Species
The balsam poplar P. balsamifera (= 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 (P. alba) which is a relative of the aspens (Populus sect. Populus).
The western balsam poplar, black cottonwood, or California poplar P. 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. It was announced on 15 September 2006 in the journal Science that P. trichocarpa became the first tree species to have its entire genome sequenced. In the mountains of interior western North America, it is replaced by the willow-leaved poplar or narrowleaf cottonwood, P. angustifolia. P. trichocarpa is sometimes treated within P. balsamifera as P. balsamifera ssp. trichocarpa.
Simon's poplar (P. 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 or Japanese poplar (P. maximowiczii) and the Ussuri poplar (P. ussuriensis = P. maximowiczii var. barbinervis) are similar, occurring in northeastern China, Japan, Korea, and eastern Siberia; they have broader leaves. Another similar species is the laurel-leaf poplar (P. laurifolia from Mongolia), which differs from its relatives in narrower leaves shaped like a bay laurel leaf. Another putative member of this group is the Korean poplar (P. koreana).
Whether the northeast Asian Sichuan poplar (P. szechuanica) and P. tristis belong here or with the aspens is not yet resolved. Likewise, the affiliation of P. cathayana and the Yunnan poplar (P. yunnanensis) are in need of further study, though these may well be balsam poplars.
There are other species not listed here. How to classify Populus into species, at least for Chinese populations, is not yet settled.
Read more about this topic: Populus Sect. Tacamahaca
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