Warming and reduced snow cover improve cheatgrass (Bromus tectorum) performance in the Intermountain West

Background/Question/Methods

Cheatgrass (Bromus tectorum) has degraded extensive low elevation areas of the Intermountain West. Indirect evidence suggests that global warming might improve the performance of this problematic invasive grass. We directly tested whether (i) the impacts of cheatgrass on Sagebrush ecosystems of the Intermountain West will be increased by climate warming and whether (ii) these impacts will be further amplified when temperature increase is large enough to reduce snow cover. We hypothesized that lack of snow would favor cheatgrass by increasing nitrogen availability through increased soil freeze-thaw events that release nutrients held within microbial biomass. We also hypothesized that cheatgrass populations from low elevations would be the most likely to take advantage of higher temperatures. To test these hypotheses, we conducted an experiment in a Northern Utah sagebrush steppe to quantify the demographic response of three different cheatgrass populations to three treatments: temperature increase using infrared heaters, snow melt (heaters only turned on to melt snow), and nitrogen addition.

Results/Conclusions

All treatments had significant effects on the population growth rate but no significant interactions were detected. Cheatgrass population growth rate progressively increased from nitrogen addition to snow melt and then to the warming treatment, with this last treatment having by far the highest growth rate. Cheatgrass seed from the lowest elevation performed consistently worse than seed from higher elevations. Contrary to expectations, soil nutrient content analyses showed that snow melt did not favor cheatgrass by increasing nitrogen availability. Instead, the snow melt treatment doubled the proportion of seedlings surviving the winter while the constant warming treatment also dramatically increased fecundity. These results suggest that there is a chance for cheatgrass performance to increase non-linearly where warming reduces snowpack depth and duration. Moreover, mid and high elevation cheatgrass genotypes from Northern Utah seem already adapted to increased temperatures, thus allowing rapid population response to climate change.