Interactions between woody and herbaceous species are central to the structure, dynamics and functioning of plant communities, particularly in semi-arid ecosystems. Empirical evidence suggests that as environmental stress increases, the frequency or intensity of positive ecological interactions (i.e., facilitation) among plants increases, and presumably so do positive spatial relationships (e.g., grass-shrub associations). From a practical standpoint, understanding spatial patterning of plants and the underlying mechanisms provides opportunities for enhancing restoration success. In the Great Basin, for example, the most effective defense against annual grass invasion may be targeted restoration of competitive herbaceous plants in existing stands of sagebrush (Artemisia tridentata ssp. wyomingensis), rather than areas where sagebrush has been removed due to fire or other disturbances. Such an approach requires a solid understanding of how target herbaceous species will grow in a sagebrush mosaic. Our objective therefore was to identify species-specific establishment and growth responses of native herbaceous species to two major spatial components of these communities: canopy vs. gap microsites associated with sagebrush. We investigated the spatial patterning of plant communities at twenty sites spanning a rainfall gradient across the Great Basin and examined species-specific patterns in establishment with respect to canopy and gap microsites.
Results/Conclusions
Our preliminary results indicate that in high rainfall (i.e., least stressful) conditions, cover of one native perennial grass, Poa secunda, is greater in gaps than canopies of sagebrush, suggesting a competitive relationship. Conversely, in the lowest rainfall (i.e., most stressful) conditions, P. secunda cover is higher in canopies than gaps, suggestive of a positive relationship. These results are consistent with predictions that positive relationships will increasingly predominate with increasing stress. Our results also indicate that positive associations between sagebrush and densities of another grass species, Elymus elymoides, appear to persist regardless of rainfall. For this grass species, different mechanisms may be driving spatial relationships in dry vs. wet areas, e.g., degradation of gap microsites in dry sites vs. reduction of competition with sagebrush in wet sites. Overall, our results illustrate that growth of herbaceous plants varies according to species, microsite, and rainfall. These results can be used to help guide restoration in sagebrush-dominated systems.