Introduction
Studies examining bee communities within temperate forests have largely
restricted sampling to the understory (Milam et al., 2022) with the
presumption that most bees remain in this lower stratum. However, recent
evidence indicates that bees are vertically distributed within temperate
forests (e.g., Ulyshen et al., 2010; Urban-Mead et al., 2021),
suggesting a potentially large knowledge gap in the ecological role of
these important forest pollinators. Despite this revelation, research
regarding the vertical distribution of bees and other pollinators within
forests is further limited by the difficulty of sampling the high canopy
(Cannon et al., 2021; Cunningham-Minnick et al., in press ).
Current sampling methods reach into the canopy (e.g., Maguire et al.,
2014; Ulyshen et al., 2010), but the canopy-aerosphere interface — a
potentially ecologically important area for bees due to copious floral
resources available — remains unexplored in temperate forests
(Nakamura et al., 2017; Urban-Mead et al., 2021). Thus, our
understanding of pollinator ecology within forests will remain
incomplete until the distribution of forest bee communities along the
entire vertical gradient of vegetation structure is documented.
Moreover, if the current understanding of bee abundance and diversity
patterns in forests are inaccurate, forest management recommendations
for bee conservation may be biased or potentially misguided (Milam et
al., 2022; Urban-Mead et al., 2021), further highlighting the importance
of understanding the distribution of bee communities along the full
vertical gradient of temperate forests, including the canopy-aerosphere
interface.
Bees are expected to be spatially and temporally distributed throughout
temperate forests in response to local resource availability. Studies
have demonstrated that forest bee communities are diverse and vertically
stratified on sun-exposed edges (e.g., Cunningham-Minnick & Crist,
2020), within forests near edges (e.g., Urban-Mead et al., 2021), and
within the forest interior (e.g., Campbell et al., 2018; Milam et al.
2022; Ulyshen et al., 2010) when floral resources of the forest are
available, as well as when they are not. Inferences and observations
further suggest that bees will both forage on floral resources and nest
at different vertical strata within forests (Cunningham-Minnick &
Crist, 2020; MacIvor et al 2014; Russo & Danforth, 2017; Smith et al.,
2019; Sobek et al., 2009; Urban-Mead et al., 2021; Wood et al. 2018).
For instance, Smith et al. (2019) and Wood et al. (2018) found support
through pollen analyses that forest bee communities rely upon floral
resources of dominant tree species. Yet floral resources of herbaceous
and woody species within temperate forests are typically limited to a
spring and early summer phenology, which has been correlated to fewer
late-season bees in the forest understory (Cunningham-Minnick & Crist,
2020). Alternatively, studies have found more bees in the forest
herbaceous layer during spring and more bees in the canopy during the
summer (Cunningham-Minnick & Crist, 2020; Ulyshen et al., 2010),
suggesting that the distribution of forest bees may also shift out of
the understory and into the higher vertical strata of the forest as the
year progresses. However, no studies have examined the bee fauna in the
aerosphere above the forest canopy. Thus, we undertook this study to
determine the extent to which bees occupy the open air above the forest
canopy, how the bee assemblages of this canopy-aerosphere interface
compare in abundance, species richness, and composition with assemblages
at other strata, and how these patterns change with seasonal phenology.