SOIL EROSION, SEDIMENT YIELD, AND DECISION SUPPORT SYSTEMS FOR IMPROVED LAND MANAGEMENT ON SEMIARID RANGELAND WATERSHEDS
Location: Southwest Watershed Research
Title: A geomorphic perspective on terrain-modulated organization of vegetation productivity: Analysis in two semiarid grassland ecosystems in Southwestern United States
Submitted to: Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 27, 2012
Publication Date: N/A
Interpretive Summary: Differences in ecosystem productivity are caused by the influence of terrain on wetting and drying of the landscape. Daily relative greenness was used to explore how plant productivity changes with landscape over time. Two semiarid grasslands with pronounced topography, one located in southeastern Arizona, AZ, with a mean annual precipitation (MAP) of 350 mm; and the other in central New Mexico, NM, with a MAP of 250 mm were analyzed. Results showed that relative greenness was more uniform after wet conditions where higher biomass was present and productivity was higher in channels. There was a clear dependence between ecosystem productivity and topography, and relative greenness was more sensitive to changes in topography at the wetter Arizona study site. The results from this study serve to provide an improved understanding of vegetation-topography dependence. Such understanding is critical for ecosystem management and testing ecohydrologic models.
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 (NDVI). We analyze two semiarid grasslands with pronounced topography, one located in southeastern Arizona, AZ, with a mean annual precipitation (MAP) of 350 mm; and the other in central New Mexico, NM, with a MAP of 250 mm. Our results indicate: (1) that rG is spatially more uniform after wet conditions (higher biomass) than after dry conditions (lower biomass); (2) differences in the relative frequency distribution (rfd) 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 (UA), curvature (Curv) and annual insolation (Rad) in more than 80% of the terrain indicate a clear dependence between ecosystem productivity and topography; (4) that rG is more sensitive to changes in topographic indices at the wetter AZ 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.