Data from: Lichens: a limit to peat growth?

1. The fruticose lichens Cladina stellaris and Cladina rangiferina, form thick mats that can cover large areas of northern peatlands (above ~ 50° latitude), including the extensive peatlands of the Hudson Bay Lowland (HBL) in Canada, where lichens may cover up to 50 % of the landscape. Despite the abundance of lichens in northern peatlands, our understanding of their role within peatland ecosystems, and peat accumulation in particular, is limited. 2. We investigate the potential effect of these mat-forming lichens on peat production and decomposition processes, using field data from an ombrogenous bog in the HBL and laboratory analyses. We hypothesise that (a) production in lichen-shrub hummocks is less than in Sphagnum-shrub hummocks; (b) the decay of lichen litter is faster than that of Sphagnum moss so the mass litter input to the peat profile is reduced; and (c) faster decomposition of the underlying peat is stimulated by lichen leachates, resulting in greater mass loss. 3. We found that thick lichen mats alter vegetation composition in peatlands, reducing Sphagnum cover and inhibiting the growth of small shrubs. Coupled with low lichen productivity that is constrained by moisture conditions, production for lichen-shrub hummocks is significantly smaller than for Sphagnum-shrub hummocks, confirming hypothesis (a). Our data also support hypothesis (b), with chemical analyses of lichen mats and leachates from lichen mats indicating faster decay of lichens compared to Sphagnum moss, and therefore reduced mass litter input to the peat profile in lichen-dominated hummocks. Although we found no evidence to suggest leachates from lichens enhance decomposition processes in peatlands (hypothesis c), larger dry bulk densities for peat under lichen mats indicate a loss of structural integrity and potential collapse of the peat column. 4. Synthesis: As production of new material added to the peat column is less in lichen-dominated hummocks, local peat accumulation slows or ceases, representing a potential temporary limit to peat growth. Our results highlight the importance of lichens as a vegetation feedback in peatland development, with thick mats probably constraining or reducing hummock height relative to adjacent, lichen-free hollows.1. 枝状地衣（fruticose lichens）星蕊鹿蕊（Cladina stellaris）和鹿蕊（Cladina rangiferina）会形成厚实的地衣垫，可覆盖北纬50°以上的大片北方泥炭地，包括加拿大境内广袤的哈得逊湾低地（Hudson Bay Lowland, HBL）泥炭地，此处地衣可覆盖至多50%的地表景观。尽管北方泥炭地中地衣分布广泛，但目前学界对其在泥炭地生态系统中的作用，尤其是对泥炭积累过程的认知仍十分有限。2. 本研究依托哈得逊湾低地一处雨养沼泽（ombrogenous bog）的野外调查数据与实验室分析结果，探究这类垫状地衣对泥炭生产与分解过程的潜在影响。我们提出如下假说：（a）地衣-灌丛丘的物质生产低于泥炭藓（Sphagnum）-灌丛丘；（b）地衣枯落物的分解速率快于泥炭藓，因此输入泥炭剖面的枯落物总质量会减少；（c）地衣浸出液会刺激下层泥炭的更快分解，进而导致更大的质量损失。3. 研究发现，厚实的地衣垫会改变泥炭地的植被组成，降低泥炭藓的覆盖度并抑制小型灌丛的生长。加之地衣生产力受水分条件限制而偏低，地衣-灌丛丘的物质生产显著低于泥炭藓-灌丛丘，验证了假说（a）。我们的实验数据同样支持假说（b）：对地衣垫及其浸出液的化学分析显示，地衣的分解速率快于泥炭藓，因此在地衣主导的丘体中，输入泥炭剖面的枯落物总质量有所降低。尽管未发现地衣浸出液可促进泥炭地分解过程的证据（假说c），但地衣垫下方的泥炭干容重更高，这表明泥炭柱体的结构完整性遭到破坏，存在泥炭柱坍塌的潜在风险。4. 综合分析：由于地衣主导的丘体中，添加至泥炭柱体的新生物质产量更低，当地的泥炭积累过程会放缓甚至停止，这代表泥炭生长可能存在暂时性的限制。本研究结果凸显了地衣作为植被反馈因子在泥炭地发育过程中的重要性——厚实的地衣垫可能会相较于相邻的无地衣洼地，限制甚至降低丘体的高度。