Divergent climate impacts on C3 versus C4 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 C₃ (cool-season) and C₄ (warm-season) grasses respond to climate change will be key to predicting future composition and functioning of grasslands. Here, we evaluate environmental drivers of C₃ and C₄ perennial distributions and assess how C₃ and C₄ 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 C₃ and C₄ 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, 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.

Main Conclusions

Findings imply C₃ and C₄ 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 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.