DIFFERENCES IN DIURNAL WATER TABLE FLUCTUATIONS IN WOODY PLANT PATCHES AND HERBACEOUS VEGETATION IN A CALCAREOUS FENS IN NORTHEASTERN PENNSYLVANIA

Calcareous fens are highly valued ecosystems due to the rich diversity of rare flora and fauna they support. Hydrogeochemical parameters are fundamental determinants of the character and function of calcareous fen communities. Water table depths control peat decomposition rates, nutrient mineralization rates, and redox conditions which in turn strongly influence plant species composition. Minor alterations in water table depth can lead to substantial shifts in biogeochemical cycles and therefore also ecological assemblages. Encroachment by woody plant species poses a threat to the hydrologic regime of these otherwise herb-dominated fens. These woody plants have greater water uptake demands than their herbaceous counterparts which could cause a lowering of the water table. However, continuous recharge from groundwater inputs in calcareous fens may exceed the demands of these encroaching woody plants thereby maintaining saturation within the upper peat layers.

Hobo water level pressure sensors were placed in shallow, peat wells in an herbaceous and a woody study area of a representative calcareous fen in order to continuously monitor water level fluctuations throughout the 2005 growing season. The four shallow wells were all screened at a depth ~10cm from the surface. Two of the shallow wells were installed in each of the respective study areas as replicates. Pressure sensors were also placed in deep aquifer piezometers pre-existing on the site. Analysis of redox state, pH, nutrient and major ion concentrations supplemented the examination of fluctuation in water levels. Ion chromatography (IC) and inductively coupled plasma mass spectrometry (ICP-MS) were used to analyze concentration of the major anions and cations, respectively. Water samples were collected from the piezometers, peat, and surface water sites at times of predicted low and high points in the plants' daily water usage.

Preliminary results show strong diurnal fluctuation in water levels in all wells that diminish with the onset of winter. Water drawdown in the woody study area is not significantly greater than in the herbaceous study area. Instead, both of the replicate sensors support a difference in the degree of fluctuation between the two study areas with the woody area experiencing less fluctuation in water level than the herbaceous area.