The pines are species of trees in the genus Pinus, of the family Pinaceae and phylum Coniferophyta, the cone-bearing plants (conifers). Relatives of the pines include other conifers such as fir, Douglas fir, spruce, hemlock, cypress, and redwood. Pines and these other conifers are all considered gymnosperms, because they bear their seeds naked, rather than within an ovary as in the angiosperms (flowering plants). There are about 100 different species of pines in the world.
All of the pines are woody plants. The mugo pine (Pinus mugo ), native to the Alps of Europe, is one of the smallest pines. At maturity, it is really more of a bush than a tree, and is often planted in gardens of Europe and North America. Many other pines which are native to North America are large trees which can grow 197-262 ft (60-80 m) or more in height.
The leaves of all pines are needle like and arise from the stem in bundles, called fascicles. Each fascicle is often associated with a fascicle sheath, a special tissue at its base. Most species have two to five needles per fascicle, but some species have as few as one and others have as many as eight needles per fascicle. The needles of pines are arranged in a spiral about the stem. Each year, as the branch of a pine tree grows, it produces a whorl of new leaves, called a candle. The needles of pines last about two years and most species are evergreen, meaning they have some needles at all times. Since pines have needles throughout the year,
they have the potential to photosynthesize whenever conditions are suitable.
The needles of pines, like those of other conifers, are well-adapted for growth in dry environments. In particular, the outer surface of pine needles has a thick waxy layer, called a cuticle, which reduces evaporative water loss. Like the leaves of all higher plants, pine needles have special microscopic pores on their surface, called stomata, which are important for exchange of water vapor, carbon dioxide, and oxygen. The stomata are usually arranged in rows on the underside of the needles, where they appear as white lines. At the microscopic level, the stomata are beneath the surface cells, so they are often called “sunken stomata.” This stomatal adaptation reduces evaporative water loss.
The pines are vascular plants, in that their trunks and stems have specialized cells, xylem and phloem, for the transport of water and food. The xylem of pines consists mainly of tracheids, elongated cells with thick walls and tapered ends. The phloem of pines consists mainly of sieve cells, elongated cells with relatively unspecialized sieve areas at the ends. Sieve cells are characteristic of gymnosperms and free-sporing plants, whereas sieve tube elements are characteristic of the more evolutionarily advanced flowering plants.
The oldest known fossil of the pine family (Pinaceae) is a cone from the Lower Cretaceous period, about 130 million years ago. The structure of this fossilized pine cone is similar to that of modern cones of the Pinus genus.
Today, there are about 100 species of pines. Pines grow throughout the Northern Hemisphere, and only one species (Pinus merkusii ) is native to the Southern Hemisphere. More than 70 species are native to Mexico and Central America, and this is their likely center of origin. Pines are distributed in North America from the subarctic of northern Canada and Alaska to the tropics. There are about 35 species of pines in the United States and Canada. Although only one species is native to the Southern Hemisphere, many pines have been introduced and cultivated there for timber or as ornamental plants.
There are two subgenera of pines, and botanists believe these are evolutionarily distinct groups. These subgenera are Diploxylon, commonly called the hard pines, and Haploxylon, commonly called the soft pines. As suggested by their names, the wood of soft pines tends to be soft, and the wood of hard pines tends to be hard.
The needles of hard pines have the following characteristics: (a) they usually arise in fascicles (bundles) of two or three; (b) they have a semicircular shape in cross-section; and (c) they have two main veins, as revealed by a cross-section. In addition, the fascicle sheaths of hard pines remain attached as the needles mature.
The needles of soft pines have the following characteristics: (a) they usually arise in fascicles (bundles) of five; (b) they have a triangular shape in cross-section; and (c) they have only one main vein, as revealed by a cross-section. In addition, the fascicle sheaths of soft pines wither away as the needles mature.
All species of pines are monoecious, in that male and female reproductive structures occur on the same plant. Once a pine tree reaches a certain stage of maturity, it forms male and female reproductive structures, termed strobili (singular: strobilus). The strobili of pines are unisexual, in that they contain either male or female reproductive organs, but not both. The male strobili are typically about 0.4-0.8 in (1-2 cm) in diameter and form on the lower part of the tree. The female strobili are much larger and form on the upper part of the tree.
The male strobilus is composed of many modified leaves, called microsporophylls, which are spirally arranged about a central axis. Each microsporophyll has two microsporangia attached. Microsporangia are organs that contain microsporocytes, immature pollen grains. The microsporocytes develop into pollen grains with four cells each. The four cells of the pollen grain are haploid, in that each contains one set of chromosomes. Thus, the pollen grain of pines is a multicellular haploid tissue, and is the male gametophyte. In the spring time, the male strobilus releases pollen into the wind, and then shrivels up and dies.
The female strobilus is larger than the male strobilus. It is composed of many scales (modified leaves) which are spirally arranged about a central axis. Each scale has a sterile bract and two ovules, egg-forming structures, attached to it. The ovule consists of two types of tissues, the nucellus and its surrounding integument. A special pore, called a micropyle, passes through the integument to the nucellus.
In pollination, a pollen grain lands on the female strobilus and sticks to a special fluid in the micropyle. As this fluid evaporates, the pollen grain is drawn into contact with the nucellus. This causes the pollen grain to germinate and form a pollen tube. Then, the female tissue produces four megaspores. The megaspores are haploid cells, in that each has one set of chromosomes. One of the megaspores develops into a megagametophyte, a multicellular haploid tissue, and the others degenerate. Then, more than one year after the pollen grain has landed on the female strobilus, the female megagametophyte forms archegonia, reproductive structures which contain egg cells.
In fertilization, the pollen tube arrives at the surface of the egg cell and releases two haploid sperm nuclei into it. One of these sperm nuclei degenerates and the other unites with the nucleus of the egg to form a cell with two sets of chromosomes. This is the zygote. The zygote develops into a seed, which contains an embryo. The entire process from pollination to formation of a mature seed typically takes two to three years. This is much slower than in the flowering plants (angiosperms).
Wind or foraging animals generally disperse pine seeds into the environment. The seed germinates following stimulation by certain environmental signals, such as exposure to light or temperature changes. Most species of pines can live for a hundred or more years and some species, such as the bristlecone pine (see below), can live for thousands of years.
Pines are very important economically. The wood of many species is used as timber for construction and furniture. Pines are also used for the manufacture of turpentine, rosin, pulp, and paper.
One of the most economically important pines of the 1800s was the eastern white pine (Pinus strobus ). This pine once dominated forested regions in Pennsylvania, New York, New Jersey, much of New England, and southeastern Canada. Most of these pines were several hundred years old and 197-230 ft (60-70 m) in height. During the 1800s, most of these pine forests were clear-cut and the lumber was used for construction in North America, or was shipped to Europe where lumber was in short supply. More recently, the eastern white pine and the red pine (Pinus resinosa ) have been used for reforestation in parts of eastern North America.
In modern times, several other species of pine are economically important. The ponderosa pine (Pinus ponderosa ) of the western United States is currently the most economically important pine of North America. The southeastern United States also has economically important pines such as loblolly pine (Pinus taeda ), short leaf pine (P. echinata ), slash pine (P. elliottii ), and longleaf pine (P. palustris ). Many of these southeastern pines are cultivated in plantations. Outside of North America, Pinus pinaster of the Mediterranean region and Pinus longifolia from India are major commercial species.
The bristlecone pine (Pinus aristata ) is an important species to scientists because it lives so long, and has tree rings can provide important clues about the climate of previous eras. This species grows in the arid mountainous regions of California, Nevada, Utah, and Colorado at an elevation of about 9,840 ft (3,000 m). Bristlecone pine grows very slowly, but can live for several thousand years. The oldest known specimen is nearly 5,000 years old. Bristlecone pines have been intensively studied by dendrochronologists, scientists who examine and interpret tree rings.
The tree rings of bristlecone pines and other trees appear as concentric rings, and are visible in a cross-section of a trunk or in a core sample. A new growth ring typically forms each year, as the tree trunk expands. Growth rings are relatively wide in years favorable for growth, and narrow in unfavorable years. Bristlecone pines grow so slowly that there can be more than a hundred rings in the space of only a few centimeters, so their tree rings must be examined with a microscope. The width and other features of these growth rings provide valuable clues to archaeologists about the prevailing local climate during the period when ancient native American cultures inhabited the western United States.
One of the most familiar feature of pines is their cones. Biologically, a pine cone is simply a fertilized female strobilus containing seeds within.
While their economic significance is not as great as that of pines, which are harvested for timber (see above), the pinyon pines (Pinus cembroides, P. monophylla, P. quadrifolia, and P. edulis ) are prolific producers of edible pine “nuts,” which are technically seeds. These seeds are often used in salads, sauces, desserts, and other foods. The pinyon pines are native to semi-arid regions of the western United States and Mexico.
The largest pine cones come from the sugar pine (Pinus lambertiana ). This species grows in western North America and its pine cones are typically 15-18 in (38-46 cm) long and 4 in (10 cm) wide. The cones of the big cone pine (Pinus coulteri ), a native of California, are somewhat smaller, but can weigh over 4.4 lb (2 kg), heavier than any other species.
One of the most interesting pine cone adaptations occurs in jack pine (Pinus banksiana ), pitch pine (P. rigida ), knobcone pine (P. attenuata ) and several other species. The cones of these species are serotinous, meaning that they are “late opening.” In particular, the pine cones remain closed long after the seeds have matured. They typically open up to disperse the seeds only after exposure to very high temperatures, such as occurs during a fire. At the biochemical level, the heat of a fire apparently softens the resins that hold together the scales of the cone. Pine trees with serotinous cones often grow in ecosystems that have a high frequency of fires. For example, the pitch pine grows in the New Jersey pine barrens, where natural or man-made fires have occurred for many centuries.
The U.S. Fish and Wildlife Service’s Division of Endangered Species List includes no pine species. However, this list does not cover non-U.S. species, and there are endangered pine species in Mexico and in Asia.
The rapid disappearance of the pine forests of Mexico and Central America have been largely due to disease, insects and human activity. Mexico’s population increases by over a million people each year, and this places heavy demand on firewood and land for agricultural uses.
There are nine Mexican pines that are considered either endangered or rare; they are:
- Pinus culminicola (Potosi pinyon)
- P. maximartinezii (large cone Martinez pine)
- P. rzedowskii (Rzedowski pine)
- P. pinceana (weeping pinyon)
- P. johannis (Johannis pinyon)
- P. radiata var. binata (Monterey pine)
- P. lagunae (Laguna pinyon)
- P. jaliscana (Jalisco pine)
- P. nelsoni (Nelson pine)
Of these, the first four are considered very rare and very endangered. The next two, P. johannis and P. radiata, are classified as rare and endangered, and the last three are considered rare.
According to the World Conservation Union-IUCN, the following Asian pines are considered to be the most endangered:
- P. dalatensis (found in South Vietnam)
- P. massoniana var. hainanensis
- P. wangii (found in small area in Yunnan Province, China)
Other endangered species on the World Conservation Union’s list are:
- P. bungeana (in northern central China)
- P. dabeshanensis (in the Dabie shan mountains of eastern central China)
- P. culminicola
- P. maximartinezii
- P. rzedowski
- P. torreyana subsp. torreyana
- P. torreyana subsp. insularis
- P. radiata var. bipinata
Tree conservationists have learned that when forests are eliminated, the trees that grow back are seldom the same ones that were there before. The pine trees felled in Michigan in the late nineteenth century never grew back, and were replaced by oaks and aspens, which the gypsy moth is fond of. The hard-woods in the southern part of the country were cut to make room for pines that could be harvested 20-40 years later. There are now pine plantations from North Carolina to Arkansas, where the trees frequently do not grow as rapidly as had been planned.
Today, enlightened foresters practice sustainable forestry, a practice that places nature ahead of timber harvests, and removes tree from the forest at a rate that can be maintained indefinitely. Models for returning land to forest come from the old stands of unmanaged forest, which have sustained themselves for thousands of years.
Cuticle— Layer of wax covering the surface of leaves and other plant parts.
Dendrochronology— Scientific examination and interpretation of tree rings.
Diploid— Nucleus or cell containing two copies of each chromosome, generated by fusion of two haploid nuclei.
Fascicle— Bundle of leaves, in the pines often associated with a fascicle sheath, a special tissue at its base.
Fertilization— Union of male and female sex cells to form a diploid cell.
Haploid— Nucleus or cell containing one copy of each chromosome.
Pollination— Movement of pollen from the male reproductive organ to the female reproductive organ, usually followed by fertilization.
Strobilus— Reproductive organ consisting of modified leaves (sporophylls) spirally arranged about a central axis, colloquially referred to as a cone.
Ecology and Biogeography of Pinus, edited by David M. Richardson. Cambridge, U.K.: Cambridge University Press, 2000.
White, John, and David More. Illustrated Encyclopedia of Trees, 2nd ed. Portland, OR: Timber Press, 2005.
Chaw, S. M., et al. “Seed Plant Phylogeny Inferred From All Three Plant genomes: Monophyly of Extant Gymnosperms and Origin of Gnetales from Conifers.” Proceedings of the National Academy of Sciences of the United States of America 97 (2000): 4086-4091.
Peter A. Ensminger