FOREST ECOLOGY AND FLUVIAL GEOMORPHIC RELATIONS IN THE VICINITY OF THE STRASBURG QUADRANGLE, VIRGINIA

The physiographic geography of plants was studied in the vicinity of the Strasburg Quandrangle, Virginia. Vegetation of the region, at scales about that of the topographic sheet (1:24,000), seems to be largely related to geomorphic variation in both fluvial and interfluvial environments. Relations of vegetation with fluvial landforms, stream grade, stream order, streamflow characteristics, and upland topographic features were documented and interpreted through plot and plotless plant ecological techniques and vegetation mapping. A strong correlation was found between woody bottomland species distributional patterns and fluvial landforms such as depositional bar, channel shelf, flood plain, and terrace. These patterns appear to result more from hydrologic processes that operate on each of the fluvial landforms than from sediment-size characteristics or depth to saturated zone. Stream grade was shown to have a profound effect upon fluvial landforms and attendant bottomland vegetation. Increased diversity, vegetation zonation, and upland species presence appears to be associated with bottomland forests adjacent to stream reaches with high stream grade. Large scale vegetation zonation in both woody and herbaceous species was found on fluvial landforms below the flood-plain level. Reparian vegetation zonation was most pronounced where the areal extent of channel shelf was great, relative to the area of flood plain, which is correlated with reaches of high stream grade. Vegetation patterns above and below the flood plain appear to be maintained by periodic flood disturbance and should not be considered successional communities. An analysis of the headward extent of bottomland vegetation and fluvial landforms showed that the channel shelf and its associated vegetation proceed farther upstream on first-order streams than flood plains and their associated vegetation. Stream grade rather than basin size appears to be more limiting to bottomland vegetation in the headward direction than basin area. A vegetation map of the quadrangle was developed and suggests that moisture availability is the most important control on upland species patterns. Overall, the results strongly suggest that water, as it operates in stream and slope processes, is the most limiting factor affecting vegetation distribution in stream eroded landscapes of the Strasburg Quadrangle.