Moss Mediates the Influence of Shrub Species on Soil Properties and Processes in Alpine Tundra

In tundra ecosystems, bryophytes influence soil processes directly and indirectly through interactions with overstory shrub species. We experimentally manipulated moss cover and measured seasonal soil properties and processes under two species of deciduous shrubs with contrasting canopy structures, Salix planifolia pulchra and Betula glandulosa-nana complex. Soil properties (seasonal temperature, moisture and C:N ratios) and processes (seasonal litter decomposition and soil respiration) were measured over twelve months. Shrub species identity had the largest influence on summer soil temperatures and soil respiration rates, which were higher under Salix canopies. Mosses were associated with lower soil moisture irrespective of shrub identity, but modulated the effects of shrubs on winter soil temperatures and soil C:N ratios so that moss cover reduced differences in soil winter temperatures between shrub species and reduced C:N ratios under Betula but not under Salix canopies. Our results suggest a central role of mosses in mediating soil properties and processes, with their influence depending on shrub species identity. Such species-dependent effects need to be accounted for when forecasting vegetation dynamics under ongoing environmental changes.

在苔原生态系统中，苔藓植物（Bryophytes）通过与上层灌木物种的相互作用，直接或间接调控土壤过程。本研究通过实验调控苔藓盖度，在两种冠层结构迥异的落叶灌木——平叶柳美丽变种（Salix planifolia pulchra）与腺毛桦-矮桦复合群（Betula glandulosa-nana complex）——冠层下，测定了季节尺度的土壤属性与过程。研究测定的土壤属性包括季节动态的温度、湿度与碳氮比（C:N ratios），土壤过程涵盖季节尺度的枯落物分解与土壤呼吸，实验周期共计12个月。灌木物种类型对夏季土壤温度与土壤呼吸速率的影响最为显著，平叶柳冠层下的上述指标均更高。无论灌木物种类型如何，苔藓的存在均与更低的土壤湿度相关；但苔藓会调控灌木对冬季土壤温度与土壤碳氮比的影响：苔藓盖度会缩小不同灌木物种间的冬季土壤温度差异，且会降低腺毛桦-矮桦复合群冠层下的土壤碳氮比，但对平叶柳冠层下的碳氮比无显著影响。本研究结果表明，苔藓在介导土壤属性与过程中发挥核心作用，其影响效应取决于灌木物种类型。在预测当前环境变化下的植被动态时，需考虑这种依赖于物种的调控效应。