Divergent climate impacts on C ₃ versus C ₄ grasses imply widespread 21st century shifts in grassland functional composition

Aim: Grasslands cover a third of Earth's landmass and provide critical ecosystem services. Anticipating how perennial C-3 (cool-season) and C-4 (warm-season) grasses respond to climate change will be key to predicting future composition and functioning of grasslands. Here, we evaluate environmental drivers of C-3 and C-4 perennial distributions and assess how C-3 and C-4 grass distributions shift in response to future climate change. Location: Western United States. Methods: We developed integrated species distribution models to identify climate and soil drivers of relative abundance of C-3 and C-4 perennial grasses. We then created projections of species abundances under future climate and evaluated when and where projected shifts in relative abundance were robust across climate models. Results: Historically, C-3 grasses occupied areas with lower temperature and more variable precipitation regimes, while C-4 grasses occupied areas of higher temperature, greater temperature variability and greater warm-season precipitation. C-4 species also occupied narrower soil texture niches. In response to future climate change, C-3 grass abundance declined across 74% of areas, while C-4 abundance increased across 66% of areas. C-3 grasses expanded in mid- to higher-latitude areas with increasing temperature and decreasing seasonality of precipitation. In contrast, C-4 grasses increased in higher-latitude regions, but declined in lower-latitude, dryer regions. Results were surprisingly robust across climate scenarios, suggesting high confidence in the direction of these future changes. Main Conclusions: Findings imply C-3 and C-4 perennial grasses will have highly divergent responses to climate change that may result in grassland functional compositional changes. Increasing temperatures and precipitation variability may favour some C-4 grasses, but C-4 habitat expansion may be constrained by soil conditions in western USA. Results provide actionable insights for anticipating the impacts of climate change on grass-dominated and co-dominated ecosystems and improving large-scale conservation and restoration efforts.