Spring wildflower phenology and pollinator activity respond similarly to climatic variation in an eastern hardwood forest

Climate warming could disrupt species interactions if
organisms’ phenologies respond to climate change at different rates.
Phenologies of plants and insects can be sensitive to temperature and timing of
snowmelt; however, many important pollinators including ground-nesting bees
have been little studied in this context. Without knowledge of the
environmental cues affecting phenologies of co-occurring species, we have
little ability to predict how species assemblages, and species interactions, will
be affected by climate change. Here, we studied a hardwood forest understory
over six years, to determine how spring temperatures, snowmelt timing, and
photoperiod influence the phenology of two spring wildflowers (Anemone spp. and Trillium grandiflorum), activity of ground-nesting bees, and their
temporal overlap. Surface degree-day accumulation was a better predictor of phenology
for Anemone spp. (plant) and Nomada (bees) than were day of year (a
proxy for photoperiod) or snowmelt date, whereas Trillium flowering appeared most sensitive to photoperiodic cues.
Activity periods of Andrena and Lasioglossum bees were equally well described by degree-day accumulation and day of
year. No taxon’s phenology was best predicted by snowmelt date. Despite these
differences among taxa in their phenological responses, timing of bee activity
and flowering responded similarly to variation in snowmelt date and early
spring temperatures. Furthermore, temporal overlap between flowering
and bee activity was similar over the years of this study and was unaffected by
variability in snowmelt date or temperature. Nevertheless, the
differences among some taxa in their phenological responses suggests that
diverging temporal shifts are a possibility for the future.