Does Drought Influence the Relationship Between Biodiversity and Ecosystem Functioning in Boreal Forests?

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 (delta C-13) 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 (BAI(S)). No biodiversity effect was found for stand delta C-13 (delta C-13(S)) during the wet year. However, there was a significant increase in delta C-13(S) between the wet and the dry year and a significant effect of biodiversity on delta C-13(S) in the dry year. The increase in delta C-13(S) in mixed stands was associated with both selection and complementarity effects. Although BAI(S) decreased significantly in the dry year, changes in delta C-13(S) did not translate into variations in BAI(S) 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.