Intraspecific variation of Quercus ilex L. seed morphophysiological traits in Tunisia reveals a trade-off between seed germination and shoot emergence rates along a thermal gradient

Abstract

Key message: Quercus ilex populations from cold habitats display a large lag between seed germination and shoot emergence time, favouring avoidance of late frost events. Populations from mild habitats show the fastest seed germination and shoot emergence rates at moderate temperatures, enabling them to synchronize germination in the late winter-early spring period and a rapid seed-to-seedling transition, during the favourable rainy period. Context: Quercus ilex is the most abundant and representative Mediterranean oak species. Identifying and describing intraspecific variation in seed traits is necessary to characterize the germination niche, and to elucidate drivers of species’ range. Aims: In order to identify adaptations to local environments that may reflect ecological strategies for stress avoidance and seed survival, we tested under common and optimal conditions whether seed functional traits vary, in Quercus ilex subsp. rotundifolia Lam., along climatic gradients within its distribution range in Tunisia. Methods: We have explored variations in seed morphological traits, desiccation sensitivity level, germination and shoot emergence rates under different controlled temperature conditions, among 15 populations of Q. ilex sampled throughout the Tunisian distribution of the species. Results: Significant between-populations differences were observed for morphological seed traits but no relationships could be established with the climate of the sampling sites. In contrast, key physiological traits varied significantly with elevation and temperature. Specifically, mild temperatures in lowland regions were associated with higher seed moisture content, fast germination and shoot emergence rates at moderate temperatures (13 °C) for germination. Seeds of Q. ilex populations from cold sites displayed the fastest germination rates at low temperatures (5 °C) as well as the greatest lag between seed germination and shoot emergence time. Conclusion: Intraspecific variation in seed physiological traits is significantly associated with local climate. This functional diversity should be considered when evaluating germplasm and predicting suitability for reforestation and assisted migration programs.