Stand structure modulates the long-term vulnerability of Pinus halepensis to climatic drought in a semiarid Mediterranean ecosystem

We investigated whether stand structure modulates the long-term physiological performance and growth of Pinus halepensis Mill. in a semiarid Mediterranean ecosystem. Tree radial growth and carbon and oxygen stable isotope composition of latewood (δ 13C LW and δ 18O LW, respectively) from 1967 to 2007 were measured in P. halepensis trees from two sharply contrasting stand types: open woodlands with widely scattered trees versus dense afforested stands. In both stand types, tree radial growth, δ 13C LW and δ 18O LW were 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 higher δ 18O LW and similar δ 13C LW values 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 on P. halepensis performance clearly outweigh potential benefits derived from enhanced infiltration and reduced run-off losses in dense afforested stands. We investigated whether stand structure modulates the long-term physiological performance and growth of Pinus halepensis Mill. in a semiarid Mediterranean ecosystem. The combined interpretation of latewood δ13C and δ18O and tree ring width series indicated that pines in dense afforested stands were consistently more water stressed and had lower stomatal conductance, photosynthetic rate and radial growth than pines in open woodlands for the last 40 years. In this semiarid ecosystem, the benefits of negligible inter-tree competition for water in open woodlands clearly outweigh potential benefits derived from enhanced infiltration and reduced runoff losses in dense afforested stands, thus rendering the latter more vulnerable to projected increases in drought frequency and duration across the Mediterranean region. © 2011 Blackwell Publishing Ltd.