Background and Aims Oaks are the foundation and dominant tree species of most Mediterranean forests. As climate models predict dramatic changes in the Mediterranean basin, a better understanding of the ecophysiology of seed persistence and germination in oaks could help define their regeneration niches. Tunisian oaks occupy distinct geographical areas, which differ in their rainfall and temperature regimes, and are thus a valuable model to investigate relationships between seed traits and species ecological requirements. Methods Seed morphological traits, desiccation sensitivity level, lethal freezing temperature, embryonic axis and cotyledon sugar and lipid composition, and seed and acorn germination rates at various constant temperatures were measured in Quercus canariensis, Q. coccifera, Q. ilex and Q. suber, using seeds sampled in 22 Tunisian woodlands. Key Results Only faint differences were observed for desiccation sensitivity in the oak species studied. By contrast, the species differed significantly in sensitivity to freezing, germination rates at low temperature and base temperature. Quercus ilex and Q. canariensis, which occur at high elevations where frost events are frequent, showed the lowest freezing sensitivity. A significant correlation was found between hexose contents in the embryonic axis and freezing tolerance. Significant interspecific differences in the time for seeds to germinate and the time for the radicle to pierce the pericarp were observed. The ratio of pericarp mass to acorn mass differed significantly among the species and was negatively correlated with the acorn germination rate. Quercus coccifera, which is frequent in warm and arid environments, showed the highest acorn germination rate and synchrony. Conclusions Seed lethal temperature, seed germination time at low temperatures, the ratio of pericarp mass to acorn mass and the embryonic axis hexose content appeared to be key functional traits that may influence the geographical ranges and ecological requirements of Mediterranean oaks in Tunisia.

Climate change models predict an increase in aridity, especially in the regions under Mediterranean-type climates such as the Mediterranean Basin. However, there is a lack of ecophysiological studies supporting the selection of the more drought-adapted ecotypes for reforestation programs. In this study, we analyzed the anatomical and functional adaptations of 18-month-old seedlings to drought on 16 Quercus suber L. populations from the southeastern rear edge of the species distribution in northern Tunisia growing in a common garden, in order to identify the most appropriate material to use in reforestations. The results evidenced that populations from more xeric habitats displayed the highest leaf dry mass per area (LMA) and lowest leaf area (LA) values, together with the largest increase in the bulk modulus of elasticity (Δε) in response to drought (i.e., elastic adjustment). On the other hand, some populations with intermediate values of aridity, LMA and LA displayed the sharpest increase in proline concentration (ΔPro), with a concomitant increase in osmotic potential at full turgor (Δπo) (i.e., osmotic adjustment). Therefore, two different strategies seem to drive the within-species variation of the studied Q. suber populations in response to water scarcity: (i) a water saver strategy for improving water stress tolerance through the maximization of the elastic adjustment; and (ii) a water spender strategy for maintaining water absorption and photosynthetic activity under moderate water stress through the maximization of the osmotic adjustment. We concluded that the higher elastic adjustment, together with reduced LA and increased LMA, implied a better performance under drought stress in the populations of Q. suber from more xeric habitats, which can be considered the most drought-adapted ecotypes and, consequently, the most appropriate for reforestation programs under an eventual increase in aridity.

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.