Pinus, the pines, is a genus of around 115-120 species of coniferous trees that grow widely around the northern hemisphere from cold boreal to tropical regions. They are abundant over large areas of the huge boreal taiga forests, but species diversity there is low, with only five species (Pinus sylvestris in Europe and Asia, Pinus sibirica and Pinus pumila in Asia, and Pinus banksiana and Pinus contorta in North America); species diversity is much greater in mountain forests at lower latitudes, being high between 20° to 45°N, and at a maximum in Mexico, California, and southern China. The northernmost and most widely distributed is Pinus sylvestris, reaching well north of the Arctic Circle at 71°N in Norway, and the southernmost is Pinus merkusii, which reaches just south of the Equator at 2°S in Sumatra. Several species are rare, and some critically endangered; the rarest is Pinus squamata, with under 40 individuals in Yunnan, SW China. The genus is divided into two subgenera, subgenus Pinus (hard pines) with a double vascular bundle in the leaves, and subgenus Strobus (soft pines) with a single vascular bundle in the leaves. Subgenus Strobus has sometimes also been divided into two subgenera, subgenus Strobus in a strict sense (white pines) with cone scales with a terminal umbo, and subgenus Ducampopinus (lacebark, pinyon and bristlecone pines) with cone scales with a dorsal umbo (in which they resemble subgenus Pinus), but this morphological subdivision does not match genetic relationships, and the two are now combined. Each of the two subgenera are further divided into several sections. The most distinct pines both genetically and morphologically are Pinus nelsonii from NE Mexico, and Pinus krempfii from southern Vietnam, both are classified in monotypic sections and probably very early separated from other pines. Pines are small to very large trees; the tallest are Pinus lambertiana and Pinus ponderosa, both from western North America, which both reach just over 80 metres tall and 2-3 metres trunk diameter. By contrast, Pinus culminicola from NE Mexico, Pinus pumila from NE Asia, and Pinus mugo subsp. mugo from central Europe, are all shrubby plants rarely exceeding 3-4 metres high. Pines have three types of leaves. Firstly on seedlings 1-2 years (rarely to 5 years or more) old, spirally arranged green or glaucous-green needle-like juvenile leaves which range from 2-6 cm long on. Then on adult foliage, two types, brown scale-leaves a few millimetres long on the branches, and clusters (fascicles) of green needles in the axils of the scale leaves, with two, three or five (rarely one, four or six) needles per fascicle; it is these fascicles that are the familiar pine needles. The needles are evergreen, with persistence ranging from 2 to 45 years; they are semicircular or triangular in cross-section, often sharply pointed, and have lines of stomata (breathing pores) on all sides or just on the adaxial side (as also in spruces Picea, but unlike other Pinaceae genera where the stomata are concentrated on the abaxial side). Needle length varies from 2 cm (in Pinus banksiana) up to 45 cm (in Pinus palustris, Pinus devoniana and Pinus engelmannii) and thickness from 0.5-3 mm. The pollen cones are 10-50 mm long, and shed soon after pollen release in spring. The seed cones are produced in spring and in most species mature over two growing seasons 18-24 months later (longer, over three growing seasons in a few, up to 36 months in Pinus pinea); they are erect at first when pollinated, then turn sideways to pendulous as they mature; length varies from 3 cm (in several species) to 65 cm (in Pinus lambertiana) and colour from yellow-green to red to dark purple. The cone scales have a distinct umbo (uniquely indistinct in Pinus nelsonii) which comprises the first season's growth and apophysis, which develops in the second season. Species with a three growing season development have an umbo with a concentric ring from the second season, and the apophysis in the third season. In several species, the cones have thick, armoured scales and spined umbos as a defence against seed predators (primarily squirrels Sciuridae); this development is most extreme in Pinus coulteri, where the ripe cones may weigh 3-4 kg. There are two winged seeds under each cone scale; the seeds are blackish-brown to golden-brown, and range from 3 mm (in Pinus banksiana) to 28 mm (in Pinus maximartinezii) long. In species with small seeds, the seed wings are long, and effective for wind dispersal, while in species with large seeds, the wings are vestigial and the seeds are dispersed by birds, mainly various genera in the family Corvidae (notably Aphelocoma, Cyanopica, Gymnorhinus, and Nucifraga). Species with bird-dispersed seeds typically have many features of the cone such as soft scales which also aid bird access. Pines are a major source of commercial wood throughout the world, with extensive plantations both throughout the native range of the genus, and also widely in the southern hemisphere. The wood is used for general construction, plywood, interior finishing, boxes, and also for pulp and paper. Pines are used to a small extent as Christmas trees, although they are not as popular as spruces or firs (Abies species). Pine resin is extensively used to produce turpentine. Several pines are widely planted as landscape and ornamental trees, and numerous cultivars have been developed, with variations in growth rates, needle colour, and form.
- Businský, R. (2008). The Genus Pinus L., Pines. Acta Pruhoniciana 88: 1-126.
- Farjon, A. (2005). Pines: Drawings and Descriptions of the Genus Pinus, 2nd ed. Koeltz Scientific.
- Gernandt, D. S., et al. (2005). Phylogeny and classification of Pinus. Taxon 54 (1): 29–42.
- Huxley, A., ed. (1992). New RHS Dictionary of Gardening. Macmillan.
- Lanner, R. M. (1996). Made for Each Other: A Symbiosis of Pines and Birds. Oxford.
- Mirov, N. T. (1967). The Genus Pinus. Ronald.
- Richardson, D. M., ed. (1998). Ecology and Biogeography of Pinus. Cambridge.
- Rushforth, K. (1987). Conifers. Helm.
- Selected literature:
- Syring, J., et al. (2007). Widespread Genealogical Nonmonophyly in Species of Pinus Subgenus Strobus. Syst. Bot. 56 (2): 163–181.
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