Seed origin determines cork oak germination: the warmer the higher, faster and more synchronized

Tree early-life stages, particularly germination, are crucial to fitness and highly sensitive to climate. Yet the effects of temperature on recalcitrant seed germination remain understud- ied, mostly because their desiccation sensitivity prevents effective storage. Mediterranean oaks may be especially vulnerable to the projected temperature increases in this region. To assess this, we investigated the effects of warming temperatures on germination using 965 acorns collected from eight range-wide Quercus suber populations. Acorns were sown at day-night alternating temperatures of 15/10, 20/15 and 25/20 °C in climate chambers, and germination was monitored over four months. We analysed germination dynamics using Cox proportional hazards models. We assessed environment (germination experiment tem- peratures), population (climate of seed origin) and their interaction effects on germination percentage, time, and synchrony using generalized linear mixed-effects models. Genetic clines in germination percentage, time and synchrony were mostly driven by temperature, with seeds from warmer origins showing higher germination, earlier timing, and greater synchrony than colder ones. Higher sowing temperatures promoted earlier germination, and this effect was higher in seeds originating from regions with stronger seasonality. Earlier and more synchronous germination found in seeds from warm origins may reduce the risk of desiccation for acorns and seedlings, while the later and less synchronous germination observed in seeds from cold origins might represent an adaptive response to unpredictable frost events that could impair seedling survival. The potential adaptive pattern on germination synchrony was unexpected, and further investigation is required to confirm this finding.