Associate Professor University of Oregon Eugene, United States
Abstract: Perhaps the most well-documented and dramatic effects of climate change on plant communities thus far are the changes in plant phenology. Extensive observations show that species are generally advancing their phenology, but that there is substantial variation in the magnitude and direction of shifts, even among co-occurring species. These species-specific phenological shifts may lead to novel temporal overlap between interacting species, but the implications of such novel overlap for species performance and coexistence are poorly understood.
In this study we conducted a unique experimental manipulation of climate to examine how climate warming may create novel temporal overlap between co-occurring plant species and ask how that may affect species interactions and coexistence. Specifically, we grew seven annual forbs native to Pacific Northwest Prairies in pairwise competition gradients in ambient and warmed conditions, followed their phenology from germination through end of reproduction, and quantified plant fitness as a function of their competitive neighborhood. We fit Beverton-Holt models to estimate both direct and indirect (via competition) effects on species performance. We hypothesized that species-specific responses to warming would cause novel temporal overlap between species, which would create changes in the competitive dynamics in the community. We found significantly different phenological shifts among species and among phenophases. Only early flowering species significantly shifted their flowering phenology with warming and later phenophases were more impacted by warming. Plagiobothyrs nothofulvus and Collinsia grandiflora both advanced peak flowering, with C. grandiflora additionally advancing first flower, and lengthening its flowering time. Plectritis congesta, the earliest flowering species, exhibited both an earlier and shorter flowering period, causing the greatest shift in flowering overlap with all other species. This species also experienced the largest shifts in competitive effects under warmed conditions, potentially due to the shifts in temporal overlap with competing species. All three early flowering species had higher fecundity in the absence of competition with warming, but with competition only C. grandiflora performed better in warmed conditions.
Overall, this work shows how warmer temperatures can shift species’ phenological overlap and alter competition. Together with direct effects of warming on species’ performance, these indirect effects via altered competition led to changes in predicted species coexistence. To understand how species will be affected by climate warming, it will be important to understand not only the phenological shifts of a species but shifts in relation to competitors to predict species coexistence.
