Soil-geomorphic basis of divergent landscape trajectories in the Chihuahuan Desert.
Bestelmeyer, Brandon1, Ward, Judy 1, Herrick, Jeffrey1, Tugel, Arlene2, 1 USDA-ARS Jornada Experimental Range, Las Cruces, NM, USA2 USDA-NRCS Jornada Experimental Range, Las Cruces, NM, USA
ABSTRACT- A major emphasis of rangeland ecology and management is to understand the factors that regulate rates of vegetation and soil change. Explanations of change have traditionally ignored subtle soil heterogeneity and emphasized single processes. We sought to employ a multi-scale, multi-factor framework to interpret variability in rates of vegetation change occurring within a Chihuahuan Desert grassland under similar management. We used a combination of ground-based measures and remote-sensed data from 1936 and 1996 to construct a scenario explaining different rates of grass loss in two adjacent grassland areas on a piedmont slope. The two areas now merge at sharp ecotone separating a high grass, low shrub cover state and a state with low grass, high shrub, and high bare ground cover. The basis for these divergent trajectories is related to differences in the spatial pattern of soil calcium carbonate content and argillic horizon development. Where high carbonate soils with weak argillic horizons are relatively continuous, single-crown and stoloniferous grasses dominate at the expense of a rhizomatous grass. Grazing appears to have contributed to higher connectivity of bare ground where single-crown grasses predominated, leading to increased susceptibility to erosion, lower soil aggregate stability, lower rainfall infiltration, and soil truncation leading to accelerated rates of grass and shrub mortality. Under these conditions, grass and shrub cover are positively correlated at fine scales even when they are negatively correlated at a broad scale. Eroded soil accumulates on the high-grass side of the ecotone, contributing to ecotone contrast. This example illustrates the need for multiscale perspectives in explaining ecosystem resistance and resilience.
Key words: resilience, thresholds, state-and-transition
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