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As we near the holiday season, a particular kind of conifer grows in popularity across North America. Known to some as a “Christmas tree,” conifer lovers would know that it is typically one of the varieties found in the pine family. How much do you know about pine trees? In Growing Conifers, John Albers and David Perry share extensive information on identifying, selecting, and cultivating conifers. Today, we share an excerpt from the book on the 12 of the most commonly seen pine genera.
Excerpt from the Book
The 614 conifer species currently found on earth represent 70 genera classified in the following eight families: Pine (Pinaceae), Cypress (Cupressaceae), Yew (Taxaceae), Plum-yew (Cephalotaxaceae), Umbrella-pine (Sciadopityaceae) , Podocarp (Podocarpaceae), Celery-pine (Phyllocladaceae), Araucaria (Araucariaceae)
Let’s look at each of these families in turn, along with the similarities and differences among the genera.
Family: Pine (Pinaceae)
The pine family Pinaceae is by far the most ecologically and economically important co- nifer family, providing most of the world’s softwood timber used for the construction and paper-products industries. The members of this family, which includes the pine genera, not only provide shade, screening, and aromatic fragrance in the garden but are also ex- cellent for wildlife habitat and soil erosion control. The family is dominant in the boreal forests of North America and the Pacific regions, and the genera and species in this fam- ily are found naturally in the three northern continents of America, Asia, and Europe, though they also occur farther south in Central America, the Caribbean, North Africa, and just one location south of the equator, Sumatra. They are primarily resinous and aromatic evergreen trees with narrow, needle-like leaves borne singly on long shoots or in tufts or tight bundles (fascicles) on short shoots in a spiral arrangement and leaves some- times twisted. Major branching usually consists of multiple tiers of horizontal branches. The seed cones and pollen cones are usually found on the same plant (monoecious, or one home), but some conifers have them on different plants (dioecious, or two homes). The seed cones may be erect or dangling. Among the 12 genera in the pine family, the most important genera commonly seen in cultivation are as follows: Pine (Pinus), Spruce (Picea), True fir (Abies), Douglas-fir (Pseudotsuga), Hemlock (Tsuga), Cedar (Cedrus), Larch (Larix).
Pines (Pinus) are evergreen trees or shrubs distributed throughout the world but native primarily to the northern temperate regions; some grow naturally in the tropics. The trees’ crowns are usually cone-shaped when young but often flat-topped or rounded with age. The leaves are needle-like, with two to five needles spirally arranged in fascicles or clustered on shoot spurs—except for the North American piñón pine (Pinus monophylla) which has a single needle in each fascicle. Pollen cones are small and numerous, arranged in dense clusters around the base of the current year’s growth. The mostly woody seed cones have overlapping cone scales (imbricate), and they hang pendent and mature in two to three years. Cone form varies widely, from egg-shaped to elongated conical or nearly cylindrical, as does size, from less than an inch (2.54 cm) to up to 16 inches (40.64 cm). The seed cones take from six months to three years to mature, depending on the species. Some seed cones open at maturity and release their seed; others have serotinous cones; that is, they remain closed and sealed with resin until opened by fire.
Spruces (Picea) are timber evergreen trees native to the northern temperate and boreal regions of the northern hemisphere. They have a spire-like or broadly conical crown, whorled branches, and thin scaly bark. Sharp-pointed needles are borne singly and spread radially from the twigs. The leaves are mostly four-sided and rigid, growing directly from the twigs without a stem or stalk, and are often forward pointing and upswept. They per- sist for up to ten years, and the woody peg-like base remains on the twig after the leaves fall. Pollen cones are single or grouped, shedding their pollen in the spring. Seed cones are usually pendent and borne primarily on upper branches.
True firs (Abies) are medium to large evergreen trees primarily found in cool, humid mountain ranges in the northern hemisphere. They have a typical spire- or cone-shaped crown, a single straight trunk, and regularly spaced horizontal branch whorls, with one whorl produced each year. The strikingly regular branch pattern is created by a single terminal with multiple lateral shoots forming each year at the tips of growing branches. The often-twisted, spirally arranged leaves have two prominent bands of stomatal bloom beneath, and they often have additional single lines of stomatal bloom on top. The leaves are borne singly and directly from the branch, persisting for five or more years. Where a leaf is detached, a flat, round leaf scar is left on the twig. Pendant pollen cones appear in dense clusters on the undersides of the current year’s growth. In contrast, barrel-shaped or cylindrical seed cones are single and attached upright on stalks in the gap between a needle and the shoot beneath it in the upper third to upper quarter of the shoots at the top of the crown. Seed cones mature in a single season, and after a mature cone falls apart to release its seed, the spike-like central core of the cone (the rachis) remain on the branch.
Douglas-firs (Pseudotsuga) are evergreen trees distributed in western North America and eastern Asia. The hyphen in the common name lets us know that Douglas-fir is not a true fir—that it’s not a member of the Abies genus. Pseudotsuga menziesii, a native to the West Coast of North America, is an economically important tree species because of its extensive use in industry. Douglas-firs have a single straight trunk and numerous spirally arranged horizontal branches. Alternate, short-stalked, linear, flat leaves with a groove on top and two stomatal bands beneath are borne singly and either radiate in all directions from the branch or are set two-ranked on the branch. Small (about 1-inch-long [2.54 cm]), catkin-like pollen cones drop large quantities of yellow pollen in spring. Seed cones hang in a pendent arrangement like that of spruce, but what makes the Douglas-fir seed cone distinctive is its three-pointed bracts extending beyond the cone scales, with a long central lobe that looks like a mouse tail.
Hemlocks (Tsuga) are distributed in eastern and western North America, eastern Asia, and the Himalayas. They are primarily cone-shaped evergreen trees with a leading shoot that usually droops, and their horizontal branches are usually arranged in flattened sprays that arch down- ward. The linear leaves have two stomata bands on the underside and, in most species, are spirally arranged and predominantly twisted into a flat row on each side of the twig (two-ranked). The leaves persist for several years. The sparse, nearly spherical pollen cones hang singly in the axils of leaves near the tips of the twigs. The seed cones, which hang at the tips of second-year twigs, mature in about six months; once they open to shed their seed, they fall intact after one year.
True cedars (Cedrus) are distributed in northwest Africa, the Mediterranean region, southwestern Asia, and the western Himalayas. They are tall evergreen trees, ordinarily with a single straight trunk, a broad crown, and an erect or sometimes bent top. As cedars reach old age, the top of the tree becomes increasingly flattened. Spirally arranged solitary needles, three- or four-sided in cross-section, are borne on long terminal shoots; tufts of nee- dles are borne on short shoots. Leaves persist for three to six years. Upright pollen and seed cones attach at the tips of short shoots. Seed cones mature in two to three years, then break apart to shed the seeds and seed scales.
Larches (Larix) are distributed across the boreal forest in the northern hemisphere and south of this region in the northeastern and northwestern United States and in the Himalayas and the Alps. They are deciduous trees with sparse, open, conical crowns and branches in a whorl with both long lateral shoots and short spur shoots. The long shoots sparsely bear well-separated, spirally arranged needles. The spur shoots are long-lived and bear many new tufts of needles (10 to 30 needles per spur) each year and often bear pollen cones and seed cones as well. Pollen and seed cones are borne upright at the tips of short shoots. The empty seed cones persist on the tree several years after seed release.
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