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Vegetation Replacement Models for Southern Nassau County, New York

Ground-water levels in southern Nassau County have declined by 3.6 m due to the installation of sanitary sewers and ground-water pumping.' As sewer building continues to completion in the area (about 1990) ground-water levels will decline further by 0.2-5.8 m. If ground-water pumping patterns are altered, according to the Master Water Plan, ground-water levels may rebound in southwestern areas by 4 m and decline further in southeastern areas by 4 m. These changes in ground-water levels should influence the plant habitats and communities. If ground-water levels change sufficiently at a site the existing vegetation will be replaced by other vegetation that the new ground-water regime will support. Based on field-sampling in the area and past vegetation studies, descriptive habitat and community replacement models have been developed to portray the kinds and magnitude of vegetation change that could occur in Nassau County given changing ground-water levels? The replacement models are implemented here for two areas in southwestern Nassau County to illustrate the potential vegetation changes that could occur. In general, to assess the vegetation replacement sequence after perturbation, first the existing vegetation at a site is determined. Next, the predicted water level change is determined from U. S. Geological Survey simulations. The habitat that the new ground-water levels will support is determined. Over successional development time, the vegetation will be replaced by those types of communities, within the habitat, that the new ground-water levels will support. Three variables in the vegetation replacement process are presented in FIGURE 1: the communities (excluding streams and managed parks), the ground-water gradient supporting each community, and the development time necessary to produce each community via secondary succession. The stream communities are omitted in the figure because their replacement process has been described.' Managed parks are upland sites controlled by human development and are not likely to increase or decrease substantially given changing ground-water levels. The models predict that southern Nassau County will experience vegetation replacement toward drier types of vegetation after the installation of sanitary sewers (TABLE 1 ). If possible changes in ground-water pumping patterns are implemented, southwestern areas may then experience water-table rebounds and replacement sequences toward more mesic types of vegetation, while southeastern areas will become even drier. The result will be a decline in community and species diversity in the area and an increase in communities with broad ranges of tolerance to environmental perturbation.