Does Drought Influence the Relationship Between Biodiversity and Ecosystem Functioning in Boreal Forests?
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In mixed forests, interactions among species influence ecosystem functioning but environmental conditions also play an important role in shaping relationships between biodiversity and ecosystem functioning. In the context of climate change, the carbon and water balance in pure versus mixed forest stands may be differentially influenced by changing soil water availability. To test this hypothesis, we compared the influence of biodiversity on stand water use efficiency (WUES) in boreal forests between wet and dry years. We assessed the carbon isotope composition (δ 13C) of tree rings in Betula pendula, Pinus sylvestris, and Picea abies growing in pure versus mixed stands. In addition, we tested whether differences in WUES affected patterns of stand basal area increment (BAIS). No biodiversity effect was found for stand δ 13C (δ 13CS) during the wet year. However, there was a significant increase in δ 13CS between the wet and the dry year and a significant effect of biodiversity on δ 13CS in the dry year. The increase in δ 13CS in mixed stands was associated with both selection and complementarity effects. Although BAIS decreased significantly in the dry year, changes in δ 13CS did not translate into variations in BAIS along the biodiversity gradient. Our results confirmed that the physiological response of boreal forest ecosystems to changing soil water conditions is influenced by species interactions and that during dry growing seasons, species interactions in mixed stands can lead to lower soil moisture availability. This illustrates that biodiversity effects can also be negative in mixed stands in the sense that soil resources can be more intensively exhausted. Overall, our results confirm that in boreal forests, the biodiversity–ecosystem functioning relationship depends on local environmental conditions.
Keywordsbiodiversity boreal forest drought δ13C mixed forest water use efficiency
We thank Leena Finér and Timo Domisch for the selection of the stands and for their help when preparing this study, and Timo Vesala for sharing the climatic data. We also thank Olivier Bouriaud, Daniel Avacaritei, Iulian Danila and Gabriel Duduman for collecting and preparing the productivity wood cores. We would like to acknowledge the two anonymous reviewers who greatly contributed to the improvement of a previous version of this manuscript. The research leading to these results received funding from the European Union Seventh Framework Programme (FP7/2007–2013) under Grant Agreement No. 265171. CG was supported by a Grant from the INRA Nancy in the framework of the FunDivEUROPE project. This work was conducted in the framework of the Laboratory of Excellence ARBRE (ANR-12-LABXARBRE-01) supported the French National Research Agency.
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