Abstract
Grasslands are one of the most threatened ecological communities globally and are under increased risk from extreme compound events, such as those experienced during heatwaves. Arbuscular mycorrhizal (AM) fungi can facilitate plants during environmental stress and may help grasses mitigate climate stressors. When stressors occur simultaneously, it is particularly stressful for plants, and the role of AM fungi in ameliorating this stress in grassland species is poorly understood. This study investigated the role of AM fungi in facilitating growth and fitness responses of temperate grassland species to heat stress, water stress and their combination. Additionally, this study explores the influence of C 3 /C 4 photosynthetic pathways and seasonal factors in influencing grass responses. AM fungi had variable effects on plant performance under ecologically relevant heat and water stress events. While the influence of AM fungi showed some positive influence on plant performance under stress, overall biomass was reduced in the presence of AM fungi. C 3 species had increased fitness when grown with AM fungi in warmer seasons but were sensitive to heat and water stress in winter, suggesting seasonal acclimation of C 3 grasses. No such trends were shown for C 4 grasses, which were resilient across all stressors and unaffected by the presence of AM fungi. Colonisation levels of AM fungi were variable, with no patterns associated with photosynthetic pathway, and colonisation tending to be lower with higher stress. The results highlight the risk of community scale shifts in temperate grassland communities, informing restoration and management of grasslands under the global increase of extreme compound events.