The ELU is a composite of several layers of abiotic information that critically influence the form, function, and distribution of ecosystems - elevation zone, bedrock geology, and landforms. Each 90m grid cell is assigned a given elevation, bedrock or surficial geology, and landform class. The three components can be viewed or queried separately or in combination.
Elevation has been shown to be a powerful predictor of the distribution of forest communities in the Northeast. Temperature, precipitation, and exposure commonly vary with changing altitude. Bedrock geology strongly influences area soil and water chemistry. Bedrock types also differ in how they weather and in the physical characteristics of the residual soil type. Because of this, local lithology is usually the principle determinant of soil chemistry, texture, and nutrient availability. Rowe (1998) contends that landform is "the anchor and control of terrestrial ecosystems." Landforms are largely responsible for local variation in solar radiation, moisture availability, soil development, and susceptibility to wind and other disturbance. We adopted the Fels and Matson (1997) system for landform modeling, in which combinations of slope and landscape position are used to define topographic units such as ridges, sideslopes, coves, and flats on the landscape.
Six ecologically relevant elevation zones were defined; over 250 bedrock and surficial geology classes were collapsed into 9 ecologically distinct geology classes; and GIS modeling gave us 13 ecologically significant landform classes. Combination of these resource grids resulted in 480 unique ELUs in the ecoregion.