Adaptation of Mediterranean forest species to climate: Lessons from common garden experiments

Mediterranean ecosystems are biodiversity hotspots located between temperate mesic climates and semi-deserts and deserts. Mediterranean climates are characterized by a drought season but its length and severity can be highly variable across regions. In this review, we explored population genetic variation in functional traits and fitness in Mediterranean oak and pine species. We tested the hypothesis that increased drought tolerance has evolved as an adaptation to xeric and warm areas of the Mediterranean climate while increased competitive abilities have been favoured in more mesic regions, following a productivity-persistence trade-off within species. We first reviewed the literature and performed a meta-analysis of studies on Pinus sp. and Quercus sp. from Mediterranean climates, and then analysed a set of unpublished data from Spanish common garden experiments of species from the Mediterranean Basin, searching for evidence of trait genetic variation among populations, local adaptation and trait-environment associations. The meta-analysis showed that 82.9% (95% CI 75.7-89.4) of traits per study showed significant population genetic differentiation and that 52.5% (35.6-69.2) of traits in studies with two or more Mediterranean environments exhibited population-by-environment interaction. Population differences in traits related to drought tolerance, cold hardiness and fitness were usually correlated with provenance precipitation and temperature gradients, suggesting that adaptation to different climatic regimes is an important process driving intraspecific genetic divergence. Within the Spanish trial network, however, predicted fitness functions based on climatic transfer distance failed to reveal significantly higher fitness of populations at experimental sites with climates closer to their provenances' ones. Neither the literature review nor the new analyses of the Spanish trial network provided evidence of a negative association between growth- and persistence-related traits at the intraspecific level. Synthesis. Population genetic differentiation in functional traits and fitness is common in Mediterranean species, driven, at least in part, by the adaptation to contrasting temperature and precipitation regimes. Because of experimental constraints or biological reality, our study failed to detect a trade-off between resource-use strategies within species, calling for future studies to investigate further whether intraspecific evolution of stress tolerance occurs at the expense of growth potential in Mediterranean environments or not.