Timing is everything: subalpine wildflower phenology is driven by snow and traits

Background/Question/Methods

What controls flowering time in subalpine wildflowers? Climate is thought to play an important role in phenological events like flowering, with different climate variables potentially important to different species. For example, recent climate change is causing life events such as migration and breeding of some (but not all) species to advance in the spring. Our understanding of which climatic variables influence phenology of different species, and whether species traits explain variability among species in responses, remains unknown. This is important, because our ability to forecast biological impacts of climate change on communities hinges on this understanding. For example, if subalpine plants are differentially sensitive to date of snow melt (a known critical driver of phenology in snow-covered regions) then future warming could result in changes in seasonal flowering assemblages that could also influence insect pollinators. To determine which climatic variables influence phenology, and whether species traits explain variability in responses, we quantified wildflower phenology of 45 species in subalpine meadows at Mount Rainier National Park. Specifically, we measured climate (air temperature, soil temperature, and snow duration) and weekly reproductive phenology in 70 plots over 4 years. 

Results/Conclusions

We found that flowering phenology is strongly related to the snow disappearance date (SDD), and that there are species-specific differences in the lag between snow melt and peak flowering, and species-specific differences in the sensitivity of flowering to date of snow disappearance. Differences in flowering and the importance of SDD were correlated with traits such as seasonality and form. For example, early-flowering plants were surprisingly less sensitive to SDD than mid- and late-flowering plants, and woody shrubs are less sensitive to SDD than herbaceous plants. Differential climate sensitivities have implications for the assemblages of flowering communities at Mount Rainier in a future, warmer world. For example, if in the future snow melts earlier (as is expected with climate change), based on simple linear models between SDD and peak flowering, the early-flowering Erythronium montanum (SDD-sensitivity 0.88) and the later flowering Aster alpigenus (SDD-sensitivity 1.30), will increase in flowering overlap. If communities flower in novel assemblages, pollinator-mediated interactions (such as facilitation or competition for pollinator services) may be altered and plant reproductive performance may be impacted.