Contrasting functional strategies following severe drought in two Mediterranean oaks with different leaf habit: Quercus faginea and Quercus ilex subsp. rotundifolia

Tree Physiol. 2020 Oct 20;tpaa135. doi: 10.1093/treephys/tpaa135. Online ahead of print.

Abstract

Nowadays, evergreen sclerophyllous and winter-deciduous malacophyllous oaks with different paleogeographical origins coexist under Mediterranean-type climates, such as the mixed forests of the evergreen Quercus ilex subsp. rotundifolia and the winter-deciduous Quercus faginea. Both Mediterranean oaks constitute two examples of contrasting leaf habit, so it would be expected different functional strategies to cope with summer drought. In this study, we analysed photosynthetic, photochemical and hydraulic traits of different organs for Q. faginea and Q. ilex subsp. rotundifolia under well-watered conditions and subjected to very severe drought. The coordinated response between photosynthetic and hydraulic traits explained the higher photosynthetic capacity of Q. faginea under well-watered conditions, which compensated its shorter leaf life span at the expense of a higher water consumption. The progressive imposition of water stress evidenced that both types of Mediterranean oaks displayed different functional strategies to cope with water limitations. Specifically, the decrease in mesophyll conductance (gm) associated to edaphic drought seems to be the main factor explaining the differences found in the dynamics of net CO2 assimilation (AN) throughout the drought period. The sharp decline in photosynthetic traits of Q. faginea was coupled with a strong decrease in shoot hydraulic conductance (Kshoot) in response to drought. This fact probably avoided extensive xylem embolism in the stems (i.e., "vulnerability segmentation"), which enabled new leaf development after drought period in Q. faginea. By contrast, leaves of Q. ilex subsp. rotundifolia showed effective photoprotective mechanisms and high resistance to drought-induced cavitation, which would be related with the longer leaf life span of the evergreen Mediterranean oaks. The co-occurrence of both types of Mediterranean oaks could be related to edaphic conditions that ensure the maintenance of soil water potential above critical values for Q. faginea, which can be severely affected by soil degradation and climate change.

Keywords: hydraulic traits; mediterranean-type climates; mesophyll conductance; photoprotection; photosynthesis; vulnerability segmentation; water stress.