Elevational Differences in Plant Growth Traits, Litter Decomposition, and Soil Bacterial Communities During Deyeuxia angustifolia Encroachment in Alpine Tundra

Graminoid encroachment in alpine tundra is often reduced to a simple rise in dominant-species cover, leaving open whether this aboveground shift is coupled to belowground soil conditioning. Here, we tracked the encroachment front of Deyeuxia angustifolia (Kom.) Y.L.Chang in the Changbaishan alpine tundra, integrating plant community surveys, growth trait monitoring, litter decomposition, phenology-matched soil physicochemical properties, enzyme activities, and bacterial 16S rRNA sequencing across slightly and largely encroached plots at 2000 and 2200 m. Abundance-weighted plant community composition differed significantly across habitats (p = 0.0003), with D. angustifolia relative cover surging from 21.8–29.9% in S to 89.9–93.0% in L, substantially displacing Rhododendron aureum and reducing community Shannon diversity and Pielou evenness. Repeated-measures mixed models revealed that D. angustifolia exhibited elevation-dependent growth trait variation (elevation × stage × date interactions for height, leaf width, stem diameter, leaf count, and tillers; all p < 0.05): plants at 2000 m produced more tillers with a higher seasonal maximum tiller count (active and seasonal maximum tillers), whereas plants at 2200 m prioritized culm reinforcement and foliar expansion (increased leaf width, length, and stem diameter) alongside reduced leaf and branch numbers. Litter decomposition linked these patterns to belowground change: rates were similar between stages at 2000 m, but at 2200 m largely encroached plots decomposed more slowly (lower k, p < 0.001; higher September mass remaining, p = 0.002) while inorganic nitrogen and enzyme activities rose, indicating active local nutrient transformation despite slower turnover. Bacterial community composition varied mainly with elevation (p = 0.001), with further contributions from encroachment stage and month, reflecting compositional restructuring rather than simple diversity change. Together, these results depict D. angustifolia encroachment as a continuous process, from establishment through local space occupancy to litter decomposition dynamics and associated soil habitat differentiation, that reorganizes both aboveground structure and belowground habitats and offers ecological evidence for forecasting tundra meadowization under continued warming.