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An Assessment of Forest Cover and Impervious Surface Area on Family Forests in the New York City Watershed

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An Assessment of Forest Cover and Impervious Surface Area on Family Forests in the New York City Watershed An Assessment of Forest Cover and Impervious Surface Area on Family Forests in the New York City Watershed Nathaniel M. Anderson, Rene´ H. Germain, and Myrna H. Hall 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 ABSTRACT 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. Keywords: parcelization, land cover, development, impervious surface, watershed management he west-of-Hudson New York City (NYC) water supply Watershed Agricultural Council (WAC; a nonprofit created to ad- system (the Watershed), spans 1,625 mi2 and includes six minister the voluntary, incentive-based Watershed Agricultural Treservoir basins and two major watersheds, the Catskill to Program), have purchased conservation easements on private hold- the east and the Delaware to the west (Figure 1 ). Approximately 1.1 ings totaling approximately 27,000 ac. In total, about 34% of the 1 billion gallons of water per day is delivered from the six reservoirs million-ac Watershed is protected permanently in public ownership through an extensive network of aqueducts connected to the east- and through conservation easement, with about 66% of the Water- of-Hudson Croton water supply system north of the city (NYC shed classified as private land in unprotected status (NYC DEP DEP 2008). The forested nature of the Watershed, combined with 2010, VanBrakle 2011). chemical treatment measures to control sediment loads, is responsi- Previous studies have shown that between 1984 and 2000, the ble for producing water of sufficient quality that the system was average size of a private parcel in the Watershed dropped from 19 to granted a 10-year filtration avoidance determination by the US En- 16 ac (LaPierre and Germain 2005, Hall et al. 2008). Tax records vironmental Protection Agency (EPA) in 2007, continuing its status show that up to 6% of the aggregate area held in parcels greater than as one of the largest unfiltered municipal water supplies in the world. 100 ac was subdivided into smaller parcels over this period. Most In economic terms, filtering the city’s water would cost taxpayers notable is the increase in the total number and aggregate area of and consumers approximately $10 billion in construction costs for a new filtration facility and $400 million per year for operating costs parcels in the 5–10-ac class, a lot size that is especially appealing to (NYC DEP 2008). This fact has provided a strong incentive for homeowners interested in the aesthetic values and privacy provided maintaining high water quality by reducing pollution from agricul- by a residence on a forested parcel. LaPierre and Germain (2005) ture and protecting forests in the region from development through reported that private forested tracts greater than 100 ac made up an land acquisition and conservation easements. estimated 43% of the land area in 2000. This figure likely declined New York State and NYC have combined land holdings of ap- further by 2010, despite the housing downturn late in the decade. proximately 282,000 ac, or 29% of the Watershed area, which in- Development following parcelization, including the construc- cludes the Catskill Forest Preserve. The balance of the watershed tion of new buildings and roads, has potential negative implications area (more than 700,000 ac) is in private holdings dominated by for water resources. Forested watersheds are associated with high family forest owners. Since signing the landmark New York City water quality (Stolton and Dudley 2007), and the negative effects of Watershed Memorandum of Agreement in 1997, NYC and the forest loss due to urbanization have been well documented in many Manuscript received March 16, 2011; accepted December 23, 2011. http://dx.doi.org/10.5849/njaf.11-009. Nathaniel M. Anderson ([email protected]), US Forest Service, Rocky Mountain Research Station, US Forest Service, Rocky Mountain Research Station, 200 East Broadway, Missoula, MT 59807. Rene´ H. Germain, State University of New York, College of Environmental Science and Forestry, Department of Forest and Natural Resources Management, 1 Forestry Drive, Syracuse, NY 13210. Myrna H. Hall, State University of New York, College of Environmental Science and Forestry, Department of Environmental Studies, 1 Forestry Drive, Syracuse, NY 13210. The authors acknowledge funding from the McIntire-Stennis Cooperative Forestry Research Program, the Edna Bailey Sussman Foundation, and the New York Department of Environmental Conservation, Division of Water. This article was written and prepared by a US Government employee on official time, and it is therefore in the public domain and not copyrightable. NORTH.J.APPL.FOR. 29(2) 2012 67 Table 1. Frequency distribution of surveyed parcels according to parcelization type and parcel size class. Stratified sampling was used to adequately represent larger parcel sizes. Parcel type Parcel size class (ac) Intact Subdivided Total 5–9.9 19 29 48 10–49.9 24 20 44 50–99.9 15 6 21 Ն100 16 8 24 Total 74 63 137 imagery, we compared subdivided parcels to parcels that remained intact from 1984 to 2005 to evaluate the impact of parcelization on these metrics. Patterns are discussed in light of observed trends in parcel-level and landscape-level forest cover inferred from spatial models of land cover and land use based on remote sensing and field data. Methods Two previous studies quantified parcelization in the Watershed using a stratified cluster sample of 4,216 parcels, 28% of which were determined to have been subdivided from a larger parcel (i.e., “par- celized”) between 1984 and 2000 (LaPierre and Germain 2005, Figure 1. Location map of the west-of-Hudson New York City Hall et al. 2008). This study uses a stratified random subsample of Watershed and component reservoir basins. The Delaware Water- parcels from the original sample of 4,216 parcels. Using parcel at- shed includes the Cannonsville (1) and Pepacton (3) basins. The tribute data, we contacted 1,267 land owners by mail to request Catskill Watershed includes the Schoharie (2), Ashokan (4), Nev- permission to complete an on-site field survey, which included a ersink (5), and Roundout (6) basins. land cover assessment and measurements of impervious surface area on each parcel. This subsample was stratified by parcel size, county, regions (Hatt et al. 2004), including the east-of-Hudson water sup- and parcel type (intact or subdivided) and did not include any ply system (Burns et al. 2005, Heisig 2009). In general, the density parcels smaller than 5 ac. The focus of this study is family forest, and of housing units is positively correlated with impervious surface area parcels smaller than 5 ac were not included in the sample because and negatively correlated with forest cover (MacDonald and Rudel they were deemed to be too small for forest management and thus 2005, Lathrop et al. 2007). In the urbanizing environment, poor are not classified as family forest. We acknowledge that some re- water quality is closely linked to runoff from impervious surface source managers believe the threshold for forest management to be cover, which intensifies surface water flow and the surface transport higher than 5 ac; however, we agree with DeCoster (1998) that of sediments and other pollutants (Schueler 1994, Arnold and Gib- traditional forestry practices will have to adapt to smaller parcels, bons 1996, Rogers and DeFee 2005, Xian et al. 2007). Low-density especially in areas with high rates of parcelization and development, residential development in suburban, exurban, and rural environ- and therefore we included parcels down to 5 ac. ments has also been linked to significant negative water quality By definition, the boundaries of parcels classified as intact did not impacts, including nonpoint source contamination from lawn change between 1984 and 2000, whereas parcels classified as subdi- chemicals and fertilizers, septic systems, sump pumps, fecal contam- vided were newly created by division of a single, larger parcel into ination, and sediment (Bolstad and Swank 1997, EPA 2006, Schiff two or more parcels sometime during that period. Using the parcel- and Benoit 2007, Conway 2007). Furthermore, research indicates ization topology presented by Donnelly and Evans (2008), 46% of that low-density patterns of development can be associated with subdivided parcels were the result of the parcelization of a large higher impervious surface area per housing unit (Stone 2004). These “parent” parcel into a relatively large number of “child” parcels of studies show that low-density patterns of development are not with- similar area—a pattern indicative of organized subdivision for resi- out risk and may be important to consider in rural watersheds. dential housing. To isolate the effects of a single occurrence of Parcelization is widely acknowledged as a precursor to develop- parcelization over the study period, none of the sampled parcels in ment (Best 2002).
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