The Midlands and South-west of England are represented by a long oak tree-ring chronology spanning approximately 4500-3900 BC (calibrated radiocarbon dates). The wood on which it is based originates in a technologically advanced trackway crossing the low-lying Somerset Levels, in a coastal submerged forest probably killed by rising sea-level, and in flood-plain oaks washed down the River Trent. Cross-matching between the growth patterns of the three groups of trees is of good quality, yet so far the chronology has failed to cross-date with the long Irish and German dated chronologies. The reasons for this, and the implications of eventual dating, are discussed.

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Le genre Quercus comprend plus de 500 espèces et est réparti sur l’ensemble de l’hémisphère nord. La phylogénie du genre, faite à ce jour à partir d’un nombre très limité de marqueurs nucléaires, n’était pas résolue. Des incertitudes demeuraient au niveau des nœuds profonds où ont divergé les principaux groupes taxonomiques aujourd’hui reconnus. L’objectif de cette thèse était d’explorer de manière plus exhaustive les ressources génomiques nucléaires et chloroplastiques pour affiner la phylogénie du genre. Les travaux sont basés sur les séquences de six gènes nucléaires et de l’ensemble du génome chloroplastique. Ces travaux confirment le caractère diffus du signal phylogénétique et le gain de résolution obtenu par l’adjonction de séquences nouvelles. Ils confirment également la subdivision du genre en six groupes infragénériques (Cyclobalanopsis, Ilex, Cerris, Lobatae, Quercus s.s. et Protobalanus), dont les relations phylogénétiques ont été précisées, même si certaines irrésolutions persistent. La thèse met très clairement en évidence l’empreinte phylogéographique dans le génome chloroplastique au niveau du genre et de sa distribution mondiale. Le signal phylogéographique chloroplastique ajouté à la phylogénie nucléaire permet d’échafauder un scénario biogéographique de diversification du genre. Ce scénario devra être corroboré par des apports d’autres disciplines (paléontologie et géologie historique). / The genus Quercus comprises more than 500 species, and is widely distributed across the Northern hemisphere. Phylogenetic reconstructions based on traditional molecular sequences were so far irresolutive at the deeper nodes where the major extant taxonomic groups have diverged. This thesis aims at improving the phylogeny of the genus by exploring the current nuclear and chloroplastic genomic resources. The phylogenetic investigations are based on sequences of six nuclear genes and the entire chloroplastic genome. The results confirm that the phylogenetic signal is rather diluted and that substantial improvements can be obtained by adding sequences from additional genes. They also confirm that the genus can be subdivided in six infrageneric groups (Cyclobalanopsis, Ilex, cerris, Lobatae, Quercus s.s. et Protobalanus). Phylogenetic relationships among these groups are refined, although not fully clarified. There is a very clear phylogeographic imprint in the chloroplast genome that extends at the macroevolutionary level at the whole genus across its entire distribution. The phylogeographic structure together with the phylogeny at the nuclear level allows to elaborate an historical scenario of the radiation of the genus. Additional elements coming from other disciplines (paleontology, historical geology) are however necessary to confirm this scenario.

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Quercus praedelavayi Xing Y.W. et Zhou Z.K. sp. nov. is reported from the upper Miocene of the Xianfeng flora in central Yunnan, southwestern China. The fossil species is identified based on the detailed leaf morphological and cuticular examinations. The primary venation is pinnate and the major secondary venation is craspedodromous with regular spacing. Stomata are anomocytic and occur on abaxial epidermis. Trichome bases are unicellular and multicellular. The new fossil species shows the closest affinity with Quercus delavayi, an extant species distributing in southwestern China. The responses of the functional leaf traits to the climate change were studied by comparing the leaf characters of fossil species and its nearest living relative. The stomatal density of Q. praedelavayi is higher than Q. delavayi, which suggests a lower palaeoatmospheric CO2 concentration during the late Miocene. The trichome base density of Q. praedelavayi is higher than the extant Q. delavayi. Considering the palaeoclimatic reconstruction of Xianfeng flora, it rejected the hypothesis that increase in trichome density is an adaptation to the drier environment.

Quercus praedelavayi Xing Y.W. et Zhou Z.K. sp. nov. is reported from the upper Miocene of the Xianfeng flora in central Yunnan, southwestern China. The fossil species is identified based on the detailed leaf morphological and cuticular examinations. The primary venation is pinnate and the major secondary venation is craspedodromous with regular spacing. Stomata are anomocytic and occur on abaxial epidermis. Trichome bases are unicellular and multicellular. The new fossil species shows the closest affinity with Quercus delavayi, an extant species distributing in southwestern China. The responses of the functional leaf traits to the climate change were studied by comparing the leaf characters of fossil species and its nearest living relative. The stomatal density of Q. praedelavayi is higher than Q. delavayi, which suggests a lower palaeoatmospheric CO2 concentration during the late Miocene. The trichome base density of Q. praedelavayi is higher than the extant Q. delavayi. Considering the palaeoclimatic reconstruction of Xianfeng flora, it rejected the hypothesis that increase in trichome density is an adaptation to the drier environment.