In arid ecosystems, grazing-induced desertification can be either associated with an increase or a decrease in the spatial heterogeneity of plant communities. We explored the grazing-associated change in the spatial heterogeneity along a gradient of plot sizes (=grains) in a Patagonian steppe with two structural phases: scattered grasses and shrubs surrounded by grasses. We compared the biomass heterogeneity (estimated from locally calibrated digital photographs) of three sites: one ungrazed for the last 26 y and two moderately grazed. On each site, we analyzed three belt transects of 50 contiguous 20- ×20-cm plots and grouped the photographs into plots of increasing size. We used the p parameter (gamma distribution) to measure the global heterogeneity of the biomass and semivariograms to calculate internal heterogeneity and spatial dependence (estimated from the y-intercept and the fractal dimension of semivariograms, respectively). We found that the global and the internal heterogeneity were higher in the grazed sites than in the ungrazed one, possibly because grazing increases the contrast between the structural phases. However, the fractal dimension did not change with grazing. The difference between grazed and ungrazed sites did not disappear when grain increased, probably because the grazing-induced biomass increase of the shrub-dominated phase was higher than the biomass decrease of the scattered grasses phase.
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Vol. 12 • No. 1