Abiotic conditions along altitude shape plant‐fungal associations by influencing both fungal availability and association strength

Abstract Plant‐fungal associations are shaped by both abiotic and biotic variables, but how abiotic variables influence the pool of available fungal species has been much more studied than how the abiotic environment structures the complex and dynamic associations between a host plant and its associated fungi. The question of whether plant‐associated fungal communities are primarily driven by fungal species availability or by the effects of the abiotic conditions on the type and strength of the association remains unclear. Here, a large‐scale field survey in Fennoscandia investigated how the selected fungal communities within different Bistorta vivipara plant compartments (bulbils, leaves and roots) varied with altitude, relative to the available fungal species pool in the soil and surrounding plants. Our results revealed that while the selected fungal species pool was largely determined by the available fungal species pool, the occurrence probability of some fungal species increased at high or low altitudes relative to the respective available fungal species pool. Furthermore, while we found that plant compartment was a major selective force shaping plant‐fungal associations, environment‐driven selection influenced the type and strength of plant‐fungal associations. While the fungi present in the environment largely determined which fungi associated with the focal plants, the type and strength of fungal species associations changed depending on the altitude‐related abiotic stressors imposed on the plant, such as lower temperatures, shorter growing seasons and high winds. Synthesis. Whether plants associate with fungal species just from the available fungal pool or differentially depending on the environmental stressors imposed on the plant, is not well understood, yet this is central for understanding both plant and microbial assembly. We conclude that plant‐fungal associations at different altitudes are not exclusively shaped by passive responses to fungal species availability but rather by more strategic responses by intensifying the associations with fungal species that may offer ecological advantages, such as improved productivity and health. These findings deepen our understanding of how plants adapt to environmental stress through selective symbiosis, with broader implications for plant‐fungal community assembly and ecosystem dynamics.