A plant for all seasons: Examining patterns of spring, summer, and fall flowering phenology across South Carolina

Background/Question/Methods

Phenology, or the timing of major life cycle events, represents a key mechanism for understanding the relationships between climate and the biosphere.  Flowering phenology in particular can be critical, as changes in the timing of flowering relative to pollinator availability or competing plant species may alter the reproductive success of plant populations.  However, most existing research on temperate flowering phenology has focused on spring flowering species, and existing phenoclimate indices have similarly been designed around spring leaf development and early season flowering.  As a result, little is known about the long-term patterns of change in the flowering phenology of summer and fall-flowering species, their similarity to the phenology of spring flowering species, or their sensitivity to variations in climate.  This research will utilize herbarium records to examine the flowering phenology of a broad cross-section of species across South Carolina that exhibit differing flowering seasonality to address the following questions; 1) Does flowering phenology show similar patterns of inter-annual (short-term) variation across taxa with differing flowering seasonality?  2) Do the phenologies of early and late flowering species show significantly different long-term trends? 3) Does spring reflect the season in which flowering phenology is most responsive to changes in climate?

Results/Conclusions

This study demonstrates that a variety of phenological patterns occur across the plant community, and that significant differences occur in both short-term inter-annual variations and in long-term trends.  While species that flower early in the growing season have shifted towards earlier flowering since 1950, summer and fall flowering species have shifted towards later flowering.  Additionally, the flowering of summer species shows not only a highly consistent pattern of inter-annual phenology, but also larger and more coherent shifts in timing in response to climate variation than occur among early flowering species.  Collectively, these results indicate that shifts in summer and fall flowering phenology may be just as significant and closely tied to annual climate conditions as spring flowering.  As patterns of mid- and late-season flowering appear quite disconnected from patterns of spring flowering, any attempt to understand the practical ecological ramification of future phenological change on the plant biosphere will need to account for differing seasonal responses to climate variation.