Prevalence and consequences of heterospecific pollen receipt in the subalpine plant communities of <em>Polemonium brandegeei</em>

Many pollinators visit multiple plant species, causing pollinators to deposit diverse pollen loads. Although heterospecific pollen (HP) transfer is widespread, few studies have tested how HP deposition affects plant fitness in natural populations. We investigated the fitness consequences of HP receipt within two floral communities surrounding Polemonium brandegeei (Polemoniaceae), a subalpine herb with diverse pollinators. First, we collected open pollinated stigmas from abundant species in flower (n = 11) and generated pollen transfer networks. All sampled species received or donated HP with distantly related species. Positive relations between HP receipt and conspecific pollen (CP) number and germination indicated facilitation due to pollinator sharing in P. brandegeei and Harbouria trachypleura (Apiaceae); however, HP receipt was low and did not affect CP germination in most species. Second, we conducted hand pollinations to test how donor-recipient relatedness affected consequences of HP receipt for P. brandegeei. A distantly related donor did not affect CP germination, seed set, or seed weight, but two congeneric donors reduced reproductive measures. This study suggests that pollinator sharing between native co-flowering species is common and can be beneficial in plant communities. However, competition may occur if closely related species begin to co-flower due to range changes or phenological shifts.