COS 26-2 - CANCELLED - Droughts promote invasion by C4 grasses, resulting in a loss of native grassland biodiversity

Background/Question/Methods

The frequency and duration of drought events are increasing globally due to climate change, resulting in dramatic shifts in ecological function and species’ interactions. It is hypothesized that increased drought may exacerbate a leading cause of ecological degradation, invasion by non-native species, by favoring those species that are tolerant of variable or extreme conditions. Grassland systems have been heavily impacted by invasive species worldwide, particularly warm-season (C₄) grasses, but it remains unknown how the relative success of non-native and native plant groups will be altered by drought conditions. Using 18 years of data from a uniquely replicated experimental grassland system in central Texas, we compared vegetative responses of three functional groups of non-native plants- warm season (C₄) grasses, cool season (C₃) grasses, and forbs- to those same functional groups of native plants, including more than 6,000 observations across both spring and fall seasons.

Results/Conclusions

In contrast to the historically dominant native forbs and C₃ grasses, non-native C₄ grasses showed a strong positive response to drought conditions, indicating that this group will be favored under climate change. Further, we found that these non-native C₄ grasses reduced native plant richness and cover of native C₃ grasses and forbs, especially during the spring. Though the most common disturbance regime in our region, winter burns, increased the cover of non-native C₄ grasses relative to other groups, the historic disturbance regime, summer burns, were most effective at suppressing nonnative species. In sum, our results highlight the capacity for C₄ grasses to be particularly effective grassland invaders given increasing drought conditions, and that their spread will likely accelerate given current distributions and future climate scenarios without targeted, ecologically informed management.