Stomatal responses to drought of mature trees and seedlings of two co-occurring Mediterranean oaks

We studied stomatal responses to decreasing predawn water potential ((psi)pd) and increasing leaf-to-air water vapour pressure deficit (VPD) of seedlings and mature trees of two co-occurring Mediterranean oaks with contrasting leaf habits: the evergreen Quercus rotundifolia and the deciduous Quercus faginea. Our objective was to define and to compare the stomatal strategies of both species for drought resistance and to identify the possible differences between growth stages in selected drought adaptation attributes. Among the mature trees, Q. rotundifolia exhibited a water-use behaviour that was more conservative than that of Q. faginea: lower maximum stomatal conductances and greater sensitivity to VPD than the deciduous species. As a result, the leaf water potential of the evergreen species never decreased along the day and along the growth season as much as in the deciduous species; this may help to guarantee longer leaf longevity by avoiding irreversible damage during the summer drought. The seedlings of the two species showed a less conservative water-use strategy in comparison with adult trees: a relatively high stomatal conductance and lower stomatal sensitivity to soil and atmospheric drought. As a consequence, leaf water potential decreased more in the seedlings along the day than in the adults. Q. rotundifolia was the species for which the most pronounced differences between growth stages were obtained. Thus, interspecific differences in response to drought disappeared in the first stages of the life of the trees, and the seedlings of the two species showed a common strategy, probably as a response to the competition from the herbaceous layer. A low stomatal sensitivity in benefit of an increase in growth would probably be a more successful strategy under the competitive conditions that seedlings experience during their establishment.