What good is a cottonwood tree anyway? (part 1)

Black cottonwood
Black cottonwood tree (also called western cottonwood). Photo: David Powerll/U.S. Forest Service. bugwood.org

Here at DNR we often talk about trees in terms of their value for habitat as well as for revenue to state trust land beneficiaries, such as public schools, state universities, many counties, and others. Then there are the trees whose main value is for habitat and beauty. Cottonwoods fall squarely into this latter category.

Cottonwoods aren’t worth much on the timber market, they can crowd out and shade new conifer plantations, and they don’t have many BTUs of energy for firewood use. They sprout when and where they aren’t wanted and form impenetrable stands. They can clog septic drain fields. They are notorious for breaking apart during minor storms and, among other annoying habits, their billowing cottony seeds can clog water intake structures and screens. Yet, they are one of the most widespread and important wildlife trees in the western United States and Canada.

Cottonwoods belong to genus Populus. There are at least four primary species of Populus in North America: eastern cottonwood (Populus deltoides), balsam poplar (P. tacamahacca), black or western cottonwood (P. trichocarpa), and quaking or trembling aspen (P. tremuloides). Two of these–western cottonwood (also called black cottonwood) and quaking aspen–are found on appropriate sites across Washington.

Balsam poplar occurs throughout the intermountain west and is most prevalent in northern Canada and Alaska while aspen, the most widespread native poplar throughout the northern hemisphere, is unique enough for an article of its own. In this article, we will focus on the black or western cottonwood.   

This poplar grows to heights and diameters larger than all other poplars in North America. It can live well over 100 years and attain heights of 150 feet and growth rates of seven feet or more in a year. Thus, it rivals most of our native conifers and certainly outgrows them in its first several years.

The growth rate of the black cottonwood obviously causes problems for young conifers that need the space, sunlight, moisture, and nutrients that are invariably captured by the faster growing cottonwoods. One thing all cottonwoods have in common is their love for high soil moisture. Although they can survive in low moisture conditions, cottonwoods do not begin to achieve their height and growth potential in arid soils. In fact, they can survive with short-term partial inundation; conditions that would kill almost all of our native conifers.

So what makes the black cottonwood so valuable to wildlife? Their twigs, barks, cambium and leaves provide food and forage for a wide array of browsing and gnawing animal, as well as birds and insects. Tomorrow, we’ll take a closer look at the habitat values that these water-loving cottonwoods provide.

This article written by Jim Bottorff, a retired DNR forest stewardship wildlife biologist, and originally published in Forest Stewardship Notes, a quarterly e-newsletter produced by DNR and Washington State University Extension.

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