Climate-change-driven shifts in C₃ and C₄ grass distributions and leaf traits could lead to changes in community-level flammability

Premise: Climate change poses challenges to grasslands, including those of the North American Great Plains Region, where shifts in species distributions and fire dynamics are expected. Our present analysis focuses on remaining grasslands within this largely developed and agricultural region. The differential responses of C₄ and C₃ grass species to future climate conditions, particularly in habitat suitability and flammability, are critical for understanding ecosystem changes. Methods: We used species distribution models to predict shifts in habitat suitability for 37 grass species under future climate scenarios and assessed flammability traits in a free-air CO₂-enrichment study, focusing on species' physiological responses to elevated CO₂, warming, and drought. Results: Our models predicted that C₄ species will retain higher habitat suitability, while C₃ species will decline. Leaf-level flammability analysis showed that species with higher water-use efficiency under elevated CO will have lower flammability than under non-elevated, potentially decreasing the predicted rate of fire spread when such species dominate. In contrast, species with higher growth rates but lower water-use efficiency may be more flammable. Species-specific responses varied within functional types. Anticipated shifts in species distributions suggest C₄ species will become more dominant, potentially altering competitive dynamics and reducing C₃ diversity. Changes in flammability under future conditions are expected to influence fire regimes, with a predicted decrease in mean community rate of spread due to the dominance of less-flammable C₄ species. Conclusions: These findings highlight the need for adaptive fire management and conservation strategies to maintain biodiversity and ecosystem function in North American grasslands under climate change.