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Quaking Aspen: The Iconic and Dynamic Deciduous Tree of the Rocky Mountains


Abstract and Figures

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 photographers 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.
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Timothy J. Assal
Department of Geography, Kent State University
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.
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?
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).
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 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
(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
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).
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.
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
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.
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
... Despite its abundance across the continent, aspen has a limited extent along the southwestern edge of its range, which extends from the southwestern United States (hereafter the Southwest) to central Mexico (Little 1971;Martínez González and González-Villarreal 2005;Perala 1990). Aspen has high ecological importance (Campbell and Bartos 2001;Rogers et al. 2020), a positive impact on local economies (McCool 2001;Rogers 2017), and important aesthetic and cultural values (Assal 2020;Dahms and Geils 1997;McCool 2001). Compared with other areas of aspen's expansive range, the importance and value of aspen ecosystems is even more pronounced in the Southwest, where they are scarce but make an outsized contribution to biodiversity, especially in comparison to neighboring coniferdominated ecosystems (Chong et al. 2001;Gitlin et al. 2006;Kuhn et al. 2011;Riva and Fahrig 2022). ...
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Quaking aspen (Populus tremuloides Michx.) has high conservation value on the southwestern edge of its range, which extends from the southwestern United States (i.e., Arizona, New Mexico, and Texas) to central Mexico. This value is driven by aspen's ecological importance, positive impact on local economies, and aesthetic and cultural values. Generally, the scant aspen populations that remain in the Southwest lack resilience and adaptive capacity, and managers are unsure how best to maintain the species in an uncertain future. This systematic review seeks to address that need by reviewing existing literature from the Southwest on which biotic and abiotic factors influence aspen forest dynamics and by synthesizing that literature with a discussion of how management can promote aspen ecosystem resilience and adaptive capacity. We found that fire and silvicultural treatments promote aspen regeneration, but chronic ungulate browse inhibits recruitment. Moreover, drought is a driver of overstory mortality and has a negative influence on recruitment. In the second half of this review, we propose three management objectives for increasing aspen resilience and adaptive capacity: (1) promote diversity in age structure, (2) mitigate ungulate impacts, and (3) enhance complexity. We consider how various management strategies could meet these objectives and highlight potential threats to aspen forest health and resilience.
... Aspen stands also provide a variety of ecosystem services, including significant contributions to carbon sequestration (Woldeselassie et al. 2012), nitrogen mineralization (Stump and Binkley 1993), water yield potential (LaMalfa and Ryle 2008), and revenue from hunting, tourism, and recreation (McCool 2001;Rogers 2017). Aspen also has cultural and aesthetic value as an iconic tree species of the American West (Assal 2020), perhaps best demonstrated by the phenomenon of ''leaf peeping,'' whereby recreationists travel to the high country in autumn to enjoy aspen's beautiful golden leaves (Johnson et al. 1985). ...
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Oystershell scale (OSS; Lepidosaphes ulmi) is an emerging invasive insect that poses a serious threat to conservation of quaking aspen (Populus tremuloides) in the southwestern US. Although OSS has been an urban pest in the US since the 1700s, it has recently spread into natural aspen stands in northern Arizona, where outbreaks are causing dieback and mortality. We quantified the ongoing outbreak of OSS at two scales: (1) local severity at two sites and (2) regional distribution across northern Arizona. Our regional survey indicated that OSS is widespread in lower elevation aspen stands and is particularly pervasive in ungulate exclosures. Advanced regeneration had the highest levels of infestation and mortality, which is concerning because this size class is an underrepresented component of aspen stands in northern Arizona. If OSS continues to spread and outbreaks result in dieback and mortality like we observed, then aspen in the southwestern US, and perhaps beyond, will be threatened. Three interacting factors contribute to OSS’s potential as a high-impact invasive insect that could spread rapidly: (1) its hypothesized role as a sleeper species, (2) potential interactions between OSS and climate change, and (3) the species’ polyphagous nature. Invasive pests like OSS pose an imminent threat to native tree species and, therefore, represent an immediate research and monitoring priority. We conclude with recommendations for future research and monitoring in order to understand OSS’s biology in natural aspen stands, quantify impacts, limit future spread, and mitigate mortality and loss of aspen and other host species.
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Riparian ecosystems provide critical habitat for many species, yet assessment of vegetation condition at local scales is difficult to measure when considering large areas over long time periods. We present a framework to map and monitor two deciduous cover types, upland and riparian, occupying a small fraction of an expansive, mountainous landscape in north-central Wyoming. Initially, we developed broad-scale predictions of predominant woody vegetation types by integrating Landsat data into species distribution models and combining subsequent outputs into a synthesis map. Then, we evaluated a 35-year Landsat time series (1985–2019) using the Mann-Kendall test to identify significant trends in the condition of upland and riparian deciduous vegetation and assessed the rate and direction of change using the Theil-Sen estimator. Finally, we used plot level data to assess the utility of the framework to detect bottom-up controls (ungulate browse pressure and management actions) on vegetation condition. The synthesis map had an overall correct classification rate of 87% and field data indicated deciduous vegetation within 45 m of coniferous forest faces increased pressure of conifer expansion. The trend assessment identified consistent patterns operating at the landscape scale across both upland and riparian deciduous vegetation; a predominant greening trend was observed for 12 years followed by a 9-year browning trend, before switching back to a greening trend for the last 13 years of the study. Our results indicate trends are driven by the climate of the measurement period at the landscape scale. Although we did not find conclusive evidence to establish a strong link between browse pressure and satellite data, we highlight examples where prevailing trends can be overridden by local disturbance or management intervention. This framework is transferable to other understudied riparian environments throughout western North America to provide insight on ecohydrological processes and assess global and local stressors across broad spatiotemporal scales.
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Quaking or trembling aspen (Populus tremuloides Michx.) forests occur in highly diverse settings across North America. However, management of distinct communities has long relied on a single aspen-to-conifer successional model. We examine a variety of aspen-dominated stand types in the western portion of its range as ecological systems, avoiding an exclusive focus on seral dynamics or single-species management. We build a case for a large-scale functional aspen typology based on the existing literature. Aspen functional types are defined as aspen communities that differ markedly in their physical and biological processes. The framework presented here describes two “functional types” and seven embedded “subtypes”: seral (boreal and montane), stable (parkland, Colorado Plateau, elevation and aspect limited, and terrain isolated), and a crossover seral-stable subtype (riparian). The assessment hinges on a matrix comparing proposed functional types across a suite of environmental characteristics. Differences among functional groups based on physiological and climatic conditions, stand structures and dynamics, and disturbance types and periodicity are described herein. We further examine management implications and challenges, such as human alterations, ungulate herbivory, and climate futures, that affect the functionality of these aspen systems. The functional framework lends itself well to stewardship and research that seek to understand and emulate ecological processes rather than combat them. We see advantages of applying this approach to other widespread forest communities that engender diverse functional adaptations.
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Quaking aspen (Populus tremuloides Michx.) is the most widespread tree species in North America, and it is found throughout much of the Mountain West (MW) across a broad range of bioclimatic regions. Aspen typically regenerates asexually and prolifically after fire, and due to its seral status in many western conifer forests, aspen is often considered dependent upon disturbance for persistence. In many landscapes, historical evidence for post-fire aspen establishment is clear, and following extended fire-free periods senescing or declining aspen overstories sometimes lack adequate regeneration and are succeeding to conifers. However, aspen also forms relatively stable stands that contain little or no evidence of historical fire. In fact, aspen woodlands range from highly fire-dependent, seral communities to relatively stable, self-replacing, non-seral communities that do not require fire for persistence. Given the broad geographic distribution of aspen, fire regimes in these forests likely co-vary spatially with changing community composition, landscape setting, and climate, and temporally with land use and climate – but relatively few studies have explicitly focused on these important spatiotemporal variations. Here we reviewed the literature to summarize aspen fire regimes in the western US and highlight knowledge gaps. We found that only about one-fourth of the 46 research papers assessed for this review could be considered fire history studies (in which mean fire intervals were calculated), and all but one of these were based primarily on data from fire-scarred conifers. Nearly half of the studies reported at least some evidence of persistent aspen in the absence of fire. We also found that large portions of the MW have had little or no aspen fire history research. As a result of this review, we put forth a classification framework for aspen that is defined by key fire regime parameters (fire severity and probability), and that reflects underlying biophysical settings and correlated aspen functional types. We propose the following aspen fire regime types: (1) fire-independent, stable aspen; (2) fire-influenced, stable aspen; (3) fire-dependent, seral, conifer-aspen mix; (4) fire-dependent, seral, montane aspen-conifer; and (5) fire-dependent, seral, subalpine aspen-conifer. Closing research gaps and validating our proposed aspen fire regime classification will likely require additional site-specific research, enhanced dendrochronology techniques, charcoal and pollen record analysis, spatially-explicit modeling, and other techniques. We hope to encourage development of site-appropriate disturbance ecology characterizations, in order to aid efforts to manage and restore aspen communities and to diagnose key factors contributing to changes in aspen.
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While clones of trembling aspen (Populus tremuloides, Michx.) in the Intermountain West of North America are expected to be large, one putative genet in central Utah, identified from morphological evidence, has garnered particular attention for its size, even gaining the nickname “Pando” (Latin for “I spread”). In order to determine if a single genetic individual coincides with the morphological boundary of “Pando,” we sampled 209 stems on a 50-m grid throughout the putative clone for analysis at 7 microsatellite loci. We have identified a single genetic entity concurrent with that described from morphological characteristics. Spatial analyses indicate that the clone covers approximately 43.6 ha. Surprisingly, an additional 40 genotypes were identified adjacent to the putative clone, indicating that genet diversity may be high in the stand as a whole. In confirming the existence of the “Pando” clone, we suggest that this organism will provide valuable opportunities to study important biological processes such as clonal growth, somatic mutation, and senescence.
Altered climate and changing fire regimes are synergistically impacting forest communities globally, resulting in deviations from historical norms and creation of novel successional dynamics. These changes are particularly important when considering the stability of a keystone species such as quaking aspen (Populus tremuloides Michx.), which contributes critical ecosystem services across its broad North American range. As a relatively drought intolerant species, projected changes of altered precipitation timing, amount, and type (e.g. snow or rain) may influence aspen response to fire, especially in moisture-limited and winter precipitation-dominated portions of its range. Aspen is generally considered an early-seral species that benefits from fire, but increases in fire activity across much of the western United States could affect the species in unpredictable ways. This study examined post-fire aspen stands across a regional climate gradient spanning from the north-central Great Basin to the northeastern portion of the Greater Yellowstone Ecosystem (USA). We investigated the influence of seasonal precipitation and temperature variables, snowpack, and site conditions (e.g. browsing levels, topography) on density of post-fire aspen regeneration (i.e. all small trees ha⁻¹) and recruitment (i.e. small trees ≥2 m tall ha⁻¹) across 15 fires that occurred between 2000 and 2009. The range of post-fire regeneration (2500–71,600 small trees ha⁻¹) and recruitment (0–32,500 small trees ≥2 m ha⁻¹) densities varied widely across plots. Linear mixed effects models demonstrated that both response variables increased primarily with early winter (Oct-Dec) precipitation during the ‘fire-regen period’ (i.e., fire year and five years after fire) relative to the 30-year mean. The 30-year mean of early winter precipitation and fire-regen period snowpack were also positively related to recruitment densities. Both response variables decreased with higher shrub cover, highlighting the importance of considering shrub competition in post-fire environments. Regeneration and recruitment densities were negatively related to proportion browsed aspen leaders and animal pellet densities (no./m²), respectively, indicating the influence of ungulate browsing even at the relatively low levels observed across sites. A post-hoc exploratory analysis suggests that deviation in early winter precipitation during the fire-regen period (relative to 30-year means) varied among sites along directional gradients, emphasizing the need to consider multiple spatiotemporal scales when investigating climate effects on post-fire successional dynamics. We discuss our findings in terms of dynamic management and conservation strategies in light of changing fire regimes and climate conditions.
Elk browsing and conifer species mixing with aspen (Populus tremuloides Michx.) present current challenges to aspen forest management in the western United States. We evaluated the effects of conifers and elk browsing on quaking aspen stands in and near Rocky Mountain National Park using tree rings to reconstruct patterns of aspen establish- ment, growth, and mortality over the past 120 years. High conifer encroachment and elk browse were both associated with decreased aspen recruitment, with mean recruitment dropping over 30% from pure aspen to mixed stands and over 50% from low-browse to high-browse stands. Maximum aspen recruitment was lower in mixed stands than in pure stands with the same tree basal area. High levels of elk browsing were also associated with a 30% decrease in stand-level growth of aspen. Neither high conifer abundance nor elk browse affected the growth of individual trees or aspen mortality. Aspen establishment was negatively influenced by conifers and elk browsing; however, aspen growth and mortality appeared to be resilient to these two external influences. Overall, these results suggest that long-term preservation of aspen forests could be achieved by enhancing aspen recruitment.
Vegetation responses to prescribed fire over a 12-year period are reported for several deteriorating aspen clones in northwestern Wyoming. This study extends earlier work by Bartos and Mueggler (1981) on a prescribed fire intended to regenerate these aspen clones. After 3 years, numbers of suckers were close to pre-burn levels ranging between 10,000 to 20,000 suckers/ha. After 12 years, 1,500 to 2,400 suckers/ha remained at a meager height averaging approximately 0.5 m. The demise of this aspen was attributed to heavy ungulate use, primarily elk. Total undergrowth production increased substantially by the second postfire year and declined slowly after that. Biomass values of 2,130 kg/ha (low burn severity), 2,140 kg/ha (moderate burn severity), and 2,190 kg/ha (high burn severity) were recorded after 12 years. This exceeds preburn production by 23 to 46%. Forbs made up approximately 75% of the undergrowth production after 12 years, which was dominated by a dramatic postburn shift to fireweed (Epilobium angustifolium L.). The remaining production was comprised of approximately 20% grasses and 5% shrubs. Most of the fluctuation in species composition occurred on the high severity burn sites.