An assessment of forest cover and impervious surface area on family forests in the New York City Watershed

Between 1984 and 2000, the parcelization of family forests in the New York City Watershed caused a decline in average parcel size from 19 to 16 ac. However, little is known about the timing and intensity of development on subdivided parcels, which has the potential to negatively affect water quality by increasing nonpoint source pollution associated with nutrient runoff and erosion intensified by increased impervious surface area. Using a combination of field measurements and analysis of digital orthoimagery, this study quantified forest cover and impervious surface area on new parcels resulting from subdivision and compared subdivided parcels to intact parcels. Measurements of buildings, driveways, and other features show that by 2005, parcels subdivided between 1984 and 2000 were developed to nearly the same intensity as intact parcels, with 68% of parcels classified as developed. Results indicate that residential development on subdivided parcels has added more than 640 ac of impervious surfaces on family forest since 1984, apparently without being accompanied by observable net reductions in forest cover at the landscape level. These trends have important implications for public policy intended to maintain a forested watershed and prevent the degradation of drinking water quality from nonpoint source pollution associated with development.