Riparian vegetation along a rural-urban gradient
Grace S. Brush, Johns Hopkins University
The riparian corridor in the Gwynns Falls watershed in Baltimore follows a gradient from rural-suburban in the upper watershed to densely urban where the Gwynns Falls enters the Patapsco River (Baltimore Harbor), and drains into the Chesapeake Bay. Before early to middle 19th century, the watershed was mixed forest and rural. In areas that remain non-urban, Piedmont streams, which characterize most of this watershed, overflow their banks every two to three years.
Consequently, seed dispersal, germination and growth of plants are adapted to frequent low magnitude floods. With the onset of urbanization, impervious surfaces increased, streams were modified to prevent overbank flooding, runoff and peak flow increased, base flow decreased, streams incised, and flooding frequency was reduced. Analysis of vegetation data in 110 plots shows a shift from wetland species in non-urban floodplains to a mix of upland and wetland species, including many exotic species, along the rural-suburban-urban gradient. Urbanization inhibits growth and regeneration of riparian species, leaving only relic populations represented by large floodplain trees and newly established regenerating populations of upland and exotic species, in many of the historical floodplains. Urban modification of Piedmont streams and floodplains has created conditions of "hydrological drought" which have triggered changes in vegetation in response to shifts in water availability.