Conifers have also been assigned to higher ranks: Phylum Pinophyta, Class Pinopsida (Burnett 1835); but in this treatment they are treated as a subclass of the seed plants, Equisetopsida, comprised of three orders and seven families:
|Pinales Gorozh. 1904||Pinaceae|
|Araucariales Gorozh. 1904||Araucariaceae
|Cupressales Link 1829||Sciadopityaceae
These distinctions are based primarily upon molecular studies that have clearly shown the Pinaceae to be sister to all other conifers, and for the latter group to be comprised of two sister clades, Araucariaceae + Podocarpaceae and Cupressaceae + Sciadopityaceae + Taxaceae; for details see Missouri Botanical Garden (2011).
Historically, some authors segregated the yews and plum-yews Order Taxales with two families, Cephalotaxaceae and Taxaceae; and it was also popular for a time to segregate the Cupressaceae into two families, Cupressaceae and Taxodiaceae; see Cupressaceae for relevant remarks. Many authors elevate the genus Phyllocladus, here assigned to the Podocarpaceae, to the rank of family, but molecular and morphological studies have provided increasing evidence against this interpretation.
Conifers usually have needle-shaped or scalelike leaves, and nearly all are evergreen. They typically have straight trunks with horizontal branches varying more or less regularly in length from bottom to top, so that the trees are conical in outline. They are characterized by having staminate or pollen-producing cones; most also bear ovulate or seed-producing cones.
Almost all conifers form a vesicular-arbuscular mycorrhiza, but the species in the Pinaceae form ectomycorrhizas (Brundrett 2008).
Cosmopolitan, excepting polar regions, the highest mountains, the driest deserts, and a few oceanic islands. Species in the Pinaceae are almost entirely confined to the northern hemisphere, while the Podocarpaceae and Araucariaceae are most widespread in the southern hemisphere and tropical northern latitudes. The Taxaceae are very widespread, although nowhere abundant, in the northern hemisphere and a bit of the southern. Sciadopitys is only native in Japan. Some of the most widespread tree species in the world are found in the northern forests, where a handful of Larix, Picea and Pinus species circle the globe across Scandinavia, Russia, Alaska and Canada.
The largest of all is Sequoiadendron giganteum, in the Cupressaceae. A hundred years or so ago the largest was probably Sequoia sempervirens, also in the Cupressaceae, but the finest stands in that species were all destroyed by the loggers long ago; only remnants remain. The third largest is Agathis australis, in the Araucariaceae. Here again the finest forests were taken by the loggers, and we can only speculate on how large some of the vanished giants were; they may have rivaled Sequoia and Sequoiadendron. Pseudotsuga menziesii representing the Pinaceae, Podocarpus totara representing the Podocarpaceae, and Taxus sumatrana representing the Taxaceae are all distinctly impressive trees. Little Sciadopitys verticillata representing the Sciadopityaceae, on the other hand, seldom attracts much attention.
The Great Basin bristlecone pine, Pinus longaeva, can live nearly 5,000 years and is the undisputed champion. It is often described as the oldest living thing. Ages of over 3,000 years have been shown for Fitzroya cupressoides and Sequoiadendron giganteum, both in the Cupressaceae, and there are probably more really ancient trees in the Cupressaceae than in any other family. Next up is the Podocarpaceae; the Huon pine, Lagarostrobos franklinii, can live for 2,500 years. None of the other families have species that have been proven to live over 1,000 years, but such ages are probably achieved by Taxus baccata in the Taxaceae, and by Agathis australis and Araucaria araucana in the Araucariaceae. I should mention that Taxus baccata could reasonably be expected to live a very, very long time - thousands of years - but it lives by growing at the cambium while dying at the heart, so that a tree might grow in one place for a thousand years and yet not have any wood in it more than a few hundred years old. Does this make the tree a thousand years old?
A great many species have received attention, and in fact, unless someone has done some work with ginkgo (I haven't heard of it), conifers are the only gymnosperms that have received attention from dendrochronologists.
Conifers are one of the world's most important renewable resources. Most economic and cultural exploitation concerns members of the families Pinaceae and Cupressaceae, with Araucariaceae (in Australia and South America) and Podocarpaceae (in scattered tropical locales) locally important. See genus and species descriptions for particulars.
Several conifers, particularly in the genera Agathis (Araucariaceae), Abies, and Pinus (Pinaceae), produce economically important resins. Resins are sticky, liquid, organic substances that usually harden when exposed to air into brittle, amorphous, solid substances. Natural resins are classified according to their physical and chemical properties into hard resins, oleoresins, and gum resins (Moussouris and Regato 1999).
Conifers have a long and complex spiritual tradition in our culture. Their evergreen nature has made them symbols of immortality; Taxus baccata is a good example. Spiritual powers were also attributed to the cedar of Lebanon, Cedrus libani. Other trees were so important to the lives of aboriginal peoples that they were personified or worshipped; examples include the western redcedar, Thuja plicata; the Bunya pine, Araucaria bidwillii; and the kauri, Agathis australis. Even in our modern secular culture, people still speak reverently about species such as the Coast redwood (Sequoia sempervirens) and the Sequoiadendron.
Conifers are very popular ornamentals, widely used in landscaping. Good places to observe the diversity of conifers include arboreta, botanical gardens, zoos, and often city parks. However, I feel they are best seen in the wild, growing and reproducing in their native habitat. The species pages on this site will give you information helpful in finding such sites.
Conifers are known from fossils more than 290 million years old. Although more species of conifers once existed, they are still a widely distributed group.
Conifers reproduce by means of seeds borne on the scales of female cones, and the pollen is produced in separate male cones. Pollination in conifers is always dependent on wind currents to blow the abundant yellow pollen from the male cones to the female cones.
Brundrett, Mark. 2008. Mycorrhizal Associations: The Web Resource. mycorrhizas.info, accessed 2009.06.09.
Cronquist, A., A. Takhtajan, and W. Zimmermann. 1966. On the higher taxa of Embryophyta. Taxon 15:129-134.
Gorozh. 1904. Lekts. Morf. Sist. Archegon., pp. 72 and 88.
Link, J.H.F. 1829. Handbuch zur Erkennung der nutzbarsten und am haüfigsten vorkommenden Gewächse. Berlin. V. 2, p. 470.
Missouri Botanical Garden. 2011. Pinales. www.mobot.org/mobot/research/apweb/orders/conifers.html, accessed 2011.03.19.
Wikipedia also has some good articles on conifers.