How shrub encroachment under climate change could threaten pollination services for alpine wildflowers: A case study using the alpine skypilot, .

Unlabelled: Under climate change, shrubs encroaching into high altitude plant communities disrupt ecosystem processes. Yet effects of encroachment on pollination mutualisms are poorly understood. Here, we probe potential fitness impacts of interference from encroaching (willows) on pollination quality of the alpine skypilot, . Overlap in flowering time of and is a precondition for interference and was surveyed in four extant and 25 historic contact zones. Pollinator sharing was ascertained from observations of willow pollen on bumble bees visiting flowers and on pistils. We probed fitness effects of pollinator sharing by measuring the correlation between pollen contamination and seed set in naturally pollinated . To ascertain whether interference occurred during or after pollination, we compared seed set under natural pollination, conspecific pollen addition, and pollen addition. In current and past contact zones and overlapped in flowering time. After accounting for variance in flowering date due to latitude, and showed similar advances in flowering under warmer summers. This trend supports the idea that sensitivity to temperature promotes reproductive synchrony in both species. pollen is carried by bumble bees when visiting flowers and accounts for up to 25% of the grains on pistils. contamination correlates with reduced seed set in nature and when applied experimentally. Postpollination processes likely mediate these deleterious effects as seed set in nature was not limited by pollen delivery.

Synthesis: As willows move higher with climate change, we predict that they will drive postpollination interference, reducing the fitness benefits of pollinator visitation for and selecting for traits that reduce pollinator sharing.