Cushion shrubs encroach subhumid rangelands and form fertility islands along a grazing gradient in Patagonia

Abstract

Purpose: We assessed changes in soils and vegetation associated with different plant life forms and increasing grazing intensity (GI) in a subhumid grassland of Patagonia (Argentina), with vast portions of these grass steppes in degraded states. Methods: We reconstructed the historic GI gradient of the study sites and characterised the vegetation cover. We sampled the soil beneath patches of grass and shrub and their bare interspaces, as well as the height of the hummocks and the thickness of superficial horizons. We used multivariate analysis, inferential tests, simple regressions and the relative interaction index to measure the effect of historical grazing in these rangelands. Results: The soil in interspaces between shrubs was the most degraded, with particle and fertility losses. While the soil of vegetation patches did not differ in any fraction, the soil beneath shrub patches was far more fertile. The soil of the sites with cushion shrubs developed the typical spatial heterogeneity of the fertility island effect, and their fertility decreased with increasing GI. With increasing GI, the relative cover of cushion shrubs grew and the total grass cover decreased, while the percentage of bare soil increased. Conclusions: The increasing grazing intensity favoured the transition of this subhumid grassland to shrubland. Grazing as an exogenous factor triggers processes of vegetation change and soil degradation, which lead to the encroachment by the cushion shrub Mulinum spinosum and fertility island development. This self-reinforced degradation process, well documented in arid and semiarid environments, also occurs in subhumid rangelands of the forest-steppe ecotone.