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Abstract：
Changes in plant communities can have large effects on ecosystem carbon (C) dynamics and long-term C stocks. However, how these effects are mediated by environmental context or vary among ecosystems is not well understood. To study this, we used a long-term plant removal experiment set up across 30 forested lake islands in northern Sweden that collectively represent a strong gradient of soil fertility and ecosystem productivity. We measured forest floor CO2 exchange and aboveground and belowground C stocks for a 22-yr experiment involving factorial removal of the two dominant functional groups of the boreal forest understory, namely ericaceous dwarf shrubs and feather mosses, on each of the 30 islands. We found that long-term shrub and moss removal increased forest floor net CO2 loss and decreased belowground C stocks consistently across the islands irrespective of their productivity or soil fertility. However, we did see context-dependent responses of respiration to shrub removals because removals only increased respiration on islands of intermediate productivity. Both CO2 exchange and C stocks responded more strongly to shrub removal than to moss removal. Shrub removal reduced gross primary productivity of the forest floor consistently across the island gradient, but it had no effect on respiration, which suggests that loss of belowground C caused by the removals was driven by reduced litter inputs. Across the island gradient, shrub removal consistently depleted C stocks in the soil organic horizon by 0.8 kg C/m2 . Our results show that the effect of plant functional group diversity on C dynamics can be relatively consistent across contrasting ecosystems that vary greatly in productivity and soil fertility. These findings underline the key role of understory vegetation in forest C cycling, and suggest that global change leading to changes in the relative abundance of both shrubs and mosses could impact on the capacity of boreal forests to store C.
摘要：
植物群落的变化可能对生态系统碳（C）动态和长期C存量产生重大影响。然而,这些影响是如何由环境背景介导的，或者是如何在生态系统中变化的，目前还没有很好的理解。为了研究这个，我们在瑞典北部的30个森林湖泊岛屿上进行了长期植物去除实验，这些岛屿共同代表了土壤肥力和生态系统生产力的强大梯度。我们进行了22年的实验，测量了森林地面CO2交换以及地上和地下C存量，该实验涉及对北方森林林下的两个主要功能组的阶乘去除，即白质矮灌木和羽毛苔藓，在30个岛屿上。我们发现，长期清除灌木和苔藓会增加整个岛屿的森林地面净二氧化碳损失，并持续减少地下碳储量，而不论其生产力或土壤肥力如何。然而,我们确实看到了呼吸对灌木清除的上下文依赖性反应，因为清除仅增加了中等生产力岛上的呼吸。CO2交换和C库存对灌木去除的反应都比对苔藓去除的反应更强烈。在整个岛屿梯度上，灌木的去除一直降低了林地的初级生产力，但对呼吸没有影响,这表明，由清除引起的地下C损失是由减少的垃圾投入驱动的。横跨岛梯度，灌木的去除持续使土壤有机层中的碳储量减少0.8kgC/m2。我们的结果表明，植物功能群多样性对碳动态的影响在生产力和土壤肥力差异很大的对比生态系统中可以相对一致。这些发现强调了林下植被在森林C循环中的关键作用，并表明导致灌木和苔藓相对丰度变化的全球变化可能会影响北方森林储存C的能力。
