Rooting depth, water relations and non-structural carbohydrate dynamics in three woody angiosperms differentially affected by an extreme summer drought

In 2012, an extreme summer drought induced species-specific die-back in woody species in Northeastern Italy. Quercus pubescens and Ostrya carpinifolia were heavily impacted, while Prunus mahaleb was largely unaffected. By comparing seasonal changes in isotopic composition of xylem sap, rainfall and deep soil samples, we show that P. mahaleb has a deeper root system than the other two species. This morphological trait allowed P mahaleb to maintain higher water potential (Ψ), gas exchange rates and non-structural carbohydrates content (NSC) throughout the summer, when compared with the other species. More favourable water and carbon states allowed relatively stable maintenance of stem hydraulic conductivity (k) throughout the growing season. In contrast, in Quercus pubescens and Ostrya carpinifolia, decreasing Ψ and NSC were associated with significant hydraulic failure, with spring-to-summer k loss averaging 60%. Our data support the hypothesis that drought-induced tree decline is a complex phenomenon that cannot be modelled on the basis of single predictors of tree status like hydraulic efficiency, vulnerability and carbohydrate content. Our data highlight the role of rooting depth in seasonal progression of water status, gas exchange and NSC, with possible consequences for energy-demanding mechanisms involved in the maintenance of vascular integrity. We compared seasonal changes in isotopic composition of xylem sap in three woody species differentially damaged by an extreme summer drought (Prunus mahaleb, Quercus pubescens and Ostrya carpinifolia), and compared this with isotopic composition of rainfall and deep cave soil samples. Deep roots allowed P.mahaleb to maintain higher water potential, gas exchange rates and non-structural carbohydrates content throughout the summer, when compared with the other species. More favourable water and carbon states also allowed P.mahaleb to maintain stable stem hydraulic efficiency throughout the growing season, while in Quercus pubescens and Ostrya carpinifolia, spring-to-summer loss of hydraulic conductivity averaged 60%. Our data highlight the role of rooting depth in seasonal progression of water status, gas exchange and carbohydrates content, with possible consequences for energy-demanding mechanisms involved in the maintenance of vascular integrity. © 2015 John Wiley & Sons Ltd.