Shades of Green: Earth's Forests -- Broadleaf Forest

The leaves of broad-leaved trees are a far more desirable food source for insects than the tough waxy needles of coniferous forests. Many kinds of insects feed exclusively on leaves. But they are not a constant food source, because in most temperate broad-leaved forests, trees are deciduous, meaning they shed their leaves every winter.

The leaves of a Douglas maple. Photo courtesy Al Walters.

A thick, nutrient-rich "litter layer" of the fallen leaves collects on the ground and is gradually decomposed, enriching the soil. Only in the warmest, moist areas do leaves remain on the trees all year. Most other food sources in the broad-leaved forests are also seasonal: flowers occur in the spring and early summer, providing food for pollinating insects, fruit ripens near the end of summer, seeds and nuts become available in the fall and are an especially important food source since they remain throughout the winter. Animals in these forests have to be able to adapt to different diets in different seasons.

Vegetation consists mainly of deciduous trees. A decrease in latitude leads to an increase in the number of broadleaf trees that are evergreen and keep their leaves all winter.

Left: The upper tree trunks in a temperate deciduous forest. Photo courtesy Naomi Woods. Below and Bottom: Mosses and lichens make up the first level of vegetation; the flowering dogwood trees are low and bushy, and often grow under the canopy. Photos by Maya Walters.

The forest vegetation can be divided in up to five layers. On the ground there are lichens and various types of mosses, which are prevalent on the trees also. Next is an herb layer, made up of perennial forbs, which are broad-leaved plants. A third layer consists of shrubs. Many shrubs, such as mountain laurel and huckleberries, are members of the heath family.

Small trees and saplings constitute another slightly taller layer, which also includes species like sourwood and dogwood. The last layer is the canopy, a combination of the species characteristic of the broadleaf forest, and forms a tree stratum sixty to one-hundred feet tall.

Tree species common in the broadleaf deciduous forest include: oak (Quercus spp.), maple (Acer spp.), beech (Fagus spp.), chestnut (Castanea spp.), hickory (Carya spp.), elm (Ulmus spp.), and basswood or linden (Tilia spp.). Different species of each tree exist on separate continents.

Most of the animals in the broadleaf deciduous forests are mast eaters, which means they eat nuts such as acorns, or are omnivores. Many have arboreal lives, and a great percentage hibernate during the winter.

Most temperate broadleaf forests are dominated by only a few species of tree and are therefore called simply "beech-maple forest" or "oak-hickory forest", depending on their most abundant types of trees.

There are, however, exceptions -- in the eastern United States, there is in fact a forest where over 80 species of tree can be found. They include oaks, beeches, magnolias, basswoods, maples, hickories, and other trees, many of them very rare. But this unusual forest is dying rapidly, and no one really knows why.

Red oaks (Quercus rubra) are a common species in North America's eastern forests, ranging from Nova Scotia to Georgia. Photo by Maya Walters.

More and more trees in this diverse forest are rotting away and falling every year. The American chestnut was once widespread in this forest, growing quite tall and forming much of the canopy, but an imported fungus from China began attacking these trees. First noticed in 1906, this fungus killed all of the large trees, since the American chestnut is completely unresistant. However, the blight didn't destroy the roots, and new shoots continue to grow, although they are usually killed by the time they reach 20 feet in height. The once dominant American chestnut has thus been completely eliminated from the forest canopy.

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