ANALYSIS OF LARGE-SCALE SPATIAL HETEROGENEITY IN VEGETATION INDEXES AMONG NORTH-AMERICAN LANDSCAPES

We analyzed the spatial heterogeneity in vegetation indices among 13 N orth American landscapes by using full Landsat Thematic Mapper images. Landscapes varied broadly in the statistical distribution of vegetati on indices, but were successfully ordinated by using a measure of cent ral tendency (the mean) and a measure of dispersion (the standard devi ation or the coefficient of variation). Differences in heterogeneity a mong landscapes were explained by their topographic relief and their l and cover. Landscape heterogeneity (standard deviation of the Normaliz ed Difference Vegetation Index, NDVI) tended to increase linearly with topographic relief (standard deviation of elevation), but landscapes with low relief were much more heterogeneous than expected from this r elationship. The latter were characterized by a large proportion of ag ricultural land. Percent agriculture, in turn, was inversely related t o topographic relief. The strength of these relationships was evaluate d against changes in image spatial resolution (grain size). Aggregatio n of NDVI images to coarser grain size resulted in steady decline of t heir standard deviation. Although the relationship between landscape h eterogeneity and explanatory variables was generally preserved, rates of decrease in heterogeneity with grain size differed among landscapes . A spatial autocorrelation analysis showed that rates of decrease wer e related to the scale at which pattern is manifested. On one end of t he spectrum are agricultural, low-relief landscapes with low spatial a utocorrelation and small-scale heterogeneity associated with fields; t heir heterogeneity decreased sharply as grain size increased. At the o ther end, desert landscapes were characterized by low small-scale hete rogeneity, high spatial autocorrelation, and almost no change in heter ogeneity as grain sized was increased-their heterogeneity, associated with land forms, was present at a large scale.