In the Northern Hemisphere, north of the temperate deciduous forests, the principal plant life is coniferous forest. Because of the commercial value of the timber from these trees (used in building and as a source of wood pulp for papermaking, for example), many of the forests are man-made. The natural coniferous forest, lying mostly between latitudes 45° N. and 70° N., is called the taiga. Farther north it gives way to tundra vegetation. To the south, it blends into deciduous woodland or grassland (on the North American prairies) and steppes (in central Asia).
There are no large natural forests comparable to those of the taiga in the Southern Hemisphere because land does not extend into the equivalent latitudes, although there are some pockets of coniferous forest south of the equator in New Zealand and Chile.
Adaptations of coniferous forests
During the winter, when the ground is frozen, tree roots are unable to obtain water from the soil. Few of the winds bring rain, and although precipitation may reach 10 to 39 inches (25 to 99 centimeters) a year, most of it falls as snow. Trees that grow in these conditions—termed winter drought—must therefore by xerophytic (drought-resistant), a feature they achieve mainly by their leaf form. The leaves of most conifers are tough, leathery, and evergreen. A waxy cuticle reduces water loss by transpiration, and the toughness prevents the leaves from wilting under water stress. Most leaves of coniferous trees are needle-shaped and highly resistant to frost.
Unlike deciduous trees, evergreen species do not need to expend as much energy putting out new green leaves each year, and those needles that persist for several years conserve scarce nutrients. Although they keep their needles, evergreens do not photosynthe-size on warm winter days, for the roots remain frozen and their stomates remain closed. The trees are conical in shape, which permits light to reach lower branches for photosynthesis and permits snow to be shed so that branches do not break under the heavy weight.
Variety within the forest
Few natural coniferous forests are homogeneous, containing only evergreens. In the Siberian forest, for example, particularly at high altitudes, the dominant species are three kinds of larches, Larix sukaczewii, L. Siberian, and L. dahurica. The larch is deciduous, but it is extremely hardy and can withstand gales, as well as being rot-resistant.
Parts of the Siberian forest, like large areas of Canada, occupy boggy ground called muskeg. Those in Canada support the tamarack or American larch (Larix laricinal, although the forests in other areas such as Labrador, New Brunswick, and Newfoundland are chiefly spruce (Picea sp.) and larch. White spruce (P. glauca) grows close in well-drained and warmer soils and black spruce (P. marina) in colder and wetter soils. Birch trees (Betuia spp.) and balsam firs (Abies balsamea) are also found in these regions. The cells of the dormant deciduous trees remain undamaged after slow freezing. As a result, the Siberian larches can withstand lower temperatures than evergreens, and in Finland the birches extend farther north than pines and spruces.
The eastern side of the North American continent has a more temperate climate than western North America and Siberia. Around the Great Lakes precipitation can fluctuate between 20 and 40 inches (50 and 101 centimeters) a year and temperatures between —50° and 104° F. (10° and 40° C). These conditions result in a variety of species. White pine (Pinus strobus) and hemlock (Tsuga canadensis) dominate the forests, and red pine (P. resinosa) and aspen (Populus spp.) are common, as are black spruce (Picea marina) in bogs and white spruce (P. glauca) in uplands.
Succession of tree species
Excluding coniferous forests, there are few virgin forests left in the world. A recent study of the northern regions of Canada and Scandinavia, however, has shown how natural succession of different species—in this case of birch, pine, and spruce—occurs in one place over a period of 300 years, if it is left undisturbed.
The birch is a broad-leaved deciduous tree, which is often found in coniferous forests. When a space is cleared in a forest, by fire or falling trees, the birch (which grows rapidly and needs considerable light) quickly invades it by means of its widely dispersed, windborne seeds. For the next 60 years a birch forest is formed. Under the canopy and in gaps between the birches, pines begin to grow and the woodland changes to a pine association.
The pines, which live for about 100 to 150 years, maintain a dense forest in which ground vegetation dies because of the lack of light. Spruce and fir seedlings have difficulty in establishing themselves, but their light requirements are minimal so that, as the pine association dies away, the spruce and fir gradually become the dominant (or climax) species, so forming a spruce-fir association. Because they need less light and create great shade, the spruce and fir can maintain their dominance over the pine and birch until fire or felling renews the succession. The cycle then begins again with birch.
Where a space in the forest does admit light, juniper (Juniperus communis) and bilberry (Vaccinium myrtillus) as well as grasses and herbaceous plants, such as wintergreen tPyroia sp.), grow.
Other less highly evolved plants also live in the coniferous forest. Fungi and bacteria cover old needles on the forest floor, feeding on the slowly decaying needles, and lichens such as the reindeer moss ICIadonia rangiferina) also grow there. Many simple plants live on the trees and are known as epiphytes. They live and grow on nutrients carried from the air by rain. They include mosses, ferns, and lichens.
The carpet of fallen needles, branches, and cones decays slowly because of the low temperatures. Beneath this layer of infertile material is a leached stratum, which the roots penetrate. It is deficient in nutrients, and so the trees depend largely on the fungi in the litter, which take carbohydrates from the tree and in turn provide it with mineral salts.