HYDROGEOLOGIC CONDITIONS THAT AFFECT RESTORED PITCHER PLANT BOGS, SOUTHEASTERN VIRGINIA

Analyses of a hydrogeologic setting that supports native pitcher plant bogs in the coastal plain of Virginia illustrate the effects that watershed-wide alterations to the vegetation have on groundwater dynamics. Once common in Virginia’s coastal plain, longleaf pine savanna and pitcher plant bog ecosystem complexes declined significantly following centuries of fire suppression. Because re-creation and maintenance of these savannas, such as the ones at the Joseph Pine Preserve in Sussex County, Virginia require considerable effort and expense, effective management plans must be based upon a comprehensive understanding of the interaction between the geologic, hydrologic, and biotic elements that formed them.

Tidal-flat deposits of the Bacons Castle Formation underlie the rolling landscape of the Preserve. Analysis of split-spoon and vibracore samples reveal bogs form at toe-slopes where groundwater seeps from extensive sand layers sandwiched between thick clay beds. Examination of bog stratigraphy across the toeslope using ground-penetrating radar reveals colluvial sand sheets overlie dense clay layers. Diurnal water table fluctuations reflect evapotranspiration cycles. Hydrographs from monitoring wells in the sand beds show relatively stable water table elevations above the bog root zone in 2010-2011. No evapotranspiration signal was observed in the bog toeslope monitoring well installed in the sand. The lack of ET signals in bog hydrographs suggests the rate of groundwater discharge there markedly exceeds drawdown effects caused by ET near the bog well. Comparisons of annual ET rates between adjacent areas with different vegetation densities indicate the average volume of groundwater withdrawn by dense mixed pine/deciduous hardwood forest is roughly 1.5 times greater than the sparsely-wooded longleaf pine savanna and bogs. These results suggest long-leaf pine savanna ecosystems managed with annual prescribed burning withdraw less groundwater than loblolly pine woodlands, effectively increasing the volume of water available to supply the pitcher plant bog. This study will provide resource managers with information critical for restoring these rare wetland habitats.