Competitive strategies for water availability in two Mediterranean Quercus species

Competition for water availability was studied in a mixed natural stand of Quercus suber L. and Quercus cerris L. growing in Sicily by measuring diurnal changes of leaf conductance to water vapour (g(L)), water potential (Psi(L)) and relative water content (RWC) in April, July and October 1997 as well as the seasonal changes in root hydraulic conductance per unit leaf surface area (K-RL). Quercus cerris behaved as a drought-tolerant species, with strong reductions of K-RL, Psi(L), and RWC in the summer. By contrast, Q. suber appeared to withstand summer drought by an avoidance strategy based on reducing g(L), maintaining Psi(L) and RWC high and K-RL at the same level as that measured in the spring. A 'conductance ratio' (CR) was calculated in terms of the ratio of g(L) to K-RL. Seasonal changes of this ratio contrasted in the two species, thus suggesting that Q. suber and Q. cerris did not really compete for available water. In the summer, when Q. suber was extracting water from the soil to maintain high leaf hydration, Q. cerris had restricted water absorption, thus suffering drought but tolerating its effects. The possibility that cohabitation of drought-tolerant with drought-avoiding species can be generalized is also discussed.