Quaking and Trembling
Perhaps no single element so eloquently captures
the aesthetic appeal of the Rocky Mountains as the Quaking Aspen (Populus tremuloides). Their white barks
with black accents, trembling leaves, and contrasting light green foliage exclaim
themselves apart from the dark green of the surrounding pines, firs, and
spruces. In the Fall, their presence is further punctuated by the aspen’s vibrant
yellow leaves that turn to flecks of gold as the leaves take to the wind.
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| Aspen stand. Photo by Whitney Tilt |
Prime
Real Estate. Aspen is the most widely distributed tree species native to
North America. In ecological terms, aspen have great “amplitude,” occupying a
wide range of elevations, aspects, and soils. Along with riparian areas, aspen
communities are considered the most biologically diverse ecosystems in the
Intermountain West and critical wildlife habitats. Aspen provide forage for
wildlife and livestock and their trunks provide a virtual apartment building for
nesting and feeding, especially as they age. The open canopy of aspen stands
allows sunlight to reach the forest floor supporting a diverse community of
forbs that are also a wildlife magnet. Aspen prefer moist soils and their
stands provide cool, shaded habitats. They are particularly adept at retaining water
relative to other forest types. As a keystone species, their presence (or
absence) significantly affects the survival and abundance of many other closely
associated species.
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| A wide range of bird species, like the Red-naped Sapsucker, depend on aspen for food and nesting sites. Photo by Mark Resendes. |
Heavy
Weight Champion of the World. Aspen also have the distinction of being the largest
living organism in the known world, by weight. This is a result of aspen’s
habitat of propagating primarily vegetatively rather than by seed. Groves of
aspen trees are commonly clones, where adult trees are all genetically
identical to each and connected by their root system. As adult trees die off,
new sprouts, called "suckers", grow from the roots and the clone
continues to survive. In some cases, these clones may be tens of thousands of years
old. The Pando (Latin for “I spread”) Clone, in Utah’s Fishlake National Forest,
is estimated to weight 13 million pounds, cover 106 acres, and have lived for 80,000
years.
Aspen in
Decline. The aspen’s valuable contribution to the ecology and esthetics of
the region, however, does not ensure its survival. Aspen stands are in
significant decline from their historical abundance across the Rockies (Arizona
-96%, Colorado -49%, Utah -51%, Wyoming -53%, Montana -64%).
As an early serial species, aspen require open
canopies and continuing disturbance events, principally wildfire, windstorms, and/or
disease, to maintain stand vigor, stimulate regeneration, and keep the
surrounding conifer forest at bay. As aspen stands age and are not renewed by
disturbance, conifer species (e.g., Douglas-fir, Engelmann spruce) encroach and
overtop the aspen, ultimately crowding the aspen out of existence. Adding to
their vulnerability, aspen are a relatively short-lived species (individual
trees as opposed to the overall clone) living an average of 60-80 years while their
thin, living bark make them susceptible to a host of insect pests and diseases.
Add to these vulnerabilities, the fact that they are also tasty, marking young
aspen shoots and saplings as a favorite browse for elk and other wildlife, as
well as livestock. To add insult to injury, decades of drought and warmer
temperatures in the Rocky Mountain West have taken their collective toll.
Managing
for Aspen. In the absence of wildfire or wind-throw, several possible treatments
are available for managers to improve aspen stand health and vitality. The
primary management tools center on creating physical disturbance and managing
herbivory.
1. Prescribed fire, wildland fire
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Use fire to burn adult
trees and encroaching conifers. Found to elicit best aspen regrowth, but comes with risks and liabilities as controlling prescribed fires or allowing wildfires to burn
is difficult.
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2. Dozing
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Mechanically uproot adult trees and
break apart root systems. Commonly generates high regeneration of stems, but eliminates adult trees until new growth matures.
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3. Cut/Harvest
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Cutting adult trees and encroaching conifers allows
for more selective harvesting, and is effective at stimulating regrowth,
although it may not yield the same kinds of stem densities as treatments 1 or
2.
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4. Ripping
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Physically breaking up root systems with a
ripping blade in the stand. Stimulates regeneration without killing existing
adult trees, but generally does not elicit same regeneration responses
as methods 1-3.
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| Successful aspen regeneration at the High Lonesome Ranch, NW Colorado. Note the remaining decadent stand in the background. |
Managing to stimulate aspen regeneration is
commonly the first step. In areas of livestock and wildlife use, however, the new growth
will need some form of protection from critters. Methods range from herd
management to physical barriers:
5. Limit Herbivory
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a. Limit or reduce livestock in treatment areas,
if possible or practical; not an option for wildlife.
b. Erect barriers to fence out wildlife and/or livestock. Erect traditional livestock fencing or other barriers (e.g., jackstrawing felled
trees). Fencing can be very effective, but costly and requires continued
maintenance.
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| Aspen in their fall colors. Photo by Whitney Tilt |
Resources
Blue Valley Ranch. Aspen Forest Management. http://bluevalleyranch.com/explore/aspen-forest/
Campbell, Robert B. and Dale Bartos. 2001.
Aspen Ecosystems: Objectives for Sustaining Biodiversity. USDA Forest Service
Proceeding RMRS-P-18.
Paige, Christine. 2017. Bring Back the Gold.
2017. Bugle Magazine, Rocky Mountain Elk Foundation, September-October: 104-112.
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