On which mark do they get set? Seed set of alpine plants is limited by a combination of temperature, seed-predation, and pollination

Background/Question/Methods

Worldwide declines in pollination services have been predicted by a number of studies, but a commonly-held assumption is that the reproduction of plants will suffer because it is most limited by access to pollinators.  To test this assumption in an alpine community, we conducted a series of field experiments involving simultaneous pollen-supplementation, pollinator-exclusion, and “open” pollination, using seven species of native perennials, over a 400 m. elevation gradient in the Coast Mountains of British Columbia, Canada.  We compared the number of viable and non-viable seeds produced per flower among plants from each of the treatments and elevations.  We also estimated the relative abundance and diversity of pollinators across the gradient, recorded hourly air temperatures throughout the growing season using iButtons (Maxim, USA), and recorded instances of seed-predation for several species.

Results/Conclusions

Our results show that six of seven species of plants were not pollen-limited, but responses to the pollen-supplementation treatments varied between early and late-flowering species.  The most important factors limiting production of viable seeds also varied among species, and ranged from temperature-mediated factors (number of frost events, and accumulation of degree-days) to biotic factors such as seed-predation.  Locations with warmer temperatures and fewer frost events were associated with higher relative abundance of insects – including both seed-predators and pollinators.  This implies that the impacts of declining pollination on potential reproduction of alpine plants might currently be over-emphasized relative to other limiting factors that are also likely to be affected by climate change.