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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.

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.


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. 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.


2. Dozing. Mechanically uproot adult trees and break apart root systems. Commonly generates high regeneration of stems, but eliminates adult trees until new growth matures.


3. Cut/Harvest. 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.


4. Ripping. 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.


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. 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.

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.


Shepperd, Wayne D.; Paul C Rogers; David Burton; and Dale L. Bartos. 2006. Ecology, biodiversity, management, and restoration of aspen in the Sierra Nevada. Gen. Tech. Rep. RMRS-GTR-178. Fort Collins, CO: U.S. Dept. of Agriculture, Forest Service, Rocky Mountain Research Station 122 p.



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