A long-term study of tree seedling recruitment in southern Appalachian forests: the effects of canopy gaps and shrub understories

We examined the importance of intermediate-sized gaps and a dense shrub lay er on tree seedling recruitment in a southern Appalachian deciduous forest. We created 12 canopy gaps under two contrasting understory conditions: 6 g aps were dominated by the dense, shade-producing shrub, Rhododendron maximu m L., while the remaining gaps were relatively open. Density of first-year and > first-year seedlings was monitored for 5 years in transects extending from adjacent undisturbed forest through the experimental gaps. We concurr ently measured the understory light environment, soil moisture, litter biom ass, and seed rain. Neither species diversity nor richness consistently inc reased following gap formation. Acer rubrum L. responded consistently to ca nopy gaps with increased seedling densities while most other species, inclu ding both shade-tolerant and shade-intolerant species, did not. Seedling de nsities were especially low and unresponsive to gap formation in areas domi nated by R. maximum. Understory light levels were consistently low beneath R. maximum and did not increase with canopy gap formation. Our results sugg est that dense shrub cover can neutralize recruitment opportunities in cano py gaps, that seed rain often limits recruitment in gaps, and that canopy g aps that are larger or include understory disturbance are needed to maintai n diversity in these forests.