Germination timing under climate change: warmer springs favor early germination of range-wide cork oak populations

Climate change is favoring the northward shift of Mediterranean species which are expanding their ranges at their leading edges, becoming natural candidates for increasing forest biodiversity in these regions. However, current knowledge on tree populations responses to climate change is mostly based on adult trees, even if tree early developmental stages are far more sensitive to climate and tightly linked to fitness. To fill this knowledge gap, we investigated the potential adaptation of cork oak range-wide populations to increasing spring temperature in germination and post-germination traits. We sowed 701 acorns from 11 populations at 15, 20 and 25{degrees}C, monitored germination daily and measured post-germination traits. We model germination timing through Coxs proportional-hazards models, assess populations adaptation to spring temperature transfer distances and quantify the effect of acorn mass and storage duration on all considered traits with fixed-effects models. We predict germination and post-germination climate niches under current and RCP 8.5 2080 scenarios. Large differences in germination timing are due to both the population origin and temperature treatment; germination and survival rates showed a sub-optimality towards warmer-than-origin temperatures and heavier acorns produced faster growing seedlings. The timing of germination is the early stage trait most affected by increasing spring temperatures, with germination in 2080 predicted to be 12 days earlier than to date in the northern part of the species range. Warmer spring temperatures will significantly accelerate the germination of other recalcitrant Mediterranean species, which could alter seedlings developmental environment and ultimately populations regeneration and species composition. As such, germination timing should receive more attention by scientists and stakeholders, and should be included in forest vulnerability assessments and assisted migration programs aiming at long-term forest regeneration to adapt forests to climate change.