Relationships among Vegetation, Surficial Geology and Soil Water Content at the Pocono Mesic Till Barrens

Abstract

EBERHARDT, R. W. AND R. E. LATHAM (Department of Biology, Swarthmore College, Swarthmore, PA 190811390). Relationships among vegetation, surficial geology and soil water content at the Pocono mesic till barrens. J. Torrey Bot. Soc. 127:115-124. 2000.-We examined relationships among vegetation, surficial geology, and soil water content in the Pocono till barrens, an assemblage of pitch pine-scrub oak-heath communities rich in rare species near Long Pond, Pennsylvania. Soil water content was measured using time domain reflectometry over three time periods in summer 1997 in four vegetation types (forest and three barrens communities) on two parent materials (Illinoian glacial till and upper Devonian sandstone bedrock). Wide variation in drought conditions among the time periods (quantified using the Keetch-Byram drought index) allowed for strong tests of differences in soil water-holding capacity among vegetation and parent material types. Soils of barrens and forests did not differ in water content at any depth or drought condition, which confirmed floristic evidence and earlier research that suggested pitch pine-scrub oak vegetation is not associated with xeric conditions. Disturbance history, and possibly the hypothesis of alternative community states, better explain the distinction between barrens and forests in the study area. Soils derived from till and bedrock had similar water contents at each time period, although bedrock soils may have been slightly wetter in the top 15 cm of the soil profile. This contradicts a presupposed relationship between parent material and soil water content and suggests that pitch pine-scrub oak vegetation might occur more extensively on mesic sites in northeastern Pennsylvania than previously thought. Contrary to expectation, barrens types did not differ in soil water content under any drought condition. These results suggest that rhodora barrens, which include diverse and abundant hydrophytes, are not restricted to sites relatively immune from drought. A testable alternative explanation is offered for the distribution of the rare rhodora barrens vegetation type.