Plant functional traits provide one tool for predicting the effects of grazing on different ecosystems. To test this approach, we compared plant traits and grazing response across analogous climatic gradients in sagebrush steppe, USA (SGBR), known to have a short evolutionary history of grazing, and Patagonian steppe, Argentina (PAT), where generalist herbivores exerted stronger selective pressures. We measured grazing response by sampling vegetation and soils across distance-from-water gradients at arid, semiarid, and subhumid study areas in both regions. Based on a previous analysis of graminoid traits, we predicted that: (1) high forage quality in all three SGBR communities would lead to high utilization and large grazing effects, whereas low quality in arid PAT would constrain utilization and grazing impacts, with semiarid and subhumid PAT intermediate in quality and grazing response; and (2) grazing in arid PAT would cause shifts in relative abundance within the graminoid functional group, due to the large range of forage quality among graminoids, but in SGBR, where all graminoids are relatively palatable, shifts in abundance would occur between grasses and shrubs.
Utilization in locations close to water was higher in SGBR than in PAT study areas. This utilization difference led to differences in grazing effects consistent with our first prediction. Abundance of graminoids increased with distance from water in all three SGBR communities and in subhumid PAT, but not in arid PAT. Shrub and total production decreased with distance in SGBR but not PAT study areas. Grazing variables explained less variation in species composition in arid PAT (43%) than in any other study area (59–74%). Grazing did not significantly influence species richness. Evidence for our second prediction was mixed. Grazing did alter the relative abundance of SGBR graminoids and shrubs, but abundance shifts among the graminoids in SGBR communities were larger than in PAT communities, counter to our prediction.
This case study demonstrates how plant traits can explain relative effects of grazing on ecosystem structure and functioning, although predicting species-specific responses remains a challenge. Regardless of their evolutionary origin, poor-quality graminoids make the arid Patagonian steppe more resistant to overgrazing than communities dominated by more nutritious species.