Mixing oak and pine trees does not improve the functional response to severe drought in central French forests.

Summary Key message: Mixing sessile oak and Scots pine in central France to reduce intraspecific competition for water resources did not improve the ability of these two species to withstand severe drought during the summer. Context: In order to reduce the impact of increasingly extreme droughts on forests, managers must adapt their practices to future climate conditions. Maintaining a greater diversity of tree species in temperate forest ecosystems is one of the recommended options. Aims: We addressed how interactions between sessile oak and Scots pine in mixed forests in central France affect their functional response to drought. Methods: We characterized the carbon isotope composition (δ 13 C) in the tree growth rings formed during wet (2001, 2007) or dry (2003, 2004) summers for each of the two species growing both in pure and in mixed stands in order to compare the effect of stand composition on variations in carbon isotope discrimination (Δ 13 C) among contrasted years. Results: The severe drought in 2003 induced a strong decrease in Δ 13 C for all trees and in all stands as compared to 2001. This decrease was greater in pine than in oak. There was no significant difference between pure and mixed stands in the response of either species to drought. Conclusion: Mixing sessile oak and Scots pine in stands in central France does not improve the ability of either species to withstand severe drought during the summer.