B. fruticosa and B. middendroffii raw sequence reads

Shrub birches are important plants of massive wetlands in northeast China, although these habitats have been lost since the past two decades. However, the taxonomy of shrub birches has been ambiguous due to their morphological variation. In this study, we incorporated morphological and molecular data to address the taxonomic challenges associated with shrub birches. We characterized morphological variation of leaves, seeds, and bracts in 11 shrub birch populations in northeast China and a population of B. ovalifolia in northern Japan. We genotyped birch specimens from northeast China at 15 microsatellite loci and using restriction-site associated DNA sequencing. The populations in northeast China consisted of B. fruticosa and B. middendorffii. These two species and B. ovalifolia differed in the width of seed wings and the angle between side lobes of bracts, despite the substantial intraspecific variation.