GIS Data Layers Used for Mapping Risk in the Catskills

Hemlock vulnerability to HWA is complex and linked to multiple site, climatic, stand, and chemical factors (Pontius and others 2006). Here, we were limited to those variables for which raster-GIS coverages are available for inclusion in a landscape-scale risk assessment of the Catskills. This included the following:

Topographic Features

Topographic variables such as slope, aspect, and landscape position can easily be derived from DEMs using the 3D Analyst available in ArcToolbox (ESRI® ArcMap v.9.1). Here we used a digital raster DEM with 10-m resolution from the National Elevation Dataset (NED) assembled by the U.S. Geological Survey (USGS). NED is designed to provide national elevation data in a seamless form with a consistent datum, elevation unit, and projection for the conterminous United States. These can be downloaded at no charge from http://seamless.usgs.gov. The selected GIS available topographic variables included aspect (calculated as the degrees from southwest), slope (degrees) and physiographic landscape position (classes of 1 to 6 representing the least to most xeric landscape positions following (Bailey and others 2004)).

Foliar Chemistry

The researchers involved in this project have used NASA’s AVIRIS instrument to predict foliar concentrations of N, lignin, and cellulose in forested areas of New England (Martin and Aber 1997) with a high degree of accuracy. Using these same methods, we have developed a map of foliar N from 2001 AVIRIS imagery for the Catskills region of New York. Such coverages can be used to inform palatability or defensive chemical-based relationships related to risk assessment.

Hemlock Species Abundance Coverage

Existing maps of hemlock abundance in the Catskills region were available from previous work (Pontius and others 2005). Using 2001 AVIRIS imagery, Mixture Tuned Matched Filtering in ENVI (v.4.2, ©Research Systems, Inc. 2005) was used to unmix spectra and quantify the hemlock signature contribution to each pixel. The availability of this distribution coverage allowed us to isolate only those areas dominated by hemlock (greater than 40 percent hemlock basal area) for the final risk coverage.