Temperate forests

Autumnal leaf fall occurs in response to changes in day length. A cork layer grows within the base of the leaf stem, cutting it off from the sap, and the leaf eventually drops off the branch.

Temperate forests cover large areas of the earth, occupying regions that have more than 35 inches (90 centimeters) of precipitation. Many regions experience snow and frost for four or five months in winter. The growing season is more than four months with summer temperatures averaging 65° to 80° F. (18° to 27° C) over large areas. Deciduous forests dominate the eastern United States, southeastern Canada, central Europe, and eastern China. Many other temperate forests of Australia, New Zealand, South America, and Asia have evergreen broad-leaved species.

Temperate forests can be divided into those that contain deciduous trees, and those that comprise evergreens, although the two types of tree are often found in the same forest. The regions where these forests occur can also be separated into cold, cool, and warm temperate climates, which determine the types of trees found there. Very few of the forests remain in an unmodified state—most have been felled and replanted, or selectively harvested for hundreds, if not thousands, of years.

In a temperate forest the dominant trees occupy the top level with smaller trees forming the shade, or second layer. Shrubs form a story of their own, below which, on the ground level, is an herbaceous layer.

Temperate deciduous forests

These forests usually occur in cold temperate regions, between latitudes 25° N. and 55° N„ on the western fringe of Europe reaching eastward, in eastern Asia, and in the eastern United States and Canada. The Southern Hemi-sphere has few areas that are climatically suitable for these trees.

One of the most striking characteristics of these forests is the loss of leaves in winter and the replenishment by a new set grown in spring. During the long summer days, photosynthesis can be sustained for 16 or more hours because light levels and the availability of water are high. Deciduous trees and shrubs loose their leaves in the autumn in response to a shorter photoperiod (longer nights). The loss of leaves presents their freezing at more northern latitudes, where the roots and branches may remain frozen for several months. With cold or frozen soils, water can not be absorbed and transported to the branches. Farther south, those trees and shrubs that retain all or part of their leaf surface (live oaks) can carry on some photosynthesis in winter.

As day length becomes shorter in autumn, chlorophyll in the leaves breaks down, revealing the secondary pigments, such as carotenes, anthocyanin, and xanthophyll. These pigments enable the leaves to turn yellow, orange, and purple. The cool nights and sunny, warm days of New England, the upper Great Lakes states, and the higher mountains provide the most colorful fall foliage. Losses of important nutrients with leaf fall are minimized by the withdrawal of nutrients into the branches and roots prior to leaf fall. A thin layer of cork forms within the base of the leaf stalk before leaf fall. The fallen leaves decompose to form humus, the nutrients of which are absorbed by the tree roots in the next growing season and recycled.

The wood of deciduous trees is also adapted to support the leaves in their vigorous seasonal growth—it contains vessels and tra-cheids that allow efficient water conduction that supports the high transpiration rate of the leaves.

A few genera of trees occur throughout these cold temperature forests, but their species vary on the different continents. They include oaks (Quercus spp.), beeches (Fagus spp.), ashes (Fraxinus spp.), birches (Betula spp.), and elms (U/mus spp.). Most of the present temperate deciduous forests in Europe are dominated by beech or oak. Other trees generally found growing in association with them include maple and sycamore (Acer spp.), ash (.Fraxinus spp.), and walnut (Juglans spp.).

Some conifers also occur in the northern deciduous forests; in the northern forests of the North American continent, for example, white pine (Pinus strobus) and hemlock (Tsuga canadensis) may be found among deciduous trees.

The cycle of leafing and leaflessness has a strong influence on the association species of deciduous woodlands. In the spring there is a period, before the canopy trees produce their leaves, when the sun can illuminate and warm the soil. At this time many of the perennial woodland herbs conduct most of their yearly photosynthesis, grow, and flower. In western Europe, species such as wood anemone (Anemone nemorosa), bluebell (Endymion non-scriptus), and oxlip (Primula eiatior) flower at this time and make most of their vegetative growth. A few then produce thin, large “shade leaves,” but most die down to their underground storage organs after midsummer. Many shrubs in these woodlands also flower early before the canopy trees are in leaf, such as hazel (Corylus avellana).

Temperate evergreen forests

These forests are found in cool and warm temperate regions and contain both broad-leaved and coniferous species. At the lower latitudes, in the warm temperate regions, deciduous forests are replaced by evergreen, mostly broadleaved forest. The cool temperate forests, which are coastal, are usually comprised of coniferous trees.

The evergreen broad-leaved trees, such as oaks (Quercus spp.) in the United States, do not need to shed their leaves because the temperatures are warm enough to keep leaves from freezing and keep soil temperatures above 39° F. (4° C); the roots can therefore constantly absorb water and sap can be supplied continually to the leaves. The leaves of these trees also have waxy surfaces and small stomata, which limit transpiration and avoid excessive water loss. Another example is the genus Eucalyptus in southeastern Australia. These trees also hold their leaves vertically so that they are parallel to the sun’s rays and, therefore, reduce the effects of its heat.

Other warm temperate regions are also inhabited by various coniferous species, such as pines (Pinus spp.), as in southeastern North America, southern China, and parts of Japan. Because the humidity is so high in these areas, there is a rich understory growth of flowering herbs, mosses, ferns, and lichens.

Some cool temperate forests grow in coastal mountain slopes where sea winds bring a high annual rainfall. They also contain pines and, in North America, giant redwoods (Sequoia sempervirens). Like most conifers their leaves are needlelike and waxy, which reduces transpiration, particularly in the cool winter. These trees grow densely and, like the evergreen broad-leaved forests, do notallow much light through, which results in little understory growth.

Bluebells (Endymion non-scriptus) carpet a birch forest for the few weeks that the warm spring sunshine can penetrate the foliage to the ground level. During this time these plants produce most of their vegetative growth. They die down to their bulbs once the leaves of the trees block out the sun, in about early summer.