Material and Methods
We selected two trees ⁓50 m apart in each of two forest patches on the
campus of the University of Massachusetts-Amherst in Amherst, Mass., USA
(Fig. A1); each pair of trees consisted of a northern red oak
(Quercus rubra L.) and a red maple (Acer rubrum L.). Both
sites were in USDA Hardiness Zone 5a and were characterized by an
herbaceous stratum of ferns (e.g., Dennstaedtia punctilobula(Michx.) T.Moore, Polystichum acrostichoides (Michx.) Schott),
wild sarsaparilla (Aralia nudicaulis L.), white wood aster
(Eurybia divaricata (L.) G.L.Nesom), star flower
(Lysimachia borealis (Raf.) U.Manns & Anderb), Canada mayflower
(Maianthemum canadense Desf.), partridge berry (Mitchella
repens L.), and Solomon’s seal (Polygonatum spp .). The
understory of these sites consisted of brambles (Rubus spp. ),
poison ivy (Toxicodendron radicans (L.) Kuntze), maple-leaf
viburnum (Viburnum acerifolium L.), witch hazel (Hamamelis
virginiana L.), glossy buckthorn (Rhamnus cathartica L.), and
seedlings of the dominant canopy trees (e.g., A. rubrum, A.
saccharum Marshall, Betula lenta L., B. papyriferaMarshall, Q. rubra and Q. alba L.). We chose A.
rubrum and Q. rubra because they are dominant species in forests
of the area and represent different flowering systems and blooming times
that span the duration of most available floral resources in these
canopies. We chose these forest patches due to their accessibility and
general representation of dominant species in forests of the area.
The bee community was sampled using blue vane traps in the understory,
midstory, canopy, and above-canopy strata of the forests at each focal
tree. Three traps were individually attached to a rope hung over a high
branch in the canopy as in Cunningham-Minnick & Crist (2020). Traps
were placed 1, 10, 20, and ⁓30 meters above the ground (Table A1) to
represent the following strata: understory, midstory, canopy and above
canopy (Fig. 1). The trap above the canopy was set one meter above the
tallest leaf bearing branch of each tree using a telescoping hanger
attached to a vertical limb in the crown of the canopy as described in
Cunningham-Minnick et al. (in press ). Traps were deployed on
April 2, 2022, and checked every 1–3 weeks until August 21, 2022, for a
total of 12 checks. Bees were sorted, pinned, and identified to species
by JM using published keys (e.g., Gibbs, 2011; Gibbs et al., 2013;
LaBerge, 1987, 1989; Mitchell, 1960, 1962) and the online source
Discoverlife.org (Ascher & Pickering, 2020); voucher specimens are in
the possession of MC-M. To distinguish differences in microclimate from
other conditions among strata, Onset HOBO® Pendant data loggers (Part
AU-002-64) were placed directly above each trap to record light
intensity and temperature every 10 minutes June 7–21, 2022, to provide
data of daily microclimate conditions and hourly from June 22–August
21, 2022, to represent seasonal change.
Data Analysis : To compare bee abundance and species richness
across vertical strata throughout the sampling season, we built
generalized linear mixed effects models with negative binomial errors
and created 95% confidence intervals of pairwise comparisons for each
response across strata. Models were made using the glmmTMB function in
the glmmTMB package (Brooks et al., 2017) with fixed effects of stratum
(understory, midstory, canopy, above canopy), sample (1–12) as a
continuous variable, and their interaction. We allowed the model
intercept to vary by tree to account for tree-specific differences, and
accounted for differences in sampling effort by including an offset term
of the log of the trap deployment duration (days). Significance of
interaction terms was evaluated by likelihood ratio tests; simulated
model residuals through the DHARMa package were used to evaluate overall
model fit (Hartig, 2020). Post-hoc comparisons were made using the
confint and glht functions in the multcomp package (Hothorn et al.,
2008). Differences in bee species composition among strata were
visualized with non-metric multidimensional scaling ordinations
performed on a species occurrence matrix of Sorensen distances using the
metaMDS function in the vegan package (Oksanen et al., 2019); statistics
and p-values were derived using the pairwiseAdonis function with a
Bonferroni adjustment for multiple comparisons (Arbizu, 2017).