While many studies have addressed the effect of individual stresses on plant—plant associations, few have addressed the effects of co-occurring stresses. We therefore investigated how associations between Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) and 2 native grasses (Poa secunda and Elymus elymoides) responded to different combinations of grazing and moisture stresses in the Great Basin, USA. Positive (i.e., facilitative) interactions between nurse plants and their beneficiaries are predicted to increase with increasing moisture limitation and grazing stress, but these interactions may break down at extreme levels of stress. We hypothesized that (1) competitive interactions and negative shrub-grass spatial associations would occur under the least stressful conditions (low grazing intensity / high precipitation); (2) positive shrub-grass spatial associations would dominate at intermediate levels of stress (high grazing intensity / high precipitation and low grazing intensity / low precipitation); and (3) negative grass-shrub relationships would dominate at extreme levels of stress (high grazing / low precipitation). We sampled 5 site pairs (high vs. low grazing intensity) that occurred over a precipitation gradient. We assessed how abundance of the 2 grasses P. secunda and E. elymoides responded to sagebrush microsite (canopy vs. interspace), grazing intensity, and precipitation. We found that both grass species were positively associated with A. tridentata canopy microsites at low annual precipitation levels. However, grazing stress appeared to weaken this effect for P. secunda, indicating, as we predicted, a potential breakdown of facilitative interactions in highly stressful conditions. Although we predicted that facilitation would dominate in moderately stressful conditions, we only found this to be true (for both grasses) in one of the 2 moderately stressful scenarios (low grazing / low precipitation). Our results provide insights into how Great Basin plant communities may respond to the separate and combined effects of grazing and drought stresses, both of which may intensify in the future.
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