Interspecific competition influences the response of oak transpiration to increasing drought stress in a mixed Mediterranean forest

Increasingly severe droughts are expected to negatively impact forest functioning in the future, especially in the Mediterranean region. Favoring mixed species stands has been advocated as a compromise between wood production and biodiversity conservation, but whether such management practices would allow forest ecosystems to acclimate to future climate conditions remains to be addressed. We tested whether the transpiration of Quercus cerris (Qc) and Quercus petraea (Qp) during droughts differ when they grow in pure or mixed forests. We measured sap flux density (FD) and leaf carbon isotope composition (δ13C), as a proxy for intrinsic water use efficiency (WUEint), in pure and mixed Qc and/or Qp forest plots in Italy during the summer 2012. Qc and Qp showed a drop in FD of 41% and 52% respectively during the summer drought independently of the mixtures, but with different seasonal trends. Qp transpiration response to soil drought did not differ between pure and mixed plots. In contrast, Qc transpiration was reduced by 56% in mixed plots at the maximum of the drought and only by 31% in the pure plot. Furthermore, we observed higher WUEint for Qc in the mixed plot and no change for Qp, supporting the higher water stress intensity in the mixed plot for Qc. Our study illustrated the negative impact of Qp on Qc when these species compete for water resources. We suggest that managing for mixed stands in the Mediterranean region might, in some cases, increase the detrimental effect of drought on species functioning.