Vegetation response to midstorey mulching and prescribed burning for wildfire hazard reduction and longleaf pine (Pinus palustris Mill.) ecosystem restoration

摘要

Dense midstorey vegetation, developed during fire exclusion, not only reduces understorey plant diversity and increases the risk of damaging wildfire but also impedes efforts to safely restore prescribed burning in longleaf pine (Pinus palustris Mill.) ecosystems. Our study examined the effects of midstorey reduction on stand structure and plant diversity in a forest treated by mulching alone and also when followed by prescribed fire during the winter, spring or summer. For trees ≥5 cm diameter at breast height (d.b.h.), mulching reduced stand density (1220–258 trees ha−1) and basal area (24–17.7 m2 ha−1) and increased mean d.b.h. (12.8–29.2 cm), with the largest reductions in loblolly pine (Pinus taeda L.), sweetgum (Liquidambar styraciflua L.) and oaks (Quercus spp. L.). Removing hardwoods and smaller pines resulted in a decline in tree species richness (8.9–4.4). Despite a modest increase in evenness (0.72–0.79), tree species diversity (H′ = 1.32–0.84) dynamics were largely driven by changes in richness. While the cover of tree seedlings initially declined from 32.4 to 16.9 per cent, rapid regrowth of hardwoods led to recovery by end of the second growing season. This, along with gains by shrubs, vines, grasses and forbs, resulted in a near doubling of understorey plant cover. Although tree seedling increases were not related to fire season, peak responses occurred for shrubs and vines after winter fire and spring fire, grasses following winter fire and forbs after summer fire. An increase in species richness (18.7–24.5) and decline in species evenness (0.86–0.70) produced only a small increase in understorey species diversity (H′ = 2.31–2.45). The greater number of understorey species following treatment were less equitably distributed as a result of differential rates of plant growth. While mulching led to a short-term increase in woody and herbaceous understorey plants, prescribed fire is needed to curtail redevelopment of the woody midstorey and further increase grasses and forbs.