Plants in time: Uncovering phenological complexity across the landscape with digital herbaria

Daniel S. Park, Biological Sciences, Purdue University, West Lafayette, IN, Goia Lyra, Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, Aaron Ellison, Harvard Forest, Harvard University, Petersham, MA, Ian K. Breckheimer, Rocky Mountain Biological Laboratory, Gothic, CO and Charles Davis, Organismal and Evolutionary Biology, Harvard

Background/Question/Methods
Phenological timing is an important component of plant fitness and reproduction and provides one of the clearest indicators of the effects of recent climatic change. Phenological disruption has already impacted species' local persistence, community diversity, and critical ecosystem processes, including carbon sequestration, ecosystem–atmosphere interactions and trophic interactions. However, the data necessary to assess the direction, magnitude, and mechanism(s) of phenological responses for most species are sparse. The hundreds of millions of plant specimens collected across centuries and stored in thousands of herbaria across the globe represent an underutilized resource that can address this impasse.
Results/Conclusions
Using data gathered from digital specimen images across the eastern United States, we demonstrate huge variation in reproductive phenological responses to climate within and among species and investigate the underlying ecological and evolutionary mechanisms. We test how i) geographic variation in climate, ii) co-occurrence with close relatives, iii) urbanization, and iv) the seasonal timing of reproduction could account for this variation. We find that early-reproducing species in warm, humid, urban environments tend to be the most phenologically sensitive to changes in spring climate. The effects of co-occurring closely-related species were strongest among congeneric species flowering close in time, but these temporal patterns were predicted to change with global warming. Overall, our results suggest that phenological responses to anthropogenic climate change will be heterogeneous within communities and across regions, with large amounts of regional variability driven by local adaptation, phenotypic plasticity and differences in species assemblages. Used carefully, the millions of imaged herbarium specimens becoming available online will play an increasingly critical role in revealing large-scale patterns within assemblages and across landscapes that ultimately can improve forecasts of the impacts of climatic change on the structure and function of ecosystems.