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 sm all-scale topographic complexity of the riverbank profile and tree spe cies-richness. A total of 153 5 m-wide transects established at 530 m intervals, containing 2568 trees, were measured along 97 km of the riv er 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 specie s dominating the community are: silver maple (Acer saccharinum), boxel der (Acer negundo), and sycamore (Platanus occidentalis), together com prising 62% of the stems and 80% of the basal area. A statistically si gnificant positive relationship was observed between tree species-rich ness and topographic complexity. Analysis of covariance indicated that adjacent land use and bedrock geology had no statistically significan t impact on species-richness, and that the simple linear-regression mo del was adequate to describe the effect of topographic complexity on s pecies-richness. A significant negative relationship was observed betw een tree species-richness and the width of the riparian zone; this rel ationship was consistent across geology. Width of the riparian zone wa s the best overall predictor of tree species-richness. Tree-species di stributions suggest that this is an edge effect, due to a greater numb er 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 riparia n sections. (C) 1998 Elsevier Science B.V.