The role of treefall gaps and forest edges on understory vegetation dynamics in a mixed deciduous forest.

摘要

Treefalls and abandoned agricultural fields are common disturbances in the deciduous forests of the northeastern United States. This study explores these two disturbance types individually and in combination to ascertain their role(s) in structuring forest understory communities both spatially and temporally. Abiotic and biotic variables are included in the analyses. Soil moisture, soil pH, soil seed bank, and light levels were measured in treefall gaps and under closed canopy, while controlling for microtopographic variation. Understory vegetation within permanent plots in forty-two natural treefall gaps were censused during the 1991-1993 growing seasons from May through September. Plant density, leaf cover, and species richness were compared between treefall gap and closed canopy plots for all three census years, as well as from one year to the next. Additionally, forest/old field edges were evaluated for their impact on both understory and treefall gap vegetation.;Mounds within treefall gaps had significantly more soil moisture than mounds under closed canopy, and mounds were consistently drier and more acid than pits regardless of overstory condition. The soil seed bank within treefall gaps had a larger number of locally uncommon species and a greater density of Eupatorium rugosum than did closed canopy sites. There was a significant difference in plant density, leaf cover, and species richness between treefall gap and closed canopy plots for most of the growing season during most census years, except around canopy leaf out. Species composition differed little between treefall gap and closed canopy plots. Gap age and gap size minimally explain species composition within treefall gaps. Several species were found more frequently in treefall gaps, but they were also found in closed canopy plots. Additionally, understory vegetation in both treefall gap and under closed canopy was highly variable during both single growing seasons and year-to-year. Species preference for microsites was altered by the presence of a treefall gap, as well as species richness, plant density, and leaf cover on mounds and pits. Abundance of tree seedlings, exotic species, and locally uncommon species were altered along the forest/old field edge, as was treefall gap vegetation.