Ectomycorrhizal fungi associated with mature white spruce (Picea glauca ) across three Alaskan treelines

Root-associated fungal (RAF) communities influence tree nutrition, yet their drivers and link to tree growth remain unclear, particularly near treelines. We characterized RAF communities, with a focus on ectomycorrhizal (ECM) fungi, associated with fine roots of 111 mature white spruce (Picea glauca [Moench] Voss) trees at three Alaskan sites (Alaska Range, Brooks Range, and Interior Alaska) across paired forest and treeline plots representing three treeline ecotones, two elevational and one drought-limited. Using ITS2 metabarcoding, we related composition, alpha diversity and guild structure to individual tree growth (raw, detrended and regionally standardized basal area increment; n = 106) averaged over 5-30 years. Our results showed that site was the strongest predictor of composition (19.3% of explained variation), followed by soil pH (11.5%), tree age (2.4%) and habitat type (2.1%). Treeline effects were small and context-dependent: 13, 17 and 2 taxa differed between habitat types in the Alaska Range, Brooks Range and Interior Alaska, respectively, independent of tree age; alpha diversity was lower at the treeline only at the drought-limited site; and ECM relative abundance was higher at all three treelines (significantly so in the Brooks Range), while plant saprotrophs declined at all three. Six of seven responding ECM taxa were enriched at the treeline. RAF composition and diversity metrics were not associated with growth over any period. Thus, RAF communities of mature P. glauca were structured mainly by site and soil chemistry, with the most consistent treeline signature being a shift towards ECM fungi. ECM richness alone did not predict tree growth.