In the Mediterranean evergreen oakwoodlands ofsouthern Portugal, the main tree species are Quercus ilex ssp.rotundifolia Lam. (holm oak) and Quercus suber L. (cork oak).We studied a savannah-type woodland where these species coexist,with the aim of better understanding the mechanisms oftree adaptation to seasonal drought. In both species, seasonalvariations in transpiration and predawn leaf water potentialshowed a maximum in spring followed by a decline through therainless summer and a recovery with autumn rainfall. Althoughthe observed decrease in predawn leaf water potential in summerindicates soil water depletion, trees maintained transpirationrates above 0.7 mm day–1 during the summer drought. Bythat time, more than 70% of the transpired water was beingtaken from groundwater sources. The daily fluctuations in soilwater content suggest that some root uptake of groundwaterwas mediated through the upper soil layers by hydraulic lift.During the dry season, Q. ilex maintained higher predawn leafwater potentials, canopy conductances and transpiration ratesthan Q. suber. The higherwater status of Q. ilexwas likely associatedwith their deeper root systems compared with Q. suber.Whole-tree hydraulic conductance and minimum midday leafwater potential were lower in Q. ilex, indicating that Q. ilex wasmore tolerant to drought than Q. suber. Overall, Q. ilex seemedto have more effective drought avoidance and drought tolerancemechanisms than Q. suber.
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