Landscape heterogeneity, soil development, precipitation regime, and growth and distribution of vegetation in a desert-grassland ecotone

Bouteloua gracilis (blue grama), Bouteloua eriopoda (black grama), and Larrea tridentata (creosote) are dominant plants on the McKenzie Flats portion of the Llano de Manzano landform within Sevilleta National Wildlife Refuge in central N.M. This area is part of the biome ecotone from the Colorado Shortgrass Steppe to the Chihuahuan Desert. In this study, I examine the hypothesis that soil heterogeneity, determined by variation in cumulic surface soil depth, carbonate accumulation, and fine-textured fraction, controls the relative dominance of the three major vegetation species across the ecotonal landscape. The area is flat, generally <1% slope; however abrupt soil differences exist even within the flattest parts of the landscape. These soil boundaries correspond to the pattern of buried channels that cut through the petrocalcic horizon (caliche) that formed in a 0.5–1.2 million year-old paleosol buried under the current surface soil. Multivariate analyses of soil-moisture-related variables suggest that B. gracilis, a Colorado Shortgrass Steppe indicator, dominates the buried paleochannels where the Holocene surface deposits are deepest and the argillic (clay-rich) B-horizon is thickest. B. eriopoda, dominant in Chihuahuan Desert grasslands, is most abundant where the buried petrocalcic horizon lies within 40 to 60 cm of the surface and the argillic horizon is weakly developed. L. tridentata, an indicator of desertified Chihuahuan Desert shrubland, is dominant where the petrocalcic horizon is exposed or near the surface. This study illustrates the strong relationship between geomorphology, soil development and vegetation patterns in arid and semi-arid environments.; A repeated-measures precipitation manipulation experiment was conducted over two years to monitor the growth and nutrient responses of the two grass species to 0%, 100%, and 200% ambient precipitation to model B. eriopoda and B. gracilis ecophysiological dynamics across the desert/steppe ecotone. Percent-cover of the two grama grasses was measured annually three times on each plot. Foliar and soil chemistry were sampled at the end of the experiment to examine physiological effects of the treatments. Univariate results suggest that the two grasses respond similarly to changes in moisture regime. Foliar phosphorus concentrations were correlated with growth trends. However, foliar nitrogen concentrations did not differ between treatments. After water, nitrogen is the limiting mineral nutrient in the ecosystem, therefore its availability does not vary with changes in precipitation. (Abstract shortened by UMI.)...