PS 37-156
Willow shrub density affects pollination of alpine forbs differentially and nonlinearly
In the Colorado Rocky Mountains, subalpine willow shrubs (Salix spp.), which serve as important early-season resources for insect pollinators, are rapidly expanding their ranges and encroaching the herbaceous alpine meadows. By increasing competition for pollination services, willow shrubs may reduce pollinator visitation to, and therefore pollen deposition on, resident alpine flowering forbs (pollinator sharing hypothesis). Willow shrubs also change the structure of alpine meadows, and may obscure forbs, making them more difficult for insects to find (structure hypothesis). To understand how the density of willow shrubs affects pollination of resident alpine forbs, we exposed experimental floral arrays of three important insect pollinator food plants to three willow density treatments (high, low, and no willow) on Niwot Ridge. We tested the pollinator sharing hypothesis with the early-blooming Polemonium viscosum, and we tested the structure hypothesis with the late-blooming Castilleja occidentalis and Trifolium dasyphyllum. We emasculated one flower per inflorescence to remove the potential for autogamous pollen deposition and then exposed the inflorescences to the willow density treatments. We subsequently dissected the stigmas from the emasculated flower and one hermaphroditic flower per inflorescence, stained the stigmas with Calberla’s fluid, and analyzed pollen deposition by donor type (conspecific, willow, and non-willow heterospecific pollen).
Results/Conclusions
Results suggest that willow shrub density affects pollination of early-season alpine forbs. The effects of willow density were nonlinear and depended on both plant species and potential for autogamy. Pollen receipt in C. occidentalis and T. dasyphyllum was unaffected by willow treatment. For P. viscosum flowers at low willow density, on the other hand, pollen receipt quadrupled. Emasculated P. viscosum flowers received more conspecific and heterospecific pollen relative to flowers in high density willow patches or in plots without willows. Low density of willows may facilitate pollination in P.viscosum through pollinator sharing. We hypothesize that for co-flowering forbs, willow shrubs may 1) attract generalist pollinators and 2) shield pollinators from strong alpine winds, allowing them to forage on forbs more effectively. These positive impacts of willows are consistent with the importance of facilitation in alpine environments. However, at high density willows tend to depress pollen receipt, consistent with the structure hypothesis. Pollination services for co-flowering alpine forbs are sensitive to the changing architecture of the willow krummholz under climate change. Understanding mechanisms for these effects will be a major goal of future work.