Habitat-specific effects of flowering advance on fruit-set success of alpine plants: a long-term record of flowering phenology and fruit-set success of Rhododendron aureum

Global warming tends to accelerate flowering phenology of alpine plants, and it may cause a decrease in fruit production due to lower pollinator activity and/or higher risk of frost damage earlier in the season. Because flowering period of alpine plants varies highly depending on snowmelt conditions, the effects of phenological variation on fruit-set success may vary among local populations. I observed the relationship between flowering time and fruit-set success in four populations of a bee-pollinated dwarf shrub, Rhododendron aureum, located in fellfield and snowbed habitats in northern Japan, for 12 or 13 years over the 25 years from 1995 to 2019. Flowering of the fellfield populations usually occurred in June, and flowering of the snowbed populations commonly started after mid-July, although there was considerable yearly variation in actual flowering time within individual populations. Generally, the fruit-set rates of the fellfield populations were low, with large yearly fluctuations, whereas those of the snowbed populations were stable and high. There was a clear trend toward a decrease in fruit-set rates with earlier flowering in the fellfield populations due to pollen limitation and occasional frost damage. The risk of frost damage increased with earlier flowering in the fellfield habitat. These results indicate that the effects of climate change on fruit-set success of alpine plants are strongly site-specific and are greatest early in the growing season.