Populus trichocarpa

Populus trichocarpa, the black cottonwood, western balsam-poplar or California poplar, is a deciduous broadleaf tree species native to western North America. It is used for timber, and is notable as a model organism in plant biology. The tree is notable for the seed-carrying cottony fluff it releases into the air each spring.

It is a large tree, growing to a height of 30 to 50 m (98 to 164 ft) and a trunk diameter over 2 m (6+1⁄2 ft). It ranks 3rd in poplar species in the American Forests Champion Tree Registry. It is normally fairly short-lived, but some trees may live up to 400 years. A cottonwood in Willamette Mission State Park near Salem, Oregon, holds the national and world records. Last measured in April 2008, this black cottonwood was found to be standing at 47 m (155 ft) tall, 8.8 m (29 ft) around, with 527 points.

The bark is grey and covered with lenticels, becoming thick and deeply fissured on old trees. The bark can become hard enough to cause sparks when cut with a chainsaw. The stem is grey in the older parts and light brown in younger parts. The crown is usually roughly conical and quite dense. In large trees, the lower branches droop downwards. Spur shoots are common. The wood has a light coloring and a straight grain.

The leaves are usually 7–20 centimetres (2+3⁄4–7+3⁄4 in) long with a glossy, dark green upper side and glaucous, light grey-green underside; larger leaves may be up to 30 cm (11+3⁄4 in) long and may be produced on stump sprouts and very vigorous young trees. The leaves are alternate, elliptical with a crenate margin and an acute tip, and reticulate venation. The petiole is reddish. The buds are conical, long, narrow, and sticky, with a strong balsam scent in spring when they open.

P. trichocarpa has an extensive and aggressive root system, which can invade and damage drainage systems. Sometimes, the roots can even damage the foundations of buildings by drying out the soil.

In 2016, the first direct evidence was published indicating that wild P. trichocarpa fixes nitrogen.

P. trichocarpa is normally dioecious; male and female catkins are borne on separate trees. The species reaches flowering age around 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, elongated to 2 to 3 cm, and are deciduous. The pollen can be an allergen. Pistillate catkins at maturity are 8 to 20 cm long with rotund-ovate, tricarpellate subsessile fruits 5 to 8 mm long. Each capsule contains many minute seeds with long, white, cottony hairs.

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 P. trichocarpa seed under natural conditions may be as short as two weeks to a month. This can be increased with cold storage.