COS 55-1 - Impacts of climate change on C3 versus C4 grasses imply 21st century shifts in grassland functional composition

Abstract: Grasslands cover a third of Earth's landmass and provide critical ecosystem services. Anticipating how perennial C3 (cool-season) and C4 (warm-season) grasses respond to climate change will be key to predicting future composition and functioning of grasslands. We evaluated environmental drivers of C3 and C4 perennial grass distributions and assess how C3 and C4 grass distributions shift in response to future climate change. We developed integrated species distribution models to identify climate and soil drivers of relative abundance of 11 dominant C3 and C4 perennial grasses in the western United States. 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.

Historically, C3 grasses occupied areas with lower temperature and more variable precipitation regimes, while C4 grasses occupied areas of higher temperature, greater temperature variability and greater warm-season precipitation. C4 species also occupied narrower soil texture niches. In response to future climate change, C3 grass abundance declined across 74% of areas, while C4 abundance increased across 66% of areas. C3 grasses expanded in mid-to higher-latitude areas with increasing temperature and decreasing seasonality of precipitation. In contrast, C4 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.

Findings imply C3 and C4 perennial grasses will have highly divergent responses to climate change that may result in functional compositional changes in grasslands. Increasing temperatures and precipitation variability may favor some C4 grasses, but C4 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.