The effect of urban hydrology on plant species
Grace S. Brush
The riparian corridor in the Gwynns Falls watershed, Baltimore follows 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 in non-urban areas. 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, less rainfall was absorbed into soil surfaces, streams incised, and flooding was reduced to rare events. Analysis of vegetation data in 110 plots shows that all vegetation layers, tree, shrub and herb in the Gwynns Falls riparian corridor show a shift to drier upland species in the lower urbanized section. The shift is most pronounced in the shorter lived herbs and shrubs but is also evident among tree species. Distributions of seedlings and saplings show only a few species regenerating in a pattern similar to established populations, indicating that urbanization inhibits growth and regeneration of riparian species, leaving only older populations represented by large floodplain trees and young regenerating populations of mostly upland and exotic species, in many of the relic or historical floodplains. This pattern of urban drought could be intensified with warming trends in climate.
Keywords: trees, shrubs, herbs, native, exotic, urbanization, hydrology, drying

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