Mature trees versus seedlings: Differences in leaf traits and gas exchange patterns in three co-occurring Mediterranean oaks

We studied gas-exchange patterns and leaf traits of seedlings and mature trees of three coexisting Mediterranean oaks with contrasting leaf habits (the evergreen Quercus ilex, and the deciduous Q. faginea and Q. pyrenaica) during the well-watered part of the growth season. Leaf life span in Q. ilex seedlings was shorter than in mature trees, whereas for the deciduous species the differences in leaf life span between both growth stages were less pronounced. In all species leaves on seedlings displayed a lower mass per unit area and lower nitrogen content than on mature trees. However, owing to their larger stomatal conductance, leaves on seedlings usually showed larger photosynthetic nitrogen-use efficiency and lower water-use efficiency than on mature trees. Stomatal conductance and CO2 assimilation rate were lower in Q. ilex than in the two deciduous species at the mature stage. However, the interspecific differences in gas exchange rates related to differences in leaf longevity disappeared at the seedling stage because in Q. ilex the seedlings showed a much higher stomatal conductance than the adults. Thus, seedlings of the three species showed a common strategy, regardless of the leaf life span, probably as a response to the competition from the herbaceous layer.