Flower-leaf sequences in temperate woody plants: interspecific variation in function and physiology determine the consequences of shifting phenological series with climate change

Daniel Buonaiuto, Organismic and Evolutionary Biology, Harvard University, Cambridge, MA and Elizabeth Wolkovich, Forest and Conservation Sciences, University of British Columbia, Vancouver, BC, Canada

Background/Question/Methods
The leaves and flowers of deciduous woody plants in temperate forests both appear in the spring, but the order and duration of flower-leaf phenological sequences (FLSs) vary among species. Species that flower before leaves are common in these regions, and several analyses suggest that this pattern is adaptive, contributing to species’ pollination biology, physiological regulation and resource allocation patterns. Further, FLSs appear to be shifting with climate change, but the direction and rate of change varies among species. Predicting FLS shifts with climate change will require an improved understanding of how the environment dictates FLS patterns, and how species differ in their responses to the environment.
Decades of experiments have confirmed that both flower and leaf phenology are cued by changes in temperature and photoperiod. However, the primary mechanism driving FLS shifts with climate change is subtle differences in how flower and leaf phenological phases respond to these cues, and these differences are poorly understood for most temperate species. Characterizing differences in how flower and leaf phenology respond to cues, and how these differences vary among species, is critical to predicting the magnitude and consequences of FLS shifts with climate change.
To address this need we compared the phenological responses of flower and leaf buds on twig cuttings of 10 temperate woody species to varying levels of temperature and photoperiod in a growth chamber experiment. We then combined these finding with simple climate change projections to examine how FLSs may shift in the coming decades, and how these shifts may vary among species.
Results/Conclusions
In our study, flower and leaf phenology responded similarly to changes in forcing (warm spring temperature), but leaf phenology was more sensitive to changes in chilling (cool winter temperatures) and flower phenology more sensitive to changes in photoperiod. These findings suggest that FLSs are likely to continue to shift with climate change. We also observed differences in cue sensitivity among species, indicating that the magnitude of FLS shifts will be species-specific.
We found that flowering-first, wind-pollinated shrubs were most sensitive to FLS shifts with climate change, with the time between flowering and leafing decreasing substantially in all of our modeled scenarios. Because the period between flowering and leafing is especially critical for successful pollen transfer in wind-pollinated taxa, the direction of this shift suggests that some wind-pollinated taxa could experience increased pollen limitation in the decades to come, affecting their overall reproductive fitness.