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1234 BioScience •December 2021 / Vol. 71 No. 12 https://academic.oup.com/bioscience
The Importance of Forests
in Bumble Bee Biology and
Conservation
JOHN M. MOLA , JEREMY HEMBERGER, JADE KOCHANSKI, LEIF L. RICHARDSON, AND IAN S. PEARSE
Declines of many bumble bee species have raised concerns because of their importance as pollinators and potential harbingers of declines among
other insect taxa. At present, bumble bee conservation is predominantly focused on midsummer flower restoration in open habitats. However,
a growing body of evidence suggests that forests may play an important role in bumble bee life history. Compared with open habitats, forests
and woody edges provide food resources during phenologically distinct periods, are often preferred nesting and overwintering habitats, and
can offer favorable abiotic conditions in a changing climate. Future research efforts are needed in order to anticipate how ongoing changes in
forests, such as overbrowsing by deer, plant invasions, and shifting canopy demographics, affect the suitability of these habitats for bumble bees.
Forested habitats are increasingly appreciated in the life cycles of many bumble bees, and they deserve greater attention from those who wish to
understand bumble bee populations and aid in their conservation.
Keywords: bumble bees, woodlands, forest conservation, insect decline, habitat complementarity
Bumble bee conservation and management has
garnered considerable attention because of bees’ role as
pollinators of economically and ecologically important crops
and wild plants. The precipitous decline of several bumble
bee species has been documented in the twenty-first century,
raising alarm about the viability of these charismatic species
(Cameron and Sadd 2020). Because of this, bumble bees
have become a focal taxon for understanding and preventing
the loss of insect biodiversity more broadly (Goulson and
Nicholls 2016, Wagner etal. 2021). Threats to bumble bee
populations include habitat loss, novel pathogen exposure,
climate change, and pressures from intensive agriculture,
such as pesticide applications (Cameron and Sadd 2020).
One of the primary tasks for bumble bee conservation is
developing a greater understanding of the habitat require-
ments of species throughout their life cycle and incorporat-
ing that knowledge into restoration and management plans.
Successful bumble bee conservation relies on an under-
standing of the parts of landscapes used throughout bees’
life cycles (figure 1). Most bumble bees have an annual social
life cycle, with queens emerging in early spring as solitary
individuals. These lone queens seek nesting sites and then
begin foraging for the initial pollen and nectar resources
needed to establish their nests. As colonies grow by pro-
ducing successive cohorts of workers across the growing
season, they demand more resources. Successful colonies
begin producing males and gynes late in the growing sea-
son. Finally, colonies senesce, with only gynes seeking sites
to establish hibernacula and overwinter. Because bumble
bees have relatively long flight seasons, they may make use
of different land cover types that contrast or complement
in their value over time by providing resources at different
points in the season (Mandelik etal. 2012) or vary in their
abiotic conditions. Forests can provide seasonally distinct
floral resources from other habitats (e.g., Mola etal. 2021)
and may be primary sites of nesting and overwintering
(reviewed in Liczner and Colla 2019). As such, forests may
serve as complementary habitats, supporting bumble bees in
ways that are less readily apparent than midsummer foraging
in open habitats but nonetheless critical.
Research on bumble bees has been primarily focused on
their midsummer stage, when workers reach peak abun-
dance and are readily found on flowers (Goulson 2009).
Understandably, this focus arises because that is when the
most individuals can be observed as colony sizes are at their
peak and numerous workers can be found foraging. These
types of studies have revealed important insights into the
habitat needs and stressors of bumble bees, such as the rela-
tionship between landscape context and bumble bee diver-
sity (e.g., Hines and Hendrix 2005) or patterns of disease
prevalence (e.g., McNeil et al. 2020). However, this focus
commonly overlooks other key points in the bumble bee
BioScience 71: 1234–1248. Published by Oxford University Press on behalf of the American Institute of Biological Sciences 2021. This work is written by (a)
US Government employee(s) and is in the public domain in the US.
https://doi.org/10.1093/biosci/biab121
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https://academic.oup.com/bioscience December 2021 / Vol. 71 No. 12 •BioScience 1235
life cycle—namely, the solitary phase of life for wild queens
and males, early season foraging, nesting, mating, and over-
wintering. Despite their lower numerical abundance, recent
studies have shown that these phases of bumble bee life his-
tory are especially important in determining the trajectory
of their populations (Crone and Williams 2016, Carvell etal.
2017). Because forests in many regions contrast with open
habitats in terms of their flowering phenology, structural
features, and abiotic conditions, these habitats may be par-
ticularly relevant to the understudied portions of the bumble
bee life cycle. When considering the bumble bee year more
broadly to include early floral resources or nesting and over-
wintering habitat, the role of forests, forest edges, and other
woody habitats becomes more central in our understanding
of bumble bee biology.
Forests can vary greatly along axes of canopy openness,
mesic versus xeric conditions, successional stage, and more.
In some instances, forests are unsuitable habitats for bumble
bees (e.g., unbroken swathes of closed canopy evergreen for-
est), but in many landscapes, a variety of forest types such as
open canopy mixed conifer forests (Mola etal. 2020a), oak
woodlands (Wray etal. 2014), aspen groves (Gonzalez etal.
2013), early successional (Taki et al. 2013), or old growth
forests (Proesmans etal. 2019) may all play a role in bumble
bee ecology for all or part of their life cycle. Readers should
interpret the term forest broadly to include a range of vari-
ability and not all types are going to function in the same
way as bee habitat (e.g., some forest types may be quality
nesting, overwintering, and foraging habitat, whereas others
may only be suitable for overwintering and offer few floral
resources). For the purposes of this review, we define forests
relatively broadly to include a variety of landcovers contain-
ing woody plant species. We consider work focusing on for-
est interiors and edges, riparian corridors, open and closed
canopy alike. We hope our discussion will allow readers to
combine knowledge from their forest type or woody habi-
tat of interest to bumble bee life history to make informed
ecological inferences.
In this article, we consider the role of forests in bumble
bee life cycles and its importance for conservation plan-
ning. First, we review evidence from landscape-level studies
about the connection between forests and the abundance
of bumble bee species. We relate these trends to the life
history of bumble bees to develop general expectations for
the relationship between forests and bumble bees. Next, we
consider threats to forests that may limit their suitability as
bumble bee habitat and explore how forests can be managed
to support bumble bees. We contextualize our discussion by
highlighting an endangered bumble bee species, the rusty-
patched bumble bee (Bombus affinis; box 1). Because there
are still many uncertainties as to the role of forests in bumble
bee conservation, we conclude with a discussion of major
research themes relating to bumble bees and forests that are
likely to inform conservation efforts and improve our under-
standing of the basic biology of bumble bees.
How bumble bees use forests
The overwhelming majority of bumble bee observations
occur in open areas, so why argue for the importance of
forests for these species? Bumble bees use forested habitat in
different ways throughout their life cycle (figure 1). Casual
observations of bumble bees and many bumble bee monitor-
ing programs do not observe these bumble bees in forests
because they concentrate primarily on summer bumble
bee communities, largely composed of foraging workers.
However, the bumble bee life cycle is complex (figure 1),
and evidence points to an outsized importance of some of
the more cryptic life stages as drivers of bumble bee demog-
raphy (Carvell etal. 2017). These life stages often occur in
forests.
Forage. Bumble bees have a long foraging season, often span-
ning the flower production of many species or habitats over
several months (Williams and Osborne 2009, Timberlake
et al. 2019). Therefore, populations are sensitive not only
to the total amount of resources but also their availabil-
ity through time (Carvell et al. 2017, Malfi et al. 2019,
Hemberger etal. 2020). In most species, hibernating queens
emerge in early spring, when the earliest flowers emerge
(e.g., willows and forest understory herbs), and complete
colony reproduction in late summer or early fall (figure 1).
Colonies require a continuous supply of floral resources
because they do not store large amounts of pollen or nectar
(Timberlake etal. 2019). The availability of floral resources
in the early season, when queens are establishing colonies
or the first workers begin foraging, is especially important
for colony success (Carvell etal. 2017, Watrous et al. 2019,
Figure 1. Bumble bee life cycle with emphasis on the role of
forests as sites of foraging, nesting, and overwintering. This
example is based on a temperate deciduous forest; forests
can provide critical sources of early season forage within
tree canopies or via forest floor ephemerals. Early summer
colonies begin developing in a variety of substrates such
as underground cavities or hollow logs. Although many
types of forests decline in their importance as foraging
sites in the summer, forests again become common sites of
overwintering queens in the fall through winter.
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Woodard et al. 2019). When a resource gap occurs at the
establishment phase colonies rarely recover from it (Malfi
etal. 2019).
Because forest herbs and trees often flower before plants in
other habitats, they may be especially important to queens,
colony establishment, and forest specialist species. In many
areas, the first flowering resources are found within forest
canopies or understories (Inari et al. 2012, Bertrand et al.
2019, Proesmans etal. 2019), and some of the last are in for-
est shrubs, edges, or in canopy gaps (Walters and Stiles 1996,
Sakata and Yamasaki 2015). In Illinois, in the United States,
the peak estimated flowering date of plants used by bumble
bees is 81 days earlier in forests than in grasslands or wet-
lands (Mola etal. 2021). In these regions, records of spring
bumble bee queens overlap most strongly with flowering in
forests (Mola etal. 2021). In Japan, bumble bee population
cycles are driven in large part by the availability of spring-
time resources in forest canopies the previous season (Inari
etal. 2012). In Europe, tree pollens represent roughly 80% of
early season pollen loads in Bombus terrestris (Kämper etal.
2016, Bertrand etal. 2019), suggesting a critical role of for-
ests in the early foraging of this generalist species. In eastern
North America, where pathogens are thought to be respon-
sible for the decline of some bumble bee species, Bombus
impatiens in habitats with higher spring floral abundance
(predominantly forests), had lower pathogen loads (McNeil
etal. 2020). As well, some species such as Bombus vagans,
Bombus. ardens, or Bombus terricola complete almost their
entire colony life cycles within forests, specializing on plants
within the canopy and understory.
Within the world’s deciduous forests, resource abun-
dance can be high early in the spring, when canopy trees
and shrubs flower and leaf-out has not yet shaded spring
ephemerals. However, within open canopy forests, flowering
phenology may have a different distribution. For example, in
the open canopy conifer forests of the western United States,
species such as mountain pennyroyal (Monardella odoratis-
sima) and waterleaf phacelia (Phacelia hydrophylloides) can
flower well into the bumble bee foraging season (Mola etal.
2020a). Some temperate deciduous forests also have late-
season herbaceous flowers used by bumble bees (e.g., Kato
etal. 1990), but these flowers may be less common than in
decades past because of degradation by deer browse and
other factors (Sakata and Yamasaki 2015). Considering both
ends of the flowering season is important for bumble bees as
the abundance of late-flowering resources is associated with
elevated gyne and male production by colonies (Rundlöf
etal. 2014) and may be important in explaining interannual
variability in colony abundance (Timberlake et al. 2020).
Given these examples, it seems likely that overall woody
habitats provide resources that are complementary or at least
supplemental to those of adjacent open habitats.
In addition to the total availability of resources, a pref-
erence for different plant species because of nutritional
composition, resource return rate, or other factors is worth
considering. For example, bumble bees selectively forage
to balance dietary protein:lipid ratios (Vaudo et al. 2016,
Woodard and Jha 2017). Rivers-Moore and colleagues
(2020) documented a preference among bees, including
bumble bees, for certain plants within woody habitats over
those available in open habitats although exactly why these
pollens were preferred was not identified, but it is possible
these patterns are driven by phylogenetically conserved
foraging preferences (Wood etal. 2021). At present, it is not
clear if colonies perform better when accessing resources in
woody environments over those in open habitats. One study
showed that B. impatiens colonies experimentally placed in
forest, open, and forest-edge habitats achieved similar nutri-
ent ratios, but the colonies located within forests did not
grow as rapidly (Vaudo et al. 2018). By contrast, Pugesek
and Crone (2021) found that wild B. impatiens colonies
monitored in forest patches had higher gyne production
than those found in meadows, but these forest fragments
were relatively small. Long travel distances limit productiv-
ity and reproductive output (Cresswell etal. 2000), but given
the permeability of forests by foragers (Kreyer etal. 2004,
Mola etal. 2020a), these limitations are likely due to total
travel distance and resource availability rather than connec-
tivity (Herrmann etal. 2017). Understanding how forested
and open habitats complement bumble bee diets beyond
raw abundance or phenological complementarity is likely of
great importance for informing habitat management plans
targeting pollinators. However, more work is needed to
understand habitat differences in resource quality and their
consequences for bumble bees.
Nesting and overwintering. Most bumble bee conservation
efforts are focused on increasing available forage in the form
of floral resources (Dicks etal. 2015, Requier and Leonhardt
2020); however, this is only one component of bumble bee
habitat. The degree to which overwintering and nesting
resources limit bumble bee populations is an ongoing area
of debate (Roulston and Goodell 2011, Liczner and Colla
2020), but the provision of at least some habitat within the
landscape is a necessity. Unlike foraging habitat, forests are
commonly recognized as sites of nesting and overwintering
within management documents and restoration initiatives
(e.g., the draft recovery plan for Bombus affinis; USFWS
2019).
Bumble bees nest both below and above ground. Bumble
bee nests, although they are cryptic, may be found through
observations of spring nest-searching queens, careful obser-
vation of workers returning from foraging bouts, scent-
detecting dogs, and radiotelemetry (Svensson et al. 2000,
Mola and Williams 2019, Liczner etal. 2021). Preferred and
actual nesting locations can be inferred indirectly on the
basis of the nest searching behaviors of bumble bee queens
and genetic mark–recapture method. On the basis of the
available evidence, forests seem to be favorable and com-
mon nesting habitats for many species (Lanterman et al.
2019, Liczner and Colla 2019). In the US, nest searching
bumble bee queen abundance was positively associated
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with the amount of forest within 1 kilometer of study sites
(Lanterman et al. 2019). Likewise, at multiple locations,
bumble bee colony density was associated with a greater
amount of forest or woodland in the landscape (Jha and
Kremen 2013, Pfeiffer etal. 2019), with the authors suggest-
ing these trends are due to the availability of nesting habitat.
In Europe, nest searching bumble bee queens are often found
within wooded areas or alongside edge habitats (Svensson
etal. 2000, Kells and Goulson 2003). A community science
project in the United Kingdom documented high nest densi-
ties along linear features such as hedgerows and forest edges
but lower density in grassland and forest interiors (Osborne
etal. 2008). However, this study did not correct for differ-
ences in detection rates between habitats that may be lower
in forests (Pugesek and Crone 2021). It is worth noting that
the taxonomic and geographic coverage of studies on nesting
and nest seeking behaviors is currently somewhat limited.
Studies conducted in Europe postagricultural intensification
(Svensson etal. 2000, Kells and Goulson 2003) may overrep-
resent subgenera that are more associated with woodlands
(e.g., Pyrobombus, Bombus sensu stricto), and surface nesting
in open habitat is common for many species as well (Liczner
and Colla 2019). Regardless, it appears that forests, forest
edges, and the many microhabitats that they provide (Ouin
etal. 2015) are common sites of nesting for many species.
These observations, both direct and indirect, suggest that
bumble bee nesting sites are often located within forests.
Do forests also increase the success of those nests? To date,
the evidence for this is sparse and contrasting. Pugesek and
Crone (2021) found that B. impatiens nests in similar densi-
ties in open habitat and forests but that the reproductive suc-
cess of colonies within forests was nearly three times higher.
In contrast, in an experimental study of the rates of preda-
tion on artificially placed bumble bee nests, nests placed in
forests experienced greater predation than those placed in
open habitats (Roberts etal. 2020). More work is needed to
understand the fitness consequences of bumble bee nests
placed in forested and open habitats.
Direct quantification of overwintering is rare, although
scattered records suggest that forests are the most common
overwintering habitats for many bumble bees (Liczner and
Colla 2019). Overwintering queens are commonly docu-
mented in shaded areas near trees (Sladen 1912, Plath 1934,
Alford 1969). On the coast of California, Bombus vosnesen-
skii queens were found overwintering in well-composted
duff layers beneath cypress trees but not in adjacent open
habitats (Williams etal. 2019). It is possible that the sheltered
environments under trees provide coverage from rain or
buffer against poor environmental conditions. Alternatively,
undisturbed litter layers may be less common in open habi-
tats, resulting in less frequent overwintering (Liczner and
Colla 2019). Rotting logs and other woody debris may be
important overwintering substrate (Frison 1926, Alford
1969), but these microhabitats may be absent from early suc-
cession forests. Generally, much is still to be learned about
the importance of different habitats to overwintering and the
success of individuals overwintering in different substrates,
but it is recognized that forests are important habitats for the
overwintering of many species.
Abiotic effects
Forested environments have distinct abiotic conditions
compared with open habitats such as grasslands and
meadows. Bumble bee abundance can vary considerably
from year to year, based in part on the direct and indirect
impacts of annual climate conditions (Ogilvie etal. 2017),
and forest microsites may buffer against this variation. At
the same time, human use of forest differs considerably
from that of open spaces that are more likely to experi-
ence impacts from agrochemicals (Bentrup etal. 2019). As
such, it is useful to consider beyond the biotic effects of
wooded habitats and consider the role that microclimates
and physical attributes of forests may play in bumble bee
biology and conservation.
Microclimates. The foraging of bees and other insects can be
strongly influenced by weather conditions, predominantly
air temperature, precipitation, and wind speed. Pollinator
energetic costs are increased in high winds, resulting in
reduced foraging efficiency and pollination success (Vicens
and Bosch 2000, Brittain et al. 2013). Forested areas can
reduce wind speed in adjacent environments and moderate
air temperatures in both natural and urban environments.
Both factors may reduce the energetic costs of foraging
for bees within or adjacent to forests (Papanikolaou etal.
2017). For example, air temperatures are warmer downwind
of windbreaks (McNaughton 1988), potentially resulting
in longer available windows of foraging. Although they do
not test abiotic conditions directly, Gonzalez and colleagues
(2013) suggested one possible explanation for their finding
that bumble bees were more common in aspen groves than
adjacent grasslands was improved microclimatic conditions
under the tree canopy—namely, reduced temperatures in
summer. Temperature differences between forested and
open habitats should be considered not only for foraging,
but also nesting and overwintering. Heat waves have been
suggested as a stressor for bumble bee colonies (Rasmont
and Iserbyt 2012). Nests within shaded forested areas may
be better protected from these extreme temperature swings
compared with open field habitats. Maintenance of exist-
ing forested areas or the planting of windbreaks within
agricultural landscapes can assist in the delivery of pollina-
tion services by bumble bees and potentially buffer against
warming temperatures and associated unfavorable foraging
conditions.
Correlations between bumble bees and forest cover
Many landscape-scale studies have looked at the relationship
between forest cover and bumble bee abundance or diversity.
Generally, increased landscape complexity or heterogeneity
is positively correlated with pollinator diversity and abun-
dance suggesting these landscapes offer more patches for
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habitat specialists (Tscharntke etal. 2012, Mallinger et al.
2016). Several studies demonstrate clear positive relation-
ships between forest cover and bumble bee abundance or
diversity (Wray etal. 2014, Rivers-Moore etal. 2020, Sõber
et al. 2020), spring queen abundance (Lanterman et al.
2019), or estimated colony density from molecular analysis
(Jha and Kremen 2013, Pfeiffer etal. 2019). Negative rela-
tionships between bumble bee abundance and forest cover
have also been reported (Winfree et al. 2007, Mandelik
et al. 2012), as well as contrasting results among species
(Richardson etal. 2019).
The variation in observed associations between bumble
bee abundance and forest cover is likely driven by variation
in the amount of forest cover considered in a study and the
bumble bee species involved (figure 2), as well as methods
differences in bumble bee surveys. A general model of
bumble bee–forest associations might consider a continuum
of forest density or fragmentation as a primary niche axis for
bumble bees and acknowledge that different species might
associate with species-specific optimal levels of forest den-
sity or fragmentation (figure 2).
Different bumble bee species have differing habitat
optima along a forest gradient, resulting in landscapes
with higher heterogeneity (i.e., intermediate levels of for-
est and open habitat) being most favorable for total spe-
cies richness (figure 2). Quantitative surveys support this
notion, with some bumble bees more associated with forest
habitats than others (Richardson et al. 2019). In cases in
which forests are very dense and few flowers are present
within the habitat, negative relationships
begin to arise (e.g., Loffland etal. 2017)
with bumble bees only found in natural
or artificial forest gaps (Kolosova et al.
2016, Moquet et al. 2017). In contrast,
the amount of forest and forest edge in
the surrounding landscape can posi-
tively predict abundance (Wray et al.
2014, Banaszak and Twerd 2018, Rivers-
Moore et al. 2020). Fragmentation of
forests can favor generalist bumble
bees associated more with open habi-
tats, resulting in an overall reduction
in species diversity and the loss of for-
est specialists (Gómez-Martínez et al.
2020). Some species may have an affinity
for forests and use woody habitats for the
majority of their colony development,
such as the aptly named tree bumble
bee (Bombus hypnorum; Crowther etal.
2014). Other species may only use for-
ested habitats seasonally. For example,
in Japan, B. ardens is found in forests
for most of its colony cycle but Bombus
diversus visits forests only early in the
season before switching to open habitat
(Ushimaru etal. 2008, Inari etal. 2012).
In addition to variation among species, differences in
survey methods can bias observed associations between
bumble bees and forest habitats. First, because bumble bees
may rely on forests the most early in their life cycle, surveys
later in the season may affect the observed relationship
between a bumble bee species and forest habitat (Proesmans
etal. 2019). Notably, even in examples in which a positive
bumble bee–forest correlation is found, surveys were con-
ducted in summer when workers are present, well after the
point in time when forests may be of most importance to
bumble bees (Mola etal. 2021). Second, the scale of surveys
may influence observed relationships. For example, Moquet
and colleagues (2017) found positive relationships between
bumble bee abundance and surrounding spruce forest cover,
but argued the increased abundance was due to concentra-
tion effects of bumble bees on limited forage resources found
only in gaps and not due to forest cover per se.
Of note is that a substantial portion of the studies docu-
menting foraging by bumble bees in forests demonstrate
the use of plants within natural or artificial forest edges
or ecotones, rather than deep within forests themselves
(McKechnie etal. 2017, Sõber et al. 2020, Lee etal. 2021).
It is unclear whether bumble bees prefer forest edges or
whether this connection is caused by modern-day changes
to forest structure. In high-quality old growth forests, there
may be rich understory resources, but in many modern or
degraded forests, there may not be sufficient solar radiation
to sustain favorable foraging temperatures or herbaceous
cover beyond the forest edge (Proesmans etal. 2019).
Figure 2. A hypothesized relationship between forest cover and the abundance
of bumble bees varying in their association with forests. Some species, such as
Bombus vagans, are strongly associated with forest throughout their range and
are expected to be present in high abundance at more densely forest sites and
then absent from open areas far from forests. Others show opposite patterns,
being associated with open habitats, such as Bombus fervidus. Generalist
species may be present across the continuum of forest types, but may reach
peak abundance at intermediate levels of forest cover or have a more uniform
distribution. Example species follow from the results of Richardson and
colleagues (2019).
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In managing forests to support bumble bees, it may be
important to consider how particular bumble bees use
forests. For example, B. terrestris is invasive throughout
Hokkaido, Japan, where it displaces native species in open
or agricultural habitats but not in forested areas (Ishii etal.
2008, Nukatsuka and Yokoyama 2010). In general, phenol-
ogy varies substantially among bumble bee species, and, at
least in temperate habitats, bumble bee species that emerge
earlier in spring are more likely to rely on floral resources
in forests (Colla and Dumesh 2010, Mola etal. 2021), sug-
gesting that there may be predictable differences in how
forest management may affect different bumble bee species.
However, more work is needed to show the generality of
a positive correlation between forest-affiliation and early
phenology. Of final note in interpreting the correlations
between bumble bees and forest cover is the need for care-
ful consideration of the problem of shifting baselines (Pauly
1995, Collins etal. 2020). In contemporary landscapes, the
large-scale elimination of grasslands (Samson and Knopf
1994, Wesche etal. 2012), and therefore, open habitat associ-
ated species may have already occurred, potentially biasing
modern surveys toward more forest associated species. As
such, some caution is warranted in interpreting a general
pattern of increased landscape-scale forest cover leading to
increased bumble bee abundance and diversity. However, it
seems fair to conclude that heterogenous landscapes com-
posed of a mix of forested and open landscapes are likely to
support abundant and diverse bumble bee communities in
most regions.
Threats to forests as bumble bee habitat
Forests throughout the world are changing rapidly. For
example, forests are affected by changing land use, climate
change, invasive species, and fires (Lindenmayer etal. 2012,
McDowell etal. 2020). Many of these changes are likely to
affect the important roles that forests play in the lives of
bumble bees, sometimes positively and sometimes nega-
tively (table 1).
Table 1. Threats to forests and their potential impact on bumble bee populations.
Threat Hypothesized impact on bumble bees Key references
Fragmentation • Loss of forest habitat specialists, increase in generalist
species
Ouin etal. 2015, Proesmans etal. 2019, Gómez-
Martínez etal. 2020
• Changes in edge microclimates affecting foraging,
nesting, and overwintering conditions
Loss of old growth forests • Change in forest floor structure suitable for overwintering Varhola etal. 2010, Lindenmayer etal. 2012,
Jackson etal. 2014, Proesmans etal. 2019
• Loss of understory herbs
• Loss of old trees, stumps, and nesting cavities
Overbrowsing by deer • Loss of bumble bee forage plants Shelton etal. 2014, Sakata and Yamasaki 2015,
Nakahama etal. 2020
• Change in forest structure may affect suitability of nesting
and overwintering, directionality unknown
Introduced earthworms • Changes to forest floor structure, moisture, and soil
compaction may affect overwintering and nesting
Bohlen etal. 2004, Laushman etal. 2018
• Loss of bumble bee forage plants
Wild and prescribed fire • Varied impacts depending on forest type, presumed
increases in floral abundance due to increased light
levels and postfire bloom
Burkle etal. 2019, Carbone etal. 2019, Galbraith
etal. 2019
• Potential mortality of queens and colonies during
overwintering or nesting
• Loss of microclimate buffering if canopy severely reduced
Logging • Varied impacts depending on logging intensity, type of
machinery used, seasonality, soil disturbance, etc., likely
increases in forage and bee abundance, especially along
edges
Pengelly and Cartar 2010, Jackson etal. 2014
• Potential long-term negative impact due to loss of
microhabitat structure
Invasive plants • Loss of floral abundance although some invaders are
suitable forage
McKinney and Goodell 2010, Hanula etal. 2016,
Gibson etal. 2019
• Increased shade reduces foraging
Changing flowering
phenology
• Phenological mismatch Burkle etal. 2013, Kudo and Cooper 2019
Pesticide concentration • Potential transfer to overwintering queens in soil Hladik etal. 2016, Bentrup etal. 2019
• Uptake into nectar and pollen
Note: The key references are not intended to be an exhaustive list. The italicized references are about the threat but do not directly study bees.
See the main text for further details.
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A common change in forests that could threaten bum-
ble bees is the loss of understory flowers. For example,
declines in floral resource availability in forests have been
documented in Illinois, in the United States, driven in
part by the loss of important spring flowering plants such
as Geranium maculatum and Hydrophyllum virginianum
(Burkle etal. 2013, Augspurger and Buck 2017, Mola etal.
2021). A decline in understory flowers may be caused by
overbrowsing by deer or cattle grazing, canopy crowding,
plant invasions, and other factors such as earthworm intro-
ductions. Overbrowsing by deer can reduce the abundance
of understory herbs, as has been documented widely across
the eastern United States (Frerker etal. 2014, Shelton etal.
2014) and Japan (Sakata and Yamasaki 2015). Although not
as widely studied within woodlands, and seemingly with no
studies focused on impacts on bees, cattle grazing can simi-
larly decrease the abundance of native perennial wildflowers
and increase exotic plant invasion (Pettit etal. 1995, Mabry
2002). Overbrowsing can reduce spring ephemeral availabil-
ity and autumn flowering plants critical for fat acquisition
by gynes before overwintering (Sakata and Yamasaki 2015).
Restoration efforts aimed at reducing deer browse may be
successful. In a study in grasslands, Nakahama and col-
leagues (2020) found the installation of deer fencing resulted
in increased floral abundance and increased bumble bee and
butterfly abundance and diversity within fenced areas about
3–8 years after installation. They caution, however, that
other efforts to install deer fencing may be unsuccessful if
the habitat has already been substantially degraded (Tamura
2010, Okuda et al. 2014). In those instances, deer fencing
may need to be combined with additional efforts such as
native plant seeding.
Encroachment from invasive shrubs can also reduce
flower production within forests with downstream effects on
pollinator populations (Miller and Gorchov 2004, McKinney
and Goodell 2010, Hanula etal. 2016). The effects of invasive
plants on pollinators can vary substantially according to the
context of the invasion. Invasive plants may, at times, be
the preferred floral resources of bumble bees (e.g., Gibson
etal. 2019), but widespread invaders such as Chinese privet
(Lingustrum sinense) and Amur honeysuckle (Lonicera
maackii) can crowd forest understories and reduce total
floral diversity or flowering duration even if the invader is a
suitable food source itself (reviewed in Hanula etal. 2016).
Experimental removal of invasive plants in these habitats
can result in improved foraging conditions for bees and
rapid recovery of bee communities (Hanula and Horn 2011).
The net benefit of biomass removal on bumble bees may be
highly context specific and requires careful consideration of
the study system (Gibson etal. 2019).
Some human activities within forests such as limited log-
ging can have positive impacts on bumble bee forager abun-
dance by opening canopies and more closely approximating
conditions within mature forests with well-established gap
dynamics (Pengelly and Cartar 2010, Jackson et al. 2014,
Proesmans etal. 2019, Lee etal. 2021). However, these same
activities may have negative effects on the availability of
microhabitats for nesting and overwintering; because of this,
the net impact of long-term changes in forest dynamics are
unknown. These effects are yet to be tested but provide clear
research pathways for understanding how changing forest
dynamics and associated management activities will affect
bumble bee populations over the next several decades.
The direct and indirect negative impacts of pesticides,
fungicides, and herbicides on bees are well documented
(Lundin etal. 2015, McArt et al. 2017, Motta et al. 2018).
Bees in agricultural landscapes may be exposed to pesti-
cides directly, and drift carries different pesticides different
distances from the places where they are applied (Hladik
etal. 2016). Pesticide residues may reach forests via surface
or subsurface water movement, airborne drift, or volatility.
Movement of herbicides via volatiles is worthy of further
consideration, because injury to plants up to 250 meters
from application sites has been documented (Soltani et al.
2020). These herbicides could injure floral resources within
forests resulting in reduced foraging opportunity for bees
(Bohnenblust et al. 2016, Florencia et al. 2017). Previous
work has suggested that forests may mitigate drift by cap-
turing agrochemicals and reducing wind speeds (reviewed
in Bentrup et al. 2019). However, the benefits of forests
trapping these compounds are only positive if the forest is
seen as matrix and not as primary habitat itself. If, instead,
forests are bumble bee habitat these effects may be negative
as drift or damage may concentrate in these areas. Future
work investigating how forest habitat quality for bumble bee
foraging, nesting, and overwintering is affected by drift from
croplands is needed to understand how pesticides affect
forests as bumble bee habitat. Overwintering may be of
particular importance, because queens may come in direct
contact with residues within soils for extended periods of
time, which has been shown to negatively affect solitary bee
development (Anderson and Harmon-Threatt 2019).
Several other factors change conditions within forests
substantially and may affect bumble bee populations, but
evidence is currently lacking to address this. Introduced
European earthworms in hardwood forests of the Upper
Midwest, in the United States, have resulted in changing
soil and leaf litter conditions with negative consequences for
understory forbs (Bohlen etal. 2004, Laushman etal. 2018)
and possibly overwintering substrates. Besides direct losses
of floral richness or abundance, shifting flowering phenol-
ogy may also threaten resource availability, with advances in
spring bloom documented widely (Kudo and Cooper 2019,
Augspurger and Zaya 2020). Changing fire regimes, either
reduced burning because of mesophication (Nowacki and
Abrams 2008) or increased fire severity from climate warm-
ing and built-up fuel loads (Jolly etal. 2015), are also likely to
affect bumble bee populations. Bumble bees often respond
positively to fire in the short term, because of postfire bloom
and increased canopy openness (Burkle etal. 2019, Galbraith
etal. 2019, Mola etal. 2020b). However, direct mortality to
queens and colonies also needs to be considered, especially
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for species of conservation concern or in areas in which spe-
cies are unlikely to be adapted to frequent or high-severity
fires. Changes in forest structure may negatively affect the
availability or suitability of nesting and overwintering sub-
strates, although this is merely speculative as no research
has been conducted on this to date. Understanding how past
and continued changes in forest conditions affect foraging,
nesting, and overwintering opportunities is critical, because
we may overlook the role of forests in bumble bee life history
if these conditions are sufficiently altered.
Incorporating forests into bumble bee monitoring
and restoration
There has been a lack of emphasis on forest habitats for
bumble bees within monitoring and restoration efforts. For
the reasons outlined above, this likely limits the effectiveness
of our conservation efforts. However, by explicitly incorpo-
rating forests into monitoring and restoration efforts we can
further understand the role of forests in bumble bee biology
and improve these habitats to support populations.
Several governmental and community science monitor-
ing programs exist to attempt to locate bumble bees, often
with a focus on rare or declining species. These efforts have
proven critical in trends and locations of rare bumble bees
(MacPhail et al. 2019). Some efforts are passive, such as
iNaturalist or BeeSpotter, whose users upload their observa-
tions as species are encountered. Others are more directed
with explicit sampling protocols. For example, the Nebraska
Bumble Bee Atlas project encourages community scientists
to survey for bumble bees by “survey[ing] for bumble bees
at least twice between June and September” (www.nebraska-
bumblebeeatlas.org/requirements-145172.html). The timing
of these surveys is likely to miss most queens. Similarly,
US Fish and Wildlife Service protocols to survey for the
endangered rusty-patched bumble bee (Bombus affinis)
intentionally avoid queens, thereby reducing observations
within early season habitats such as forest canopies and
understories (box 1). Of course, efforts such as this yield
tremendous value for detecting species presence or persis-
tence through time but may paint an incomplete picture of
the importance of different habitat types to species by focus-
ing predominantly on peak flight season. Given the lower
numerical abundance of queens and early worker cohorts,
and the difficulty of locating nests or overwintering queens,
detectability of bumble bees within forests may be lower as
well (Liczner and Colla 2019, Graves etal. 2020, Pugesek and
Crone 2021). However, low numerical abundance should
not be confused with low demographic importance, because
these earliest individuals are key to colony establishment
and success even long after the initial colony phases (Carvell
et al. 2017, Woodard et al. 2019). Future monitoring and
research efforts to explicitly include forests in search efforts
along with a focus on early season surveys could greatly
enhance our understanding of bumble bee habitat use.
Restoration programs or pollinator habitat creation
efforts follow a similar pattern, with a strong focus on
midsummer flowering resources (Dicks etal. 2015, Requier
and Leonhardt 2020). Although many pollinator planting
guides (examples at http://millionpollinatorgardens.org/
resources) encourage the availability of floral resources
all season long or encourage the use of trees or shrubs,
this is generally not the focus of public-facing materials.
Although pollinator plantings clearly increase peak season
resources (Wood etal. 2018), greater consideration needs
to be placed on nesting and overwintering habitat as well
as resource availability during the tails of the season. The
most cost-effective way to achieve this may be through the
management and preservation of forested areas (Bentrup
etal. 2019). Forest restoration is a costly and lengthy pro-
cess, so protection of existing forests and restoration efforts
targeted at reducing canopy crowding or the impacts of
overbrowsing may be even more cost-effective means of
increasing the services that forests provide to bumble bee
conservation.
Promisingly, the management of forests for bumble bee
populations is synergistic with other wildlife management
goals and is often an unintended effect of other efforts
(Williams 2011, Hanula et al. 2016). For example, in one
study, forests that were managed for the red-cockaded
woodpecker were also the most favorable long-term habitat
for bees (Hanula etal. 2015). Similarly, management aimed
at opening forest canopies to control pests and disease
(Fettig et al. 2007, Simler-Williamson et al. 2019), is also
likely to benefit bumble bees because favorable conditions
for flowering are often found in mature forests with canopy
gaps (Proesmans etal. 2019). However, changes in canopy
cover from management activities can also affect forest floor
temperatures, snowpack accumulation, and water infiltra-
tion and may influence the suitability of overwintering sub-
strates (Varhola etal. 2010, Simler-Williamson etal. 2019),
but this is yet to be studied for bumble bees or arthropods
broadly and the directionality of the effects is unknown.
Forest management efforts such as burning also seem com-
patible with bumble bee conservation goals as the effects
of fire on bumble bees generally remains positive across a
variety of habitats and species (reviewed in Carbone et al.
2019). Hedgerows, often containing woody plant species,
have also been a mainstay of pollinator restoration efforts
(Hannon and Sisk 2009). Forest edge plants can be favor-
able forage and may also serve multiple purposes in creat-
ing physical structure as well as providing protection from
browsing mammals. For example, Bombus dahlbomii queens
forage on Chilean box thorn (Vestia foetida) which is a nox-
ious plant that can poison browsing mammals and so may
provide protected forage (Polidori and Nieves-Aldrey 2015).
Creative opportunities for managing habitat for bees may
exist that make these efforts compatible with broader forest
management goals.
Future research
There are many avenues of future research on the relation-
ship between bumble bees and forests that are likely to be
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Box 1. The potential of forests in conservation of the endangered rusty patched bumble bee (Bombus affinis).
In 2017, the rusty patched bumble bee (Bombus affinis) became the first bumble bee listed under the Endangered Species Act in the
United States. Bombus affinis was once fairly widespread in North America but has suffered population declines and range contraction
in the past few decades (Giles and Ascher 2006, Colla etal. 2012, Williams etal. 2014, USFWS 2019). As a sort of conservation flagship
species for bees more generally, the development of this species’ recovery plan presents an opportunity to “get it right” from the start
and apply lessons learned as a model for other pollinator species that face similar threats.
Current conservation efforts by US Fish and Wildlife Service (USFWS), state, and local monitoring predominantly focus on mid-
summer populations. For example, USFWS surveys “must be conducted between early June and mid-August, for the highest
detection probability and to reduce potential impacts to B. affinis queens” (www.fws.gov/midwest/endangered/insects/rpbb/pdf/
Survey_Protocols_RPBB_12April2019.pdf ). Although this is a laudable goal to avoid negatively affecting queen nest establishment, it
also means information on spring queens and early workers is underreported. Intentionally avoiding surveys during these times of the
year leaves us without data-driven management actions and may be undervaluing the importance of woody land covers.
Earlier natural history surveys suggest B. affinis queens use a range of woody and nonwoody species early in the season. Fye and
Medler (1954) document B. affinis queens using several fruit trees such as Pyrus and Prunus species as well as early flowering shrubs
such as Salix and Lonicera. In a similar investigation, Macior (1968) documented 156 B. affinis queens foraging with more than half
of them captured from Berberis, Pyrus, and Lonicera. In contrast, Wood and colleagues (2019) found only 14% of pollen species from
museum specimens of B. affinis were from woody plant species. However, these samples had a median date of August 6th, which is
relatively late in the flight season of B. affinis (Mola etal. 2021).
To extend on prior understanding and make use of limited data, we examined records compiled originally for Bumble Bees of North America
(Williams etal. 2014) and updated annually by Dr. Leif Richardson to understand the potential importance of forests for this species recovery.
We found records of spring and early summer queens (April–June) foraging on 13 plant genera, of which 10 were associated with forest
habitat (figure 1a). Two species of forest-associated flowering plants (Dicentra cucullaria, Mertensia virginica) account for nearly half of
the observations (figure 1a) and are known to be especially early blooming (Mola etal. 2021). In contrast, gynes foraging between July
and September were found on nine floral genera of which only two are primarily associated with forests, suggesting the importance of
forests as forage habitat declines as the season progresses (figure 1b).
Figure 1. Landcover and floral associations of Bombus affinis spring foundresses (panels (A) and (C)) and gynes
(panels (B) and (D)). (A) Tally of landcover types within which each record of B. affinis spring foundresses was
collected within the study region. (B) Tally of landcover types for B. affinis gynes (queen records after day of year
150). (C) Tally of floral species identified from photos of B. affinis foundresses. (D) Tally of floral species identified
from photos of B. affinis gynes. Forest-associated plant species and land covers are colored green.
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fruitful (box 2). Although many studies demonstrate a cor-
relation between bumble bees and surrounding forest cover,
few set out with this intention in mind. Future studies seek-
ing to understand exactly why these correlations arise, either
because of nesting, overwintering, or foraging habitat, across
a variety of forest types could help land managers incorpo-
rate forests and woody habitats in species plans thoughtfully.
There is a pressing need to understand how changing
conditions within forests from the threats discussed above
are likely to affect bumble bees in the future and how man-
agement activities intended to counteract those threats will
affect bumble bees. Of significant importance is likely to be
the role of forests as thermal refugia under climate warm-
ing and understanding if, for example, species more reliant
on open habitats may be more susceptible to the effects of
warming as forests may offer refuge from heat waves and
extreme weather events. Finally, perhaps underlying all
these research needs, is greater capacity to study the role of
forests in bumble bee biology. We argue above that our lack
of understanding often comes from the difficulty of detect-
ing bumble bees within forests (i.e., visual blockage, canopy
foraging, time of year). Efforts to better coordinate commu-
nity scientists, improve detection methods, and overcome
the difficulty of identifying nesting and overwintering sites
are all needed to improve our ability to study bumble bees
within forested habitats.
Finally, although we review studies conducted in a wide
range of forest types, there is also a strong bias toward
research in temperate deciduous forests, predominantly
in eastern North America, Europe, and Japan. A notable
species lacking from our discussion is the tropical lowland
rainforest specialist B. transversalis, which lives its entire life
under deep canopy and makes use of twigs and leaves in its
nest construction (Olesen 1989). Although it is an outlier
in bumble bee life history, learning more about that species’
origins and behaviors may reveal general patterns. Broadly,
it remains to be seen whether the associations between
bumble bees and woody habitats described in the present
article are relevant to forest types, such as tropical montane
forests, that are both understudied and important habitats
for bumble bees.
Conclusions
In this article, we focused on the value of forests for ful-
filling habitat requirements for bumble bees. However, it
is important to note that these factors are not necessarily
restricted to forests but are likely most often found within
forests. For instance, orchards and gardens can also offer
similarly early resources as natural or seminatural forest
habitats (Watson etal. 2011, Nakamura and Kudo 2019,
Nikkeshi etal. 2019). As we show in the case study with
B. affinis (box 1), developed lands can offer substantial
foraging opportunities for bumble bees and other studies
demonstrate urban habitats can be suitable landscapes
(McFrederick and LeBuhn 2006, Glaum etal. 2017, Reeher
et al. 2020). In addition, nesting within anthropogenic
habitats seems to be fairly common (Medler and Carney
1963, Liczner and Colla 2019). Overall, we have demon-
strated that forests are often critical bumble bee habitat,
but it may be possible that the benefits of forests are sub-
stitutable to some extent with other environments such as
developed landcovers containing early season species or
other types of early blooming natural habitats. We hope
our perspective does not provide the idea that forests are
required for bumble bees but instead that they offer a cost
effective means to provide foraging, nesting, and overwin-
tering habitats that are compatible with conservation goals
of other organisms (Williams 2011, Bentrup et al. 2019)
and may be overlooked in studies of bumble bee biology. A
recurrent problem in bumble bee conservation is the lack
of informed demographic models or an understanding of
basic aspects of species biology (i.e., nesting and overwin-
tering). Increasing our capacity to incorporate forests into
these efforts is likely to produce rich data sets that better
inform conservation efforts and lead to the development of
useful demographic models.
Box 1. Continued.
We also examined land cover associations of queen records to assess habitat associations of B. affinis. In total, we overlaid 139 records
with USDA Cropland Data Layer (NASS 2019) and extracted the landcover type each record was collected in. Although developed
land cover types were the primary habitat association (figure 1c and 1d), as was expected given the dominance of community science
records, deciduous forests were the second most common landcover (figure 1c and 1d).
The associations described in the present article are preliminary but suggest that the relationship between forests and B. affinis warrants
rigorous scientific assessment, particularly to inform the species recovery plan and targeted conservation efforts. It seems unlikely a
loss of forest plants was a driving factor in the decline of B. affinis (Mola etal. 2021), especially with other forest-associated species
such as B. vagans remaining stable within the range. However, foraging associations from historical studies and from contemporary
community science observations suggest that early season forest plants may be important areas of focus for habitat management. In
addition, it is likely that nesting and overwintering habitat for B. affinis is favorable within forested landscapes, as was evidenced from
several community science and anecdotal observations. Although data is limited at this time, the available evidence suggests that for-
ests may play an important part in conservation and recovery planning for this endangered species.
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Acknowledgments
We would like to thank all of the researchers who conducted
the studies this article relies on. Thank you for your work
and sharing your knowledge. Any use of trade, firm, or
product names is for descriptive purposes only and does
not imply endorsement by the US government. This work
was supported by the US Geological Survey Science Support
Program and the Environmental Protection Agency Great
Lakes Restoration Initiative.
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John M. Mola (jmola@usgs.gov; ORCID 0000-0002-5394-9071) and
Ian S. Pearse (ORCID 0000-0001-7098-0495) are affiliated with the US
Geological Survey, Fort Collins Science Center, in Fort Collins, Colorado, in
the United States. Jeremy Hemberger (ORCID 0000-0003-3648-4724) is affili-
ated with the Department of Entomology and Nematology, at the University
of California Davis, in Davis, California, in the United States. Jade Kochanski
(ORCID 0000-0001-8693-2404) is affiliated with the Department of Integrative
Biology and with the Department of Entomology at the University of Wisconsin
Madison, in Madison, Wisconsin, in the United States. Leif L. Richardson
(ORCID 0000-0003-4855-5737) is affiliated with the Xerces Society for
Invertebrate Conservation, in Portland, Oregon, in the United States.
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