Effect of acorn fall phenology on seedling establishment success in two mediterranean oak species. Implications facing climate change

Abstract

This study focuses on the effect of differences in acorn dropping time on seedling establishment in two abundant Mediterranean oak species: Quercus ilex subsp. ballota and Q. suber. These species show extended seed dropping seasons (i.e., 4 to 5 months) and the fallen acorns thus experience variable conditions, including differences in climate and microclimate, as well as differences in biological interactions (mainly pre- and post-dispersal predation by insect larvae and rodents, respectively). We conducted two field experiments and a field survey on acorn infection by insect larvae and analysed the ability of early and late dropped acorns to achieve success (i.e., remaining healthy, germinating, and emerging as seedlings). The results indicated that pre- and post-dispersal predation of propagules changed over time. However, these factors had low effect on final seedling success. In opposition, warmer (i.e., autumn) conditions at acorn dropping and sowing time decreased seedling establishment in Q. suber, likely by a lack of cold stratification, while the conditions had no effect on Q. ilex subsp. ballota seedlings. We conclude that climate warming can selectively decrease the number of seedlings that are established in late springtime, before the onset of the characteristic Mediterranean summertime drought, thus negatively affecting the population dynamic in these species, which exhibit a high level of physiological dormancy as they depend on cold conditions to maximise acorn germination and epicotyl emergence.