Coniferous Trees

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Sometimes called evergreens, most coniferous trees keep their foliage year-round. There are over 600 living species of conifers, and while there is some debate over how many are native to Canada, the number is approximately 30. Conifers include the oldest and tallest trees. The oldest, the Bristlecone pine, can live to be nearly 5,000 years old. The tallest, the Coast redwood, grows to over 100 m high. Both of these conifers grow in California. Products made from coniferous trees include paper, many kinds of lumber, furniture and anti-cancer drugs. In large part because of their usefulness, conifers are in danger. Exploitation, forest degradation and habitat destruction have placed 34 per cent of conifers under threat of extinction.

Classification

Conifers are a large group of resinous, cone-bearing trees and shrubs. According to the biological classification system, conifers comprise the order Coniferales of the Gymnosperms. Gymnosperms are woody plants that have naked seeds and do not produce flowers. There are seven families of conifers, which are sub-classified into 67 groups called genera, and sub-classified further into over 600 living species.

Range

Conifers have an extensive range, but are found primarily in the Northern Hemisphere, as far north as the Arctic Circle. They can also be found in Central America and South America. Conifers are widespread in Europe and Asia, and several species can be found in Africa. Few are tropical.

Conifers native to Canada include the Douglas fir, pine, spruce, larch, true fir, hemlock, cedar, cypress, juniper and yew. The greatest diversity of conifers occurs in the western provinces, with British Columbia having 25 native species. The second greatest diversity is in the eastern and Atlantic provinces, with relatively few species in the central provinces. Most species grow in limited geographical regions, but the black spruce and white spruce species extend from coastal British Columbia to the Atlantic Ocean.

Anatomical Features

Leaves

Most conifer species are evergreen, meaning they retain most of their leaves throughout the year. However, a few genera, such as larch, are deciduous, meaning they shed all their leaves every autumn. Most evergreens shed leaves (or branches, in cedars) that grew two or more years earlier, so that newer branches are never bare of leaves. Most conifers have needle-like leaves such as the fir, pine, spruce and larch. Some, like cedar, cypress and juniper trees, have scale-like leaves and do not shed individual leaves, but shed short branches bearing one or more years growth.

Seeds

Most conifers have seeds on the surface of their scales, forming seed cones. This is why they are called Gymnosperms, which means naked, seeded plants. Seed cones also have smaller leaf-like structures called bracts below each scale. Bracts may be small and entirely hidden under the scale as in true firs or partially hidden as in hemlock. In some conifers, such as Douglas fir or larch, bracts are long and extend beyond the scales. In these examples, the bracts are not attached to the scale. In other conifers, such as pines, the bract and scale may be partly fused forming a bract-scale complex. In still others, such as cedar and cypress, the bracts and scales are totally fused into a single structure.

Mature seed cones are commonly large and woody as in pines, spruce, larch and firs. In others, like cedar and cypress, mature seed cones are small and woody, but they can also be non-woody and soft as in junipers. In a few conifers, seeds are borne singly, occasionally with a berry-like covering as in Podocarpus and yew.

Resin Ducts

Resin ducts are found in all conifers and conifer structures, including the roots, stems, leaves, cones and even some seeds. Resin ducts are tiny tubes lined with cells that secrete sticky pitch into the ducts and often to the surfaces of the tree. It serves as a protection mechanism to seal wounds or areas where leaves, cones, branches or bark have been naturally shed. The resin is harvested from some conifers, especially pine trees, for its many commercial uses. Resin’s earliest use was as caulking for wooden sailing ships. It is still used to obtain terpenes, a group of chemicals used for the extraction of turpentine, and related oils and compounds.

Surviving Winter

During late summer and early fall, when days become shorter and temperatures decrease, temperate conifers undergo several changes in their leaves, stems and roots. Cell divisions and cell growth stop. There is no active growth forming new tissues or organs such as wood or leaves, respectively. Many complex chemical changes also occur, ranging from increased amounts of soluble salts and sugars, and in some, the production of anti-freeze compounds, all of which increase the cold resistance of the living tissues. These complex physiological changes lower the ice-forming temperature by several degrees allowing the living tissues to survive long periods of sub-freezing temperatures.

Reproduction

All conifers have separate seed cones and pollen cones. These may be borne either on the same tree (monoecious, one home) or on different trees (dioecious, two homes). Pollen cones produce abundant yellow pollen, which is dispersed by wind every spring and enters the seed cones (pollination) where fertilization, embryo and seed development occur.

In most conifers, pollination, fertilization, and embryo and seed development occur in one growing season, from spring through autumn. However, in pines and a few other genera, there is a delay of one year between pollination and fertilization, or fertilization and seed development, and the reproductive cycle is extended over two growing seasons. In both types of reproductive cycles, the seed cones mature in the autumn and seeds are shed either when the dry cone opens or disintegrates.

Most species have seeds with either one or two wings that slow their fall, helping in seed dispersal. Seeds are commonly dispersed by wind, but squirrels and other rodents may disperse them as well. In yew and a few species of pine, birds disperse the seeds. In several conifers, entire cones are shed rather than individual seeds.

Families

The classification and taxonomy of conifers has changed in recent years as a result of new molecular technologies and studies of reproductive biology. There are now considered to be seven living families and one extinct family, the Lebachiaceae. It is believed that all modern conifers evolved from the extinct Lebachiaceae during the Mesozoic Era over 200 million years ago, around the time of the rise of the dinosaurs. Of the seven living families, Araucariaceae and Podocarpaceae are the oldest families. They are distributed primarily within the Southern Hemisphere. The Sciadopityaceae family has but one genus and one species (Sciadopitys verticillata), which is native to Japan. The 15 species (in two genera) of Cephalotaxaceae are all native to eastern Asia. Three families have species native to Canada, the Pinaceae, Cupressaceae and Taxaceae.

Pinaceae

The pine family is the largest, most familiar and widely-distributed family containing 10 genera and about 220 species. Most are large trees, like firs, spruce and larches, but some species are shrubs. The Pinaceae arose soon after the Podocarpaceae and Araucariaceae, and are distributed across the Northern Hemisphere, mostly in temperate regions, with the exception of a species native to Indonesia. Pinaceae are also found along ocean shores and in high alpine and dry desert regions across North America, Europe and Asia.

They all have needle-like leaves and small-to-large woody cones with two seeds per cone scale. The largest genera are Pinus (pines) with 109 species. Abies (true firs) have 49 species and Picea (spruce) are comprised of 34 species. Other genera have fewer species, like Larix (larch) with 10 species; Tsuga (hemlocks) with nine species; Pseudotsuga (Douglas fir) with four species; Cedrus (true cedars) with four species; and Keteleeria with three species.

The Pinaceae are commercially the most important conifer family in Canada. Nine species of pine, five species of spruce, three species of hemlock, three species of larch, three species of true firs and one species of douglas fir are found in Canada. They are most abundant in western and eastern Canada, but some species of pine, larch and spruce can be found almost coast-to-coast.

Cupressaceae

The Cypress family (cypresses, cedars and junipers) were combined with the former Redwood family (Taxodiaceae) because of their similarities in cone structure, reproductive development and new molecular evidence. The current Cupressaceae family consists of 28 genera and 118 species.

They are quite variable in leaf structure. Some have deciduous needle-like leaves. Most have small scale-like leaves and a few, like junipers and interior redwood (Sequoiadendron), have short-to-awl-shaped leaves. The considerable variation in leaf form (morphology) was one of the main reasons for the original separation of the two original families. The new classification is based on more conservative reproductive and molecular information.

The Cupressaceae vary greatly in size. They may be huge trees, like redwoods, or shrubs, like many junipers. Mature seed cones are usually small, woody, soft or berrylike, and all have completely fused bracts and scales, each of which may bear several seeds. The seeds usually have two small wings or no wings.

The Cupressaceae is the most widely distributed family of conifers. They are found throughout the Northern Hemisphere from the arctic tundra to high mountains and desert areas. They are also found in the Southern Hemisphere, in South America, Africa and Australia. A few species also grow in tropical regions of Southeast Asia. The following species can be found in Canada: the Western red cedar (Thuja plicata) and Eastern white cedar (T. occidentalis), the yellow cypress (Chamaecyparis nootkatensis), and three species of JUNIPERS (Juniperus communis, J. horizontalis and J. virginiana).

Taxaceae

The yew family is represented by five genera and about 22 species. Four of the genera, including the largest and most familiar, Taxus, are distributed predominantly in the Northern Hemisphere and one genus, Austrotaxus, extends into the Southern Hemisphere.

The Taxaceae was previously considered to be a separate order, the Taxales, comparable to the order Coniferales because of their single-seeded berry-like seed-cone structure. They are now included as a family within the Coniferales based on new molecular evidence showing a close genetic relationship with other conifers. Also, new reproductive evidence shows that the seed, which is enclosed by a fleshy additional seed covering called an aril, actually begins development within a reduced compound seed-cone structure. The seed-cone structure is similar to that of other conifer families, but in yew, the cone has become reduced to a single small scale and a fleshy red aril that encloses most of the seed.

The wood of Taxus is very hard and beautiful, and is used for furniture, woodcarvings and archery bows. A chemical, taxol, can be extracted from the bark or leaves to be used as a medicine to treat some cancers. This chemical was synthesized in the laboratory in the early 1990s, saving Taxus species from the rapid harvest that began in the late 1980s. Two yew species are native to Canada: ground hemlock (T. canadensis) and western yew (T. brevifolia).

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