In the genus Quercus, species of the subgenus Cyclobalanopsis, which occur in tropical and subtropical habitats, show a higher seed sensitivity to low germination temperatures than species of the subgenus Quercus, which have wider habitat ranges in the northern hemisphere with low average overwinter temperatures ( Xia et al., 2015). In a study that included 49 species of various plant families originating from contrasting climatic conditions, the minimum temperature required for seed germination was shown to be significantly correlated with habitat temperature and was proposed as a direct proxy for predicting plant species distribution ranges along a temperature gradient ( Rosbakh and Poschlod, 2015). For instance, seed germination functional traits appeared to be of primary importance in explaining alpine species distribution in calcareous and siliceous habitats ( Tudela-Isanta et al., 2018).
Several studies, however, showed that seed functional traits may significantly limit species distribution across habitats. By contrast, whether seed traits that govern seed persistence and germination niche breadth influence the geographical ranges and ecological strategies of Mediterranean oaks remains largely unexplored. Acherar and Rambal, 1992 Cavender-Bares et al., 2005 Quero et al., 2006 Baldocchi et al., 2010 Castagneri et al., 2017). Numerous studies have been carried out to understand how vegetative functional traits affect Mediterranean oak ecological ranges (e.g. The vegetation in this drier area is mostly an evergreen Q. To the south, the northern part of the country is separated from the Sahara by the Tunisian Ridge, a range of hills running north-east to south-west for ~220 km. coccifera only occurs and is confined to the fixed dunes. This area is moderately wet (800 mm annual rainfall), but the upper mountains are very wet (1300 mm annual rainfall) with low winter temperatures. canariensis occurring in the highest coldest sites. In the northwest region of Tunisia, where the climate is influenced by the proximity of the Mediterranean Sea with its mild temperatures, vegetation is mostly an evergreen Q. suber), but are found in different areas that are characterized by contrasted environmental conditions ( Hasnaoui, 1992 Sakka et al., 2015). These habitat specificities of Mediterranean oaks also exist in north Tunisia, where four dominant oak species occur ( Q. Quercus ilex is known to have the widest ecological amplitude, from semi-arid to per-humid bioclimates, and from warm to cold conditions depending on the elevation ( Barbero et al., 1992). canariensis, it grows in humid and cold Mediterranean areas ( Maire, 1961 Mir et al., 2006). While the type of substrate is not important for Q. suber has strict humidity and soil requirements and is restricted to the warmer humid and sub-humid Mediterranean areas ( Maire, 1961 Gil-Pelegrín et al., 2017). coccifera becomes dominant under warm and arid conditions ( Gil-Pelegrín et al., 2017).
However, they occupy distinct habitats and their respective distribution is related to altitudinal variation in pluviothermic factors and the minimum temperature of the coldest month, as early reported by Emberger (1930).
Mediterranean oak species are usually seen to share the capacity to cope with water stress through specialized adaptive features such as sclerophylly, restricted leaf area and thick cuticles. Oaks ( Quercus, Fagaceae) are the foundation and dominant tree species of many temperate and Mediterranean forest ecosystems.
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