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 both plants and insects are known to be sensitive to temperature and/or timing of snowmelt, with warmer temperatures and early snowmelt 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 cues affecting activity of these pollinators, we have little ability to predict whether pollinators will continue to be well synchronized with flowering as the climate changes. In this study, we 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, Canada. From 2013 to 2017, we characterized bee activity phenology (using pan traps) and flowering phenology of understory plants at ten plots, focusing on Anemone spp. (the earliest-flowering plants at our plots) and later-flowering Trillium grandiflorum.
Results/Conclusions
Bee abundance was dominated by Andrena, Lasioglossum and Nomada spp, all of which have similar activity periods. Degree-day accumulation was the best predictor of Anemone phenology, but, despite significant interannual variation in bee and plant phenologies, neither temperature nor snowmelt day significantly predicted phenology of T. grandiflorum or the three bee taxa. Bee activity and plant flowering phenologies responded at similar rates to snowmelt but at different rates to April temperature; specifically, Lasioglossum activity advanced faster than plant flowering. Temporal overlap between flowering and bee activity was similar over the years of this study, and the number of overlapping days was affected neither by snowmelt day nor by temperature. These results support the idea that early-flowering plants are more responsive than later-flowering species to temperature variation. Furthermore, interacting plant and bee taxa may respond at different rates to the same environmental variables but still maintain their synchrony under the conditions we have recorded so far. Early snowmelt and warmer springs could increase temporal overlap between bees and flowers because of the extended period of bee activity, but early-emerging bees could experience a disruption in food supply.