Populus trichocarpa (black cottonwood; also known as western balsam poplar or California poplar) is a tree species native to western North America. It is used for timber, and is notable as a model organism in plant biology. Its full genome sequence was published in 2006. It is the first, and so far only, tree species to be sequenced and contains the largest number of genes ever discovered in any organism.
It is a large tree, growing to a height of 30-50 m and a trunk diameter of over 2 m, 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 (Forbes 2006). A cottonwood discovered in Haines, Alaska
set the national record at tall and 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 (see leaf terminology). 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.
Flowering and Fruiting
is normally dioecious
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
, 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
, but frequently not until mid-July in Idaho
. 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 2 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 5 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
The native range of P. trichocarpa
covers large sections of western North America
. It extends northeast from Kodiak Island
along Cook Inlet
62° 30° N., then southeast in southeast Alaska
and British Columbia
to the forested areas of Washington
, to the mountains in southern California
and northern Baja California
(lat. 31° N.). 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
, 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
Use as a model species
P. trichocarpa has several qualities which makes it a good model species for trees:
- Modes genome size (although signicantly 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 (ESTs) have been sequenced from it. The wide range of topics studied by using P. trichocarpa include the effects of ethylene, lignin biosynthesis, draught tolerance and wood formation.
wood is light-weight and although not particularly strong, is strong for its weight. The wood material has short, fine cellulose
fibres which are used in the production of high-quality book and magazine paper
. The wood is also excellent for production of plywood
. Living trees
are used as windbreaks
P. trichocarpa contains salicin, and has been used medicinally as an antipyretic, analgesic and to control inflammation.
P. trichocarpa grows very quickly; trees in plantations in Great Britain have reached tall in 11 years, and tall in 28 years (Mitchell 1996). It can reach suitable size for pulp production in 10-15 years and about 25 years for timber production.
It is also 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 (Mitchell 1996).
Branches can be added to potted plants to stimulate rooting.
Commercial extracts are produced from the fragrant buds for use as a perfume in medicines and cosmetics.
The sequence of P. trichocarpa
is that of an individual female
, named after the Nisqually River
in the Washington
state in the USA
, where the specimen was collected. The sequencing
was performed at the Joint Genome Institute
by using the shotgun
method. The depth of the sequencing was approximately 7,5 x (meaning that each base pair
on average 7,5 times). Genome annotation
was done by primarily by the Joint Genome Institute
, the Oak Ridge National Laboratory
, the Umeå Plant Science Centre
and the 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. trichocarpa 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.
"Trichocarpa" is Greek for "hairy fruits". The following scientific names are now considered synonymous with Populus trichocarpa
- P. balsamifera subsp. trichocarpa
- P. balsamifera var. californica
- P. hastata
P. trichocarpa subsp. hastata
P. trichocarpa var. hastata
P. trichocarpa var. cupulata
P. trichocarpa var. ingrata |