Mediterranean trees coping with severe drought: Avoidance might not be safe

Plants coexisting in Mediterranean regions differ widely in their sensitivity to drought and in their ability to maintain carbon assimilation and hydraulic function ranging from avoidance to tolerance of stressful periods. We studied the response of three coexisting Mediterranean tree species (Quercus ilex L. spp. Ballota (Desf.) Samp., Quercus faginea Lam. and Pinus nigra ssp. Salzmannii J.F. Arnold) under natural conditions for three years to test their functional strategies in dealing with extremely dry years. Despite P. nigra following a drought-avoiding strategy, it was unable to effectively avoid the hampering effects of a severe drought in the driest year, and experienced significant drops in the efficiency of the hydraulic system, in gas exchange at the leaf level and in tree water use estimated from daily sap flow. In contrast, the two Quercus species showed a more drought-tolerant strategy by maintaining a more profligate water use at the leaf and tree level, with Q. ilex having a larger tolerance threshold to severe drought. The main decrease in carbon assimilation was due to stomatal closure, with a minor effect of photochemistry damage or down regulation in the three species. In the case of the two oaks, seasonal osmoregulation was inferred from changes in the osmotic potential at full turgor (πo), bringing about the lowest leaf osmotic potentials at turgor lost (πtlp) at late summer in both species, and maintenance of leaf turgor under the lowest midday leaf water potential (Ψmid). Seasonal changes in osmotic potentials were observed in the three years, more during the driest year. Plasticity in modulating the tolerance to leaf dehydration as the season advances, especially in response to the intensity of inter-annual summer droughts was higher for the evergreen Q. ilex than in the semi-deciduous Q. faginea. In spite of the intensity of the drought and the associated stress experienced by the three forest tree species, especially by P. nigra, mortality or severe defoliation were not observed after the extreme dry year. This reveals a large operational range of coping with water stress by the three species regardless of the hydric strategy, which is crucial for survival under the increasing drought expected in the most likely climate change scenarios for the region.