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. Use by humans 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, does not polish or plane 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 developed for lumber production or for use as shade trees. These hybrids are selected for exceptionally fast growth and disease resistance. Poplars are also of potential use for biofuels because of their fast growth. Researchers are aiming to use genetic techniques to make poplars grow fatter, with a smaller canopy, so that more trees can be grown more quickly in a small space, and to make the plants contain a higher proportion of cellulose to lignin. The increased cellulose content would make them easier to convert into sugars and ethanol for biofuel or for pulp in paper mills. The buds of various balsam poplars have long been combined with a lard base to make the vulnerary ointment Balm of Gilead. See also Balm of Gilead Protium heptaphyllum, South-American tacamahaca. References sect.