Stand structure modulates the long‐term vulnerability ofPinus halepensisto climatic drought in a semiarid Mediterranean ecosystem

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Abstract
We investigated whether stand structure modulates the long‐term physiological performance and growth ofPinus halepensisMill. in a semiarid Mediterranean ecosystem. Tree radial growth and carbon and oxygen stable isotope composition of latewood (δ13CLWandδ18OLW, respectively) from 1967 to 2007 were measured inP. halepensistrees from two sharply contrasting stand types: open woodlands with widely scattered trees versus dense afforested stands.In both stand types, tree radial growth,δ13CLWandδ18OLWwere strongly correlated with annual rainfall, thus indicating that tree performance in this semiarid environment is largely determined by inter‐annual changes in water availability.However, trees in dense afforested stands showed consistently higherδ18OLWand similarδ13CLWvalues compared with those in neighbouring open woodlands, indicating lower stomatal conductance and photosynthesis rates in the former, but little difference in water use efficiency between stand types. Trees in dense afforested stands were more water stressed and showed lower radial growth, overall suggesting greater vulnerability to drought and climate aridification compared with trees in open woodlands.In this semiarid ecosystem, the negative impacts of intense inter‐tree competition for water onP. halepensisperformance clearly outweigh potential benefits derived from enhanced infiltration and reduced run‐off losses in dense afforested stands.

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