POLLINATION DYNAMICS OF ARCTIC DWARF BIRCH (BETULA-GLANDULOSA BETULACEAE) AND ITS ROLE IN THE LOSS OF SEED PRODUCTION

At the northern limit of its distribution, arctic dwarf birch, Betula glandulosa, shows nearly complete absence of sexual reproduction (i.e., <0.5%) viable samaras) and maintains its populations vegetatively. To investigate the possible role of pollination dynamics in the loss of sexual reproduction, pollination biology of arctic dwarf birch was compared at two locations: Tarr Inlet on Baffin Island, Northwest Territories near the northern limit of the species, and Kuujjuaq, Quebec in the center of its distribution where sexual reproduction is the primary mode of reproduction. Relative production of staminate and pistillate flowers, pollen rain, pollen viability, and stigmatic pollen loads throughout pollen dispersal were compared. Plants at Tarr Inlet produced 15%-30% of pollen produced at Kuujjuaq, both as a result of a lower density of staminate catkins and less pollen per catkin. Potential seed productivity is limited al the northern limit because pistillate catkins produce 50% fewer flowers in the north than in the south. While stigmatic pollen loads were similar at both sites, lower pollen viability (68% vs. 93%) and a higher probability of geitonogamous pollen due to clonal growth pattern reduced fertilization success at the northern site. These data suggest that lack of sexual reproduction in B. glandulosa at its northern limit is in part due to pollen limitation.