Bryophyte functional traits regulate root and soil microbial responses to reduced snow cover in boreal forests

Boreal forests store substantial soil carbon yet are experiencing climate change characterized by reduced snowpack and increased soil freeze-thaw cycles. Mosses, particularly Sphagnum spp. and Pleurozium schreberi , dominate boreal forest floors, contributing to soil insulation and moisture retention, but their role in buffering belowground carbon processes under reduced snow cover remains poorly understood. This study examined functional traits of these moss types and their capacity to mitigate snow-cover loss effects on root biomass, soil microbial biomass, and microbial diversity and community composition. Thirty-six experimental plots were established across three mature black spruce stands in Nord-du-Québec, Canada, with treatments varying in moss type, moss presence, and snow cover. Sphagnum spp. colonies under snow were near 36% denser (2.25 ± 0.14 vs. 1.65 ± 0.14 g/cm 3 ) and had approximately 44% greater water absorption capacity (2.24 ± 0.08 vs. 1.56 ± 0.08 ml/cm 3 ) than those without snow. Root biomass was approximately 2-fold lower under Sphagnum spp. with reduced snow, with no corresponding difference observed in P. schreberi across treatments. Soil bacterial diversity was stable across treatments, though community composition shifted; Actinobacteria were more abundant under reduced snow in P. schreberi plots, and Acidobacteriota were more abundant in Sphagnum spp. plots with snow. Soil fungal diversity decreased significantly under reduced snow in P. schreberi plots but remained stable in Sphagnum spp. plots. Microbial biomass was 22% higher in Sphagnum spp. plots with snow by late summer (1827 ± 451 vs. 1495 ± 369 μg C/g soil without snow). These findings suggest that as snow cover reduces, forest floors dominated by Sphagnum spp. may shift from stable carbon sinks to disturbance-sensitive systems, while P. schreberi- dominated floors may offer greater resistance, at the cost of reduced thermal protection and long-term carbon accumulation.