Precipitation regime controls bryosphere carbon cycling similarly across contrasting ecosystems

In arctic and boreal ecosystems, ground bryophytes play an important role in regulating carbon (C) exchange between vast belowground C stores and the atmosphere. Climate is changing particularly fast in these high-latitude regions, but it is unclear how altered precipitation regimes will affect C dynamics in the bryosphere (i.e., the ground moss layer including senesced moss, litter, and associated biota) and the closely associated upper humus layer, and how these effects will vary across contrasting environmental conditions. Here, we set up a greenhouse experiment in which mesocosms were assembled containing samples of the bryosphere, dominated by the feather moss Hylocomium splendens, and the upper humus layer, that were collected from across a boreal forest chronosequence in northern Sweden which varies strongly in nutrient availability, productivity, and soil biota. We tested the effect of variation in precipitation volume and frequency on CO2 exchange and dissolved organic carbon (DOC) export, and on moss growth. As expected, reduced precipitation volume and frequency lowered net CO2 efflux, DOC export, and moss growth. However, by regulating moisture, the lower bryosphere and humus layers often mediated how precipitation volume and frequency interacted to drive C dynamics. For example, less frequent precipitation reduced moss growth only when precipitation volume was low. When volume was high, high moisture content of the humus layer helped avoid moss desiccation. Variation in precipitation regime affected C cycling consistently in samples collected across the chronosequence, despite large environmental variation along the sequence. This suggests that the bryosphere exerts a strong buffering effect on environmental variation at the forest floor, which leads to similar responses of C cycling to external perturbations across highly contrasting ecosystems. As such, our study indicates that projected increases in droughts and ground evapotranspiration in high-latitude regions resulting from climate change will consistently reduce C losses from moss-dominated ecosystems.

在北极与北方森林生态系统中，地表苔藓植物在调控巨量地下碳库与大气之间的碳（C）交换过程中发挥着关键作用。这些高纬度区域的气候变化速率尤为显著，但目前尚不清楚降水格局改变将如何影响苔藓圈（bryosphere，即包含枯落苔藓、凋落物及相关生物群的地表苔藓层）与紧密关联的上层腐殖质层的碳动态，且这类影响会如何随环境条件的差异而变化。本研究通过温室实验开展相关研究：我们构建了中型生态系（mesocosms），其中包含以塔藓（Hylocomium splendens）为优势种的苔藓圈样本，以及上层腐殖质层样本；所有样本均采自瑞典北部一条北方林演替序列样地，该样地的养分有效性、生产力及土壤生物群落均存在显著差异。我们探究了降水量与降水频率的变化对二氧化碳交换、溶解性有机碳（dissolved organic carbon, DOC）输出以及苔藓生长的影响。正如预期，降水量与降水频率的降低会减少净二氧化碳排放通量、溶解性有机碳输出量与苔藓生长速率。然而，通过调控土壤湿度，下层苔藓圈与腐殖质层通常会介导降水量与降水频率的交互作用对碳动态的调控效应。例如，仅在降水量较低时，更低的降水频率才会抑制苔藓生长；当降水量较高时，腐殖质层的高含水量可避免苔藓发生干旱胁迫。尽管该演替序列样地存在显著的环境异质性，但降水格局变化对所有样本的碳循环过程均产生了一致的影响。这表明苔藓圈对林地表层的环境异质性具有较强的缓冲作用，使得碳循环过程在差异显著的生态系统中对外部扰动的响应趋于一致。据此，本研究表明，气候变化导致的高纬度区域干旱与地表蒸散发加剧，将持续降低以苔藓植物为优势的生态系统的碳流失量。