Pinus L.
Pinus is a large genus of ca. 110 species. Most are confined to north temperate regions. At least 24 species are cultivated in Europe, either as ornamentals, for windbreaks or for timber (Page & Bennell 1986).
In addition to those keyed out below, several other taxa have been reported as escapes from cultivation in western Europe. Lambinon & al. (2004) cite Pinus mugo Turra as cultivated in Belgium but subspontaneous occurrences are not known so far. Van der Meijden (2005) further reports about Pinus banksiana Lambert in the Netherlands (sometimes intermixed with P. sylvestris).
The actual degree of naturalisation and distribution of the genus Pinus in Belgium is insufficiently known. Young, self-sown specimens of Pinus are often not reported and but very rarely represented in herbarium collections.
Moreover, the identification of seedlings or juvenile specimens is often quite difficult Seedlings of many species are often much alike (see drawings in Vidaković 1991) and needle length in immature specimens can vary considerably.
Juvenile specimens are often found in the vicinity of parental trees. Collecting mature leaves and cones of the (presumed) parental plants usually much facilitate identification.
Self-sown specimens of Pinus are often better represented in suitable habitats near solitary parental trees (especially in urban habitats) than in mass-plantations. The number of seedlings in or near plantations (of all species used in forestry in Belgium, especially Pinus nigra and P. sylvestris) is usually remarkably restricted.
Needles in Pinus are surrounded at base by a membranous sheath. The length of this sheath often provides a good feature to distinguish between closely related species. However, it is easily damaged and should be investigated on young, undamaged branches.
At least 21 species of Pinus are considered to be invasive worldwide. No other speciose genus of plants has yielded important insights on so many facets of invasion ecology (Richardson 2006).
1. Needles with one vascular bundle, grouped in fascicles of (4-)5(-6), thin, soft, blue-green, 6-14 cm long. Bracts at base of needle fascicles not deflexed === 5. Pinus strobus
1. Needles with two vascular bundles, grouped in fascicles of 2-3, thin or rigid, of variable length. Bracts at base of needle fascicles deflexed === 2
2. Most needles in fascicles of 3 === 3
2. Most needles paired === 4
3. Needles 15-30 cm long, sheaths 22 mm long. Cones abscising === 3. P. ponderosa
3. Needles 6-14 cm long, sheaths 9-12 mm long. Cones not abscising === 4. P. rigida
4. Mature needles 3-7 cm long, often twisted. Cones up to 6 cm long === 6. P. sylvestris
4. Mature needles 8-25 cm long, twisted or not. Cones 3-22 cm long === 5
5. Needles very rigid, 12-25 cm long, slightly concave abaxially, convex adaxially in cross section, sheaths 20-25 mm long. Cones 8-22 cm long. Buds not resinous === 2. P. pinaster
5. Needles less rigid, 8-16 cm long, slightly convex abaxially, convex adaxially in cross section, sheaths 10-12 mm long. Cones 3-8(-9) cm long. Buds resinous === 1. P. nigra
Literature:
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Farjon A. (2005a) Pines. Drawings and descriptions of the genus Pinus (2e ed.). Brill, Leiden & Boston: 235 p.
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Vidaković M. (1991) Conifers, morphology and variation. Grafički Zavod Hrvatske, Zagreb: 754 p.