European colonists found most of the eastern seaboard of North America covered with deciduous trees-trees that shed their broad leaves in the fall. Similar forests once blanketed most of Europe, the temperate coasts of Asia, and smaller areas of Australia and South America.
The Plant Community
Mature deciduous forests often contain dozens of tree species, but in any given area one or two kinds of oak, hickory, chestnut, maple, basswood, buckeye, or beech are common enough to give the forest their name.
Big trees dominate the community by providing most of the food supply and by modifying the physical environment through shade and windbreak action. The largest trees are widely spaced, but the forest includes individuals of all ages. Such a forest is a stable community that can maintain itself through continued reproduction.
Under the trees of a deciduous forest, shrubs and herbaceous (nonwoody) plants are scattered about, but few are abundant except in clearings.
The density of the leafy canopy overhead and the time each year that the canopy is present regulate growth of understory (below the canopy) plants.
Many of these are small herbs or “spring flowers” that grow rapidly as days lengthen. They reproduce and carry out much of their photosynthesis before the trees leaves open and reduce the sunlight available on the forest floor.
Soils of some deciduous temperate forests are brown and rich- looking, but most of the available minerals cycle through the plants each year. Only a small portion of the minerals absorbed are retained in the wood of the tree.
The remainder returns to the ground as twig and leaf litter. Not all the returned materials can be reused the following year, because decomposition takes a long time. Of the minerals released from the litter, some invariably leach away in rainwater. Humus from decomposing leaves and twigs helps hold both water and minerals.
Temperate deciduous forests are highly productive, and they support a large number of organisms, as suggested by Table. Such forests are also quite diverse.
An often overlooked part of the community lives in the litter on the forest floor. Much of the forest enters this decomposer food chain yearly. Litter decomposition involves the activity of multitudes of tiny animals and microorganisms.
Insects such as springtails and other near-microscopic arthropods eat dead leaves. These primary consumers absorb less than 10 percent of the nutrients in the litter. However, as a result of their digestive process, the remans that become feces are easily attacked by bacteria and fungi.
Animals not only consume and alter litter; they also mix it into the soil. Earthworms are among the most active litter mixers. The carnivores that eat the litter feeders also stir litter and mix it into the soil as they endlessly seek their prey.
Adaptation of Climate
Temperate deciduous forests have moderate temperatures and rainfall (about 70-100 cm or 28-40 in /yr) but are characteristically subject to forest. Shedding leaves in the fall protects the trees against cold damage. Otherwise the large leaf surface that favours transpiration and photosynthesis during the warm season would allow heavy evaporation and severe water loss during the winter.
You will recall that the absorption of water by roots relies on diffusion, as well as on energy-consuming activities of root ceils. Both are slowed by cold, and of course, frozen soil yields no water. Cold weather can dehydrate plants more than hot weather, especially when it is combined with strong winds. Furthermore, leaves are difficult to protect against freezing. Deciduous trees have evolved an effective strategy to prevent damage from winter cold.
First they concentrate sugars and other organic compounds from their leaves into roots and stems. Here these large molecules lower the freezing point of cell fluids and prevent formation of ice crystals that could rapture cells. Then the trees simply shed their leaves.
Leaf fall is a preprogrammed process coded into the genetic structure of the plant and only slightly affected by the weather any particular year.
In response to a decrease in auxins, an abscission (cutting off) layer forms where the leaf stalk joins the stem, and the cement between the cells in that area softens. In the absence of strong intercellular cements, the short cells of the abscission layer separate, permitting the leaf to drop off or blow away in a gentle breeze.
The colour changes that foretell leaf drop in deciduous forests result from cessation of chlorophyll formation. Because chlorophyll breaks down spontaneously in light, leaves remain green only through continued synthesis of chlorophyll. With the coming of fall, chlorophyll manufacture ceases, and this green pigment gradually bleaches in sunlight. In the absence of chlorophyll we see yellow or reddish pigments, mainly carotenoids that aid in light absorption for photosynthesis. Another source of red colour results from interruption of phloem by the developing abscission layer. This causes sugars to accumulate in the leaves, where some are converted into reddish compounds.