Vital connections and altered ecology: Flowering phenology shifts increase co-flowering overlap and heterospecific interaction potential

Matthew W. Austin, Living Earth Collaborative, Washington University in St. Louis, St. Louis, MO, Nicole E. Miller-Struttmann, Department of Biological Sciences, Webster University, St. Louis, MO, Kyra N. Krakos, Department of Biology, Maryville University, St. Louis, MO, Kenneth M. Olsen, Department of Biology, Washington University in St. Louis, St. Louis, MO and Adam B. Smith, Center for Conservation and Sustainable Development, Missouri Botanical Garden, St. Louis, MO

Background/Question/Methods
Global change has altered phenological timing across ecologically diverse taxa. Species interactions, which require temporal coordination of life histories between interacting species, are prone to being affected by phenology shifts. This is particularly manifest in flowering plant communities, where interactions depend on the timing of flowering. While the literature has demonstrated that phenology shifts have altered plant-pollinator interactions, comparatively little is known about how flowering phenology shifts have affected heterospecific plant-plant interactions. One manner in which phenology shifts may increase the potential for interaction between heterospecific flowers is via greater co-flowering overlap. Here, we explore how flowering phenology shifts have affected the potential for heterospecific plant-plant interactions by studying a community of >70 species at Shaw Nature Reserve, a site with a mix of restored prairie and bottomland hardwood in the North American Midwest. We ask the following questions: How have phenology shifts over the past century affected (1) community-wide co-flowering richness and (2) the duration of heterospecific co-flowering overlap? To answer these questions, we integrated a long-term field-collected dataset (i.e., initial survey of flowering dates in 1938-41, followed by our resurvey in 2009-2012) with >3,000 herbarium records spanning the past century.

Results/Conclusions
We find that flowering phenology shifts over the past century have resulted in a higher richness and longer duration of co-flowering overlap. Specifically, we find that co-flowering richness in our community has increased by an average of 36%. This effect is greater in late-blooming species than in early-blooming species, with phenology shifts having induced a 52% increase in co-flowering richness after mid-June, compared to a 10% increase before mid-June. This increase in co-flowering richness is attributable to a general protraction of species-specific flowering times, with the onset of flowering having advanced by ~6.5 days and the cessation of flowering having delayed by ~10.5 days at the community-level. As interactions among species depend on co-occurrence in time and space, the higher richness and longer duration of co-flowering overlap observed in this community likely confer increased potential for heterospecific floral interactions. Species interactions have considerable ecological and evolutionary effects. Our results suggest that by driving phenology shifts, global change is increasing the potential for heterospecific plant-plant interactions and, resultantly, their eco-evolutionary consequences.