Competitive strategies for water availability in two Mediterranean Quercusspecies

Competition for water availability was studied in a mixed natural stand of Quercus suber L. and Quercus cerris L. growing in Sicily by measuring diurn al 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 we ll as the seasonal changes in root hydraulic conductance per unit leaf surf ace 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 o n 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 contr asted in the two species, thus suggesting that Q. suber and Q. cerris did n ot really compete for available water. In the summer, when Q. suber was ext racting water from the soil to maintain high leaf hydration, Q. cerris had restricted water absorption, thus suffering drought but tolerating its effe cts. The possibility that cohabitation of drought-tolerant with drought-avo iding species can be generalized is also discussed.