Coniferous Forests

views updated

Coniferous Forests

Coniferous forests are dominated by gymnosperm trees such as pines, spruces, and firs. Conifers were the first plants to evolve seeds. Gymnosperms (from the Greek words gymnos, meaning "naked," and sperma, meaning "seed") have seeds exposed to the environment on cones. In most species, male and female cones occur on the same tree, but the Juniperus (juniper) and Taxus (yews) genera have species with separate male and female trees. Male cones are smaller than female cones and produce pollen in the springtime. The larger female cones are able to be fertilized only when they are young and often unnoticeable. Most conifers rely on wind to carry their beautiful and diversely shaped pollen grains to the female cone.

The phylum Coniferophyta is organized into two orders. Older classification schemes included a third, Ginkgoales, containing only one species (Ginkgo biloba ); more recent classification schemes now place Ginkgo into its own phylum, Ginkgophyta. Coniferales, with five families and over six hundred species, including the species most often identified with coniferous forests, is the most populous order. Some of the world's most remarkable plants are found in Coniferales. Bristlecone pine (Pinus aristata ) can live to be over six thousand years old; coastal redwoods (Sequoia sempervirens ) grow to be over one hundred meters tall; and Monterey pine (Pinus radiata ) is one of the most productive timber species. The Taxales order contains two families and over thirty species but is best known for the poisonous yew (Taxus ) genus.

Conifer Leaves

Most conifers are evergreen, meaning that they maintain green leaves, usually needles, year-round. Needles exist in all families. Scalelike leaves often obscuring the woody portion of the shoot exist in the Cupressaceae, Podocarpaceae, and Taxodiaceae families. The Podocarpaceae family contains the only broadleaf conifers. Two genera, the celery pine (Phyllocladus, found in the Southern Hemisphere) and the Japanese umbrella pine (Sciadopitys ), do not contain true leaves and instead carry out photosynthesis using specially adapted shoots.

In climates with mild, wet winters and warm, dry summers, drought adaptations and the ability to conduct photosynthesis all winter give evergreen conifers a distinct advantage over deciduous angiosperms . In the boreal forest, conifers succeed due to a combination of factors. First, growing seasons are short and conifers are able to begin photosynthesis with a full canopy as soon as temperatures warm. Second, because needles last from two to ten years, conifers need to replace fewer leaves each year than deciduous trees. Since leaves require large amounts of nutrients, nutrient-poor areas (such as the boreal forest and the southeastern United States) are often dominated by conifers. Third, conifers are more able to resist periodic drought stresses common in the boreal forest. Finally, in climates where temperatures dip below -45°C, conifers can survive where angiosperms cannot.

Nearly all conifers are evergreen but there are four deciduous genera: Larix, Pseudolarix, Metasequoia, and Taxodium. The Larix and Pseudolarix (common name larch) live in the boreal forest. In addition to possessing good cold-resistance, larches have high photosynthetic rates, flush early in the spring, and use nutrients very efficiently. Metasequoia, the dawn redwood, grows well on damp sites. Taxodium, the swamp cypress, grows in standing water in the southeast United States and parts of Mexico.

Distribution of Coniferous Forests

Coniferous forests exist in many climates around the world. The Podocarpaceae family is distributed in tropical and subtropical climates in South America and Southeast Asia. Small areas of southern Chile and western Argentina have coniferous Araucaria species living with evergreen broadleaf species. Mexico and Central America have pine forests in high elevation mountain ranges. Western North America and Japan support one million square kilometers of coastal coniferous rain forests. With nearly sixteen million square kilometers, the northern latitude boreal forests contain the vast majority of coniferous forest area. The Eurasian boreal forest begins in Scandinavia and extends east in a widening band all the way to the Kamchatka Peninsula in eastern Russia. The forest reaches its northernmost boundary at 73°30′ N in Siberia but is usually found no farther north than 68°N. In North America, the eastern boreal forest ranges from 45°N to 55°N; the western forest extends from 55°N to 69°N. Forested areas called subalpine forests cover about three million square kilometers in the U.S. Rocky Mountains, mid-elevation areas in the Himalayas, and other temperate mountain ranges.

Coniferous Forests in the United States and Canada

U.S. and Canadian coniferous forests follow a general rule found worldwide: as temperatures cool, species diversity declines. In Alaska and northwestern Canada, the boreal forest is primarily composed of black spruce (Picea mariana ), white spruce (Picea glauca ), and larch (Larix laricinia ). Farther south and in isolated warm northern areas, aspen and birch intermingle. In central Canada, lodgepole pine (Pinus contorta ), jack pine (Pinus banksiana ), and balsam fir (Abies balsamea ) appear. East of the Great Lakes, red pine (Pinus resinosa ), eastern white pine (Pinus resinosa ), oaks, and maples are common.

The Rocky Mountains resemble the boreal forest but are distinguished by the presence of subalpine fir (Abies lasiocarpa ). Engelmann spruce (Picea engelmannii ) replaces black and white spruce. In the central Rockies, drier regions of the northern Rockies, and high elevations of the southern Rockies, Douglas-fir (Pseudotsuga menziesii ) and ponderosa pine (Pinus ponderosa ) are common. In the southern Rockies, Engelmann spruce remains at higher elevations. Piñon pine (Pinus edulis ) and Rocky Mountain juniper (Juniperus scopulorum ) occupy the grassland-forest boundary. Trembling aspen exists throughout the Rocky Mountains.

The temperate rain forest, stretching along coastal North America from northern California to southern Alaska, contains western red cedar (Thuja plicata ), Douglas-fir, Pacific silver fir (Abies amabilis ), Sitka spruce (Picea sithcensis ), and hemlock (Tsuga heterophylla ). Redwoods (Sequoia sempervirens ) indicate the southern limit of the temperate rain forest. The giant sequoia (Sequoia gigantea ), one of the largest trees in world, grows well on the western Sierras in California.

Plant-Animal Interactions

Most conifers do not rely on insects, birds, or mammals to distribute their seeds and therefore have fewer readily observable examples of plant-animal interactions than flowering plants. Nonetheless, insects, birds, and mammals maintain strikingly diverse interactions with the coniferous trees in their habitat.

With few exceptions, insects in conifer forests are pests. Moths and butterflies are highly destructive, as are spruce budworms. All coniferous forests have some level of insect infestation. Vigorous forests use sap and other compounds to defend themselves against insects and are rarely catastrophically damaged. Forests in decline as a result of fire suppression or improper management are much more susceptible to insect outbreaks.

Birds in coniferous forests eat seeds and sometimes inadvertently help to plant trees. The Clark's nutcracker, for example, collects seeds from whitebark pine (Pinus albicaulis ) and limber pine (Pinus flexilis ) and brings them to nesting areas up to 45 kilometers away. The birds collect more seeds than they eat and the leftovers germinate. Insect-eating birds such as chickadees, nuthatches, and woodpeckers help to control insect populations. Owls and hawks live in coniferous forests and many, such as the spotted owl, use dead coniferous trees for nesting sites.

Mice and squirrels are the most common mammals in the coniferous forest. During the summer, these animals eat buds, berries, seeds, and even bark. Squirrels plan ahead for winter by collecting cones. As with birds not all the seeds are eaten, and some germinate into new trees. Deer, elk, mountain lions, bears, and other large mammals found in coniferous forests do not consume significant amounts of seeds or foliage. By chewing completely around a tree, porcupines interrupt the flow of sugars from leaves to roots. They are the only mammal besides humans known to kill coniferous trees.

Natural and Human-Managed Coniferous Forests

Coniferous forests exist along a gradient from purely natural to purely human created. The boreal forest, because it is so inhospitable and often contains commercially undesirable trees, contains the largest natural coniferous forests. Wildfires, insect outbreaks, and other disturbances are usually uncontrollable in remote boreal forests. In these forests, there is a variety of tree and undergrowth species; abundant animal, insect, and microbial life; and a natural fire cycle.

For most of the twentieth century the U.S. Forest Service pursued a policy of total fire suppression. Without fire, open stands of ponderosa pine were invaded by dense thickets of Douglas-fir and lodgepole pine. Insect outbreaks became common and fuels began to accumulate on the forest floor. Unmanageable and devastating fires such as the 1988 Yellowstone National Park fire caused a shift in public and scientific opinion; forest managers began to reincorporate fire through controlled burns and forests are now beginning the long process of regaining their natural relationship with fire.

In plantation forests, timber companies are interested in producing the maximum possible amount of commercial timber, not maintaining a diverse forest community. Many areas are planted with a single species at the same time. Conifers such as Monterey pine and slash pine (Pinus caribaea ), because they grow straight and quickly, are popular plantation trees. The lack of species diversity and geometrical forest arrangement make plantations very different from natural or partially managed forests. Plantations do not support diverse ecosystems nor are they are desirable for recreation. Society, however, has a large demand for forest products and maximizing plantation production reduces the need to exploit other forests.

see also Biome; Conifers; Deciduous Forests; Ecology, Fire; Forester; Forestry; Ginkgo; Sequoia; Trees.

Michael A. White