Pine, Linneaus  1753
Pinaceae

Pinus - Pine description

Evergreen single or multistemmed trees or shrubs. Bark smooth at first, nonfibrous and flaking in small irregular patches, highly varied among the species at maturity, in some remaining smooth, in others becoming variously ridged and furrowed, and in others divided into regular or irregular plates, sometimes massive, shieldlike ones. Saplings and young trees initially bearing regular annual or twice yearly tiers of about three to five rising or horizontal branches from the base to form a symmetrical, conical or cylindrical crown that may broaden and flatten with age, becoming irregular and sometimes even tattered. Branchlets strongly differentiated into two kinds, short shoots and long shoots. Short shoots are the most extreme ones among the conifers, reduced to a bundle (fascicle) of (one or) two to eight needles on an abortive growth tip and surrounded at the base by a persistent or shed sheath of papery scale leaves. Long shoots are ordinary woody shoots extending the growth of the tree but without green needles (except in seedlings and trunk sprouts), instead clothed with papery scale leaves, smooth or furrowed between the attached bases of these scales. Most scale leaves with short shoots in their axils, the rest with either pollen or seed cones, or nothing at all. Winter buds well developed, scaly, usually resinous. Foliage leaves in spirally arranged bundles radiating all around the long shoots, needlelike, each bundle circular in cross section and the individual needles then shaped like pie pieces in cross section. Needles usually longer than those of any other Pinaceae, straight or twisted, uniform in thickness for most of their length, abruptly narrowed to the pointed tip and the slightly narrowed base.

Plants monoecious. Pollen cones numerous and crowded all around the base of new long shoots as they emerge from the terminal buds, single in the axils of scale leaves and surrounded at the base by other scale leaves. Each cone cylindrical, with numerous spirally arranged pollen scales, each scale bearing two pollen sacs. Pollen grains small to large (body 30-75 µm long, 40-80 µm overall), with two round sir bladders diverging from the larger spherical to oval body at 90° (a right angle) to each other or more. Pollen body minutely warty, the bladders with a more wrinkled sculpture and often set off from the body by a distinct ridge. Seed cones quite varied in size and shape, from almost spherical to nearly cylindrical, often strongly asymmetrical, with or without a scaly stalk, often in a circle of two to five around the branchlet near the tip of a year’s growth. Each cone single in the axil of a scale leaf, standing straight out from the twigs at pollination, stiffly outstretched, angled forward or backward, or hanging at maturity. Maturing in two (or three) seasons and falling after releasing the seeds or remaining intact many years until opened by fire. Seed scales numerous, densely spirally arranged, with a tiny bract attached to the base and never protruding between the scales, woody, either thin and relatively uniform (often in subgenus Strobus) or with a diamond-shaped, shieldlike thickening (umbo) on the variably thickened exposed face near the tip of the scale (subgenus Pinus and some subgenus Strobus). The umbo, when present, usually ending in a varied point, from short, to prickle-like, to massive claws, but slender points may be shed or break off, and in some species they never form and the umbo has a flat top. Seeds two per scale, oblong, the asymmetrical wing derived from the seed scale and clasping the seed body loosely or tightly via two thin arms running the length of the body, sometimes reduced to just these two arms and hence the seeds seemingly wingless. Cotyledons 2-15(-18), each with one vein. Chromosome base number x = 12.

Wood soft (subgenus Strobus) to hard (subgenus Pinus), with a more or less pronounced resinous odor and reddish brown to brown heartwood well differentiated from the light brown or yellowish brown sapwood. Grain even to uneven, with a gradual (subgenus Strobus) to abrupt (subgenus Pinus) transition between earlywood and latewood. Vertical resin canals numerous and large, with horizontal water-conducting cells (tracheids) in the rays.

Lines of stomates confined to the inner faces adjoining the other needles of a bundle (some subgenus Strobus) or also present on the outer face (subgenus Pinus and some subgenus Strobus). Each stomate sunken deeply beneath and sometimes almost hidden by the 4-11 (depending on the species) surrounding subsidiary cells, which are often topped by a variously formed (again depending on the species) Florin ring whose height may be enhanced by an encircling furrow. Leaf cross section with a variable number of resin canals in various positions with respect to the epidermis and a single- (subgenus Strobus) or double-stranded (subgenus Pinus) midvein embedded in a cylinder of transfusion tissue. Photosynthetic tissue without a well-defined palisade layer, uniform all around the needle between the large central vascular cylinder and the epidermis and adjacent layers of hypodermis that are continuous except beneath the stomatal lines.

One hundred twenty-one species, throughout much of the northern hemisphere from the boreal zone to the tropics, extending south to Nicaragua and Cuba in the New World and Mediterranean North Africa, the Himalaya, and Malesia in the Old World.

Pinus is the largest and most widespread genus of conifers in the northern hemisphere and one of the two largest in the world, with (like the southern hemisphere Podocarpus) about one hundred species, almost twice as many as the next largest genus (Juniperus). Its ecological range is astonishingly broad, from boreal forests and alpine shrubberies to lowland tropical savannas, and from swamp margins to desert slopes. It is the most ecologically diverse genus, not only among conifers but perhaps among all woody plants. There are few areas of the northern hemisphere out of sight of some species of pine. Nevertheless, there are some common elements in the ecology of the species, one of the most important being that the majority of the species grow in areas subject to periodic fires. They are typically pioneer species with a high light requirement for establishment. The disturbance that provides these open conditions more often than not is fire. Once established, a number of species can live for hundreds of years and become part of a closed, mature forest. Even these long-lived species will eventually disappear from the canopy in the absence of fire. The longest-lived individual trees of all, the Great Basin bristlecone pine (Pinus longaeva) of California and Nevada, with ages exceeding 4,000 years, are denizens of very open subalpine woodlands of desert mountain ranges and thus never experience intense competition for light. Another ecological feature common among pine species is growth on soils even poorer in nutrients than those that support the majority of conifer species. This includes sandy soils within a wide variety of vegetation types as well as special conditions, like the hardpan soils of Californian pygmy forests. When there are higher nutrient levels in cool climates, pines are often replaced by more competitive conifers of other genera, like Abies or Picea.

In addition to their ecological importance and considered on a worldwide basis, pines have the greatest economic value and cultural significance among all the conifers. They are sources of many forest products, including construction timber, plywood, pulp, resins, and edible seeds. The latter gives the genus its name, Pinus being classical Latin for the Mediterranean stone pine (Pinus pinea), exploited throughout the Mediterranean region for food. Resins or naval stores, once so important for caulking wooden ships, are obtained both as by-products of pulp production and lumber production and by directly tapping the trees, taking advantage of the resin canals which run both vertically and horizontally throughout pine trunks. Pines have become the most important timber-producing conifers far beyond their natural distribution, having been planted throughout the temperate southern hemisphere and even in the tropics, where few other conifers are amenable to plantation culture. They are so successful in some places, as in South Africa and New Zealand, that some species, Monterey pine (Pinus radiata) in particular, can become invasive pests in natural vegetation. Scots pine (Pinus sylvestris), which sees use primarily as a Christmas tree in Canada and the United States, is likewise widely naturalized in northeastern North America. The economic and cultural importance of pines also makes them popular subjects of scientific and technological study, and there is about as much technical literature on pines as on all of the other conifer genera combined.

Not surprisingly, given their cultural significance, ornamental long needles compared to other conifers, and tolerance of neglect, many pine species have also entered into general cultivation for horticulture. Some, such as Austrian pine (Pinus nigra), Scots pine (Pinus sylvestris), and mugo pine (Pinus mugo), are among the most commonly cultivated ornamental conifers in temperate North America and Europe. Since these and others have been cultivated for centuries here and in eastern Asia and elsewhere, there has been very extensive cultivar selection, including cultivars of hybrid origin. Pine cultivars pretty much cover the range found in other conifers, but selections with modified habit, shape, and size are much more common than shifts in needle color. There are blues, silvers, and variegated golds in several species, but even more species have seed selections with twisted or contorted needles or shorter or longer ones than the norm. Growth form variations include columns and cones, weepers and spreaders, umbrellas and tabletops. There are many rock garden or larger landscape dwarfs of different sizes and shapes, and a good number of these originated as witches’-brooms although most were from chance seedlings in the nursery bed or propagated from naturally occurring runts. While there are hundreds of named pine cultivars, no one pine species has given rise to anywhere near the numbers found in such other conifers as Port Orford cedar (Chamaecyparis lawsoniana), sugi (Cryptomeria japonica), or northern white cedar (Thuja occidentalis), so there is plenty of room for further experimentation and selection in this genus.

It is clear that Pinus is closest to Picea, Cathaya, Pseudotsuga, and Larix, and some authors place all these genera together in a subfamily. Pinoideae, but the relationship is not close, and other authors restrict subfamily Pinoideae to Pinus alone while some even make it the sole genus of a restricted Pinaceae, with all other genera in a family Abietaceae. Few go so far, but all recognize the comparatively isolated position of this genus, which has almost as many species as the other ten genera combined and occupies more of the Earth’s surface.

Because there are so many species of pines (for a conifer anyway, nothing like 500 species of oaks, 750 species of figs, or 1,700 species of the potato genus), there has also been much effort expended on elaborate classifications of the genus involving botanical subgenera, sections, subsections, and sometimes series. In broad outline, the classification presented by Little and Critchfield (1969) has been widely followed and is used here, with modifications and simplifications demanded by more recent studies. The subsection names given after each species name below show which species are related closely to each other. Generally speaking, facility of species hybridization follows these subsections and few species will hybridize outside of their own subsection, so the great majority of reported natural and synthetic pine hybrids are between species within a subsection. There are commonly crossing barriers between species within subsections as well, so members of a subsection are often not freely intercrossable. DNA studies and other biochemical and molecular investigations shed new light on relationships among pine species, confirming many traditional groups, including the two subgenera of hard or yellow (subgenus Pinus) and soft or white (subgenus Strobus) pines, but also suggesting revisions. Perhaps the most dramatic of these changes concerns the uniquely flat-needled Pinus krempfii of Vietnam, so unusual that some authors placed it is a separate genus, Ducampopinus, while Little and Critchfield used the same name for a subgenus containing only this species. DNA studies confirm a minority opinion based on traditional structures, including the undivided midvein, that this two-needled pine belongs among the largely five-needled species of subgenus Strobus.

DNA studies have not yet made much of an impact on the most vexed problems in pine taxonomy, those involving the recognition of botanical species, subspecies, and varieties. Even in the well-studied pine floras of North America and Europe, there are extensive disagreements about whether to recognize certain kinds of pines as species, varieties or subspecies, or even as variants not worthy of formal recognition. The situation is even worse in the rich pine floras of China and Mexico, which need extensive new investigations at the population level involving the full array of modern botanical techniques. Floristics studies of both of these regions (Fu et al. 1999c for China, and Perry 1991 and Farjon and Styles 1997 for Mexico), while presenting important advances over previous work, still leave many unanswered questions. At least some of the taxonomic complexity in Mexican pines is probably due to natural hybridization between species, but this has scarcely been investigated and even then in only a very few species. So, despite the fact that pines are the most studied of conifers and the most widely cultivated on both a small scale and in massive plantations, there is still much to learn about the species and their relationships.

Discerning these relationships is not made any easier by the fact that Pinus is the oldest modern genus of Pinaceae, as far as known, with many fossils from throughout the northern hemisphere, beginning in the early Cretaceous, over 100 million years ago. By the Eocene, some 50 million years ago, many modern sections and subsections were in evidence, so there has been a great deal of time for evolution and confounding extinction within the genus. Its age obscures its relationships to the other genera of Pinaceae, and many extinct Cretaceous species (placed in the fossil genus Pityostrobus) combine characters of Pinus and various other genera.

References

• Farjon, A. (2010). A Handbook of the World's Conifers. Koninklijke Brill, Leiden.
• Eckenwalder, J.E. (2009) Conifers of the World: The Complete Reference. Timber Press, Portland.
• IUCN Red List of Threatened Species, International Union for Conservation of Nature and Natural Resources. Cambridge, UK /Gland, Switzerland

Copyright © Aljos Farjon, James E. Eckenwalder, IUCN, Conifers Garden. All rights reserved.