Warming promotes cheatgrass invasion in mixed-grass prairie

Background/Question/Methods

In the western great plains of North America, cheatgrass (Bromus tectorum) is a common and often troublesome species, but does not commonly dominate landscapes as it does in many parts of the intermountain west. Global change, however, could make great plains grasslands more susceptible to cheatgrass invasion. A variety of global changes have already been linked to the success of cheatgrass in other regions. Elevated CO₂ can strongly increase cheatgrass growth in controlled environments, and invasion by a closely related winter annual (Bromus rubens) in the field. Increased nitrogen availability can also promote cheatgrass invasion. Biogeographic models have predicted that cheatgrass success will depend strongly on changes in the seasonality of precipitation, with cheatgrass being favored in areas with decreased summer precipitation. Little is known, however, about how the combination of elevated CO₂ and warming will influence cheatgrass invasion. We hypothesized that the distinct phenology of cheatgrass would cause it to respond differently than native species to CO₂ and warming. We tested this hypothesis by seeding cheatgrass into partially disturbed subplots within the Prairie Heating and CO₂ Enrichment Experiment, in southeast Wyoming. This experiment increases CO₂ enrichment (to 600 ppm) and temperature (+ 1.5C in the day, + 3C at night).

Results/Conclusions

Warming dramatically increased cheatgrass biomass, from 0.89 g/m² in control plots, to 6.96 g/m² in warmed plots. Repeated measurements of height and leaf number revealed that warming increased growth rates during the late fall and early spring, when native species were dormant. We speculate that warming may have so strongly promoted cheatgrass growth by improving growing conditions during cooler months when competition from native species for water and nitrogen was least intense. In contrast to previous studies, elevated CO₂ had no effect on cheatgrass, nor did it interact with warming to influence cheatgrass. The absence of CO₂ effects may be due to positive effects of CO₂ effects on native competitors. In sum, this study suggests that warming will reduce biotic resistance to cheatgrass invasion in mixed-grass prairie, and highlights the importance of phenological differences between native and invasive species in predicting invasive species responses to global change.