Jacob A. Cowan1, Catherine A. Gehring2 and Kevin C. Grady1
1School of Forestry, Northern Arizona University, Flagstaff, AZ, 86011‐5018 USA
2Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ, 86011 USA
The displacement of high diversity big sagebrush (Artemisia tridentata) dominated communities by cheatgrass (Bromus tectorum) invasion, an annual grass which supports much less diverse communities, is widely recognized to represent a major threat to regional biodiversity in the shrub-steppe communities of the western US. Once established, cheatgrass maintains dominance by altering fire regimes and because of its early germination, which makes it a very effective competitor for soil water and nutrients. Additionally, there is increasing recognition that cheatgrass may be altering soils in a way that suppresses sagebrush germination and regrowth. To assess the role that cheatgrass-induced plant-soil feedbacks may be playing in the maintenance of dominance by this invasive, we grew sagebrush seedlings without direct competition in soils conditioned with either cheatgrass, Elymus elymoides (a common native perennial bunchgrass) or conspecifics. Seedling growth was compared with controls to determine the direction and magnitude of feedbacks being exerted by the different conditioning plants. We found that cheatgrass and Elymus exerted negative feedback on sagebrush, and the effect of the invasive was not significantly different than that of the native. Surprisingly, negative feedbacks did not increase with increasing biomass of the conditioning plant (i.e. there was no density-dependent effect). Despite averaging six times larger than Elymus during our conditioning phase, cheatgrass had no greater suppressive effect on sagebrush growth. Conspecific feedbacks were neutral to slightly negative, which is not unusual for a climax species. To explore some potential mechanisms driving these feedback patterns, we looked at nutrient depletion and reduction in colonization by arbuscular mycorrhizal fungi (AMF), important symbionts of sagebrush. Neither of these seems to be playing a role in sagebrush suppression. Additionally, we are in the process of using next-generation amplicon sequencing to characterize root microbial communities (fungal and bacterial), to allow for the comaprison between the conditioning treatments. These data will likely be included in the presentation. Alteration of root microbial communities away from compositions beneficial to natives has been implicated in other invasions and could be driving the observed feedback patterns. We discuss aspects of cheatgrass biology that allow it to extensively alter soils that contain native bunchgrasses. Finally, we consider implications for restoration.