Temporal Synchrony between Ground-Nesting Bees and Spring Ephemerals in an Eastern Hardwood Forest Ecosystem

Abstract

Changes in phenology due to climate warming could disrupt temporal overlap between interacting organisms when previously synchronized species respond to climate change at different rates. Phenologies of plants and insects are known to be sensitive to temperature and/or timing of snowmelt, with warmer temperatures and earlier snowmelt generally advancing spring flowering and emergence; however, some groups of pollinators, such as solitary bees, have been little explored in this context. One striking aspect of eastern hardwood forests is the emergence of understory wildflowers each spring, most of which rely, at least to some extent, on wild native pollinators for seed set. Without an understanding of the environmental drivers of phenology of these species, we have little ability to predict whether pollinators will continue to be well synchronized with flowering as the climate changes. In this study, I determined how spring temperatures and timing of snowmelt influence the phenology of spring wildflowers, activity of bees, and their temporal overlap in Gatineau Park, Québec. From 2013 to 2018, I characterized bee activity phenology and flowering phenology of understory plants in multiple study plots, focusing on early-flowering Anemone spp. and later-flowering Trillium grandiflorum. The sampled bee community was dominated by Andrena, Lasioglossum, and Nomada, all of which have similar activity periods. Degree-day accumulation was a better predictor of Anemone and Nomada phenology than were day of year or snowmelt date, whereas T. grandiflorum appeared to be more sensitive to photoperiodic cues; since day of year was the variable that best described its phenology. Activity periods of Andrena and Lasioglossum were equally well described by degree-day accumulation and by day of year. No taxon’s phenology was best predicted by snowmelt date. Despite these differences among taxa in the identities of the best predictors of phenology, bee activity and plant flowering phenologies responded at similar rates to interannual and among-site variation in snowmelt date and early spring temperature. Temporal overlap between flowering and bee activity was similar over the years of this study and was affected neither by snowmelt date nor by temperature. These results suggest that interacting plant and bee taxa may respond to different environmental variables but still maintain their synchrony under the conditions recorded so far.