A geomorphic perspective on terrain-modulated organization of vegetation productivity: Analysis in two semiarid grassland ecosystems in Southwestern United States

Spatial patterns of ecosystem productivity arise from the terrain-modulated wetting and drying of the landscape. Using a daily relative greenness (rG) index, we explore the relations between spatial variability of plant productivity and landscape morphology, and how these relations change over time. The rG index is defined as a measure of local vegetation greenness relative to the site's mean greenness, calculated from remotely sensed normalized difference vegetation index. We analyse two semiarid grasslands with pronounced topography, one located in southeastern Arizona, with a mean annual precipitation of 350 mm, and the other in central New Mexico, with a mean annual precipitation of 250 mm. Our results indicate that (1) rG is spatially more uniform after wet conditions (higher biomass) than after dry conditions (lower biomass); (2) differences in the relative frequency distribution of rG among different landscape morphologies (ridges, unchanneled valleys and channels) indicate higher productivity in channels, similar coefficient of variation in all process domains, and higher skewness in the ridges; (3) relatively high correlations between the binned average rG with respect to upstream area, curvature, and annual insolation in more than 80% of the terrain indicate a clear dependence between ecosystem productivity and topography; (4) rG is more sensitive to changes in topographic indices at the wetter Arizona study site. Such improved understanding of vegetation-topography dependence is critical for ecosystem management, testing ecohydrologic models, and offers ideas for the downscaling of coarse-scale satellite-derived vegetation indices.