© 2007 Heron Publishing—Victoria, Canada
Water-use strategies in two co-occurring Mediterranean evergreen oaks: surviving the summer drought
T. S. David (1,2), M. O. Henriques (3), C. Kurz-Besson (3), J. Nunes (4), F. Valente (3), M. Vaz (4), J. S. Pereira (3), R. Siegwolf (5), M. M. Chaves (3), L. C. Gazarini (4) and J. S. David (3)
1. Estação Florestal Nacional (INIAP), Quinta do Marquês, 2780-159 Oeiras, Portugal / 2. Corresponding author (teresa.david@efn.com.pt) / 3. Instituto Superior de Agronomia, Tapada da Ajuda, 1349-017 Lisboa, Portugal / 4. Universidade de Évora, Dep. Biologia, 7002-554 Évora, Portugal / 5. Paul Scherrer Institute, Laboratory of Atmospheric Chemistry, CH-5232 Villigen-PSI, Switzerland / Received May 29, 2006; accepted July 25, 2006; published online March 1, 2007
Summary
In the Mediterranean evergreen oak woodlands of southern 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 of tree adaptation to seasonal drought. In both species, seasonal variations in transpiration and predawn leaf
water potential showed a maximum in spring followed by a decline through the rainless summer and a recovery with autumn rainfall.
Although the observed decrease in predawn leaf water potential in summer indicates soil water depletion, trees maintained
transpiration rates above 0.7 mm day–1 during the summer drought. By that time, more than 70% of the transpired water was being taken from groundwater sources.
The daily fluctuations in soil water content suggest that some root uptake of groundwater was mediated through the upper soil
layers by hydraulic lift. During the dry season, Q. ilex maintained higher predawn leaf water potentials, canopy conductances and transpiration rates than Q. suber. The higher water status of Q. ilex was likely associated with their deeper root systems compared with Q. suber. Whole-tree hydraulic conductance and minimum midday leaf water potential were lower in Q. ilex, indicating that Q. ilex was more tolerant to drought than Q. suber. Overall, Q. ilex seemed to have more effective drought avoidance and drought tolerance mechanisms than Q. suber.
Keywords:
canopy conductance, groundwater, hydraulic conductance, leaf water potential, Quercus ilex, Quercus suber, sap flow.
