Interpreting the Chart:

Climate, Landform, and Soil Diagrams: These images show, like a contour map in niche space, the distribution of species importance value with two variables each for climate, landform, and soil (variables are described below). The entire envelope for all the species (the domain of our study area) is shown in gray (which is the same for all species). The various shades inside reflect the importance value of the species, with lighter shades indicating higher importance within the total space. All species are plotted with the same axes, so that direct comparisons among species can be made. It should be noted that for species bounding the northern border of the United States, the range of ecological space would likely extend beyond that portrayed here. In these cases, the ecological space and resulting regression tree models would reflect these artificial boundaries as well, and should be interpreted accordingly.

Annual Temperature, C: average annual temperature for the counties occupied by the species. Data from USEPA (1993).

Precipitation, mm: average annual precipitation for the counties occupied by the species. Data from USEPA (1993).

Maximum Elevation, m: average maximum elevation for the counties occupied by the species. Data from USGS 1:250,000 Digital Elevation Model (DEM) files obtained from the USGS internet site (US Geological Survey 1990; available at http://edcwww/cr/usgs.gov/glis/hyper/guide/1_dgr_demfig/states.html

Slope, percent: average percent slope for the counties occupied by the species. Data from the State Soil Geographic Data Base (STATSGO), developed by the US Natural Resource Conservation Service on 1:250,00 scale maps (Soil Conservation Service 1991); processing included calculating an area-weighted average for each map unit based on the percent composition for each soil series in the map unit, and then calculating another area-weighted average for all map units within each county (Iverson et al. 1996).

pH: average pH for soils in the counties occupied by the species. Data are from STATSGO (Soil Conservation Service 1991), and because tabular data are reported by layer and soil series, three weighted averages were needed to calculate a county average (Iverson et al. 1996). First, a weighted average, based on the thickness of each layer to a depth of 60 inches (152 cm) or the depth to bedrock, was calculated for each soil series. Then, area-weighted averages were calculated for map unit and county as described for slope.

Permeability, cm/hr: the property of the soil that permits transmission of water through the soil, and is related to hydraulic conductivity of the soil. The finer the material (less porous), the slower the permeability. Reported for the counties occupied by the species, the data are processed from STATSGO, as discussed above for pH.

Literature Cited:
Iverson, L.R.; Prasad, A.; Scott, C.T. 1996. Preparation of forest inventory and analysis (FIA) and state soil geographic data base (STATSGO) data for global change research in the eastern United States. In: Hom, J.; Birdsey, R.; O'Brian, K. Proceedings, 1995 meeting of the Northern Global Change Program; 1995 March 14-16; Pittsburgh, PA. Gen. Tech. Rep. NE-214, Radnor, PA:, U.S. Department of Agriculture, Forest Service, Northeastern Forest Experiment Station: 209-214.

Soil Conservation Service. 1991. State soil geographic data base (STATSGO) data users guide. Misc. Publ. 1492. Washington, DC.: U.S. Department of Agriculture, Soil Conservation Service. 88 p.

U.S. Environmental Protection Agency. 1993. EPA-Corvallis model-derived climate database and 2xCO2 predictions for long-term mean monthly temperature, vapor pressure, wind velocity and potential evapotranspiration from the Regional Water Balance Model and precipitation from the PRISM model, for the conterminous United States. Digital raster data on a 10 x 10 km, 470 x 295 Albers Equal Area grid, in "Image Processing Workbench" format. Corvallis, OR: U.S. Environmental Protection Agency, Environmental Research Laboratory.