Cottonwood trees 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 storm events and have even clogged water intake structures and screens with their billowing cottony seeds, among other annoying habits. Yet, they are one of the most widespread and important wildlife trees in the western United States and Canada. We’re referring to cottonwood trees or more technically, the genus Populus.
There are at least four primary species of Populus in North America, with two of those commonly found across Washington. These four include the eastern cottonwood (Populus deltoides), black or western cottonwood found in Washington (P. trichocarpa), balsam poplar (P. tacamahacca), and quaking or trembling aspen (P. tremuloides) also found on appropriate sites across Washington.
Balsam poplar occurs throughout the intermountain west and is most prevalent in northern Canada and Alaska. Seldom do you hear it called anything other than a cottonwood. Aspen are true poplars but with enough differences to be considered here separately. They are the most widespread native poplar occurring throughout the northern hemisphere and are subject to an article of their own. Not to be forgotten is the more recently created hybrid poplar; a cross between the eastern cottonwood and black or western cottonwood.
In this article we are considering only the black or western cottonwood. This poplar attains 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. This growth rate 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 most all our native conifers.
So what makes them so valuable for wildlife? Well, for starters it seems like every browsing and gnawing animal thrives on young cottonwood twigs, bark, cambium, and leaves. This includes a host of insects as well as the predatory birds and mammals that feed on them. Obviously, the water-loving cottonwood does well in riparian environments and, consequently, so do beaver, which use cottonwood for food, dam, and lodge building. Being a deciduous species, cottonwood will root and stump-sprout when felled. Rabbits and hares feed extensively on cottonwood shoots and small stems; deer, elk, and moose are particularly fond of them as well. Ruffed grouse and poplar trees go hand-in-hand. Cottonwoods have large naked terminal buds that develop and persist through the winter months. During high snow events, grouse literally survive in those trees, roosting at night and feeding on these highly nutritious buds during the day.
The older large cottonwoods make excellent nest platforms for a variety of predatory birds. Eagles and ospreys commonly select large branches or broken-top cottonwoods as platforms for nest construction. Eagles frequently use cottonwoods for night roosts and for hunting perches. Great horned owls will commandeer other bird and squirrel nest platforms in cottonwoods and use them as their nest sites, as will red-tailed hawks. In drier environments, cottonwoods will be relegated to stream bottoms and are often the only large tree for long distances. In these environments, turkeys (in particular the Rio Grande subspecies) select cottonwoods for night roosts. Without suitable night roosts some flocks of turkeys would likely disappear.
As cottonwoods age and show obvious signs of senescence and decay, they are far from done as an important wildlife habitat. In fact they may harbor more wildlife than when they were young and robust. When branches snap off and expose cambium, they are usually attacked by a variety of fungal species. They are susceptible to heart-rot and other decay issues as a result. This renders them highly valuable to a large variety of cavity-dependent birds and mammals. Several species of woodpeckers not only feed on the insects that the tree supports, but excavate nest cavities in them. More than 40 other species of birds and mammals use the abandoned woodpecker cavities for their nesting and roosting activities. The larger hollow cottonwoods have been used as winter hibernation chambers by black bears and smaller openings by some bat species.
As exasperating as cottonwood trees can be to some forestland owners, they are an invaluable wildlife habitat resource throughout their range. Where they can be protected –do it! And if they don’t occur within suitable habitat on your property, they are easily planted. In fact they are one of the easiest species to propagate, and one of the fastest to deliver results.
by Jim Bottorff, Forest Stewardship Wildlife Biologist, retired
Washington Department of Natural Resources