Relationships among vegetation, surficial geology and soil water content at the Pocono mesic till barrens

EBERHARDT, R. W AND R. E. LATHAM (Department of Biology, Swarthmore College , Swarthmore, PA 19081-1390). Relationships among vegetation, surficial geo logy and soil water content at the Pocono mesic till barrens. J. Torrey Bet . Sec. 127:115-124. 2000.-We examined relationships among vegetation, surfi cial geology, and soil water content in the Pocono till barrens, an assembl age of pitch pine-scrub oak-heath communities rich in rare species near Lon g Pond, Pennsylvania. Soil water content was measured using time domain ref lectometry over three time periods in summer 1997 in four vegetation types (forest and three barrens communities) on two parent materials (Illinoian g lacial till and upper Devonian sandstone bedrock). Wide variation in drough t conditions among the time periods (quantified using the Keetch-Byram drou ght index) allowed for strong tests of differences in soil water-holding ca pacity among vegetation and parent material types. Soils of barrens and for ests did not differ in water content at any depth or drought condition, whi ch confirmed floristic evidence and earlier research that suggested pitch p ine-scrub oak vegetation is not associated with xeric conditions. Disturban ce history, and possibly the hypothesis of alternative community states, be tter explain the distinction between barrens and forests in the study area. Soils derived from till and bedrock had similar water contents at each tim e period, although bedrock soils may have been slightly wetter in the top 1 5 cm of the soil profile. This contradicts a presupposed relationship betwe en parent material and soil water content and suggests that pitch pine-scru b oak vegetation might occur more extensively on mesic sites in northeaster n Pennsylvania than previously thought. Contrary to expectation, barrens ty pes did not differ in soil water content under any drought condition. These results suggest that rhodora barrens, which include diverse and abundant h ydrophytes, are not restricted to sites relatively immune from drought. A t estable alternative explanation is offered for the distribution of the rare rhodora barrens vegetation type.