Evaluation of coarse woody debris and forest vegetation across topographic gradients in a southern Ohio forest

Abstract

In highly dissected landscapes, topographic gradients and their associated microhabitat conditions have been shown to greatly influence the distribution of woody vegetation. However, little is known about the influence of these gradients on the spatial distribution of coarse woody debris (CWD), especially in unglaciated eastern North America. The main goal of this study was to examine the influence of topography (slope aspect, percent slope, and slope position as calculated from a landform index (LI)) and plot characteristics (tree height, age, and canopy cover) on the distribution of trees (diameter at breast height, dbh≥10 cm), saplings (dbh<10 cm), and CWD in a mixed-oak forest of unglaciated southern Ohio (USA). CWD was defined in this study as any downed bole or limb with a diameter≥10 cm over a length≥0.5 m. Redundancy analysis (RDA) of the three structural layers revealed that the measured parameters explained a significant ( P<0.05) percentage of the variation in individual species distributions. In all three structural layers, slope aspect and LI were consistently important in explaining individual taxa distributions. Total CWD density (pieces ha −1) was significantly ( P<0.05) correlated with LI ( r=0.299) and percent slope ( r=−0.433). Total tree density was significantly negatively correlated with slope position ( r=−0.469), slope aspect ( r=−0.328), and canopy cover ( r=−0.391), and total sapling density was significantly negatively correlated with height of the dominant tree in each plot ( r=−0.283). According to an analysis of similarities between the three structural layers, species assemblages were significantly ( P<0.05) different between all three layers. The tree and CWD strata showed the highest similarity ( rho, R=0.216) while the CWD and sapling layers showed the lowest ( R=0.762). Oak ( Quercus spp.) dominated the tree and CWD layers while mesophytic species (maple ( Acer spp.) and beech ( Fagus grandifolia)) dominated the sapling layer. Analyses of CWD distributions in relation to both living vegetation and topographic gradients suggest that a complex interplay of factors dictate the distribution of CWD loads across the landscape. Differences in distributions were observed for total and individual CWD taxa. Given its importance in forested ecosystems, a better understanding of CWD spatial distribution is necessary if CWD dynamics and function (habitat and nutrient cycling) are to be fully understood.