Deciduous Forests

Deciduous Forests

The temperate deciduous broadleaf forest (TDBF) is composed of broad-leaf angiosperm trees like the oaks, maples, and beeches familiar to many Americans and Europeans. The forests exist best in moderate climates that are neither too hot nor too cold and neither too wet nor too dry. In addition to the temperate zone, deciduous forests are found in tropical and sub-tropical climates in open savannas and/or in closed forests. While there are roughly thirty families and sixty-five genera in the TDBF, variation in the precise definition and defined area of the forest make absolute numbers impossible. With thousands of species, the TDBF is a highly diverse biome.

North American Temperate Deciduous Broadleaf Forest

Worldwide, there are five major groups of TDBFs. Within each group, botanists define TDBFs by the species that tend to occur in a given area. These collections of species, together with their environment, are called associations. Eastern North America today contains the most extensive TDBF. The forest reaches from about longitude 95°W (just west of the Mississippi River) to the Atlantic coast and from 30 to 45°N, thereby forming a quadrant that includes most of the northeastern quarter of the United States. The eastern United States TDBF was almost completely deforested for agricultural purposes by 1850. At that time, land was opened for agriculture in the Mississippi valley, and many farms were abandoned. Pines grew well in the remaining grassy fields, but after a catastrophic hurricane in 1937, the TDBF grew back. Today, there is much more TDBF in the United States than there was one hundred years ago (though still less than before the arrival of European settlers).

There are nine generally recognized associations in the United States TDBF, each defined by differences in vegetation (see accompanying table). Though the species are representative of common dominants, many other species exist. TDBF associations are not completely separate. Many species, such as northern red oak and sugar maple, exist in more than one association. Nor are the boundaries between the associations sharp and easily identifiable. In particular, the Western Mesophytic association can be difficult to distinguish from its neighbors to the east (Mixed Mesophytic) and west (Oak-Hickory). Associations can change with time too. The Oak-Chestnut association is now almost completely devoid of chestnut, but many people still use the association name even though it is now mostly oak and maple. Association names in North America and elsewhere are most useful for distinguishing broad differences in forest type often associated with variation in soils, topography, and climate.

European, Asian, and South American Temperate Deciduous Broadleaf Forest

The European TDBF, stretching through most of Europe (except for very hot and cold areas) from the Atlantic Ocean to the Ural Mountains, is the second-largest TDBF. Due to the moderating influences of the Gulf Stream, the TDBF exists as far as 60°N in northwestern Europe. Forests in Europe have been extensively modified by humans for more than two thousand years and are some of the most manipulated forests in the world. In the northern European TDBF, birch species are common, while in the middle European latitudes, beech (Fagus sylvatica ) is widely distributed and commercially valuable. Towards the south, various oak and maple species abound. As in North America, much of the once-cleared TDBF is now regrowing. The European TDBF is replaced by drought-resistant shrubs and evergreen broadleaf trees in the south and the boreal coniferous forest in the north.

The last three TDBF areas are much smaller than the first two. East Asia, from 30° to 60°N and from central Japan to longitude 125°E in the northwest and longitude 115°E in the southwest, originally maintained very large forests. Today, even though the species mix is still very diverse, much of the East Asian forest outside of Japan is currently under cultivation and most existing forest fragments are protected refuges or in areas unsuitable for agriculture. Nearly all TDBF genera are present in East Asia, especially China. The Near East between 35° and 45°N, including areas around the eastern Black Sea and mountainous regions in Iran and near the Caspian Sea, supports a diverse TDBF. Finally, a narrow strip of South America including southern Chile and Argentina contain TDBF. Acacia caven and seven Nothofagus species are also found there. In nearly all cases, the deciduous trees of South America occur in mixtures with evergreen broadleaf species.

Of the three major TDBFs, East Asia has by far the greatest diversity, followed by North America and Europe. East Asia was glaciated less severely than America and Europe, so most species were able to survive with little difficulty. In North America, the north-to-south orientation of major mountain ranges allowed species to migrate, and species diversity here is only slightly lower than in East Asia. In Europe, on the other hand, the east-to-west mountains caused the TDBF to be trapped by advancing glaciers. Many modern European TDBF species survived only in the Near East TDBF and migrated back after the glaciers retreated. Consequently, Europe has very low-species diversity.

Climate and Soils

TDBFs are generally restricted to a warm temperate climate with four identifiable seasons in which the average temperature of the coldest month is between 3 and 18°C and the average temperature of the warmest month exceeds 10°C. The length of the frost-free period ranges from 120 to over 250 days. Precipitation is year-round and averages between 80 and 200 centimeters per year. Snowfall can range from nonexistent in the southeastern United States to extremely heavy in northern habitats. Climates that are wet and warm all year are occupied by tropical forest consisting of broadleaf evergreen trees. As climates become drier, as occurs at the western edge of the Oak-Hickory association, drought stresses are too extreme for TDBF and grasses become dominant. To the north of the major TDBF, extreme cold, short growing seasons and poor soils favor evergreen coniferous forests. TDBF soils tend to be deep and fertile and, unlike some soils in the northern coniferous forest, do not freeze year-round. For this reason, TDBFs have historically been popular for agricultural use.

Leaves and Phenology

Deciduous leaves are the most distinctive feature of the TDBF. In the fall, spectacular reds, oranges, and yellows produce breathtaking displays across the TDBF. Why does this occur? During autumn, as temperatures cool and days shorten, trees send hormonal signals to their leaves causing them to turn colors and fall off the branch. First, leaves form a barrier between the leaf and the branch, known as the abscission layer. At the same time, chlorophyll, the compound that gathers light for photosynthesis, begins to degrade in the leaf. Many of the nutrients in the leaf are sucked back into the tree for next year's leaves. Chlorophyll is responsible for the usual green leaf color: once it is gone, yellow and orange pigments that were there all along become visible. Some of the sugar in the leaves of oaks and maples may be converted into red colors. Once the leaf is totally shut down and no longer conducting any photosynthesis, the abscission layer becomes very brittle. Any small breeze can snap the leaf off at this point. In the spring, using carbon from special storage cells in the trunk, trees grow a new batch of leaves. In an evolutionary adaptation designed to maximize the amount of light received, shrubs and small trees growing in the understory will begin growth before the overstory.

The study of any recurring biological cycle and its connection to climate is called phenology. Patterns of bird migration and insect outbreaks are examples of phenological cycles. For centuries, scientists have been studying phenology in the TDBF. In the deciduous forest, phenology refers to the timing of spring leaf growth and fall leaf drop and their relationship to climatic variation. Observational evidence has shown that TDBF phenology is highly sensitive to variation in weather. Warm springs will cause leaves to grow earlier, sometimes by up to as much as one month. Conversely, plants respond to a cold fall by dropping their leaves earlier. Phenological cycles in the forest are one possible indication that plants are responding to global warming. If temperatures are warming, the growing season should be getting longer. Most evidence between 1950 and 2000 suggests that the duration of the growing season has lengthened by several days in many forests. TDBF leaves are not only beautiful, they can also provide very useful scientific observations.

Plant-Animal Interactions

The TDBF supports rich plant-animal interactions in all three of the classic ecological relationships between two species or organisms: mutualism, commensalism, and antagonism.

Mutualism.

In a mutualistic relationship, both participants receive a benefit. Mutualisms are quite a bit like bargains or trades. In flowering plants, most pollination does not take place with wind. Pollen grains must be physically transported from the male stamen to the female stigma where the process of fertilization begins. In the past, angiosperms relied on the chance event that an insect would happen to brush against the stamen on one flower and then on the stigma of another flower. This was very inefficient and plants eventually evolved in such a way as to greatly increase the chances of successful pollination. While a few TDBF trees are wind-pollinated, most use nectar, a sweet sugary substance, to lure pollinators. While they eat the nectar, pollinators brush the stamen and collect pollen. When they visit a female flower, the pollinator brushes the stigma, transporting the pollen and beginning fertilization. Brightly colored flowers and vivid aromas also attract pollinators, as do ultraviolet markings on some flowers. In the TDBF, insects (especially bees) are the most important pollinators. Other pollinators in the TDBF include moths, butterflies, wasps, flies, and birds.

Commensalism.

In a commensal relationship, one participant gains a benefit without harming the other. As in many forests, small mammals such as raccoons, squirrels, and mice as well as such birds as owls use trees for habitat in a commensal relationship. Some insects have evolved to look almost exactly like twigs or leaves. This makes it difficult for predators to locate them in the trees.

Antagonism.

Organisms in an antagonistic relationship benefit at the expense of the other organism. Antagonistic relationships are common. Herbivory by insects, in particular the gypsy moth, can cause extensive damage to the TDBF. White-tailed deer and other ungulates eat leaves and can be the most destructive animals in the forest. In a response to browsing pressures, some trees have evolved leaves with distasteful toxins . The black-tailed deer, though, has developed special chemicals in its saliva to neutralize these toxins. In the future, it is likely that trees will evolve new defensive toxins. This process of back- and-forth evolution between herbivores and plants is an example of a process called coevolution.

Threats to the Temperate Deciduous Broadleaf Forest

Farming has historically represented the greatest threat to the TDBF. Today's TDBF has extensively regrown in the eastern United States, but in Europe and East Asia, the other two major areas, the forest is still highly fragmented. Species diversity fortunately tends to remain high even in highly scattered groups. Future regrowth can occur rapidly from these isolated or protected areas.

Fungal diseases are currently much more serious threats. Plants all over the world have fungal pests to which they are usually well adapted. Serious problems arise when these diseases are transported to forests that have no defenses. The chestnut blight, introduced to America from Europe in 1904, is the best-known example. In four decades, the chestnut blight eliminated a popular and valuable tree from an entire continent. Dutch elm disease reached the United States by 1930 and to date has killed millions of elms. Humans are almost always the cause of these introduced diseases. Fungi can easily store themselves on ships, cars, or trains. As global commerce increases, it is likely that humans will continue to accidentally introduce diseases into the TDBF.

see also Biome; Chestnut Blight; Coevolution; Coniferous Forests; Deciduous Plants; Dutch Elm Disease; Forester; Forestry; Trees.

Michael A. White

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