COMPARISON OF VEGETATION-TOPOGRAPHY RELATIONSHIPS AT THE ALPINE TREELINE ECOTONE

Relationships were examined between topographic gradients, vegetation types, and values of the normalized difference vegetation index (NDVI) at the alpine treeline ecotone within Glacier National Park (GNP), Montana, and Rocky Mountain National Park (RMNP), Colorado. An objective approach to characterizing the treeline elevation within a given area is presented. Non-parametric logistic regression, a form of generalized additive modeling, was used to examine the strength and nature of relationships between the presence or absence of the vegetation types, derived from Landsat Thematic Mapper digital data, and terrain-related variables. Linear and multiple regression models were constructed to characterize the relationships between NDVI and topography. Vegetation-topography relationships were very similar across study areas at the scale of analysis. Comparison of treeline elevations in the two areas revealed a lower-than-expected treeline in Glacier National Park, compared with published latitude-treeline curves. Observed deviations between study areas resulted from differences in the seasonality of the satellite images, rock types, and precipitation regimes. The observed relationships suggested a more active disturbance regime in GNP resulting from climatic, structural, and topographic differences.