Deciduous and evergreen oaks: unravelling the co-occurrence of different functional strategies in Quercus L. species under Mediterranean type climates

Abstract

Mediterranean-type climates present mild or cold and humid winters, while the summer is affected by high pressures that bring dry and hot air (summer aridity), the latter is considered the genuine characteristic that differentiates it from other types of climates. In this way, the vegetative period is affected by summer drought, which, together with the low temperatures in winter, can limit vegetative growth to essentially two seasons: spring and autumn. Even with these restrictions, Mediterranean ecosystems are highly species-rich and represent one-sixth of the planet's flora. This is the case of the genus Quercus, which, with more than 400 species distributed throughout the northern hemisphere, is the dominant element in a wide variety of habitats, especially, in Mediterranean-type climates. Nowadays, two well-differentiated types of oaks coexist in Mediterranean regions: the sclerophyllous evergreen oak with an origin in the Paleotropical Flora and the deciduous malacophilous of Arctotertiary origin. These two models may be very different at morphological, phenological and physiological levels. In this way, the two types of oaks must constitute two very different operating models in the Mediterranean climate. Thus, the main objective of this thesis is the comparative study of the functioning of the two types of oak species with different foliar habits and degrees of sclerophylly that coexist in the Mediterranean climate. Therefore, this thesis consists of five chapters corresponding to five articles already published or under review that aim to shed light on the abovementioned issue.

Thus, a review of sclerophylly in the genus Quercus was carried out to define the mechanisms behind it and its economic consequences for the leaf. Despite the wide range of variation in the leaf functional traits and their contrasting adaptive strategies, it cannot be explained by itself by any of the ecological factors considered (drought, nutrient scarcity, low temperatures and physical damage). The study suggests that these restrictions may have a synergistic effect, and from a functional point of view, we can conclude that leaf habit greatly modulates the physiological implications of sclerophylly in oaks. Using a phylogenetic approach, anatomical, compositional, and climatic variables were quantified in 85 deciduous and evergreen Quercus species that showed fundamental leaf morphological differences revealing a diverse adaptive response. While LMA in deciduous seems to have diversified in coordination with thickness modulated mainly by aridity, LMA diversification in evergreen seems to depend on the infrageneric group. Thus, in the latter, leaf thickness diversification was modulated by both aridity and cold, while leaf density diversification is only modulated by aridity.

Furthermore, the relative importance of each leaf characteristic on the mechanical properties of 25 Quercus species was studied. It was suggested that Quercus sclerophyllous leaves are tougher and stronger due to the thicker external wall of their epidermis and/or a higher concentration of cellulose. moreover, the species in section Ilex shared common traits although they occupy quite different climates. Similarly, evergreen species living in Mediterranean-type climates shared common leaf traits regardless of their different phylogenetic origin. In addition, changes dependent on leaf age in morphological, anatomical, chemical and photosynthetic traits were evaluated in Quercus ilex subsp. rotundifolia Lam., an evergreen oak with a high leaf life span. Decreases in all photosynthetic traits were observed with increasing leaf age. Changes in leaf anatomy and biochemistry were responsible for age-dependent changes in net CO2 assimilation and CO2 conductance through the mesophyll. These findings revealed a gradual physiological deterioration related to the dismantling of the photosynthetic apparatus in older leaves of Q. ilex subsp. rotundifolia.

Finally, the photosynthetic, photochemical and hydraulic characteristics of different organs of Q. faginea and Q. ilex subsp. rotundifolia under severely dry conditions. The strong decrease in the photosynthetic traits of Q. faginea was accompanied by a strong decrease in the hydraulic conductance of the leaves in response to drought. This fact probably prevented a xylem embolism in the stems (“vulnerability segmentation”). On the contrary, the leaves of Q. ilex subsp. rotundifolia showed effective photoprotective mechanisms and high resistance to cavitation induced by drought, which would be related to the greater longevity of the leaves of Mediterranean oaks.