Limpens Juul, Holmgren Milena, Jacobs Cor M J, Van der Zee Sjoerd E A T M, Karofeld Edgar, Berendse Frank
Nature Conservation and Plant Ecology Group, Wageningen University, Wageningen, The Netherlands.
Resource Ecology Group, Wageningen University, Wageningen, The Netherlands.
PLoS One. 2014 Mar 14;9(3):e91748. doi: 10.1371/journal.pone.0091748. eCollection 2014.
Raised bogs have accumulated more atmospheric carbon than any other terrestrial ecosystem on Earth. Climate-induced expansion of trees and shrubs may turn these ecosystems from net carbon sinks into sources when associated with reduced water tables. Increasing water loss through tree evapotranspiration could potentially deepen water tables, thus stimulating peat decomposition and carbon release. Bridging the gap between modelling and field studies, we conducted a three-year mesocosm experiment subjecting natural bog vegetation to three birch tree densities, and studied the changes in subsurface temperature, water balance components, leaf area index and vegetation composition. We found the deepest water table in mesocosms with low tree density. Mesocosms with high tree density remained wettest (i.e. highest water tables) whereas the control treatment without trees had intermediate water tables. These differences are attributed mostly to differences in evapotranspiration. Although our mesocosm results cannot be directly scaled up to ecosystem level, the systematic effect of tree density suggests that as bogs become colonized by trees, the effect of trees on ecosystem water loss changes with time, with tree transpiration effects of drying becoming increasingly offset by shading effects during the later phases of tree encroachment. These density-dependent effects of trees on water loss have important implications for the structure and functioning of peatbogs.
高位沼泽积累的大气碳比地球上任何其他陆地生态系统都要多。当与地下水位下降相关联时，气候导致的树木和灌木扩张可能会使这些生态系统从净碳汇转变为碳源。树木蒸腾作用导致的水分流失增加可能会使地下水位加深，从而刺激泥炭分解和碳释放。为了弥合模型研究与实地研究之间的差距，我们进行了一项为期三年的中宇宙实验，让天然沼泽植被处于三种桦树密度条件下，并研究了地下温度、水平衡组成部分、叶面积指数和植被组成的变化。我们发现树木密度低的中宇宙中地下水位最深。树木密度高的中宇宙保持最湿润（即地下水位最高），而没有树木的对照处理地下水位处于中间水平。这些差异主要归因于蒸腾作用的不同。虽然我们的中宇宙实验结果不能直接扩大到生态系统层面，但树木密度的系统性影响表明，随着沼泽被树木殖民化，树木对生态系统水分流失的影响会随时间变化，在树木入侵的后期阶段，树木蒸腾导致干燥的影响会越来越被遮荫效应所抵消。树木对水分流失的这些密度依赖性影响对泥炭沼泽的结构和功能具有重要意义。
