Experimental evidence that root-associated fungi improve plant growth at high altitude
1 The dynamics and structure of ecological communities are shaped by the combined effects of abiotic and biotic filtering. The abiotic environment may impact the outcome of biotic interactions, consequently affecting the community's response to the environment. Therefore, understanding how species communities change along environmental gradients requires a dual understanding: the direct responses of the species to their abiotic surroundings and the indirect variation of these responses through biotic interactions.      2 Here, we focus on the interactive relationships between plants and their symbiotic root-associated fungi (RAF) along stressful abiotic gradients. We investigate whether variations in RAF community composition along altitudinal gradients influence plant growth at high altitudes, where both plants and fungi face harsher abiotic conditions.     3 We established a translocation experiment between pairs of high and low altitude populations of Bistorta vivipara along arctic altitudinal gradients. To separate the impact of shifting fungal communities from the overall influence of changing abiotic conditions, we used a root barrier to prevent new colonizations by RAF following translocation. To characterize the RAF communities, we applied DNA barcoding to the root samples using the ITS2 region. Through the utilization of joint species distribution modeling, we assessed the relationship between changes in plant functional traits resulting from experimental treatments and the corresponding changes in the RAF communities.      4 Our findings indicate that RAF communities influence plant responses to stressful abiotic conditions. Plants translocated from low to high altitude grew more when they were able to associate with the resident high-altitude RAF compared to those plants that were not allowed to associate with the resident RAF. In particular, leaf length increased and flowers were more often present when fungi from certain Ascomycota clades were present or abundant.     5 We conclude that interactions with RAF impact how plants respond to stressful abiotic conditions. In particular, our results provide experimental support that interactions with RAF improve plant stress tolerance to altitudinal stressors such as colder temperatures and less nutrient availability.