?13C and tree-ring width reflect different drought responses in Quercus ilex and Pinus halepensis

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Abstract
Holm oak (Quercus ilex L.) and Aleppo pine (Pinus halepensis Mill) are representative of two different functional types of trees extensively found in the Mediterranean: evergreen sclerophyllous and drought-adapted conifers. The former is considered a partially drought-tolerant species, whereas the latter is a typically drought-avoiding, water-saving species. We postulated that contrasting strategies in response to water deficits in Q. ilex and P. halepensis would lead to a differential sensitivity to changes in water availability. To test this hypothesis, we compared the response of both species in growth rate (measured as radial increments) and intrinsic water use efficiency [WUEi, as inferred from carbon isotope discrimination (Δ13C) in wood samples] among sites from different provenance regions in NE Spain. We found significant differences in Δ13C and growth among provenance regions, partly explained by contrasting water availability. Wood Δ13C was positively related with precipitation and the ratio between precipitation and potential evapotranspiration (P / E). However, these relationships were stronger in P. halepensis (for P / E, r 2=0.67, P Q. ilex (r 2=0.42, P 13C in P. halepensis (r 2=0.32 and r 2=0.35, respectively, P Q. ilex. We concluded that P. halepensis was more sensitive than Q. ilex to water availability, showing faster increase in WUEi in response to water stress. We also found that the effect of north/south aspect on Δ13C and growth was site-specific, and unrelated to climatic variables.