Elevated [CO₂] and temperature augment gas exchange and shift the fitness landscape in a montane forb

Climate change is simultaneously increasing carbon dioxide concentrations ([CO₂]) and temperature. These factors could interact to influence plant physiology and performance. Alternatively, increased [CO₂] may offset costs associated with elevated temperatures. Furthermore, the interaction between elevated temperature and [CO₂] may differentially affect populations from along an elevational gradient and disrupt local adaptation. We conducted a multifactorial growth chamber experiment to examine the interactive effects of temperature and [CO₂] on fitness and ecophysiology of diverse accessions of Boechera stricta (Brassicaceae) sourced from a broad elevational gradient in Colorado. We tested whether increased [CO₂] would enhance photosynthesis across accessions, and whether warmer conditions would depress the fitness of high-elevation accessions owing to steep reductions in temperature with increasing elevation in this system. Elevational clines in [CO₂] are not as evident, making it challenging to predict how locally adapted ecotypes will respond to elevated [CO₂]. This experiment revealed that elevated [CO₂] increased photosynthesis and intrinsic water use efficiency across all accessions. However, these instantaneous responses to treatments did not translate to changes in fitness. Instead, increased temperatures reduced the probability of reproduction for all accessions. Elevated [CO₂] and increased temperatures interacted to shift the adaptive landscape, favoring lower elevation accessions for the probability of survival and fecundity. Our results suggest that elevated temperatures and [CO₂] associated with climate change could have severe negative consequences, especially for high-elevation populations.