Betula ovalifolia (Betulaceae) is a multi-stemmed shrub growing in boreal wetlands of Northeast Asia. In Japan, B. ovalifolia is only known from Betsukai and Sarabetsu in eastern Hokkaido, and is considered a glacial relict species. In Betsukai, B. ovalifolia is found in three locations, which used to be at the periphery of a continuous mire now fragmented by agriculture development. Two locations are still in mires, one of which is protected as a prefectural natural monument. The remaining one location is in the national forest designated as shelterbelt, which is now dominated by Alnus japonica but also includes a scrub of B. ovalifolia. In this national forest, we established a vegetation sample plot (30 m length and 20 m width) spanning from B. ovalifolia scrub to A. japonica swamp forest, and enumerated all trees (all are A. japonica) of 3 cm or more in diameter at breast height and all B. ovalifolia individuals of 1.3 m or more in height. We examined the change in vegetation structure across the scrub-forest ecotone by subdividing the plot into six subplots of 5 m length and 20 m width to infer the fate of B. ovalifolia in the hydrosere succession through terrestrialization.