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No fully satisfactory infrageneric classification of spruces has been made, and classifications based on morphological data (e.g. Rushforth 1987, Farjon, 1990, Huxley 1992) conflict with those based on genetic data (e.g. Sigurgeirsson & Szmidt 1993, Ran et al. 2006). Species that co-occur may hybridise; the resulting genetic introgression complicates classification. The most distinct spruces morphologically are Picea neoveitchii from NW China, Picea torano from southern Japan, Picea breweriana from the western USA, and Picea martinezii from NE Mexico.
Spruces are medium-sized to very large trees; the largest is Picea sitchensis from western North America, which reaches up to 96 m tall and 5 m trunk diameter. By contrast, Picea mariana can be reproductively mature at just 1 m tall, and in many parts of its range rarely exceeds 10 m tall (though can occasionally reach 30 m tall). They have spirally arranged needle-like leaves which range from 5 mm long (in Picea orientalis) to 50 mm (in Picea smithiana) long; the needles sit individually on a stem projection called a pulvinus. The leaves are evergreen, with persistence ranging from 4 to 12 years; they are rhombic or flattened in cross-section, often sharply pointed, and have lines of often glaucous stomata (breathing pores) on all sides or just on the adaxial side (as also in Pinus, but unlike other Pinaceae genera where the stomata are concentrated on the abaxial side). The seed cones are produced in spring and mature in the autumn 4-8 months later; they are erect at first when pollinated, then turn pendulous as they mature; length varies from 2 cm (in Picea mariana) to 20 cm in (Picea abies) and colour from yellow-green to red to dark purple. There are two winged seeds under each cone scale; the seeds are blackish-brown, and range from 2 mm (in Picea mariana) to 7 mm (in Picea neoveitchii) long. The pollen cones are 10-35 mm long, and shed soon after pollen release in spring.
Spruces are a source of food and nesting sites for various species of birds and mammals: deer and rabbits eat the twigs and needles; porcupines feed on the bark; rodents eat the buds and seeds; and grouse and songbirds eat the seeds.
Spruces are a leading source of commercial wood in many north temperate countries. The wood is an important pulpwood, and is also used for general construction, plywood, interior finishing, boxes, and musical instruments. Spruces are also widely used as Christmas trees, although they are not as popular as firs (Abies species). Spruce resin has been used to produce turpentine. Several spruce species are widely planted as landscape and ornamental trees, and numerous cultivars have been developed, with variations in needle colour and form. Spruce needles can be used to produce spruce beer (alcoholic or non-alcoholic), which is high in vitamin C, and the inner bark can be used to make tea. Spruce resins have been used in North American and European traditional medicine to treat coughs, colds, and respiratory ailments, as well as aches and pains (Plants for a Future 2011).