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 nutrition and performance, yet their environmental drivers and relationship with tree growth remain poorly understood, particularly near treelines. We characterized RAF communities, with a focus on ectomycorrhizal (ECM) fungi, associated with fine roots of white spruce ( Picea glauca [Moench] Voss) across paired forest and treeline plots representing two elevational and one drought-limited treeline ecotones in Alaska. Using ITS2 DNA metabarcoding, we assessed fungal community composition and alpha diversity and related these metrics to basal area increment (BAI) over 5–30 years of individual trees. Sampling site was the strongest predictor of RAF community composition, explaining 19.6% of variation, followed by soil pH (11.7%), whereas habitat explained only 2.1%. Treeline effects were context-dependent: RAF composition differed between forest and treeline only in the Alaska Range, alpha diversity was lower at the treeline in Interior Alaska, and ECM relative abundance was higher at the Brooks Range treeline. In contrast, RAF community composition did not differ between fast- and slow-growing trees within sites. Interestingly, alpha diversity indices were significantly associated with BAI over the previous 5, 10 and 15 years, with lower diversity associated with faster tree growth. These relationships weakened with increasing BAI averaging period. Our results indicate that, in mature P. glauca , tree growth is associated with lower RAF and ECM diversity but not with distinct fungal community composition, suggesting that growth may be linked to greater dominance by a subset of fungal partners rather than higher fungal diversity.