Warmer springs favour early germination of range-wide Quercus suber L. populations

Climate change is favouring the northward shift of Mediterranean oaks which are expanding their ranges at their leading edges. However, Mediterranean oaks have recalcitrant seeds (desiccation sensitive) that do not form seed banks, depending on climate conditions after seed fall and interactions between genetic determinants to germinate. Here we investigate the potential adaptation of Quercus suber range-wide populations to increasing spring temperature in germination timing and rates. We sowed 701 acorns from 9 populations from 10 mother trees each, at 15, 20 and 25 °C and monitored germination daily during 90 days. We modelled germination timing through Cox’s proportional-hazards models, assessed populations’ adaptation to spring temperature transfer distances and quantified the effect of acorn mass and storage duration on germination probability and timing with fixed-effects models. We used the mixed-effects models to predict germination climatic niches under current and RCP 8.5 2080 scenarios. Differences in germination timing were due to both the population origin and temperature treatment; germination rates showed a sub-optimality towards warmer-than-origin temperatures. The timing of germination decreased along with spring temperatures increment, with germination in 2080 predicted to occur 12 days earlier than to date in central Iberia. Warmer spring temperatures significantly accelerate the germination of recalcitrant Mediterranean species, which could alter seedlings’ developmental environment and ultimately populations’ regeneration and species composition. As such, germination timing should receive more attention from 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.