Black Cottonwood

Appearance

It is a large tree, growing to a height of 30 metres to 50 metres and a trunk diameter of over 2 metres, which makes it the largest poplar species in the Americas. It is normally fairly short-lived, but some trees may live for up to 400 years. A cottonwood discovered in Haines, Alaska set the national record at 101 ft tall and 32.5 ft around.

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 7–20 cm long with a glossy dark green upper side and glaucous light grey-green underside; larger leaves, up to 30 cm long, may be produced on stump sprouts and very vigorous young trees. The leaves are alternate, elliptic 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.* Size: 485 million base pairs.
⤷  Proportion of heterochromatin to euchromatin: 3:7
⤷  Number of chromosomes: 19
⤷  Number of putative genes: 45,555, the largest number of genes ever recorded
⤷  Mitochondrial genome: 803,000 base pairs, 52 genes
⤷  Chloroplast genome: 157,000 base pairs, 101 genes

Naming

"P. trichocarpa" has several qualities which makes it a good model species for trees:
⤷  Model genome size
⤷  Rapid growth
⤷  Reaches reproductive maturity 4–6 years
⤷  Economically important
⤷  It represents a phenotypically diverse genus

For these reasons the species has been extensively studied. Its genome sequence was published in 2006. More than 121 000 expressed sequence tags have been sequenced from it. The wide range of topics studied by using "P. trichocarpa" include the effects of ethylene, lignin biosynthesis, drought tolerance and wood formation.

Distribution

The native range of "P. trichocarpa" covers large sections of western North America. It extends northeast from Kodiak Island along Cook Inlet to latitude 62° 30° N., then southeast in southeast Alaska and British Columbia to the forested areas of Washington and Oregon, to the mountains in southern California and northern Baja California. It is also found inland, generally on the west side of the Rocky Mountains, in British Columbia, western Alberta, western Montana, and northern Idaho. Scattered small populations have been noted in southeastern Alberta, eastern Montana, western North Dakota, western Wyoming, Utah, and Nevada. It grows up to elevations of 2100 m.

"Populus trichocarpa" has been one of the most successful introduction of trees to the otherwise more or less treeless Faroe Islands.

The species was imported from Alaska to Iceland in 1944 and has since become one of the most widespread trees in the country.

Reproduction

Flowering and fruiting

"P. trichocarpa" 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, and are deciduous. Pistillate catkins at maturity are 8 to 20 cm long with rotund-ovate, three carpellate subsessile fruits 5 to 8 mm 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 "P. trichocarpa" seed under natural conditions may be as short as two weeks to a month. This can be increased with cold storage.

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. "P. trichocarpa" 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 five because the weaker seedlings of this shade-intolerant species are suppressed.

Vegetative reproduction

Due to its high levels of rooting hormones, "P. trichocarpa" sprouts readily. After logging operations, it sometimes regenerates naturally from rooting of partially buried fragments of branches or from stumps. Sprouting from roots also occurs. The species also has the ability to abscise 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.