| Zusammenfassung: | Disturbance plays an important ecological role in determining the structure and species composition of grassland communities. Xeric limestone prairies, a rare and threatened habitat in Pennsylvania, are currently being invaded by woody species, such as Juniperus virginiana (Eastern red-cedar), Elaeagnus umbellata (autumn olive), Lonicera spp. (honeysuckles), and Crataegus spp. (hawthorns). Woody plant invasion threatens the long-term persistence of the 10 known remaining limestone prairies in Pennsylvania. Repeat aerial photographs were rectified in a Geographical Information System and were used to calculate rates of woody plant invasion within two limestone prairies in central Pennsylvania. The average invasion rate from 1949 to 1994 was 1,350 m²/year in the Great Plains prairie and 325 m²/year in the Westfall Ridge Prairie Preserve (The Nature Conservancy). Extremely droughty soils are a major impediment to rapid woody invasion and are the sole contributors to prairie maintenance at present. Historically, however, Indian-set fires and other anthropogenic disturbances promoted establishment and maintenance of xeric limestone prairies. Agricultural activities, such as plowing and livestock grazing (and associated soil erosion), forest clearing, and quarrying activities may have opened up areas for colonization by prairie species. This study highlights the importance of exogenous disturbances in determining xeric limestone prairie structure and species composition and suggests management strategies for restoring and maintaining the threatened prairies of Pennsylvania.
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