Groundwater withdrawals in a shallow unconfined aquifer can reduce water levels in connected wetlands and lead to changes in vegetation. I simulated the effect of withdrawals on the vegetation of intermittent ponds in the New Jersey Pinelands by first characterising the vegetation communities and associated hydrologic regimes of 15 ponds. A model, which was based on these results, was used in conjunction with simulated water-level drawdowns to predict changes in plant communities. Five dominant vegetation communities found at the study ponds included Carex striata, Chamaedaphne calyculata, and Vaccinium corymbosum patch types and aquatic-herbaceous and wetland-herbaceous types, which represent combinations of other vegetation types. A vegetation-hydrology model related mean vegetation cover for the five patch types to water level in 5-cm classes. Groundwater withdrawals were simulated by reducing pond-water depth by 5-cm intervals, and the resulting changes in modelled vegetation were assessed. Aquatic-herbaceous and wetland-herbaceous patch types displayed reductions in area beginning at the smallest simulated drawdowns evaluated. C. striata patch area changed only slightly for drawdowns ≤10 cm, but decreased steadily in response to greater drawdowns. C. calyculata and V. corymbosum patch areas increased for water-level reductions ≤15 cm and then decreased at drawdowns ≥20 cm. The area of combined pond-vegetation types, which represents the entire pond as a single vegetation community, consistently decreased at reductions of ≥10 cm. These simulations suggest that groundwater withdrawals favour the expansion of woody species into intermittent ponds, with eventual replacement of pond vegetation by adjacent forest vegetation if extreme hydrologic modifications occur. Copyright © 2011 John Wiley & Sons, Ltd.
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