Flowering phenology and flower production of alpine plants
There are many studies on the flowering phenology in alpine plant communities (e.g., Holway & Ward, 1965; Thórhallsdóttir, 1998; Molau et al., 2005; Jabis et al., 2020), and some of them focused on the relationship between plants and flower visitors (e.g., Bosch et al., 1997; Makolaos et al., 2008; Iler et al., 2013; Pelayo et al., 2019, 2021). However, comparative studies of flowering patterns between different pollination types are limited as mentioned before (Kudo, 2016; Mizunaga & Kudo, 2017). In the present study, the number of blooming bee-visited species was positively related to the seasonal dynamics of bee abundance, but there was no correlation between the number of blooming fly-visited species and fly abundance. This difference may reflect the temporal availability of pollinators for bee-visited and fly-visited plants. A long-term study on the flowering phenology and pollination success of Japanese alpine plants (Kudo, 2022) demonstrated that the fruit-set rates of bee-visited plants increased as the season progressed, reflecting the increasing pollination service by worker bees. In contrast, the seasonal trend in the pollination success of fly-visited plants was less clear, where the variations in fruit-set success among species and between years were independent of flowering time. This result indicates a small seasonal restriction of pollination service by flies in alpine plant communities. Although the pollination efficiency of flies was generally lower than that of bees (Herrera, 1987), frequent visits of fly pollinators might compensate for the low efficiency per visit (Kearns & Inouye, 1994; Mizunaga & Kudo, 2017). Also in the present study, the visitation frequency of dipteran insects was 2.5 times larger than that of hymenopteran insects.
The seasonal patterns of flower production also differed between bee-visited plants and fly-visited plants. Similarly in the Japanese alpine communities, the seasonal pattern of flower production was moderate in fly-visited plants, whereas flower production of bee-visited plants tended to be maximum in the middle of the season although actual flower production of bee-visited plants highly fluctuated from year to year (Kudo, 2016; Mizunaga & Kudo, 2017). Interestingly, a recent dataset of flower color diversity among 727 species (ca. 20% of Taiwan flora) sampled along elevational gradient of Taiwan also found evidences that the majority of Taiwan flora evolved to match bee’s innate color preference (Tai et al., 2020). In particular, the floral color diversities of bee-visiting plants in high altitude are over-dispersed among related species perhaps to facilitate their co-existence (Tai et al., 2020). Together, our data and the previous finding suggest the bee-visited plants in alpine Taiwan may have shifted their flowering times and colors to encourage bees’ visiting. These results suggest that the flowering structure of alpine plant communities, i.e., temporal dynamics of floral diversity and floral resources, may be influenced by the relative composition of bee-visited and fly-visited species.