Relationships between flowering phenology and fruit-set of dwarf shrubs in Alpine Fellfields in northern Japan: A comparison with a subarctic heathland in northern Sweden

Relationships between flowering phenology and fruit-set of 10 ericaceous species were studied in three alpine fellfields in the Taisetsu Mountains, northern Japan. Flowering season varied from late May to early August among species. There was a positive correlation between flowering time and fruit-set under natural conditions. Pollinator exclusion by net-bagging and hand-pollination revealed that very low fruit-set in early-flowering species was caused by severe pollen limitation, while late-flowering species tended to have high fruit-set with slight pollen limitation. Such seasonal patterns reflect the changes in seasonal activity of pollinating insects, which are highly influenced by ambient temperature. A similar research program was conducted in a subarctic heathland in northern Sweden with almost similar species composition. In Sweden, however, there was no positive correlation between flowering time and fruit-set. The difference between the alpine fellfields and the subarctic heathland may be caused by the specific seasonal pattern of temperature change in either region. A steep increase in air temperature from early to mid-summer in the alpine fellfields in Japan may cause all increase in pollinator availability with progress of season.