The Pocono till barrens: shrub savanna persisting on soils favoring forest1

LATHAM, R. E., J. E. THOMPSON, S. A. RILEY AND A. W. WIBIRALSKE (Department of Geology, University of Pennsylvania, Philadelphia, PA 19104-6316). The Pocono till barrens: shrub savanna persisting on soils favoring forest. Bull. Torrey Bot. Club 123: 330-349. 1996.-A previously undescribed shrub savanna community, which we refer to as the Pocono till barrens, occurs mainly on soils weathered from glacial till of Illinoian age on the southern Pocono Plateau of Pennsylvania. Unlike most "barrens" in east-central North America, its soils are not sandy or shallow to bedrock; the same deep, fine-loamy soil series underlie the barrens and nearby northern hardwoods forest. An unusual abundance of rare plant and animal species in and near the barrens has attracted the attention of scientists and biodiversity conservation professionals. In an effort to understand the ecological dynamics of the Pocono till barrens and why the barrens are different from their mostly forested surroundings, we undertook studies of vegetation history, landscape patterns, plant species distributions and water table depth. The Pocono till barrens (1) are old, pre-dating European settlement of the area; (2) have burned frequently and over large areas throughout their recorded history; (3) presently cover more than 22 km2 adjacent to a belt of xeric ridgetop barrens totaling approximately 18 kM2; (4) have in abundance plants usually found in moist or wet soil conditions living side-by-side with those normally associated with xeric habitats; and (5) include several vegetation types which are distributed on the landscape, in part, in association with a soil moisture gradient. The Pocono till barrens appear to flout the conventional wisdom that barrens vegetation reflects droughty, nutrient-poor soils. Although the Pocono till barrens substantially pre-date European settlement of the area, fire is clearly the key to maintenance of barrens vegetation in this system. We hypothesize that biotic factors are more important than abiotic factors in determining distributions of barrens and forest vegetation on the southern Pocono Plateau.