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

Abstract

The early life stages of trees, particularly germination, are crucial to fitness and highly sensitive to climate. The influence of temperature on recalcitrant seed germination has rarely been studied due to their desiccation sensitivity, which hampers storage. However, Mediterranean recalcitrant oaks would be particularly affected by the expected increased temperature in this region. Here we investigated the effect of warming temperatures on germination of 975 acorns from 8 range-wide Quercus suber populations. We sowed the acorns at 15, 20 and 25 {degrees}C in climatic chambers, and monitored germination during 4 months. The germination dynamics in each chamber was explored by a Cox proportional hazards model. We assessed environment (germination experiment temperatures), population (climate of seed origin) and their interaction effects on germination percentage, time, and synchrony using generalized linear mixed-effects models. Genetic clines on germination percentage, time and synchrony were mostly triggered by temperature, with seeds from warmer origins showing higher germination, earlier timing, and greater synchrony than colder ones. Higher sowing temperatures promoted advanced germination, and this effect was higher in seeds originating from regions with stronger seasonality. Earlier and synchronous germination found in seeds from warm origin may reduce the desiccation probability for acorns and seedlings, while late germination and low synchrony found in seeds from cold origin might be an adaptive response to unpredictable frost events that would impair seedling survival. The germination synchrony adaptive response was unexpected and further investigation on recalcitrant seeds germination dynamics in response to increased temperatures is needed to confirm it.