Populus trichocarpa

Black cottonwood


The Basics

Taxonomy: Kingdom - Plantae (plants). Subkingdom - Tracheobionta (vascular plants). Superdivision - Spermatophyta (seed plants). Division - Magnoliophyta (Flowering plants). Class - Magnoliopsida. Order - Salicales. Family - Salicaceae (Willow family). Genus - Populus L. Species - Populus trichocarpa (Torr. & A. Gray)

Ecology: Black cottonwood (Populus trichocarpa) is the largest of the American poplars and the largest hardwood tree in western North America. Known also as balsam cottonwood, western balsam poplar, and California poplar, it grows primarily on moist sites west of the Rocky Mountains. The most productive sites are the bottom lands of major streams and rivers west of the Cascade Range in the Pacific Northwest. Pure stands may form on alluvial soils. Black cottonwood is harvested and used for lumber, veneer, and fiber products. Many kinds of wildlife use the foliage, twigs, and buds for food, and the tree is planted for shade and in windbreaks and shelterbelts. Black cottonwood is a pioneer and early seral species. It and other members of the Populus genus are some of the fastest growing temperate trees, an adaptation useful for pioneering disturbed sites. Any activity that exposes moist, mineral soil in full sunlight creates a favorable habitat for black cottonwood seedlings, particularly when seed trees are nearby. Black cottonwood is very shade intolerant. The species can tolerate flooding and sediment deposition; when young stems are covered by sediment during floods, sprouts grow from the stem towards the current.


Black cottonwood typically has a straight, branch-free trunk for more than half its length and a broad, open crown. In closed stands, the crown tends to be narrow, with few branches growing into the lower 2/3rds of the trunk. Establishment by seed is episodic, often creating stands of several well-defined age groups. Black cottonwood is a fast growing, native deciduous tree, growing to 100 feet (30 m) high [36,159] though occasionally as high as 160 feet (50 m). Basal diameter is commonly 3.5 to 5 feet (1-1.5 m) [36,159]. Black cottonwood commonly lives 100 to 200 (or occasionally more) years. Flowers grow in catkins. Staminate catkins are usually about 1 inch (2-3 cm) long, though occasionally up to 2 inches (5 cm) long. Pistillate catkins are 3 to 8 inches (8-20 cm) long. Capsules are subsessile and 0.2 to 0.32 inch (5-8 mm) long; each capsule contains 3 (occasionally 2 or 4) carpels. Seeds bear long, white, "cotton" fibers that aid in dispersal via wind and/or water. Each cottonwood seed weighs approximately 0.3 to 0.6 mg.


Fire effects- Black cottonwood is frequently damaged by fire with low-severity burns even often causing "considerable injury". Young black cottonwood trees and seedlings are usually killed by fire regardless of severity. Severe fire kills or top-kills even older trees. In low- and moderate-severity fires older trees with thick bark may not be top-killed. In members of the Populus genus stem bark remains thin for longer than in other trees. Though old trees have increased fire resistance due to thicker, they have higher fuel loading and more heartrot, which can increase fire severity. Trunks that are not top-killed may be more susceptible to Cytospora spp., and other fungal pathogens.

Environtmental: Young saplings are frequently injured and sometimes killed by unseasonably early or late frosts. Frost cracks also lower quality of wood and provide an entrance for decay fungi. Ice storms and heavy snowfall cause considerable breakage and permanent bending. Wind damage is common, especially in stands where black cottonwood trees are much taller than surrounding vegetation; top breakage and bending result. Erosion along rivers and major streams also takes its toll in adjacent black cottonwood stands. The species is highly susceptible to fire damage.

Mammals: Mammals can create serious problems in black cottonwood plantations, especially at time of establishment or soon after. Meadow voles and meadow mice can cause severe losses in young plantations; such damage occurs most commonly on grassy or herb-covered sites. The voles feed on roots and sometimes girdle the lower stem. In some locations, rabbits and hares cause losses in young cottonwoods via clipping and basal girdling damage. Damage also results when beavers use cottonwood for food and construction of dams. Browsing and trampling of saplings by elk and deer sometimes decimate small, isolated plantings. Slugs have girdled cottonwood stems and presumably have eaten buds and newly emergent leaves of recently planted cuttings in the lower Columbia River valley.

Pathogens and fungus: At least 70 fungal species cause decay in cottonwood, but only six fungi cause significant losses in British Columbia; two of these (Spongipellis delectans and Pholiota destruens) cause 92 percent of the loss (15,17,33). A leaf rust (Melampsora spp.) has been observed in young plantations, and susceptibility to the rust appears to vary greatly across the geographic range of the species. This disease limits photosynthesis and causes leaves to fall prematurely, thereby decreasing tree growth and vigor. Severe Melampsora infections have been observed when clonal material from relatively dry areas (e.g., east of the Cascade Range in Washington or Oregon and northern California) was planted in western Washington (9); in one instance, such infections resulted in death of the clones. Other foliage diseases include leaf-spot syndrome (Venturia populina) and yellow-leaf blister (Taphrina populisalicis). A deformity of catkins is caused by Taphrina johansonii. Cytospora canker (Cytospora chrysosperma) is widespread under forest conditions but rarely causes significant damage in vigorous cottonwood stands. It may cause problems, however, to cuttings in nurseries and plantations. Stem cankers in various areas have been reported as caused by Dothichiza populea, Fusarium spp., Hypoxylon mammatum, Nectria galligena, and Septoria musiva. None appear to be of great significance in management of black cottonwood, but severe attacks of a bacterial canker have reportedly limited planting of the species in Europe. Black cottonwood is also subject to the condition known as wet wood, which leads to wood collapse during drying.


Flowering and Fruiting - Black cottonwood is normally dioecious; male and female catkins are borne on separate trees. The species reaches flowering age at about 10 years. Flowers may appear in early March to late May in Washington and Oregon, and sometimes as late as mid-June in northern and interior British Columbia, Idaho, and Montana. Staminate catkins contain 30 to 60 stamens, elongate to 2 to 3 cm (0.8 to 1.2 in), and are deciduous. Pistillate catkins at maturity are 8 to 20 ern (3.2 to 8 in) long with rotund-ovate, three carpellate subsessile fruits 5 to 8 min (0.20 to 0.32 in) long. Each capsule contains many minute seeds with long, white cottony hairs.

Seed Production and Dissemination - The seed ripens and is disseminated by late May to late June in Oregon and Washington, but frequently not until mid-July in Idaho and Montana. Abundant seed crops are usually produced every year. Attached to its cotton, the seed is light and buoyant and can be transported long distances by wind and water. Although highly viable, longevity of black cottonwood seed under natural conditions may be as short as 2 weeks to a month. There is some evidence, however, to suggest a somewhat longer lifespan under apparently adverse conditions. With proper drying and cold storage, viability and capacity to germinate can be maintained for at least 1 year.

Seedling Development - Moist seedbeds are essential for high germination, and seedling survival depends on continuously favorable conditions during the first month. Wet bottom lands of rivers and major streams frequently provide such conditions, particularly where bare soil has been exposed or new soil laid down. Germination is epigeal. Black cottonwood seedlings do not usually become established in abundance after logging unless special measures are taken to prepare the bare, moist seedbeds required for initial establishment. Where seedlings become established in great numbers, they thin out naturally by age 5 because the weaker seedlings of this shade-intolerant species are suppressed.

Vegetative Reproduction - Black cottonwood sprouts readily from stumps, and in one study, satisfactory coppice reproduction was obtained four times in 2-year cutting cycles. After logging operations, black cottonwoods sometimes regenerate naturally from rooting of partially buried fragments of branches. Sprouting from roots has also been reported. The species also has the unusual ability to abscise small shoots complete with green leaves. These shoots drop to the ground and may root where they fall or may be dispersed by water transport. In some situations, abscission may be one means of colonizing exposed sandbars.

Species Distribution


The Jepson Herbarium
The Jepson Manual: Vascular Plants of California. B.G. Baldwin, D.H. Goldman, D.J. Keil, R. Patterson, T.J. Rosatti, and D.H. Wilken [editors]. 2012. 2nd edition, thoroughly revised and expanded. University of California Press, Berkeley, CA.

USDA Plants Database
USDA, NRCS. 2016. The PLANTS Database. National Plant Data Team, Greensboro, NC 27401-4901 USA.

USFS Plant Database
Habeck, R. J. 1992. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory.

Burke Museum of Natural History and Culture
Burke Museum. 2016. Abies amabilis [Online]. University of Washington.