Tree species-richness and topographic complexity along the riparian edge of the Potomac River

We studied the riparian edge of a central portion of the Potomac River in order to test the hypothesis of a positive relationship between small-scale topographic complexity of the riverbank profile and tree species-richness. A total of 153 5 m-wide transects established at 530 m intervals, containing 2568 trees, were measured along 97 km of the river corridor between Harpers Ferry, WV and Hancock, MD. Standardization of species-richness between plots was done by rarefaction. Small-scale topographic complexity was calculated using the standard deviation of slope measurements along the length of each transect. The tree species dominating the community are: silver maple ( Acer saccharinum), boxelder ( Acer negundo), and sycamore ( Platanus occidentalis), together comprising 62% of the stems and 80% of the basal area. A statistically significant positive relationship was observed between tree species-richness and topographic complexity. Analysis of covariance indicated that adjacent land use and bedrock geology had no statistically significant impact on species-richness, and that the simple linear-regression model was adequate to describe the effect of topographic complexity on species-richness. A significant negative relationship was observed between tree species-richness and the width of the riparian zone; this relationship was consistent across geology. Width of the riparian zone was the best overall predictor of tree species-richness. Tree-species distributions suggest that this is an edge effect, due to a greater number of tree species being able to take advantage of a combination of an increased light gradient and a stable, well-drained substrate (the C & O Canal towpath) directly adjacent to the river, in narrower riparian sections.