Do abiotic differences explain contrasting graminoid functional traits in sagebrush steppe, USA and Patagonian steppe, Argentina?

摘要

Previous work in climatically similar sagebrush steppe, USA and Patagonian steppe, Argentina suggested that a more intense evolutionary history of grazing in Patagonia selected for graminoids of lower forage quality, resulting in a plant community more resistant to livestock grazing. Here we consider whether subtle abiotic differences might create greater water or nutrient limitation in Patagonian steppe, which would offer an alternative explanation for the observed contrasts in graminoid functional traits. Simulations of soil water dynamics showed that observed differences in temperatures and wind speeds canceled out, but differences in soil texture were important. The sandier soils typical of Patagonian steppe reduced the ratio of evaporation to transpiration, and may have contributed to a longer growing season in Patagonia, as measured by a satellite index of vegetation activity. Thus, we did not find strong evidence for greater water limitation than in sagebrush steppe, although the larger arid zone within Patagonia might still favor the evolution of xerophytic traits. On the other hand, sandy soils accumulate less total nitrogen than the loamier soils common in sagebrush steppe. Older, more weathered soils are also more regionally common in Patagonia. Nitrogen limitation created by coarse textured, weathered soils offers an alternative working hypothesis to explain the evolution of lower forage quality in Patagonian steppe graminoids.