Balsam poplar
The balsam poplars Populus sect. Tacamahaca are a group of about 10 species of poplars, indigenous to North America and eastern Asia, distinguished by the balsam scent of their buds, the whitish undersides of their leaves, and the leaf petiole being round (not flattened) in cross-section. They are large deciduous trees, 30-60 m tall, with leaves with a rounded base, pointed apex, and a whitish waxy coating on the underside of the leaf; this latter distinguishes them from most other poplars. The name is derived from the pleasant balsam smell of the opening buds and leaves in spring, produced by a sticky gum on the buds which also helps protect the buds from insect damage. The balsam poplars are light-demanding trees that requires considerable moisture but are tolerant of very cold conditions, occurring further north than other poplars except for the aspens.
The Ontario Balsam Poplar Populus balsamifera (syn. P. tacamahaca, P. candicans) is a native of North America, where it grows on alluvial bottomlands in the northeastern United States and Canada. It grows to a height of 30 metres and has yellow-grey bark, thick and furrowed, and coloured blackish at the base of the trunk. The twigs are yellow-brown to brown, the buds covered with a layer of balsam resin. The flowers and fruit are very much like those of the White Poplar.
The Western Balsam Poplar or Black Cottonwood Populus trichocarpa is native to western North America, from Alaska south to northern California. It is the largest species of poplar, recorded to 65 m tall. It is also a very important species in plant biology. In the mountains of interior western North America, it is replaced by the Willow-leaved Poplar or Narrowleaf Cottonwood Populus angustifolia. It was announced on the 15 September 2006 in the journal Science that Populus trichocarpa became the first tree species to have its entire genome sequenced.
Simon's Poplar (Populus simonii), a native of northwestern China, is frequently planted as a shade tree in northern European cities. It is an attractive ornamental tree with whitish bark, and nearly rhombic, 6-10 cm long leaves which appear on the tree in early spring. Maximowicz' Poplar P. maximowiczii is similar, occurring in northeastern China, Japan, Korea and eastern Siberia; it has broader leaves. Another similar species from Mongolia is the Laurel-leaf Poplar P. laurifolia, which differs from the other two in narrower leaves shaped like a bay laurel leaf.
Balsam poplars are cultivated mainly in parks for their ornamental, light-coloured bark and pleasant scent in spring. Western Balsam Poplar is also planted as a timber crop. The wood is soft, very light in weight but strong for its weight, coarse and fibrous, not polishing or planing easily, and is used for pallet boxes and other similar rough uses.
Several hybrids between balsam poplars (particularly Western Balsam Poplar) and the cottonwoods have also been produced for wood production. These hybrids are selected for exceptionally fast growth and disease resistance.
Poplars are also interesting from the point of view of producing biofuels because of their fast growth.  Researchers are aiming to use genetic techniques to make poplars grow fatter and with a smaller canopy, for example, so that more trees can be grown more quickly in a small space. And they would like to make the plants contain a higher proportion of cellulose to lignin. This makes them easier to convert into sugars and ethanol for biofuel.