The effects of environment and life stage on Quercus abundance in the eastern deciduous forest, USA: are sapling densities most responsive to environmental gradients?

Most studies that describe tree species distributions across environmental gradients focus only on the abundance of adults. This approach may overlook the response of younger life stages (i.e., seedlings and saplings) and thus may fail to identify the life stage that is most sensitive to a particular environmental gradient (i.e., site characteristics that vary across landscapes such as elevation or aspect). Alternatively, seedling and sapling abundance may be unrelated to environmental gradients and may be entirely dependent on adult abundance. In order to effectively manage eastern hardwood forests to promote species compositions that maximize economic or ecological value, we need to understand the relative roles of site characteristics and adult abundance on all life stages. To address this need, we examined the relationships between five site characteristics (aspect, slope position, site index, stand age and elevation) and the abundance of seedlings, saplings and adults of three species of oak (Quercus rubra, Quercus alba, and Quercus prinus). We assessed these relationships by censusing seedlings, saplings, and adults in large plots (500 m^2) and quantifying local site characteristics in 21 forest stands in the Monongahela National Forest in North Central West Virginia. Adult white oak densities peaked on southwestern slopes (R^2 = 0.24); adult chestnut oak densities peaked on ridge tops (R^2 = 0.58); and adult northern red oak densities were not correlated with any site characteristics (R^2 = 0.00). Densities of white oak and chestnut oak saplings were highly correlated with a combination of site characteristics and seedling densities (R^2 = 0.75, 0.81, respectively), and northern red oak densities were highly correlated with site characteristics solely (R^2 = 0.70). For seedlings, white oak and chestnut oak densities were correlated only with adult abundance (R^2 = 0.53, 0.56, respectively), and northern red oak densities were correlated solely with site characteristics (R^2 = 0.45). These data suggest that in the absence of disturbance, site characteristics have the strongest effects on abundance patterns of these oaks during the sapling stage. For each of the three species, seedling, sapling and adult densities were correlated with a different subset of factors meaning that different biotic and abiotic conditions promoted peak densities for each life stage. Such a pattern indicates that important “demographic conflicts” exist (sensu [Battaglia, L.L., Fore, S.A., Sharitz, R.R., 2000. Seedling emergence, survival and size in relation to light and water availability in two bottomland hardwood species. J. Ecol. 88, 1041–1050]).