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 Salix (willows) on pollination quality of the alpine skypilot, Polemonium viscosum. Overlap in flowering time of Salix and Polemonium 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 Polemonium flowers and on Polemonium pistils. We probed fitness effects of pollinator sharing by measuring the correlation between Salix pollen contamination and seed set in naturally pollinated Polemonium. To ascertain whether Salix interference occurred during or after pollination, we compared seed set under natural pollination, conspecific pollen addition, and Salix pollen addition. In current and past contact zones Polemonium and Salix overlapped in flowering time. After accounting for variance in flowering date due to latitude, Salix and Polemonium showed similar advances in flowering under warmer summers. This trend supports the idea that sensitivity to temperature promotes reproductive synchrony in both species. Salix pollen is carried by bumble bees when visiting Polemonium flowers and accounts for up to 25% of the grains on Polemonium pistils. Salix 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 Polemonium and selecting for traits that reduce pollinator sharing.

salix and pv phenology dataFlowering phenology data (Julian day) for Salix spp. and Polemonium viscosum specimens collected from 25 sites in the Colorado Rocky Mountains between 1907 and 2009. The location, altitude, mean minimum June temperatures, and the latitude and longitude estimated using Google Earth are provided for each specimen.2016 population surveyPollen counts on stigmas from naturally pollinated P. viscosum flowers. Pollen identified as Salix and P. viscosum (denoted by “pv” in the data file) were quantified using light microscopy. Salix pollen contamination was estimated as the frequency (proportion) of total pollen identified as Salix pollen. The site name, unique sample number (“sample ID), and the distance (m) from the focal plant to the nearest Salix spp. plant are included.pollen purity vs seed setAverage seed set per flower for naturally pollinated plants at Pennsylvania Mountain, Colorado. Pollination success (conspecific pollen deposition and germination rates) and contamination (proportion of pollen from Salix spp.) were quantified for one flower per inflorescence.pollination experimentData from the pollination contamination experiment. Individual plants found within groups of P. viscosum plants were randomly assigned to one of three pollination treatments: no pollen supplementation, conspecific (P. viscosum) pollen supplementation, and Salix spp. pollen supplementation. The plant number, group number, number of flowers pollinated, number of seeds produced, and average seeds per flower are provided.