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THE ROCKY MOUNTAIN WEST: A COMPENDIUM OF GEOGRAPHIC PERSPECTIVES
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QUAKING ASPEN: THE ICONIC AND DYNAMIC
DECIDUOUS TREE OF THE ROCKY MOUNTAINS
Timothy J. Assal
Department of Geography, Kent State University
Introduction
Perhaps no other species of tree jumps to mind as
quickly as Quaking aspen (Populus tremuloides
Michx.) when one waxes poetically about the iconic
vistas of the Rocky Mountain West. Quaking aspen,
so called for its trembling leaves, have been the focus
of countless paintings and photographs, from Ansel
Adams to John Fielder to scores of amateur photo-
graphers whose hashtags identify the species on
Instagram. The ecological setting of many upland
aspen settings provides a sharp contrast against
exposed rock, snow and the dark green blanket of
conifer forests that many of us seek out in the autumn.
Such scenes drive tourism revenue and are used in
many recreational activity advertisements by western
resorts. Beyond their aesthetic value, aspen
communities are valued for the biological diversity
and habitat they provide in western landscapes.
Furthermore, aspen face a number of ecological and
management challenges that likely go unnoticed by
the tourist or casual recreator. Above all, aspen
communities are dynamic, so there is a good chance
your favorite aspen grove might look different at
some point in your lifetime. The goal of this essay is
to provide a broad overview of the unique niche of
aspen ecosystems in the interior Rocky Mountain
West, highlight its dynamic nature, and shed light on
the challenges associated with stewardship of this
iconic species.
Aspen Biogeography
Quaking aspen, the most widely distributed
deciduous tree in North America, are largely limited
to mountainous areas, high plateaus and riparian
zones in the West where moisture, soil and topo-
graphy provide a favorable setting (Bartos 2001)
(Figure 1). The total aspen organism is more than
meets the eye, as aspen are clonal in nature, with
many trees often sharing the same root system.
Individual aspen trees or aboveground shoots are
known as ramets. Ramets that are connected together
through underground roots share the same genotype
and are known as a genet (DeByle 1985) (Figure 2).
Figure 1. The potential distribution of aspen in the western
United States (Little 1971).
Figure 2. Retired U.S. Forest Service Ecologist, Bob Campbell,
displays the rhizome or underground root that connects two aspen
shoots known as ramets. Photo by the author.
QUAKING ASPEN: THE ICONIC AND DYNAMIC DECIDUOUS TREE OF THE ROCKY MOUNTAINS
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The characteristic white bark of aspen is unique in
that, unlike most trees, it is living tissue, capable of
stem photosynthesis that contributes significantly to
aspen’s over-wintering survival capabilities
(Berveiller, Kierzkowski, and Damesin 2007).
Aspen reproduction by vegetative suckering is
much more common than sexual reproduction via
pollination, fertilization, seed development, seed
dispersal and finally seed germination (Bartos 2001).
During normal growth, the hormone auxin found in
the canopy of aspen suppresses vegetative suckering
that is triggered by cytokinin, a root hormone. How-
ever, when a disturbance removes the canopy or some
portion of it, the auxin-cytokinin hormone ratio
changes. An increase in the balance of cytokinin
induces suckering and aspen are able to quickly
regrow, effectively outcompeting most other species
for some period of time after a disturbance (Bartos
2001). Clones vary in size and are capable of growing
quite large in the western United States. In south-
central Utah, scientists have measured one aspen
stand that is nearly 180,000 m2 (one genet) and
estimated to contain over 44,000 stems (ramets)
(Kemperman and Barnes 1976; DeWoody et al.
2008). It is known as the “Pando clone” and poten-
tially represents the largest living organism on Earth
(Rogers et al. 2014). The large size of western aspen
clones is likely a result of older stands that have
vegetatively propagated from suckering after fire.
Initiation of new clones through sexual reproduction
is thought to be rare in the Rocky Mountain region
over the last several thousand years due to suboptimal
climate conditions (Romme et al. 2001). Although
documentation of sexual regeneration from seed is
rare, it may be a more widespread trait than
previously recognized, as advances in genetic
research suggest sexual reproduction is a stronger
contributor to aspen genetic variation than previ-
ously thought (Elliott and Baker 2004; Mock et al.
2008). There is a research need to address the ques-
tion of sexual regeneration that produces aspen
seedling recruitment events: are these a brief post-fire
phenomenon, or will these populations achieve tree
status, produce new ramets, and develop into
functioning aspen stands (Turner et al. 2003)?
There are generally two functional types of aspen
systems, seral and stable, with several sub-types of
each (Rogers et al. 2014). Aspen in seral systems
typically occur with coniferous trees and the
dominance is governed by the time since the last
stand-replacing disturbance. After a disturbance,
aspen resprout from root systems and might dominate
a stand for several decades, or even a century, before
conifer species eventually overtake aspen trees
(Shinneman et al. 2013). Conversely, in stable
communities, aspen maintain dominance for ex-
tended periods (> 100 years) with incremental stand
replacement (Kashian, Romme, and Regan 2007;
Kurzel, Veblen, and Kulakowski 2007).
Why is aspen important?
Biodiversity
Aspen communities provide a number of eco-
system services that may not be obvious to the casual
observer. Although they only occupy small portions
of the western landscape, aspen stands contribute
disproportionately to plant and animal diversity,
making these forests hotspots of biodiversity (Figure
3). Many more species of birds, mammals, and
invertebrates are found in aspen stands than in conifer
dominated forests, including a disproportionate
number of vascular plant species (Chong et al. 2001;
DeByle 1985). It is not surprising that aspen forests
provide high value forage for native ungulates
including, elk, moose and deer (DeByle 1985).
Social Values
Aspen are a broad-leaf deciduous species that
drop their leaves each autumn. The seasonal phenol-
ogy, or timing of biological events, associated with
aspen stands are what sets them apart from so many
other elements on the landscape, especially their
evergreen, coniferous neighbors (Assal, Anderson,
and Sibold 2015). The phenomenon of leaf
senescence is a major reason why so many people
value aspen for aesthetic reasons. Early autumn is a
great time to get a sense of the spatial extent of aspen
clones as leaves of genets, with their single,
connected root system, typically turn color at the
same time. The leaves of neighboring genets often
senesce at slightly different times, creating the
kaleidoscope of color that so many of us seek to
capture in photographs each autumn. In addition to
aesthetic reasons, many of us seek refuge in and
around aspen as part of our outdoor experience be it
skiing, camping, hiking or hunting. Many of these
experiences contribute to economic benefits of state
and local businesses, including hunting licenses,
which benefit funding of state wildlife agencies
(Rogers 2017).
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Figure 3. Sapsuckers, a species of woodpecker, create holes in
aspen bark to create sap wells (top; Steamboat Mountain, WY),
the base of an aspen tree provides a substrate for lichen (center;
Wind River Range, WY), moose congregate at the edge of a
riparian aspen stand (bottom; Bighorn Mountains, WY). Photos
by the author.
Forage for Domestic Livestock
Aspen forests provide high value forage for
domestic livestock (DeByle 1985) and livestock
producers have long depended on the diversity and
biomass of aspen understory communities to feed
their cattle and sheep. Aspen are found at higher
elevations in the West, and provide for herds during
hot dry winter months (Bartos 2001). Given this land
use, ranchers receive a direct economic benefit from
aspen, as well as the indirect benefits to their home
municipalities (Rogers 2017), although these are
rarely quantified.
Aspen Dynamics
Disturbance plays a prominent role in shaping the
vegetation of western landscapes. A disturbance is a
relatively discrete event that changes the resource
availability or physical environment which shapes
forest ecosystems by influencing their composition,
structure, and functional processes (White and Pickett
1985). Forests of the Rocky Mountain region are
molded by their history of disturbance and prior land-
use (Dale et al. 2001) and this plays a large part in the
dynamic nature of aspen ecosystems. Seral aspen are
often considered a disturbance driven species, as
stands quickly resprout from root systems following
a disturbance. Forest disturbances are both human-
induced and natural, although humans are capable of
influencing natural processes. The major disturbance
types of aspen forests include fire, climate
change/drought, insect/fungal agents, while ungulate
herbivory is a primary stressor of regeneration. For
the purposes of this discussion, management
treatments are included here although they do not
always have the same effect as natural disturbances
(due to size, severity, frequency, etc.).
Fire
Fire is widely recognized as one of the main
disturbance drivers in aspen ecosystems (Romme et
al. 2001); however, few fire history studies have been
conducted that are specific to aspen dominated forests
(Kulakowski, Veblen, and Kurzel 2006). The main
difficulty reconstructing fire history in aspen forests
is that trees are easily killed by fire and few fire-
scarred trees can be found to date past fires (Romme
et al. 2001). Fire frequency and severity differs with
elevation and location. In western Colorado, high
elevation subalpine forests experience infrequent
stand-replacing fires, whereas, woodlands and
shrublands typically experience more frequent fires
QUAKING ASPEN: THE ICONIC AND DYNAMIC DECIDUOUS TREE OF THE ROCKY MOUNTAINS
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(Kulakowski, Veblen, and Kurzel 2006; Sibold,
Veblen, and Gonzalez 2006). Given their clonal
nature, aspen are able to outcompete conifers and
other species that do not have the ability to resprout
after disturbance.
Climate Change & Drought
Climate projections over the next century indicate
aspen habitat will be reduced in western North
America (Rehfeldt, Ferguson, and Crookston 2009),
largely as a result of increased drought and heat stress
(Allen et al. 2010). Productivity is limited by carbon
dioxide fixation imposed by leaf stomatal resistance
during soil or atmospheric water deficits and
photosynthetic productivity is greatly reduced (Hogg,
Brandt, and Michaelian 2008). Recent drought events
have caused widespread aspen mortality (Michaelian
et al. 2011; Worrall et al. 2008) and low elevation
aspen forests along the forest-shrubland ecotone are
particularly vulnerable to drought (Assal, Anderson,
and Sibold 2016) (Figure 4). Furthermore, trees that
are not killed during a drought experience hydraulic
deterioration, which damages the xylem, leaving
surviving trees more vulnerable to future drought and
other stressors (Anderegg et al. 2013). The frequency
of drought events in the western United States is
expected to increase in the future (Dale et al. 2001)
and represents a likely pathway for climate change to
fundamentally alter the composition, structure, and
biogeography of aspen forests in many regions
(Anderegg et al. 2013).
Fungal and Insect Damaging Agents
Aspen are susceptible to a number of fungal
diseases and insects that can damage plants and
reduce the health and vigor of aspen stands (Shepperd
et al. 2006). Fungal canker diseases attack and girdle
aspen trees, root diseases found in the soil weaken
plants, and several insect species defoliate aspen
trees. Each of these agents are capable of killing
individual trees, although widespread disease and/or
repeated defoliation events can cause stand level
mortality (Shepperd et al. 2006). The outbreak of
mountain pine beetle (Dendroctonus ponderosae) in
northcentral Colorado represents an interesting case
for aspen forests. Mountain pine beetle outbreaks
have caused widespread conifer mortality in western
North American since the late 1990s (Raffa et al.
2008). Although aspen are not entirely congruent
with the range of these conifer species, this repre-
Figure 4. A low elevation stand with high levels of mortality
from recent drought (Middle Mountain, CO). Photo by the
author.
sents an opportunity for aspen expansion and is an
area of needed research (Pelz and Smith 2013).
Ungulate Herbivory
Domestic livestock (sheep, cattle) and wild un-
gulates (moose, elk, and deer) both consume young,
regenerating aspen plants that can have long-term
effects on the viability of the stand. Heavy and un-
controlled grazing by domestic livestock was wide-
spread during the first half of the 20th century and
effects of this time period likely linger (DeByle
1985). Herbivory by native ungulates can also have
major implications on aspen regeneration and survi-
val, especially in the animal’s winter ranges. Aspen
cohorts in Rocky Mountain National Park only re-
generated over the last century when elk populations
on the winter range were low (Baker, Munroe, and
Hessl 1997). Likewise, in western Wyoming, periods
of low to moderate elk populations corresponded with
frequent aspen ramet regeneration (Hessl and
Graumlich 2002). Other studies in the West found
high browse pressure reduced stand growth but did
not preclude stand-replacing regeneration across elk
winter ranges (Barnett and Stohlgren 2001; Kaye,
Binkley, and Stohlgren 2005). Nevertheless, high
browse pressure is a major concern on aspen
regeneration dynamics. Browsing reduces sapling
growth, vigor and abundance, particularly when
terminal buds are removed, resulting in stunted
growth (Kaye, Binkley, and Stohlgren 2005). A
browse line is often visible in stands with high browse
pressure, indicative of little regeneration (Figure 5).
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Figure 5. Heavy bark damage and no regeneration indicate high
ungulate use in this stand (Sierra Madre Mountains, WY). Photo
by the author.
Management Treatments
A number of management techniques are used to
change the hormonal balance of an aspen stand to
promote a flush of suckering and regrowth (Bartos
2001; Shepperd et al. 2006) (Figure 6). Mechanical
treatments which remove the majority of above-
ground vegetation, but leave the root systems intact,
are commonly used (Shepperd et al. 2006). Pre-
scribed fire (Bartos, Brown, and Booth 1994;
Shinneman et al. 2013) and removal of competing
vegetation in a stand, typically large conifers, are also
employed as management techniques (Bowen et al.
2010).
Long-term Trends
The scientific literature on long-term trends is not
conclusive regarding the expansion or contraction of
aspen across any given region of its range in western
North America (Rogers, Eisenberg, and St. Clair
2013; Kashian, Romme, and Regan 2007). Some
studies have concluded aspen is in decline, largely as
a result of changes in the fire regime, herbivore
population and climate (Baker, Munroe, and Hessl
1997; Bartos 2001; Di Orio, Callas, and Schaefer
2005; Kay 1997). However, other studies have found
expansion (Kulakowski, Veblen, and Kurzel 2006) or
both expansion and contraction in the same area
(Brown et al. 2006; Kashian, Romme, and Regan
2007). A growing number of studies suggest the
temporal scale of study be expanded beyond the 20th
century to consider the full range of variability of
climate conditions and fire regime found across aspen
communities (Barnett and Stohlgren 2001;
Kulakowski, Veblen, and Kurzel 2006; McIlroy and
Shinneman 2020). In addition, localized studies are
not always appropriate to extrapolate to regional
trends (Brown et al. 2006) and multiple spatial scales
should be incorporated where possible (McIlroy and
Shinneman 2020). Variable ecological conditions
affect community dynamics and long-term aspen
resilience for a given aspen functional type in dif-
ferent ways (Rogers et al. 2014). However, these
divergent viewpoints shed light on the complexity of
aspen ecology, stewardship and conservation across
the West (Rogers, Eisenberg, and St. Clair 2013).
Management Challenges
Aspen are of great concern for many state and
federal agencies charged with managing viable
populations; however, a number of factors make the
stewardship of this species incredibly difficult.
Climate warming, herbivory and inappropriate man-
agement are the biggest threats to aspen resilience
(Rogers et al. 2014). Although aspen is adept at
vegetative reproduction, it is a sexually challenged
species and may only be able to sexually regenerate
(from seed set through establishment of mature
stands) in very favorable climates (Turner et al. 2003;
Romme et al. 2001). We do not know the age of the
genets on our landscapes, but they were likely
established during a cooler and wetter climate.
Vegetative reproduction is a very effective strategy to
recolonize areas after disturbance, but it poses chal-
lenges to migration if climate profiles shift to higher
elevations as a result of climate change. In an un-
certain future, Aspen’s strategy will be tested.
Managers must decide where and when to con-
duct treatments with limited resources and diverse
aspen functional types require a targeted manage-
ment prescription (Rogers et al. 2014). Although
aspen typically respond positively to treatments, the
herbivory pressure is so great in some areas that it
could preclude young aspen saplings from growing
out of the browse zone and developing into mature
trees to sustain a stand. Managers face logistical
challenges to appease multiple stakeholders, as a
substantial amount of aspen occur on lands managed
by numerous federal and state agencies, including
private lands. Yet another concern is conducting
active management in high profile areas that will
change the landscape for many camera-toting
QUAKING ASPEN: THE ICONIC AND DYNAMIC DECIDUOUS TREE OF THE ROCKY MOUNTAINS
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Figure 6. A prescribed burn was applied to this aspen stand several decades ago. Dense regeneration has occurred where higher
severity fire induced greater mortality (Little Mountain, WY). Photo by the author.
tourists of today in an effort to ensure aspen are main-
tained on that landscape.
Conclusion
Although aspen face many ecological and man-
agement challenges, these provide a great opportunity
for scientists and managers to work alongside each
other to conduct applied science to guide manage-
ment of these systems. Indeed, these needs have
galvanized both communities over the last couple of
decades to work toward a common goal of aspen
resilience through dedicated conferences (see
proceedings associated with (Bartos 2001)), special
journal issues (Rogers, Eisenberg, and St. Clair 2013)
and field workshops (Figure 7). We often forget the
vegetation of a given landscape is dynamic, with a
number of forces acting to re-shape and reorganize it
as we move into an uncertain future. Our viewpoint is
nearly a snapshot. Take a moment to think about the
history of the landscape when you line up your next
iconic photo of quaking aspen, and remember, it
could look very different in the future.
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Timothy J. Assal
Department of Geography
Kent State University
Kent, OH 44242
tassal@kent.edu
