Delimiting 33 (Betulaceae) species with a further phylogenetic inference.

(Betulaceae) has approximately 52 species distributed in the Northern Hemisphere, with many species of found in China. However, the species boundaries and phylogenetic relationships remain poorly understood. This study reported ITS sequences for 225 individuals of 33 species, mainly from China. We also included eight species in our analyses, the closely related sister group of . We aimed to delimit these species based on ITS sequences and clarify their phylogenetic relationships by constructing tree-like topology and networks at population level. We found that only 17 of 33 species could be delimited from the closely related ones based on species-specific mutations in ITS sequence variation, including all species of sect. , and sect. subsect subsect contained two endangered species, although one seemed to be a recently originated allopolyploid species with genetic additivity from two likely parents in the ITS sequence variation. Sixteen species of sect. subsect. were classified into three species complexes, in each of which two or more could be not distinguished from each other. The closely related species of these complexes may still diverge at the early stage without genetic distinction in the nuclear ITS sequences because of too short of divergence time and frequent gene flow. Otherwise, some species may be established based on the intraspecific variations without genetic bases for an independently evolving unit.