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Contributed Talk

Session: : Climate Change: Plants 6

COS 122-5 - Is cheatgrass fitness in western North America limited primarily by climate or competition?

Tuesday, August 8, 2023
4:30 PM – 4:45 PM PDT
Location: B118
Abstract: Cheatgrass (Bromus tectorum) is an invasive annual grass that has decimated sagebrush steppe habitat in western North America by accelerating the fire regime. Currently, these impacts are greater in low-elevation, hot and dry areas than in cooler, wetter, higher-elevation areas. Is the greater invasion “resistance and resilience” of higher-elevation sites due to a climate that is unfavorable to cheatgrass or to competition from native perennials? Answering this question is essential to predict how climate change will alter the trajectory of the cheatgrass invasion. Participants in the Bromecast network conducted a distributed experiment involving demographic observations and removal of resident vegetation to answer the following two research questions: 1) What climate drivers predict cheatgrass fitness (observed in the absence of competition)? 2) Is the effect of competition independent of climate, or does it interact with climate?

On our preliminary analysis of 13 sites across the Intermountain West, the probability of a cheatgrass individual producing seeds ranged from 0 to 77%, with a mean of 43%. Probability of reproduction was not affected by the presence or absence of competition. Mean fecundity of plants that produced seeds ranged from 2 to 93 with a mean of 28 in the absence of competition (removal treatment). Competition significantly reduced fecundity to a range of 2 to 17 with a mean of 5. We failed to definitively answer either of our research questions because we found no significant relationships between climate drivers and cheatgrass fitness in the absence of competition, or between climate drivers and the proportional reduction of fitness caused by competition. However, we did find that proportional reductions in fitness caused by competition were greatest in sites with the highest cheatgrass fitness in the absence of competition. This pattern suggests that environmental factors may be more important than competition in determining whether climate change will allow cheatgrass to expand into currently marginal, higher-elevation habitats. As data from more sites in our network becomes available, our ability to identify the critical environmental drivers will increase.