Grass-shrub spatial pattern response to a moisture gradient in Great Basin sagebrush communities

Background/Question/Methods

Shrub-grass interactions play an important role in the structure and function of plant communities, especially in semi-arid ecosystems of the Great Basin. Evidence suggests that as environmental stress increases, facilitative plant interactions predominate, resulting in positive plant-plant spatial associations.   Canopy microsites may facilitate growth of native perennials; conversely, seedlings may thrive in interspaces due to reduced woody plant competition. In the Great Basin, evidence suggests that positive perennial grass-shrub associations occur between Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) and native grasses.  We investigated the response of shrub-grass spatial associations between two native grasses and sagebrush to a rainfall gradient.  We hypothesized that shrub-grass spatial associations would become more pronounced at lower rainfall levels.  We sampled 25 sagebrush at each of 29 sites across five Great Basin ecoregions.  We selected sites with loamy soils, a shrub component dominated by WY big sagebrush, and an understory comprised of native perennial grasses.  At each shrub, we recorded the percent cover of two perennial grasses, Poa secunda and Elymus elymoides, along 200cm transects extending from the shrub base.  We analyzed the percent cover of Poa and Elymus in shrub canopies and interspaces over a continuous rainfall gradient using a mixed-effects ANCOVA.

Results/Conclusions

Our preliminary results indicated that as rainfall increased, percent cover of Poa increased significantly in interspace microsites.  However, its percent cover remained constant in sagebrush canopy microsites across rainfall levels.  At low rainfall levels, Poa cover was significantly greater in canopy versus interspace microsites, while at high levels of rainfall, cover did not differ between canopy and interspace microsites.  Our results suggest that because Poa cover remained constant in canopy microsites regardless of rainfall, canopy microsites provide positive benefits to Poa at any rainfall level.  The strong increase in Poa cover over increasing rainfall levels also suggests that interspace regions provide good microsites for Poa at high rainfall levels. Percent cover of Elymus was significantly greater in canopy versus interspace microsites.  Its percent cover remained constant in interspaces.  Our results for Poa support our hypothesis that positive grass-shrub spatial associations are stronger at low rainfall levels; results for Elymus indicate a positive grass-shrub association regardless of rainfall levels. Our results can be used to help guide restoration efforts in sagebrush-dominated systems, in particular selection of planting microsites that are most likely to yield establishment success.