Effects of white sweetclover invasion on the pollination and berry production of Vaccinium sp. in Alaska

Background/Question/Methods

In Alaska, important subsistence foods such as blueberries (Vaccinium uliginosum) and cranberries (Vaccinium vitis-idaea) require insect pollination to produce fruit. Invasive white sweetclover (Melilotus albus) has spread along roadsides and rivers throughout Alaska adjacent to easily accessible berry-picking habitat, and may have the potential to alter pollinator services to native berry plants. To experimentally determine the influence of white sweetclover on Vaccinium sp. pollination and fruit production, we selected 17 sites with abundant reproductive Vaccinium sp. within the Bonanza Creek Long Term Ecological Research areas near Fairbanks, Alaska. 125 flowering V. vitis-idaea and/or 75 flowering V. uliginosum focal plants located 1-2 m, 3-5 m, 8-10 m, 15-20 m or 25-40 m from the center of the site were monitored throughout the growing season. At 11 sites, we transplanted 40 flowering 2^nd-year white sweetclover plants at the center of the site. Six sites were established as control sites. To assess the pollen quality and quantity being delivered to the Vaccinium sp. at sites with and without M.albus, we collected stigmas from 75 non-focal plants growing adjacent to focal plants, then counted and identified pollen grains present. To assess the influence of white sweetclover on Vaccinium sp. fruit set, we tracked unopened buds through fertilization and fruit formation on all focal plants.

Results/Conclusions

In sites with M. albus added, we were able to detect the invasive plant’s pollen on Vaccinium sp. stigmas over 40 m from the infestation patch. Compared to control sites, M. albus addition increased the percent flower visited, the amount of pollen deposited on stigmas, and the fruit set of plants growing directly adjacent to the sweetclover patch (1-2 m away). 8-10 m away, the sweetclover addition reduced the percentage of flowers visited, the amount of pollen deposited on stigmas of flowers from the white sweetclover patch.  Similarly, both Vaccinium species showed reduced fruit set in mid-range distances away from the sweetclover patch (8-20 m away for V. uliginosum and 15-20 m away for V. vitis-idaea) compared to control sites. Our data suggest that the impact of M. albus addition on V. uliginosum and V. vitis-idaea pollination and fruit set varies with distance from the infestation, with facilitative interactions occurring close to the invasive plant patch, and competitive interactions occurring at mid-range distances.