Direct and indirect effects of frost events on plant growth and reproduction in montane regions of Colorado
Frost is an important weather event that damages plant tissues through formation of ice crystals at or below freezing temperatures. In montane regions where climate change is expected to cause earlier snowmelt, plants that bud earlier might be directly impacted by frost through damage to flower buds and ovaries. However, the indirect effects of frost mediated through changes in pollinator interactions have rarely been studied. We addressed the following questions: 1) How do pollinators respond to plants that have been exposed to frost? 2) Does frost affect plant reproduction through pollen limitation?
We tested effects of frost on three plant species in southwestern CO, Delphinium barbeyi (Ranunculaceae), Erigeron speciosus (Asteraceae), and Polemonium foliosissimum(Polemoniaceae), using a two-way factorial design by manipulating frost events (frost/no frost) crossed by pollen supplementation (supplementation/open-pollinated). All three species require pollinators to transfer pollen within and/or between flowers. We simulated moderate (-1 to -5°C) frost events by placing foam coolers containing dry ice over plants for 4 hours when buds formed. Upon flowering, we monitored pollinator visitation and applied pollen to stigmas of flowers in the pollen-supplementation treatments. Additionally, we measured plant growth, bloom duration, and floral display. After senescence, we measured fruit and seed set.
We found that frost events can have direct effects on plants, but the indirect effects mediated through changes in pollination are less clear. For example, for P. foliosissimum, we found that frost significantly reduced plant survival by 24%, but we found no effect on estimates of plant growth or floral or flowering traits. Frost also significantly reduced seeds per fruit by 5%, but the effect of frost on seeds per fruit was a direct effect, and we found little evidence that frost affected P. foliosissimum via changes in pollination. Patterns and magnitudes of direct and indirect effects of frost will be compared across the three plant species, which exhibit variation in how susceptible buds and ovaries are to the effects of frost damage.
Our results indicate that frost can have severe direct implications on plant survival and reproduction. If climate change causes snow to melt out earlier each spring, then plants might be more susceptible to being damaged by frost. The next step is to assess the effects of frost events on bee pollinators through direct temperature effects as well as indirect effects via reduction of flower forage.