Climate adaptation across space and time: lessons from oak populations

Tree populations can respond to climate change through migration, phenotypic plasticity, or genetic evolution. Despite long generation times of forest tree species, recent studies suggest that their evolutionary responses may occur rapidly. Using oaks as a model system, we synthesize evidence from 88 common garden studies and from historical, retrospective and longitudinal approaches to explore how populations have adapted to climatic variability across different biomes, and assess the consistency and pace of evolutionary responses across spatial and temporal climatic gradients. We found that approximately 61% of the studies exhibited significant differences among populations but climatic drivers and adaptive strategies differed among biomes. Temperature-related clines predominated in temperate regions, with populations from warmer origins showing longer growing seasons and higher growth potential. In seasonally dry biomes, aridity favored increased drought tolerance in Mediterranean populations but drought avoidance in tropical populations. Allochronic studies revealed genetic changes over decades to millenia in response to climate changes, with warming associated with increased growth and reduced specific leaf area in temperate oaks. Thus, spatial differentiation and temporal evolution were generally congruent in direction for most traits except for leaf unfolding, while short-term evolutionary rates exceeded long-term estimates by two to three orders of magnitude. In summary, provenance trials can provide useful information on the direction of climate-driven evolution for some traits, but may underestimate its contemporary pace. More studies are needed to evaluate whether standing genetic variation of forest tree species is sufficient to track current climate change.