General Description
Temperate deciduous forest is found in essentially all of the eastern United States, much of Europe, and throughout Asia and Japan, in those areas that receive somewhere between 30-75" of rain per year. These forests also require a fairly even distribution of rainfall throughout the year, even though some may fall as snow in the winter months. Temperatures are warm in the summer, and cold in the winter, and frost can occur throughout the range of this biome type. In the southern hemisphere, where there is less land mass at the equivalent latitudes, smaller extents of deciduous forest are found.
One difference between U.S. forests and those in China and Japan is in the understory. In the U.S., the understory is characterized by numerous herbs, and shrubs, particularly Rhododendron in the southern ranges. But in Japan and China, in the shrub layer is often missing, and bamboo species take their place. Sometimes, the bamboo is so thick that it inhibits tree reproduction.
Species diversity is lower in the higher latitudes, and in the U.S., reaches its peak in the Smokies, where there can be 125 species of trees. Diversity is lower in Europe, because the main mountain chains run east-west, which formed a barrier to migration during the glaciations, and many species were eliminated. England, for example, has only 12 native tree species!
Most of the biome is divided into associations, based on the dominant tree species, such as northern hardwoods, mixed mesophytic, southern hardwoods, etc. We won't go into much detail about this, but you can pursue this in either Whittaker's or Bolen's books.
In much of the range for this biome, glaciation has been a factor, particularly at the higher latitudes. This has altered the soils, which in turn influences which species can grow in these areas. It also provides evidence for Gleason's individualistic concept, since some of the species assemblages we see today are of very recent origin. For example, chestnut (now reduced to just sprouts due to blight) had only migrated northward into Connecticut within the past two thousand years, which is not a long time (for trees at least) to co-evolve with their neighbors. Thus it is hard to imagine that these were highly co-evolved communities. Instead, species associations have fluctuated throughout the years, as the climate has changed.
Pine forests can be found interspersed within the deciduous trees, including such species as white pine in the north, pitch and virginia and table mountain pines on dry ridges, and loblolly and shortleaf pines in the south. These are usually earlier successional stages in this biome, and if left without fire as a major disturbance, will revert to deciduous forest. Fire, though, is an important disturbance factor, and keeps many of these species in the landscape. Some have even evolved adaptations to fire, such as cones that open only after being heated by surface fires (pitch pine and table mountain pine; this condition is called serotiny).
The coastal plain of the southeastern U.S. is often dominated by extensive pine forests, which are commercially important. These forests are usually located on deep, sandy soils, which are nutrient poor. This favors evergreenness. Evergreenness is a condition wherein the tree maintains foliage year round. However, individual leaves may or may not be evergreen, that is, not all evergreen trees have leaves that last more than one season. They may simply produce new leaves all year, so that the tree is evergreen, but leaf lifespans are actually less than one year. See papers by Kikuzawa on this subject if you are interested.
If fire is kept out of coastal plain forests, they will eventually revert to oak-hickory forest. During the 1920s and 30s, fire was thought to be bad for all forests, and the U.S. Forest Service developed its Smoky the Bear campaign, one of the most successful ad campaigns in history (over 95% of all children knew who Smoky was, and what he advocated). But beginning after WWII, it was realized that in some cases fire was actually good for the trees. Take the case of longleaf pine. Longleaf pine begins its life by producing a tuft of needles (these needles can be up to 18" long!) and an extensive tap root. Aboveground shoot growth is delayed for up to 7 years, and the tree grows no taller than 1-2' during this time. This is called the grass stage. After this period, the tree puts on tremendous shoot growth, with little or no branching, and the tree extends upwards very rapidly. Why does this tree grow this way? These habitats are characterized by severe fires every 5-10 years, which destroys most ground-level vegetation. However, longleaf pine has, as an adult, thick bark which protects it against most ground fires. But it could be susceptible to fire as a seedling, if not for the fact that it forms the grass stage. The sensitive apical meristem is buried deep within the cluster of needles, and when a fire moves along, is kept from being damaged by these needles. The tap root continues to develop, and the tree stores extensive starch reserves which it uses for its rapid shoot growth. Rapid shoot growth is one way it grows up and beyond the reaches of most ground fires. Finally, there is a needle fungus that can kill this tree, but which is wiped out when there are fires. So, during the Smoky the Bear campaign, thousands of hectares of longleaf forest were lost to fire and disease, and the realization that this species needed fire to survive led to the abandonment of the Smoky the Bear campaign. Longleaf pine forests are characterized by widely spread trees, interspersed by mainly grasses and herbs, giving it a park-like appearance, or savannah-like look. This association is one of the more rare and endangered ones in the southeast. It also harbors endemic plants and animals, making it critical in the fight to preserve biodiversity.
Prior to the introduction of the chestnut blight, at the turn of the century, chestnut was the dominant tree in the southern forests, making up to ½ of all the basal area in these forests. Some of these stumps can still be found, and some of these trees reached truly huge sizes for an eastern tree. It's a shame most of us will never know the grandeur of the eastern forest when chestnuts were king.
In the north three species of hardwoods dominant - sugar maple, beech, and yellow birch. These are so-called climax tree species, and in older forests can make up over 90% of the tree species present.
Deciduousness, which is common occurrence in this biome, may have evolved as a mechanism for trees to avoid freezing damage in the winter months. Leaf fall is also a major factor influencing nutrient cycling in these forests. In New Jersey, litter fall produces 6480 kg/ha (5767 lb/acre!) of material, of which nearly 75% is leaves! In New Hampshire, where leaves contributed 45% of the litter input, that material contained 140 kg/ha (125 lb/acre) of nutrients. Branches also fall, but they decompose more slowly, over many years. Large trunks may take decades before they completely disappear. Even then, they may leave signs of their past presence. Some tree seeds prefer to germinate on the decaying logs, and if a lot of themsurvive to adulthood, they will appear as a straight line of trees in the forest, and straight lines are not common in nature. These logs are known as nurse logs. Woody debris is also important habitat for many small animals and insects, particularly salamandars.
In addition to leaf litter, oak forests also produce mast, a vast crop of acorns and other seeds, on an irregular interval. This may be a way to avoid seed predation. If in one year the forest is saturated with huge inputs of seed, some may escape predation and live to germinate. If the trees then produce few seed the next year, many of the seed predators will die, and the year following this, even a small to moderate seed crop may have a much higher probability of surviving, since the predators can not increase their population sizes instantly. The irregularity of seed masting may prevent predators from adapting their life cycle to fit that of seed production, thereby insuring that some seeds will always escape predation. Smart trees!
In some areas, the elimination of top predators is having devastating effects on tree reproduction. Deer are very abundant in the east these days, having increased since the extirpation of the wolf and other predators. These deer populations are exceeding their carrying capacities, eating all available vegetation, which is anything green within about 8' of the ground. In some areas in Pennsylvania, there are no leaves or tree seedlings within 8' of the ground, and the continued existence of these forests is threatened by the lack of reproduction.
Decomposition is very seasonal. Leaves decay rapidly during the growing season, but little during the winter months. This tends to lead to a build up of leaf litter on the forest floor, and the development of well defined layers in the soil profile. The litter layer is composed of fresh material, still identifiable to species, a decayed layer, and a layer that is recognizable as litter, but so decayed that species can no longer be identified. From there, there is a well developed set of soil horizons, A, B, and C. Most roots are located in the upper 1-2' of soil, and most nutrient cycling processes are accomplished in that zone. However, new evidence suggests that deep roots might be more important than previously thought, and a lot of research is being done to determine their impacts. In Texas, and other areas, some tree roots may reach down 25 meters!
Other habitats of interest, but which we will not discuss in this class, include the New Jersey Pine Barrens, Carolina Bays, Balds, and Swamps.
Many bird species migrate from temperate deciduous forests to tropical locations in the winter. These are called neo-tropical migrants. Many are endangered due to logging in temperate forests, and destruction of their tropical overwintering sites. Fragmentation of the forests in the temperate zone is also having its effects, since this favors those types of birds common along forest-field edges, such as the brown cow bird, a nest parasitizer.
Old-growth forest, forest that is considered in the latest stages of succession, are very rare in these areas. Most temperate forests worldwide have been logged some time in the past 100-200 years. Only hard to reach sites escaped the loggers, or sites held by private families that did not permit logging. Thus, it is hard to determine just how these forests appeared prior to alteration by humans. Indians probably burned large areas to promote grass and attract deer, which they hunted. Charcoal can be found in many forests in the soil layers, indicating fire in the past.
Temperate forest comprise 12 x 106 km2 of land worldwide, with a mean NPP of 1250 g m-2 yr-1 (range 600-2500). Worldwide NPP for all temperate forests is 14.9 Gt yr-1, which is nearly 9% of all NPP.