Effects of single spring and fall fires and a fire surrogate (clipping) on seedling layers in oak-dominated forests in southern Ohio

Background/Question/Methods

Oak-dominated forests throughout the eastern deciduous forest biome are shifting from oaks to more shade-tolerant species, especially maples (Acer spp.), driven, in part, to decades of fire suppression. Thus, prescribed fires have been studied as a means of impeding this oak-to-maple transition and restoring oak-dominated ecosystems. However, few have examined the relative effects of burning season and heating on seedling responses. The goal of this study was to test the overall effectiveness of fire in filtering shade-tolerant seedlings, especially maples, and shifting seedling communities towards greater relative abundance of oak. Further, this study examined the relative effects of fire season (fall versus spring) and a fire surrogate (fire versus clipping) on seedling layer community trajectory and functional group abundance. Densities of tree seedlings were evaluated before and after treatments in 30 circular (19.6-m²) plots at each of three forest sites along the Appalachian escarpment in southern Ohio. Different treatment seasons (spring or fall) were applied to half of each site, while different treatment types (burned, clipped, or untreated) were applied to plots within each treatment season.  The effects of site and treatments on seedling community trajectory were evaluated using a combination of ordination, compositional vectors, and PERMANOVA. The effects of site and treatments on seedling density were evaluated using ANCOVA.

Results/Conclusions

Results from this study did not support the hypothesis that fire would alter the current oak-to-maple shift across these sites. Although fire reduced maple seedling densities, primarily at one site, oak seedlings were equivalent across season and fire/fire surrogate treatments. However, fire appeared to play a role in generating seedbeds for shade-intolerant competitor species (especially Liriodendron tulipifera and Sassafras albidum). In this study, the effects of season were confounded by variation in burn management units. Thus, small scale studies in topographically diverse landscapes have limited utility in testing seasonal effects of fire. Further, there was little difference between the effects of topkill by fire and topkill by clipping on seedlings, suggesting fires were low in intensity. Despite the short temporal scale and confounding effects of burn units, these results are consistent with the literature suggesting that single and repeated low-intensity prescribed fires alone have generally been ineffective in reversing the shift from oak to maple dominance. Future studies seeking to regenerate oaks at these sites should consider restoration of both structure (canopy thinning) and processes (repeated prescribed fire) to simultaneously increase understory light levels and reduce oak competitors.