Get to Know the Aspen

Aspens in fall. Photo: Don Hanley.
Aspens in fall. Photo: Don Hanley.

It is almost impossible to deny that aspen (Populus tremuloides) is one of our most beautiful trees of the fall season. Its bright yellow leaves against creamy white bark and a backdrop of blue sky makes walkers stand still; the hypnotizing commotion of the quaking leaves excites even the hard-of-hearing landowner. No other tree species casts this autumn spell as broadly as aspen.

Aspen grow in a variety of climates, environments, and physical conditions. They occur almost everywhere, from dry ridges to stream sides to rocky slopes. In almost every mountain vegetation zone, aspen can be found scattered across the valleys upward to the subalpine tree line. From The USDA Silvics of North America manual:

“Quaking aspen grows singly and in multi-stemmed clones over 111° of longitude and 48° of latitude for the widest distribution of any native tree species in North America (48). The range extends from Newfoundland and Labrador west across Canada along the northern limit of trees to northwestern Alaska, and southeast through Yukon and British Columbia. Throughout the Western United States it is mostly in the mountains from Washington to California, southern Arizona, Trans-Pecos Texas, and northern Nebraska. From Iowa and eastern Missouri it ranges east to West Virginia, western Virginia, Pennsylvania, and New Jersey. Quaking aspen is also found in the mountains of Mexico, as far south as Guanajuato. Worldwide, only Populus tremula, European aspen, and Pinus sylvestris, Scotch pine, have wider natural ranges.”

aspen's natural range
The aspen’s natural range includes a large swath of North America.

Aspen can occupy a site as the primary intermediate species present, until more shade tolerant conifers reclaim climax dominancy. Or, in the absence of more shade tolerant species, aspen stands can occur as an uneven-aged climax stand – self-perpetuating with seemingly no requirement for disturbance in order to regenerate.

Role of fire

East of the Cascade Mountains, fire is the dominant natural disturbance agent.  While aspen can exist as a climax species composed of uneven-aged trees, recurring fires typically create even-aged stands where all the trees are more or less the same size.  Humongous fires can create a landscape of even-aged stands, and these recurring fires tend to halt natural plant succession by periodically killing the more shade-tolerant conifers.  In other words, the absence of intermittent fire generally leads to the loss of aspen stands.

Aspen grows as a clone, and assuming a conifer seed source is present, the natural succession from an aspen stand to a conifer stand will develop until the less fire-enduring conifers burn. Fire stimulates shoot-sprouting from the roots and once again the aspen stems will thrive in wide-open sunlight for years until conifers once more topple the shade intolerant aspen – or the system is reset by another fire. It is possible for aspen stands to dominate a site for 100-200 years, sometimes longer, until they are replaced by conifers.

Aspen stand
Photo: Justin Haug.

Because aspen have moist leaves and thick twigs – as opposed to conifers that typically have thin, dry needles – aspen do not readily burn. It is common for a coniferous crown fire to drop to the ground when it reaches a stand of aspen, where it sometimes is all but extinguished as a result of moist ground fuel and low heat. If a fire is sufficiently intense to burn a stand of aspen, it can be expected that significant sucker-sprouting will occur, producing as many as 50,000-100,000 shoots in one acre!

Aspen stand
Aspen stands to dominate a site for 100-200 years, until eventually replaced by confers. Photo: Dave Powell, USDA Forest Service, Bugwood.org

Like most tree species, aspen will self-thin over time until the stand has about as many trees growing as the stand previous to the fire disturbance. Likewise, it is to be expected that the plants, insects, and wildlife associated with aspen re-occupy the site following the fire.

Wildlife

Compared to coniferous forests, aspen ecosystems host a tremendous variety and abundance of animals. Aspen stands also contain a vast amount of forage – equal to the healthiest grassland ecosystems, and ten times that of coniferous stands. Because of this richness in forage, aspen stands have long been grazed by sheep and cattle throughout the West. Intense browsing by both domestic livestock and wild ungulates such as deer and elk can seriously impact successful aspen regeneration; it is not uncommon that animals have reduced or eliminated aspen sprouts (suckers) due to heavy browsing.

wild turkey in aspen stand
Aspen leaves and buds are a favorite food for grouse and turkeys, particularly in winter when other food items are scarce. Photo” Justin Haug.

Aspen are also perfect prey for beaver, which use aspen for food as well as to build dams. This “cutting” stimulates aspen to sprout, and the beaver consume the bark and small twigs of the felled tree. Larger branches may be used for dam building, and the subsequent flooding can change the entire stream system, often creating conditions too wet for aspen to thrive. Other types of water-loving vegetation then develop, thus increasing the plant community diversity.

Aspen have very thin bark and are extremely sensitive to nibbling and chewing by voles, hares, deer, elk, moose, bears, and porcupines. Aspen leaves and buds are also a favorite food item for wildlife such as grouse and turkeys, particularly in the winter when insects and other food items are scarce. For more information on deer, elk, grouse, turkey, and other species that utilize aspen ecosystems, visit www.woodlandfishandwildlife.com

Aspen health

Recently scientists have been concerned about aspen health through its broad range. While aspen decline is a condition which has been attributed to a variety and probably a combination of damage agents, aspen decline appears to be an acute condition most frequently observed on the edge if its life zone, where aspen may or may not have existed centuries ago. Landscape-level die off has been particularly witnessed in Colorado, Utah, Wyoming and Arizona.

Biotic and abiotic damage can lead to weakened or dead trees.  Insect pests include aphids, sawflies, leaf miners, borers, and a host of defoliators. While damaging insects can have serious to incidental impacts on the tree health, impacts from disease are generally a lesser risk to aspen stands. Wind and snow damage can weaken or break trees, reducing tree vigor and rendering them vulnerable to further damage by insects or decaying fungi and canker organisms. As a result of the opening, sunscald can occur on the remaining trees if abruptly exposed to sunlight – which is a bad thing. But the disturbance may also stimulate suckering – which is a good thing! Other weather related damage can occur from extreme temperatures, severe drought, hail and ice.

Healthy aspen stand
A healthy aspen stand will have prolific sprouting, an abundance of sapling- and pole-sized trees and plenty of overstory mature trees. Photo: Dave Powell, USDA Forest Service, Bugwood.org

Is your aspen stand healthy? How do you know? A healthy aspen stand will have prolific sprouting, an abundance of sapling/ pole-sized trees as well as plenty of overstory mature trees. If you can see through the stand there is probably something wrong and you as the landowner can determine the management action to take. Your tolerance to your trees dying or absence may be high and you may elect to do nothing. Or you can take corrective actions to improve the health of the stand. If the stand is not sprouting suckers it may require some disturbance to stimulate regeneration. Or perhaps there is sprouting, but over-browsing by livestock or wildlife may be occurring. New sprouts are the best gauge of a healthy stand of aspen.

Aspen management

Most landowners do not manage their aspen stands for wood products. Rather, they tend to be used for other objectives such as enhancing wildlife habitat, forest grazing, recreation, or simply aesthetic beauty. As a result of fire exclusion or other disturbances which stimulate continued suckering (sprouting), natural succession occurs with grasses, forbs, shrubs, and more shade-tolerant trees capturing the site. Drought is probably also a factor leading to aspen decline, particularly at the edge of the aspens life zone, or ecotone. The optimal condition for an aspen stand would look like this:

  • periodic disturbance,
  • continuous sprouting, and
  • an uneven-aged structure that would develop containing lots of suckers, sapling-pole sized trees, and mature trees.

Landowners wishing to achieve this healthy forest condition should inventory what currently exists, taking before and after treatment photographs. Be sure to locate a monument for your photo point monitoring. These activities could be great learning opportunities for youth groups, friends, and family.

Once the inventory and before pictures are obtained, landowners should consider the various treatments options they have and evaluate the “doabiltiy” of the activities.  These can all be documented as part of your overall Forest Stewardship Plan. The plan will also assist in record keeping and scheduling of treatments.

Disturbance or restoration treatments are most safely accomplished through some mechanical means, whether it’s by hand or equipment. Often landowners do not have the knowledge, skill, ability, or necessary equipment to execute these treatments. The Washington Department of Natural Resources (DNR) or your local WSU Extension Forester can help you contact qualified contractors that have the capacity to implement your desired practices.

Some owners may wish to use fire (called a prescribed burn) as a treatment method. Fire behaves differently than bulldozers and chainsaws – it’s a chemical process rather than a mechanical process – though the results of a fire may render similar appearing results and serve landowner objectives. Care must be taken when using fire; escape and smoke control are risks that most landowners would rather not incur. An additional risk with a prescribed burn is the chance that the fire will get too hot, killing the shoots, roots, and potentially sterilizing the soil.

Inviting your local DNR Stewardship Forester to take a walk in the woods with you to discuss your intentions and to provide his/her professional opinion is highly recommended. The stewardship forester can also discuss “clean-up” methods regarding the slash that will be created by the thinned-out conifers. Remember, you want light and water to penetrate the forest floor to stimulate sprouting, and if too much slash is accumulated, successful regeneration may take much longer to attain. After treating a stand to stimulate sprouting and reduce conifer encroachment, regeneration is typically observed within 3-4 years, so be patient if you do not see immediate results.

Aspen exhibits unique regeneration and growth. Intermittent disturbance, either through active management or in the presence of infrequent fire, can control competing vegetation and stimulate continuous propagation of sprouts. A healthy stand of aspen benefits wildlife, watershed protection, and your recreational and aesthetic enjoyment.

By Andy Perleberg, WSU Extension Forester, andyp@wsu.edu

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