水青冈属是系统植物学家长期关注的类群。然而，该类群存在狭域种的物种地位不明、广布种可能存在形态学隐性种、属下系统发育关系混乱以及两个竞争的生物地理学假说尚未得以验证等问题。本研究拟通过北半球范围的群体采样，运用多个体-多基因位点（10–15个核基因）的策略，采用基于多物种溯祖模型的物种界定方法（BPP和spedeSTEM）对水青冈物种的界限予以界定；采用两种物种树构建方法（*BEAST和BEST）重建水青冈属的物种树；基于新的物种树，采用迁移-隔离分化分析方法（DIVA）重新分析水青冈属生物地理格局；采用ABC模拟检验两个相互竞争的生物地理假说（out-of-China scenario与northern hemispheric ancestor scenario）。本研究不仅可以解决水青冈属长期存在的系统学和生物地理学问题，同时有助于增进对中国第三纪孑遗植物区系形成的认识。

Beeches (Fagus L.) are one of the most important components of the temperate deciduous forests in the Northern Hemisphere. The genus includes ca. 13 species broadly distributed in three isolated regions: East Asia, Europe and West Asia, and North America. China, harboring five beech species, is the center of species diversity of the genus. Because Fagus has significant importance in ecological and biogeographical studies, this genus has received much attention from evolutionary botanists. However, the species status of several isolated populations (such as Fagus multivernis in South Korea) is questionable and whether some widespread beech species contain morphologically cryptic species remains open, possibly due to the morphological homogeneity within the genus. Further,because of a paucity of informative morphological traits, chloroplast haplotype sharing and intraindividual nrITS polymorphism, phylogenetic relationships among beech species are still equivocal. With regard to the species diversity of China, there are two competing hypotheses: out-of-China scenario and northern hemispheric ancestor scenario, however, these hypotheses have not been statistically tested.. In this project, we plan to conduct comprehensive population sampling across the Northern Hemisphere, including all putative beech species. Ten to fifteen nuclear loci will be sequenced for all population samples of Fagus as well as some outgroup individuals within Fagaceae. Then, under the unified species concept of de Queiroz (2007) and multi-species coalescent framework, we will determine the species boundaries of all candidate species using two species delimitation methods, BPP and spedeSTEM. The species tree of the genus will be reconstructed by means of *BEAST and BEST. The biogeographical history of Fagus will be first inferred by dispersal-vicariance analysis (DIVA) and then two competing biogeographical hypotheses (out-of-China scenario and northern hemispheric ancestor scenario) will be statistically evaluated using approximate Bayesian computation simulation. When the species tree is available and the species boundaries are determined, we will establish a new taxonomic system of this important genus. We believe that the results of this project would not only resolve the taxonomic, phylogenetic and biogeographical problems of the beeches, but also provide new insights into the formation of China's relict Tertiary flora.