Introduction

During the past 50 years urban and suburban areas in the United States have been expanding rapidly at the expense of agricultural land and natural ecosystems (Richards,1990; Douglas, 1994). Between 1960 and 1990, 12.6 million ha of cropland, forest and pasture in the United States were converted to urban and suburban land (Frey, 1984; Dougherty, 1992). An additional 4.5 million ha of rural land were developed in the 5 years between 1992 and 1997 (USDA National Resources Inventory, 2000), indicating that the pace of land conversion in the United States has been accelerating. While cities and towns now cover 3.5% of the conterminous United States, their associated sprawl into adjacent counties designated as Metropolitan Areas has resulted in 24.5% of the area of the United States being categorised as urban land cover (Dwyer et al., 2000). These areas, including their natural components like forested land, are thus becoming increasingly exposed to the effects of diverse urban activities. The states that have experienced the greatest population growth per unit land area between 1990 and 1996 occur in the eastern third of the country, where human settlement is expanding mostly into forested land (Dwyer et al., 2000). These eastern rural forests have been increasing in area in the past 100 to 150 years because of secondary succession following farmland abandonment in the nineteenth and early twentieth centuries (Foster, 1993).