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.

Description

It is a large tree, growing to a height of and a trunk diameter over , which makes it the largest poplar species in the Americas. 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 tall, 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.Ewing, Susan. The Great Alaska Nature Factbook. Portland: Alaska Northwest Books, 1996. 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 long with a glossy, dark green upper side and glaucous, light grey-green underside; larger leaves may be up to 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.

Reproduction

Flowering and fruiting

''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. 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.

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. The pollen can be an allergen. 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 bottomlands of rivers and major streams frequently provide such conditions, particularly where bare soil has been exposed or new soil laid down. Germination is epigeal (above ground). ''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.

Taxonomy

"''Trichocarpa''" is Greek for "hairy fruits". These scientific names are now considered synonymous with ''P. trichocarpa'':

Distribution and habitat

The native range of ''P. trichocarpa'' covers large sections of western North America. It extends from Southeast Alaska's Kodiak Island and Cook Inlet to latitude 62° 30° N., through British Columbia and the forested areas of Washington and Oregon, to the mountains in southern California and northern Baja California (31°N). It is also found inland, generally on the west side of the Rocky Mountains, in British Columbia, southwestern Alberta, western Montana, and north-to-central Idaho. Scattered small populations have been noted in southeastern Alberta, eastern Montana, western North Dakota, western Wyoming, Utah, and Nevada.

Black cottonwood grows on alluvial sites, riparian habitats, and moist woods on mountain slopes, from sea level to elevations of . It often forms extensive stands on bottomlands of major streams and rivers at low elevations along the Pacific Coast, west of the Cascade Range. In eastern Washington and other dry areas, it is restricted to protected valleys and canyon bottoms, along streambanks, and edges of ponds and meadows. It grows on a variety of soils from moist silts, gravels, and sands to rich humus, loams, and occasionally clays. Black cottonwood is a pioneer species that grows best in full sunlight and commonly establishes on recently disturbed alluvium. Seeds are numerous and widely dispersed because of their cottony tufts, enabling the species to colonize even burn sites, if conditions for establishment are met. Seral communities dominated or codominated by cottonwood are maintained by periodic flooding or other types of soil disturbance. Black cottonwood has low drought tolerance; it is flood-tolerant but cannot tolerate brackish water or stagnant pools.

''P. trichocarpa'' has been one of the most successful introductions of trees to the otherwise almost 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.

Ecology

Although the most populous cottonwood of the Pacific Northwest, it hybridizes with the region's three other species: balsam poplar, plains cottonwood, and narrowleaf cottonwood; all four have similar appearances and provide habitats for various animals.

Cottonwoods are shade intolerant. Black cottonwood thrives by colonizing disturbed sites, but can be replaced by conifers. The wood is relatively weak and waterlogged, often splitting during freezes. It is susceptible to rot as well. Woodpeckers create cavities which various animals can use for nests. Larger birds nest in the large upper branches. Beavers use the trees as food and dam-building material.

Cultivation

It is grown as an ornamental tree, valued for its fast growth and scented foliage in spring, detectable from over 100 m distance. The roots are however invasive, and it can damage the foundations of buildings on shrinkable clay soils if planted nearby (Mitchel 1996).

Branches can be added to potted plants to stimulate rooting.

Uses

The wood, roots and bark have been used for firewood, canoe making, rope, fish traps, baskets and structures. The gum-like sap was used as a glue or as waterproofing.Pojar, J. & MacKinnon, A. Plants of the Pacific Northwest Coast. Vancouver, British Columbia: Lone Pine Publishing, 2004. Native Americans consumed cottonwood inner bark and sap, feeding their horses the inner bark and foliage. Commercial extracts are produced from the fragrant buds for use as a perfume in cosmetics.

Lumber

''P. trichocarpa'' wood is light-weight and although not particularly strong, is strong for its weight. The wood material has short, fine cellulose fibres that are used in pulp for high-quality book and magazine paper. The wood is also excellent for production of plywood. Living trees are used as windbreaks.

This species grows very quickly; trees in plantations in Great Britain have reached tall in 11 years, and tall in 28 years. It can reach suitable size for pulp production in 10–15 years and about 25 years for timber production.

As a model species

''Populus trichocarpa'' has several qualities that makes it a good model species for trees:
* Model genome size (although significantly larger than the other model plant, ''Arabidopsis thaliana'')
* Rapid growth (for a tree)
* 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 (see "Genome" below). 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.

Genome

Characteristics

* Size: 485 million base pairs (human genome: 3 billion 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 (estimate in September 2008)
* Mitochondrial genome: 803,000 base pairs, 52 genes
* Chloroplast genome: 157,000 base pairs, 101 genes

General information

The sequence of ''P. trichocarpa'' is that of an individual female specimen "Nisqually-1", named after the Nisqually River in Washington in the United States, where the specimen was collected. The sequencing was performed at the Joint Genome Institute using the shotgun method. The depth of the sequencing was about 7.5 x (meaning that each base pair was sequenced on average 7.5 times). Genome annotation was done primarily by the Joint Genome Institute, the Oak Ridge National Laboratory, the Umeå Plant Science Centre, and Genome Canada.

Prior to the publication of ''P. trichocarpa'' genome the only available plant genomes were those of thale cress and rice, both of which are herbaceous. ''P. trihocarpa'' is the first woody plant genome to be sequenced. Considering the economic importance of wood and wood products, the availability of a tree genome was necessary. The sequence also allows evolutionary comparisons and the elucidation of basic molecular differences between herbaceous and woody plants.

Doty et al. (2016) found the first direct evidence of nitrogen fixation in the wild ''P. trichocarpa'' (a nonleguminous tree).

Somatic mosaicism

Genome-wide analysis of 11 clumps of ''P. trichocarpa'' trees reveals significant genetic differences between the roots and the leaves and branches of the same tree. The variation within a specimen is as much as found between unrelated trees. These results may be important in resolving debate in evolutionary biology regarding somatic mutation (that evolution can occur within individuals, not solely among populations), with a variety of implications.

References

Further reading

Populus genome at the JGI website
* Popgenie: The Populus Genome Integrative Explorer

Plants for a Future: ''Populus trichocarpa''
* Forbes, R. D. (2006)
Morrisey Old Growth Cottonwood Forest
(pdf file)
* Mitchell, A. F. (1996). ''Alan Mitchell's Trees of Britain''. Collins. .
* Davis, T. Neil. (1981)


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Garden plants of North America
Trees of the West Coast of the United States
Trees of the Northwestern United States
Trees of Alaska
Trees of Alberta
Trees of British Columbia
Trees of the Southwestern United States
Flora of the Sierra Nevada (United States)
Plant models
Ornamental trees
Plants used in traditional Native American medicine
trichocarpa
Trees of Subarctic America
Trees of Western Canada
Flora without expected TNC conservation status