Urban dogwood
by Camilo et al.
Cities often present
challenges to natural and seasonal interactions between native pollinator
populations and native tree species. In particular, the urban heat island
effect may cause native trees to bloom earlier than the emergence of their
pollinators. Over a two-year period, we recorded daily temperatures and
collected bees as they foraged on two native and spring-flowering tree species
(redbud, Cercis canadensis and dogwood Cornus
florida) at two urban sites in Saint Louis, Missouri and two woodland sites
within 55 kms outside the city limits. As anticipated, the ambient temperatures
in the city were higher compared to the exurban sites, and both tree species
began blooming earlier in the urban sites. We collected a total of 54 bee
species. The diversity of pollen-carrying bees (native and naturalized species)
was lower on both tree species at our city sites. The urban redbuds, which were
the earliest to bloom, had a lower bee species diversity compared to rural
redbuds and urban and rural dogwoods. We caught more pollen-carrying bees on
redbud flowers (n = 254) than on dogwood flowers (n = 180). More
pollen-carrying bees were captured in rural sites (n = 333) than in urban sites
(n = 101). Finally, pollen-carrying female bees (n = 365) outnumbered
pollen-carrying males (n = 69) but the percentage of pollen-carrying male to
female bees was disproportionately greater on both tree species in our city (redbud, 38%; dogwood 20%)
compared to our rural sites (redbud, 12%; dogwood 13%). Due to urban plantings of mass-flowering but
non-native trees and shrubs from Eurasia, bees collected on city dogwoods
were more likely to carry the pollen of more than two co-blooming species compared to rural redbuds and
dogwoods. Although the flowering period of both tree species always
overlapped at urban and rural sites, only 34 out of 434 bees captured were
found to carry the pollen of both tree species at the same time.
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