Controlling site to evaluate history: Vegetation patterns of a New England sand plain

The widespread and long-lasting impact of human activity on natural ecosystems indicates that land-use history must be treated as an integral aspect of ecological study and a critical component of conservation planning. The New England landscape has undergone a complete transformation as forests were converted to agriculture in the 18th and 19th centuries followed by succession to woodland as a result of widespread agricultural abandonment. Despite the prevalence of human impacts, the effect and longevity of land-use practices on modern forest conditions are poorly understood. In the present study of pitch pine-scrub oak vegetation on a sand plain in the Connecticut Valley of Massachusetts, we address the following questions: (1) what is the relative importance of human and natural disturbance and environmental factors in controlling vegetation composition, structure, and landscape patterns; (2) what are the mechanisms underlying human impacts on vegetation, and what is the duration of these impacts; and (3) what are the implications of land-use history for the interpretation and conservation of these communities? Sand plain vegetation was selected for investigation because the homogeneity of site conditions facilitates the interpretation of land-use and natural disturbance impacts, and because the uncommon vegetation and constituent species are priorities for conservation efforts. Paleoecological data suggest that pre-European fires were common on the study area, perhaps ignited by a large regional Indian population. The area was noted historically as an extensive pine plain and was used for wood products from the 18th to the mid-19th century. Eighty-two percent of the area was subsequently plowed for agriculture before being abandoned in the early 20th century. Soil analyses confirm the homogeneity of site conditions and suggest that land uses (plowing, woodlot/pasture) were determined according to ownership pattern rather than site factors. Previously cultivated parcels have distinct Ap (plow horizons) 15-33 cm deep, whereas uncultivated parcels have A horizons 3-10 cm in depth. Soil physical and chemical characteristics are similar among land uses and modern vegetation types. Aerial photographs document a dramatic transformation in plant cover over the last 50 yr. In 1939, the vegetation was grassland or shrub-heath (49%), open-canopy forest (29%), and scrub-oak shrublands (15%). In 1985, 73% of the study area was forested with pitch pine (40%), hardwood (12%), or mixed stands (21%), 9% was in open-canopy stands, and 3% was covered by grass or shrubs. Vegetation/land-use relations are striking. Pitch pine occurs almost exclusively (97%) on former plowed sites, whereas scrub oak stands occur preferentially (89%) on sites that have not been plowed. Land use explains the greatest variation in modern vegetation as well as the distribution and abundance of many taxa. Fire has been common across the study area but has influenced vegetation largely within patterns resulting from prior land use. Land-use patterns and factors controlling vegetation composition and structure are broadly paralleled at similar sites elsewhere in the Connecticut Valley. The study indicates that conservation biologists interested in preserving species, communities, and landscape patterns on sand plains in the northeastern United States need to incorporate a dynamic perspective of biological systems that includes the overriding impact of prior land use. In order to appreciate, study, and display these land-use and vegetation patterns it is essential to conserve the mosaic of assemblages and historical uses within a landscape setting.