Riparian vegetation along a rural-urban gradient (presentation)

Kristen Holt, Johns Hopkins University, Dept. Geography and Environmental Engineering, Baltimore, MD 21218

Grace Brush, Johns Hopkins University, Dept. Geography and Environmental Engineering, Baltimore, MD 21218

Wayne Zipperer, U. S. Forest Service, c/o SUNY-ESF, 1100 Irving Ave., Syracuse, NY 13210

Riparian corridors in the Gwynns Falls watershed follow a gradient from rural/suburban in the upper watershed to densely urban where the Gwynns Falls enters the Patapsco River. Before early to middle 19th century, the watershed was mixed forest and rural. Piedmont streams, which characterize most of this watershed, overflow their banks every two to three years. Consequently, growth and seed dispersal of plants occupying these areas are adapted to frequent, low magnitude floods. Tree species include green ash, sycamore, box elder, silver maple, willow and black walnut. With the onset of urbanization, streams were modified to prevent overbank flooding, which resulted in stream incision with steep banks adjacent to the streams.

A series of transects were laid perpendicular to each stream in the Gwynns Falls watershed at 240 meter intervals. Locations of transects were determined using USGS topographic maps. The width of the floodplain was based on the presence of riparian tree species, or if the floodplain was cutoff by development or some other obstruction, the width was measured to the point of cutoff. A series of 10 m2 plots were located along each transect. Plots were located on each side of the stream channel and every 30 meters to where the floodplain ended. Samples included 125 plots along 56 transects. In each plot all stems > 2.5 cm dbh were identified and the dbh recorded. All stems < 2.5 cm dbh were counted and identified. Herbaceous species were identified and percent cover recorded in 4 1m2 quadrants in each plot.

Analysis of the data collected in the 125 plots show a shift in vegetation, from riparian species to a mix of upland and exotic species along the gradient from rural/suburban to urban land use in the watershed. Riparian species are seen in all stages of growth, from seedlings to large trees, in the upper suburban areas. In contrast, slopes along the streams in the lower watershed are colonized by an assortment of upland and exotic species, including black locust and Norway maple. Land surfaces adjacent to the steep slopes associated with urban streams, represent historical floodplains occupied by very large riparian trees, but few or no seedlings and saplings. As streams become incised and banks steepen, floodplains are no longer actively flooded. These conditions inhibit regeneration of riparian species, leaving only older populations represented by large floodplain trees and young regenerating populations of upland and exotic species, in many of these dry relic floodplains. This trend is expected to continue in the upper watershed, as development continues into rural/suburban areas.