Shades of Green: Earth's Forests -- Temperate Forests

Just south of the northern boreal forests is a deciduous forest community thought of as a "transition" between the boreal forest and the warmer broad-leaved forests. In eastern North America, this transition forest is dominated by maples, beeches, and birch trees, which draw crowds of visitors every year for their unusually impressive display of fall colors.

A variety of maple species. Photo by Maya Walters.

Sugar maples, which are commonly tapped for their sap that is boiled for maple syrup, grow in this forest. This area is also inhabited by the sapsucker woodpecker, which as their name suggests, have an unusual diet that includes tree sap.

These forests are not exclusively broad-leaved, however, and many coniferous trees grow here that also grow slightly farther north in the boreal forests. There are few areas anywhere in the temperate zone where forests consist only of broad-leaved or only of coniferous trees.

Fallen trees are an important part of the temperate forest ecosystem, especially in temperate rainforests. When large trees fall, they create openings which allow sunlight down through the canopy of branches and leaves. The huge fallen logs are vital to the growth of young trees. The wood begins to decompose with the help of fungi and bacteria, and is soon covered with mosses, lichen, and tree seedlings.

The fallen trunk is a rich source of nutrients and an ideal habitat for new seedlings. In fact, several thousand young trees may begin to grow on a fallen log only fifty feet long.

A fallen log provides a rich habitat in plenty of sunlight. Photo by Maya Walters.

Most of these seedlings never reach full size, and the log itself has completely rotted away by the time that the few successful trees reach maturity. Several full-grown trees are sometimes growing in such a straight line that they appear to have been planted -- in fact it is because they grew along the straight trunk of a fallen tree. Some species are more dependent on this kind of situation than others -- some trees almost never begin growing directly on the ground, and require a fallen log to provide nutrients and keep them up off the layer of fallen leaves and needles on the forest floor.

A rainforest in the temperate zone? Tropical rainforests are well known, but rainforests also exist in North America. True, there are some very significant differences between those of the tropics and these temperate forests, and nothing can match the tropics for variety of animal and plant species, but still there are enough similarities to call these northern areas rainforests.
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The temperate forest experiences warm, enjoyable summers, and cold, often harsh winters. With these winters comes snow toward the northern range of the forests, rapidly decreasing in amount near the southern end. The growing season lasts about six months, and the twenty to sixty inches of precipitation is distributed fairly evenly throughout the year. Conditions such as heavy tree cover and mild, damp summers allow organic materials to build up on the forest floor. These materials eventually turn into humus, a brown or black material resulting from the decay of organic materials. It forms the organic portion of the soil.

Right: The top of a small fir tree covered in snow. Below: In a thick temperate forest, little light reaches through the branches. Photos by Maya Walters.

Broad-leaved forests occur throughout the temperate zone where the climate is moist. In colder areas the trees are deciduous and many animals hibernate or migrate south for the winter. During the summer these forests are the temporary home of many migratory birds which come for the variety of food sources available at this time of year, such as insects and berries. Insects are extremely abundant in these forests because of the huge supply of leaves. In more southern areas, the trees keep their leaves all year long and forests have a lush almost tropical appearance.
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Temperate forests are often interrupted by mountain ranges, and at high elevations, the climate becomes too severe for forests to grow. At the same latitude as a warm temperate broad-leaved forest, but several thousand feet higher, is an alpine tundra of small shrubs and wild flowers. The tree-line on mountain slopes is easy to spot -- above this elevation, no trees can grow. At these heights, trees are replaced by shrubs, or in some cases, stunted shrub-like forms of trees such as willows. Moving just a few hundred feet up a mountain slope reveals such a change in vegetation types that in some areas is comparable to moving over a hundred miles north.

A mixed broad-leaved and coniferous forest on a riverbank. Photo by Maya Walters.

To a far lesser extent than mountains, riverbanks and floodplains also alter habitat and growing conditions, and are home to different species than in drier areas. However, trees from surrounding forests often invade the floodplain forest, and often there appears to be little difference between the two. Since the water level of many larger rivers is now controlled, flooding does not occur so often, and this has changed many floodplain habitats.

Coniferous trees are not limited to the northern boreal forests, and there are many forests in warmer temperate regions that are mixed broadleaf and conifer. In still warmer temperate forests, the coniferous trees dominate. This is because they are adapted for dry habitats, not just cold ones. The needles of the coniferous trees do not let nearly as much water vapor escape as the leaves of broadleaf trees. Therefore, conifers require much less water to begin with and can survive in dry, sandy soils.
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The dry coniferous forests are quite unlike other temperate forest communities, in that they are so dry, sunny, and open, with an appearance more like a savanna than a forest. Moist coniferous forests are more common, and have quite a different character than the dry woodlands.

At high elevations where the climate is cool and moist, forests of spruce and fir are common. These forests are dark with closely spaced trees, and are often difficult to move through because of many fallen trees and branches. These high-elevation coniferous forests are home to many of the same animal species as the boreal forests.

Of course, some of the most famous coniferous forests are the redwoods of California. Redwoods are the tallest trees on earth, and the tallest individual measured was 367.8 feet tall. They require great amounts of moisture and the range of this forest is limited to within 35 miles from the ocean, where it is often covered by coastal fog. There is usually a well-developed understory of shrubs such as Western azalea and Pacific rhododendron, ferns, and wildflowers.

The remaining redwood forests of California have become a large tourist attraction, drawing visitors to parks where some of the tallest trees in the world can be seen. These forests were once much more extensive, but are confined to coastal areas. Photo by Maya Walters.

Douglas-fir is a common tree throughout much of western North America. This conifer thrives in the temperate rain forests, but also grows in slightly drier sites, occasionally in the same areas as Ponderosa pine, which is the most abundant tree species in dry coniferous woodlands. The Douglas-fir is not a true fir, and this impressive tree can reach heights of over 200 feet.

Some insects that damage forest plants are aphids, which live on stems and leaves and feed on the plant's sap. They are often found in association with ants, which feed on a special liquid that the aphids secrete. Here they can be seen clustered on a new tree shoot, with an ant at the lower right. Photo by Maya Walters.

The seeds of coniferous trees are one of the most important food sources. Rodents, such as red squirrels, depend heavily on these seeds. In the summer when food is abundant, squirrels eat a variety of things including bark, flowers, berries, insects, and occasionally meat. In the fall they can be seen collecting mushrooms, which they leave to dry on the branches of trees, and then store for the winter. But the main type of food collected by squirrels for the winter are seeds. They make caches of seed cones in holes or hollow trees, which they can return to throughout the winter. These caches are particularly important because red squirrels don't hibernate. Of course, they occasionally forget to return to a cache, and the seeds are left to sprout.
An animal which feeds heavily on tree bark is the porcupine. In the summer, porcupines prefer to eat fresh leaves and grasses. But these are unavailable in the winter, so porcupines instead gnaw large patches of bark away from the trunks and branches of conifers. These areas can be distinguished from patches of insect damage because marks are left by the porcupine's teeth.
Some insects do damage trees, but unless the forest has been unnaturally disturbed, there is usually a predator ready to catch the pests. Sometimes the predator can be a very unassuming animal. For example, shrews, which have a similar appearance to mice, voraciously consume the cocoons of sawflies. These insects resemble caterpillars when they first hatch, and feed on many species of trees. Insects are also an important food source to the many songbirds which migrate to temperate forests each summer.

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[broadleaf forests] [coniferous forests] [temperate rain forests]

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