Elevated <scp>CO₂</scp> and drought modify plant–plant and plant–mycorrhizal interactions in two codominant grasses

Abstract Plant–plant interactions play a critical role in shaping plant communities and influencing ecosystem services. However, how these interactions shift between positive (facilitation) and negative (competition) in response to environmental factors, including changes in symbiotic relationships with arbuscular mycorrhizal fungi (AMF), remains less understood. To address this knowledge gap, we conducted an experiment investigating the plant–plant interactions and AMF root colonization of two codominant grasses of tallgrass prairie, Andropogon gerardii and Sorghastrum nutans . We established three neighbor treatments (no neighbor, interspecific, and intraspecific interactions), and exposed the grasses to a combination of water and CO 2 treatments: drought with ambient CO 2 , well‐watered with ambient CO 2 , drought with elevated CO 2 , and well‐watered with elevated CO 2 . We hypothesized that elevated CO 2 would ameliorate the negative effect of drought on biomass and AMF root colonization in these grasses, and that competition would be most prominent under less stressful conditions (well‐watered with ambient or elevated CO 2 ), decreasing as stress increased (drought with ambient CO 2 ), eventually leading to facilitation under more stressful conditions. Our findings demonstrated that elevated CO 2 ameliorated the negative effects of drought on the aboveground biomass of both grasses. Additionally, drought with ambient CO 2 treatment resulted in competition between plant individuals, which decreased as stress levels increased. Facilitation was observed under the least stressful condition (well‐watered with elevated CO 2 ) for belowground biomass. Interestingly, AMF root colonization was higher under drought with ambient CO 2 treatment and decreased under drought with elevated CO 2 treatment in the presence of a neighbor, suggesting a stress‐dependent response in AMF colonization. Our study revealed a shift in plant–plant and plant–AMF interactions driven by the combined effects of drought and elevated CO 2 . These findings have important implications for understanding how codominant grasses and their symbiotic relationships with AMF may respond to changing climatic conditions in tallgrass prairie.