Less is more? Faster growth is associated with lower ectomycorrhizal diversity in mature Picea glauca at the Alaskan treelines

Root-associated fungal (RAF) communities can influence tree nutrient acquisition and performance, yet their structuring factors and relationship with tree growth remain poorly understood, particularly near environmental treelines. We characterized root-associated fungal (RAF) communities on fine roots of white spruce (Picea glauca [Moench] Voss) in paired forest and treeline plots across two elevational and one moisture-limited treeline ecotones in Alaska. Using ITS2 DNA metabarcoding of fine root tips, we characterized individual RAF community alpha and beta diversity and tree growth based on basal area increment (BAI) over 5-30 years. As a result, sampling site was the strongest predictor of RAF composition, explaining 19.6% of variation, while soil pH explained an additional 11.7%. Treeline effects on RAF communities were weaker and context-dependent, with treeline trees showing distinct community composition in the Alaska Range, lower alpha diversity across all Hill numbers in Interior Alaska, and higher ECM relative abundance in the Brooks Range. RAF composition did not differ between fast- and slow-growing trees within sites. In contrast, alpha diversity was negatively associated with tree growth: OTU richness, Shannon and Inverted Simpson indices significantly predicted BAI over the previous 5, 10 and 15 years, with fast-growing trees supporting less diverse RAF and ECM communities. This relationship was strongest for recent growth and weakened with longer BAI averaging periods. Our results suggest that high growth of mature P. glauca is not necessarily associated with greater RAF or ECM diversity or specific taxa but may instead rely on fewer dominant or functionally effective fungal partners.