Differential drought resistance strategies of co-existing woodland species enduring the long rainless Eastern Mediterranean summer

摘要

In anticipation of a drier climate and to project future changes in forest dynamics, it is imperative to understand species-specific differences in drought resistance. The objectives of this study were to form a comprehensive understanding of the drought resistance strategies adopted by Eastern Mediterranean woodland species, and to elaborate specific ecophysiological traits that can explain the observed variation in survival among these species. We examined leaf water potential (Psi), gas exchange and stem hydraulics during 2-3 years in mature individuals of the key woody species Phillyrea latifolia L., Pistacia lentiscus L. and Quercus calliprinos Webb that co-exist in a dry woodland experiencing similar to 6 rainless summer months. As compared with the other two similarly functioning species, Phillyrea displayed considerably lower Psi (minimum Psi of -8.7 MPa in Phillyrea vs -4.2 MPa in Pistacia and Quercus), lower Psi at stomatal closure and lower leaf turgor loss point (Psi(TLP)), but reduced hydraulic vulnerability and wider safety margins. Notably, Phillyrea allowed Psi to drop below Psi(TLP) under severe drought, whereas the other two species maintained positive turgor. These results indicate that Phillyrea adopted a more anisohydric drought resistance strategy, while Pistacia and Quercus exhibited a more isohydric strategy and probably relied on deeper water reserves. Unlike the two relatively isohydric species, Phillyrea reached complete stomatal closure at the end of the dry summer. Despite assessing a large number of physiological traits, none of them could be directly related to tree mortality. Higher mortality was observed for Quercus than for the other two species, which may result from higher water consumption due to its 2.5-10 times larger crown volume. The observed patterns suggest that similar levels of drought resistance in terms of survival can be achieved via different drought resistance strategies. Conversely, similar resistance strategies in terms of isohydricity can lead to different levels of vulnerability to extreme drought.