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

Abstract

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.

Keywords: Polemonium viscosum; Salix; climate change; heterospecific pollen transfer; plant–climate interactions; reproductive ecology; selection on floral traits; shrub encroachment.

Figure 1

(a) Locations of four contact zones where surveys were conducted for Polemonium and Salix in the central Colorado Rocky Mountains (USA). (b) Photographs of the Pennsylvania Mountain contact zone in 1981 and 2012 showing infilling by encroaching Salix in alpine meadow habitat of Polemonium

Figure 2

Relationship between Julian days when flowering specimens of Salix and Polemonium were collected in each of 25 historic contact zones in the Colorado Rocky Mountains (USA). Sites were sampled between 1907 and 2007 (r = 0.93, df = 23, p < .0001)

Figure 3

Julian day of collection for flowering Polemonium and Salix as a function of average daily low temperature in June of the collection year. Open circles and solid line show data for Salix while triangular symbols, and dashed line show data for Polemonium (y = 228.0−3.70x and y = 235.2−4.26x; respectively). Overlapping points are offset (ggplot with jitter width = 0.05 in R Statistical Software) in order to show all data points

Figure 4

Variation among sites in contamination of Polemonium stigma pollen loads with Salix pollen (proportion of grains on the stigma belonging to Salix). Boxes show upper and lower quartile, and lines denote median values. Whiskers on each box indicate extremes, and solid circles denote outliers. Sites with different superscripts differ at p < .05

Figure 5

Relationship between contamination of Polemonium stigma pollen loads with Salix pollen and seeds per flower averaged for the plant. For the relationship, Spearman's r = −0.39, df = 32, p = .0272

Figure 6

Seed set per flower for plants of Polemonium under natural pollination, conspecific pollen addition, and Salix pollen addition treatments. Boxes show upper and lower quartile, and lines denote median values. Whiskers on each box indicate extremes. Dagger indicates treatment difference at p < .07, and asterisk indicates difference at p < .05