Timing of flowering affects pollination and fruit set in Viburnum edule in boreal forests of Alaska

Abstract

Spring flowering in Alaskan boreal forests is happening earlier on average; how this relates to the pollination of plants and their pollinator community is unknown. Highbush cranberry (Viburnum edule) is one of the first herbaceous understory plants to flower every year, and in years when it flowers early there are fewer other species in bloom compared to years when it flowers at a more average time. Highbush cranberry is also important as a subsistence food and many boreal animals consume these fruits as a regular part of their diet. The potential for change may lead to differences in the response of pollinators and plants under early season conditions which could alter resources for pollinators and impact fruit production. This research looks at the impact of flowering timing on pollen deposition on Viburnum edule, and the composition of the pollinator community visiting the available flowers. Using an experiment with flowers placed in boreal forest sites either at an early time or at a peak flowering time across two years, we found that early flowering highbush cranberry received fewer pollen grains than peak flowering highbush cranberry and were visited less. V. edule was primarily visited by syrphid flies, native bees, and muscoid flies. We also observed a lower total number of visitors, and a lower proportion of visitors that were bees during an early flowering time than at peak flowering time. Floral visitors were more abundant during the advanced flowering year than during an average flowering year. We do not currently think that pollen limitation is causing a reduction in fruit set of early flowering V. edule because at all flowering times observed, we found over 50% of flowers to have been presumed visited while less than half of flowers in an inflorescence form fruits on average. More information on boreal pollinators triggers for diapause break and floral visitation is necessary to make more reliable predictions of the future impacts of phenology shifts in flowering plants and insect pollinators.