Pinus

Linnaeus 1754

Common names

Pine, pinyon [English], pinheiro [Portugese], pino, piñon [Spanish], pino [Italian], pin, pignon [French], pijn, den [Dutch], Kiefer [German], fyr [Danish, Norwegian], tall [Swedish], mänty [Finnish], sosna [Russian], bor, mura [Bulgarian], bora, molike [Serbo-croat], peuke, pitys [Greek], çam [Turkish], chir, kail [Hindi], thong [Vietnamese], 松属 matsu [Japanese], 松属 song shu [Chinese].

Taxonomic notes

Syn: Apinus Necker; Strobus Opiz; Caryopitys Small; Ducampopinus A. Cheval. (Farjon 1998).

The pine genus is the largest in the family, with 114 species treated here. Pines include two subgenera, the typical pines in subgenus Pinus and the white pines in subgenus Strobus. Each of these subgenera has been treated as a genus in its own right, and other subgenera have been proposed, but the great majority of morphological evidence, terpene data, and, more recently, molecular phylogenetic data have all firmly established the species composition and monophyletic origin of these two subgenera. The genetic distance between the subgenera may be as large as or larger than that between e.g. Keteleeria Carrière and Abies Miller (Price et al. 1987), and if strict genetic criteria were used they should perhaps be treated at generic rank. However, the pines as a whole form a clear monophyletic unit and retention as a single genus remains the best treatment. The nomenclatural upheaval of splitting the genus would also be a serious problem.

The subgenera are chiefly distinguished by the following criteria (Richardson 1998):

Subgenus Pinus L., also called the hard pines. About 70 species. Cone scales with umbo dorsal and mucronate, scales with a sealing band adjacent to the apophysis where the scales meet on the closed cone. Two leaf vascular bundles per needle. Fascicles with 2-6 needles, stomata more or less equally distributed on all surfaces, resin ducts variable; sheath persistent except in P. leiophylla and P. lumholtzii. Decurrent pulvini at the fascicle bases.

Subgenus Strobus Lemmon (syn. genera Apinus Necker; Caryopitys Small; Ducampopinus A. Cheval.; Strobus Opiz), also called white or soft pines. About 44 species. Cone scales without a sealing band. Seedwing articulate to strongly adnate. Leaf vascular bundle single. Fascicles with 1-5 needles, stomata all or most on inner faces, rarely equal, resin ducts medial or external; fascicle sheath deciduous except in P. nelsonii. Non-decurrent pulvini at leaf bases.

Each of the subgenera can be further subdivided into a number of sections and subsections. There has never been a very widespread consensus on the composition of most of the sections and subsections, although of course some closely related species or groups of species have been long and widely recognized (e.g. the closed-cone pines, the bristlecone pines, or the piñon pines). The sections and subsections presented below are mainly those of Gernandt et al. (2005) (amended to include subsection Attenuatae) and are based on a recent cladistic analysis using both character state and molecular phylogenetic data. The cladogram below summarizes my current conceptual model for relationships between the species in Pinus. It was initially based on the system proposed by Price et al. (1998), but has been substantially modified to respect molecular phylogenetic studies since published by Liston et al. (1999), Wang et al. (1999), Geada López et al. (2002), Gernandt et al. (2005), and Syring et al. (2005). In the dendrogram, black lines connect groups identified on the basis of molecular studies, and blue lines identify probable groupings for taxa that I do not yet have molecular data on, but which have been studied using more traditional taxonomic methods.

Cladogram of the genus Pinus

Data collected thus far support the idea that the two subgenera are valid, clearly distinct, and are each monophyletic. These data also indicate that subgenus Strobus is probably of earlier derivation, with the species in section Parrya the most primitive in the genus. Each of the sections appears to be monophyletic, with very ancient divisions drawn between the subsections.

Description

Trees or shrubs, aromatic, evergreen; crown usually conic when young, often rounded or flat-topped with age. Bark of older stems variously furrowed and plated, plates and/or ridges layered or scaly. Branches usually in pseudowhorls; shoots dimorphic with long shoots and dwarf shoots; dwarf shoots borne in close spirals from axils of scaly bracts and bearing fascicles of leaves (needles). Branchlets stout, ending in a compound bud with many bud scales. Buds ovoid to cylindric, apex pointed (blunt), usually resinous. Leaves dimorphic, spirally arranged; foliage leaves (needles) (1)2-5(6) per fascicle, persisting 2-12 or more years, terete or ± 2-3-angled and rounded on abaxial surface, sessile, sheathed at base by 12-15 overlapping scale leaves, these (at least firmer basal ones) persisting for life of fascicle or shed after first season; resin canals mostly 2 or more (rarely 0-1; max. c. 20). Plants monoecious (rarely semi-dioecious); staminate cones numerous and small, in a dense, spikelike cluster around base of current year's growth, mostly ovoid to cylindric-conic, tan to yellow, red, blue, or lavender. Ovulate cones solitary to few, maturing in 1.5-2(-3) years, shed early or variously persistent, pendent to ± erect, at maturity conic or cylindric, sessile or stalked, shedding seed soon after maturity or variously serotinous (not opening upon maturity but much later, usually in response to fire); scales numerous, persistent, woody or pliable, surface of exposed apical portion of each scale (apophysis) thickened, with umbo (exposed scale surface of young cone) represented by a scar (sometimes apiculate) or extended into a hook, spur, claw, or prickle; bracts included. Two seeds at the base of the cone scale, winged, in some the wing vestigial; cotyledons (3)6-14(24). x=12 (Kral 1993, Little 1980).

Pine anatomy differs from other conifers in several respects, and the resulting anatomical differences are helpful in identification. Here are the major points:

Cones: Cones take two or three years to mature. The time requirement varies between species. Young cones may have a distinctive color, but normally identification requires mature cones. Cones may have relatively stiff, woody scales (the norm in subgenus Pinus) or more flexible scales (subgenus Strobus). The portion of the cone scale that is exposed before the mature cone opens is thickened and is called the umbo; it may be unarmed, or armed with a spine or prickle; and it may be formed into a woody pyramid called an apophysis. The shape and texture of the apophysis varies between species and is an important character for identification. Cones scales may remain closed and sealed by resin long after cone maturity; such cones are adapted to open when the resin is melted by fire and are called serotinous. Normally, cones open at maturity and release seed. The seed may be wind-carried, in which case it is normally small and light with a wing longer than the seed; or it may be dispersed by animals (primarily birds of the family Corvidae, jays and their allies), in which case the seed is normally larger and heavier, and the wing may be reduced to inconspicuity.

Foliage: Pine foliage is of four types: cotyledons, primary leaves, cataphylls and needles. Cotyledons are the first leaves produced when the plant emerges from the seeds. There may be from 4 to 24 of them (24 cotyledons, from Pinus maximartinezii, is the largest number known in any plant). Primary leaves are single, alternate (usually helically arranged), acicular leaves that are usually produced only for the first year of growth but that may be produced for many years in some species (such as Pinus juarezensis) or may be produced on a mature plant in response to a wound. Cataphylls are alternate (helically arranged) non-chlorophyllous primary leaves produced on shoots; they are typically small, subulate or lanceolate, with erose-hyaline to ciliate margins, leaving a distinctive pattern when they fall off the shoot; they are often a useful character in identification. Needles, of course, are the most common pine leaves. They are borne on dwarf shoots axillary to cataphylls in clusters or fascicles of one to eight needles, initially bound together by a basal sheath that may then fall off or may persist, falling with the needles. The number of leaves per fascicle, the length of needles, the number of sides of the needles (only Pinus monophylla has a round needle), the distribution of stomata (waxy white specks on the leaf surface), and the color and stiffness of the needles can all be useful characters for identification. The interior structure of the leaf also may be important for identification, but this requires a microscope and so leaf anatomical characters are rarely used by field workers.

Bark: Bark characters are usually not too useful for pine identification except after a species has been learned thoroughly in the field. The reason is that many species bear similar bark, and the bark characters change with the age of the tree. However, some species have highly distinctive bark, so it is occasionally an important character in identification. Generally, I provide bark characters for mature trees. Important points to note include the bark color, the size and pattern of fissures in the bark, and whether the bark is scaling or flaking.

See Farjon and Styles (1997) for a much more thorough discussion of pine anatomy.

Volume 1 of the Flora of North America (Kral 1993) offers the following advice to those attempting to identify pine specimens:

• Stated leaf measurements are based on healthy, fully expanded growth, especially that of cone-bearing branches.
• Fascicle-sheath measurements are based on fully developed, unbroken sheaths, not on sheaths as they later break up.
• After pollen is shed, pollen cones may lengthen considerably. Measurements given for pollen cones are those of the cones at the time that pollen is released.
• Colors of seed cones are those of mature, closed or newly opened cones, not of old, open, persistent cones or of weathered serotinous cones. Mature, open cones may be hygroscopic, closing partially or completely when wet.
• Descriptions of apophyses are based on mature, closed or newly opened cones. Unlike characters of umbos of most species, characters of apophyses are much altered as the cone grows.

Range

Native to all continents and some oceanic islands of the northern hemisphere, chiefly in boreal, temperate, or mountainous tropical regions; reaching its southernmost distribution shortly below the Equator in southeast Asia (Sumatra; P. merkusii). Introduced as ornamental and timber trees in much of the southern hemisphere (Mirov 1967, Kral 1993).

Some of the sections and subsections are geographically quite limited:

Section Quinquefoliae: east Asia, North America and Mexico

Subsection Strobus: east Asia, North America and Mexico
Subsection Krempfianae: Vietnam
Subsection Gerardianae: China and the Himalaya

Section Parrya: Western United States and Mexico

Subsection Cembroides: Western United States and Mexico
Subsection Balfourianae: Western United States
Subsection Nelsoniae: Mexico

Section Trifoliae: North America, Mexico and the Caribbean

Subsection Attenuatae: Western United States (California) and Mexico (Baja California Norte)
Subsection Australes: United States, Mexico and the Caribbean
Subsection Ponderosae: Western United States and Mexico
Subsection Contortae: North America and Mexico (Baja California Norte)

Section Pinus: Europe, Asia, Mediterreanean Africa, eastern North America and Cuba

Subsection Pinus: Europe, Asia (south to Sumatra), eastern North America and Cuba
Subsection Pinaster: Mediterreanean and western Himalaya

Most pines are fire-adapted, meaning that the recurrence of fire permits pines to maintain a dominant role in forest successions that lead to dominance by non-pines. The precise nature of this fire adaptation varies widely, with some pines tolerant of frequent low-intensity fires and others tending to produce high fuel accumulations that permit stand-destroying fires, after which the pines regenerate quickly. In habitats with infrequent or no fire, pines tend to occur on nutrient-poor sites such as serpentine barrens, lithosols, or bogs. Their low shade tolerance typically precludes growth beneath a closed forest canopy. Many species are very drought tolerant.

In many areas Pinus is a forest dominant, either early successional or longer-lived, persisting in the late successional forest. Certain fire successional species have a "grass stage," i.e., the stem of the young seedling elongates little during the first several years (meanwhile developing a large taproot) and bears many long, curved leaves, the plant then resembling a bunchgrass (Kral 1993). Other species have cones that are long persistent and remain closed, opening only when heated by wildfire (such cones are called serotinous); seeds are released soon after, once the fire is out.

Range maps have been published for the genus, many of its subsections and most of its species by Critchfield and Little (1966).

Big tree

Two genera in the Pinaceae, Pseudotsuga and Picea, contain larger trees. For stem volume, diameter and height, the largest pine species is Pinus lambertiana. The second- and third-largest pine species is either P. ponderosa subsp. benthamiana or P. jeffreyi; both species have held the title in the past, but the largest measured trees later died, and it is not certain which species now hosts a larger tree (Van Pelt 2001, and pers. comm. with Van Pelt at various later times). All three of these pines reach their greatest size in the mixed conifer forest of the Sierra Nevada mountains of California. Other species known to attain diameters of greater than 2 meters and proportionate heights include P. brutia, P. canariensis (largest pine in the old world), and P. radiata.

Conversely, the world's smallest pine is P. contorta subsp. contorta var. bolanderi, which grows on extremely nutrient-poor soils and has been known to bear cones when only 20 cm tall. Other pines small enough to not qualify as trees include P. culminicola, P. pumila, and P. mugo. A wide variety of other species may also be less than a meter tall when found growing at the alpine timberline.

Oldest

Unquestionably, the oldest pine is Pinus longaeva, of which many individuals over 4,000 years old are known. Due to their occurrence in some very cold and dry environments where disease and stand-destroying disturbance are rare, pines are collectively the most long-lived of conifers. Ages of over 1000 years have been encountered in P. albicaulis, P. aristata, P. balfouriana, P. flexilis, and P. longaeva, all species native to western North America. Comparable ages may occur in several old world species native to desert, arctic and alpine areas, but have not been demonstrated. The strongest candidate is probably P. heldreichii var. leucodermis, with a confirmed age of 963 years.

Dendrochronology

The science of dendrochronology originated with observations of Pinus ponderosa rings by astronomer Andrew Ellicott Douglass during travels in northern Arizona in 1904. Ever since then, pines have been preferred subjects for dendrochronology. Douglass used tree rings to date the construction of Anasazi ruins in the American Southwest, many workers have used scores of different pines to reconstruct past variations in climate, geochronologists have used pines to determine changes in the rate of atmospheric carbon-14 production over the past 7,000 years, and the rings of pines have been used to address a wide spectrum of other technical problems; for example, the tree-ring pattern in a pine board proved to be a crucial piece of evidence helping to convict the killer of the kidnapped Lindberg baby in 1923 (before O.J. Simpson, perhaps the most celebrated American criminal trial of the 20th century) (Graham 1997).

Ethnobotany

Pines are economically important for their timber, pulp, tar, and turpentine. When the world was tied together by sail, pines often assumed strategic importance as naval stores, thereby influencing patterns of Western colonialism. They were the first timber resource exploited in much of North America. They have long been a principal source of timber for all purposes, including firewood, construction and woodworking. They continue to be a leading genus in agroforestry production, dominating plantations in the U.K. (P. contorta, P. nigra), New Zealand (P. radiata) and Brazil (P. elliottii).

Stone pines (P. pinea and the species in subsection Cembroides) have an edible seed gathered by indigenous peoples (and sometimes commercially) and often comprising a major seasonal food source. By many accounts, they are also an aphrodisiac (Santesson 2000; see also the Ethnobotany section for P. pinea).

Many pines have been used to produce turpentine, a semi-fluid, yellow or brownish resin (oleoresin). "Pine resin flows on the external surface of a tree after a wound is inflicted to form a protective coat that seals the wound to pathogenic microorganisms and prevents loss of sap. To obtain resin commercially, a tapping cut is made in the pine bark and the resin drops are collected into buckets or bags. The principal products of pine resin are rosin and turpentine oil. The most significant hard resin from a commercial point of view is rosin, which is obtained by distillation of pine resin. Rosin is used in paper glue and soap manufacturing, as a constituent of varnishes and paints, and for coating bows of stringed musical instruments. Oil of turpentine is also produced by pine resin distillation and is used for thinning and dissolving paint and varnish, as well as for shoe polish and sealing wax manufacturing. It also has medical properties and can be used as stimulant, antispasmodic, astringent, diuretic and anti-pathogenic. In the past, crude pine resin had been used in sailing vessels as packing material and for waterproofing" (Moussouris and Regato 1999).

Pines are among the most popular of ornamental conifers, particularly in temperate and cold climates, for which many hardy species exist. Although most species see little or no ormamental use, the catalog of those that do is long. This may be because many pines are slow-growing and develop contorted forms that are pleasing to gardeners. Examples of the type include Pinus sylvestris, P. mugo, P. densiflora and P. contorta. Others tend to the opposite extreme, quickly developing into majestic landscape trees. Examples include white pines such as P. strobus and P. armandii, and many pines in subsections Ponderosae (in the new world) and Pinaster (in the old), such as P. ponderosa, P. coulteri, P. montezumae, P. pinaster, P. brutia, and P. canariensis. For most of these popular ornamental species, various cultivars have been developed to emphasize differences in growth form and foliage.

Observations

Comparative study of members of the genus is most easily accomplished at large arboreta; some are listed at the Links page. You should also try to visit the "hot spots" for the genus; the principal ones are Mexico, California, and the southeast United States.

Remarks

Pinus was the Roman name for pine.

Citations

Gernandt, D.S., G. Geada López, S.O. Garcia and A. Liston. 2005. Phylogeny and classification of Pinus. Taxon 54(1):29-42.

Graham, S.A. 1997. Anatomy of the Lindbergh kidnapping. Journal of Forensic Sciences 42(3):368-377.

Liston, A,, W.A. Robinson, D. Pinero, and E.R. Alvarez-Buylla. 1999. Phylogenetics of Pinus (Pinaceae) Based on Nuclear Ribosomal DNA Internal Transcribed Spacer Region Sequences. Molecular Phylogenetics and Evolution 11(1):95-109.

Santesson, Johan. 2000. Pine nuts as aphrodisiacs. Accessed 2002.04.24 at http://www.santesson.com/aphrodis/pine.htm. Site is now defunct.

See Also

Axelrod, D. I. 1980. History of the maritime closed-cone pines, Alta and Baja California. Univ. Calif. Publ. Geol. Sci. 120: 1-143.

Axelrod, D. I. 1986. Cenozoil history of some western pines. Ann. Mo. Bot. Gard. 73: 565-641.

Axelrod, D. I. and Cota, J. 1993. A further contribution to closed-cone pine (Oocarpae) History. American Journal of Botany 80(7): 743-751.

Bailey (1970) provides a classic and still very useful treatment of the pines in subsection Balfourianae.

Bailey, D.K. 1987. A study of Pinus subsection Cembroides I: The single-needle Piñons of the Californias and the Great Basin. Notes of the Royal Botanical Gardens Edinburgh 44:275-310.

Bailey, D.K. and F.G. Hawksworth. 1979. Piñons of the Chihuahuan Desert region. Phytologia 44:129-133.

Burns and Honkala (1990) provide detailed descriptions, with a focus on silviculture, for all economically significant pines native to the United States.

Cai, Qing, Daming Zhang, Zhan-Lin Liu and Xiao-Ru Wang. 2006. Chromosomal Localization of 5S and 18S rDNA in Five Species of Subgenus Strobus and their Implications for Genome Evolution of Pinus. Annals of Botany 97(5):715-722.

Critchfield, W. B. 1962. Hybridization of the southern pines in California, pp. 25-27. Proc. Forest Genetics Workshop. Macon, GA.

Critchfield, W. B. 1967. Crossability and relationships of the closed-cone pines. Silvae Genetica 16(3): 89-97.

Critchfield and Little (1969) is still the most encyclopedic source of range maps, though some notable range extensions have since been reported.

Denevan, W. M. 1961. The upland pine forests of Nicaragua. Univ. Cal. Publ. Geol Sci. 12: 251-320.

Duffield, J.W. 1952. Relationships and species hybridization in the genus Pinus. Silvae Genetica 1: 93-97.

Farjon (1984), or the second edition in 2005, provides a good overview with lots of interesting supplemental information and excellent line drawings.

Farjon, A. 1996. Biodiversity of Pinus (Pinaceae) in Mexico: speciation and paleo-endemism. Bot. J. Linn. Soc. 121: 365-384.

Farjon and Styles (1997) is the most authoritative reference on pines of Mexico and Central America.

Furman, B. J., Grattapaglia, D., Dvorak, W. S. and O''Malley, D. M. 1997. Analysis of genetic relationships of Central American and Mexican pines using RAPD markers that distinguish species. Mol. Ecol. 6: 321-331.

Hong, Y. P., Krupkin, A. B. and Strauss S. H. 1993. Chloroplast DNA transgresses species boundaries and evolves at variable rates in the California closed-cone pines (Pinus radiata, P. muricata, and P. attenuata). Genetics 135(4): 1187-1196.

Krupkin, A. B, Liston, A. and Strauss, S. H. 1996. Phylogenetic analysis of the hard pines (Pinus Subgenus Pinus, Pinaceae) from chloroplast DNA restriction site analysis. Am J. Bot. 83(4): 489-498.

Liston, A., W., Robinson, A., Piñero, D., and Alvarez-Buylla, E. R. 1999. Phylogenetics of Pinus (Pinaceae) based on nuclear ribosomal DNA internal transcribed spacer region sequences. Molecular Phlyogenetics and Evolution 11(1): 95-109.

Little and Critchfield (1969) established a taxonomic benchmark for the species.

Malusa (1992) is a wonderful little paper if you suffer the common problem of being baffled by the diversity of the piñons.

Millar, C. I. 1993. Impact of the Eocene on the evolution of Pinus L. Ann. Mo. Bot. Gard. 80: 471-498.

Millar, C. I., S.H. Strauss, T.H. Conkle, and R.D. Westfall. 1988. Allozyme differentiation and biosystematics of the California closed-cone pines (Pinus subsection Oocarpae). Syst. Bot. 13: 351-370.

Mirov (1967) is the recognized "bible" of pines, while Mirov & Hasbrouck (1976) offer a more readable introduction to the genus.

Peattie (1950) is among the more 'literate' writers, a man not afraid to confess his love of fine trees.

Perry (1991) is an invaluable reference for the pines of Mexico and Central America.

Preston, R.J. 1976. North American Trees (Exclusive of Mexico and Tropical United States), ed. 3.

Price, R.A. 1989. The genera of Pinaceae in the southeastern United States. J. Arnold Arbor. 70: 247-305.

Richardson (1998) provides a wealth of ecological information.

Sargent, C.S. 1922. Manual of the Trees of North America (Exclusive of Mexico), ed. 2. Boston and New York. [Facsimile edition in 2 vols. 1961, reprinted 1965, New York.]

Shaw (1909, 1914) and Sudworth (1908) provide excellent drawings, though the taxonomy is inevitably very dated.

Sudworth, G.B. 1917. The Pine Trees of the Rocky Mountain Region. Washington. [U.S.D.A. Bull. 460.]

Wu, J.K., V. Krutovskii, and S.H. Strauss. 1999. Nuclear DNA diversity, population differentiation, and phylogenetic relationships in the California closed-cone pines based on RAPD and allozyme markers. Genome 42: 893-908.