Spatial and temporal relationships among fire frequency, vegetation, and soil depth in an eastern North American serpentine barren

The serpentine barrens of Pennsylvania and Maryland are increasingly recognized as a global hotspot for rare and endemic species, and are the focus of management efforts by government agencies and conservation organizations. Surrounded by forests, farmland, and housing developments, the typical serpentine barrens community may be characterized as savanna-like; its presence is commonly attributed to the unusual chemical and physical conditions of serpentine-derived soils. However, recent research and anecdotal evidence suggest that disturbance, particularly fire, also plays an important role in maintaining the serpentine ecosystems. To better understand the various factors involved in maintaining these ecosystems, I investigated the spatial and temporal patterns of disturbance and vegetation change in the Nottingham Serpentine Barrens over the past 150 years. Between 1937 and 1993, savanna area was reduced by half and hardwood forest area increased from 2% to 25% of total area. Fire is one factor responsible for maintaining che savanna conditions that support the rare and endemic species. In the case of less frequent fire (or its absence) the savannas and open woodlands convert to closed hardwood forest. Soil depth also appears to influence the rate and direction of succession: where soil depth is shallow, native serpentine vegetation has a competitive advantage. Frequent fire most likely maintains shallow soil conditions by reducing litter which might otherwise contribute to soil development. This suggests that a feedback relationship among disturbance, soil development, and vegetation may best explain current conditions at Nottingham. These findings have important implications for land managers interested in using prescribed fire to protect these ecosystems.