in year 4. The disturbance process of fire, which altered the litter layer, and stimulated both seed germination and higher productivity of some species, produced an increase in herbaceous cover and richness that was not achieved by structural alteration alone.
