Pollen-limited production of viable seeds in an endemic dwarf birch, Betula apoiensis, and incomplete reproductive barriers to a sympatric congener, B. ermanii

An endemic dwarf birch, Betula apoiensis, is critically endangered, and two populations of this species are restricted to the ridges of Mt. Apoi in Hokkaido, Japan. We observed the flowering phenology, pollen dispersal, and viable seed production and conducted pollination experiments in order to examine pollen limitation and hybridization with a sympatric congener, B. ermanii. B. apoiensis flowered earlier than B. ermanii but had a more variable flowering time among trees than B. ermanii. The female flowering of B. apoiensis temporally overlapped with the male flowering of B. ermanii as well as with that of B. apoiensis. Pollination experiments demonstrated that seed set and seed germination were higher in female flowers outcrossed than in those that were non-pollinated, selfed, hybridized with B. ermanii pollen, or pollinated naturally. A few selfed or hybrid seeds were filled and germinated, which indicates that self-incompatibility and reproductive barriers are not complete. Logistic regressions of local density of conspecific trees on natural seed set and seed germination were significantly positive. These results suggest that B. apoiensis is pollen-limited.