Temperate deciduous forests

Temperate deciduous forests exist in the mid-latitude regions around the globe, including North America, Europe, Asia, and southern South America. Four seasons characterized by cold, relatively dry winters and warm, wetter summers are found in these forests. The cycle of seasons influences the shedding of leaves by deciduous tree species. These forests provide important resources to civilizations around the world, but many modern activities and urbanization threaten to decrease the extent of these forests. Conservation efforts focusing on increasing management and awareness of temperate deciduous forests are working around the world to preserve these ecosystems in the face of global changes in climate and land use.

Temperate deciduous forest regions are characterized by relatively warm summers and cold winters with growing seasons ranging from 140 to 200 days with four to six frost-free months. Annual temperatures in temperate deciduous forests range from minus 22 degrees F to 86 degrees F (minus 30 degrees C to 30 degrees C) with an average of 50 degrees F (10 degrees C), and precipitation ranges from 29 to 59 inches (750 to 1,500 millimeters) per year and mostly occurs during warm months. Temperate forest species typically thrive on neutral to slightly basic soils, and soils are typically very fertile due to the continuous decomposition of newly deposited litter such as falling leaves and woody debris. The four distinct seasons that occur in all temperate deciduous forests trigger the characteristic shedding of leaves by deciduous, broadleaved species and subsequent regrowth of leaves during spring. The cyclical adaptation of shedding leaves during the cold, drier periods allows plants to survive harsh winters in a rather dormant stage and reanimate during more favorable spring conditions.

Distinguishing Characteristics

The four distinct seasons and the changes that broadleaved trees experience during the seasonal cycle are the most defining characteristics for the temperate deciduous forests. The changes experienced physiologically by trees are unique across the seasons and represent a significant adaptation to the harsher conditions experienced by trees. For comparison, the trees in warm, wet climates such as the tropics never experience a dormant period in which leaves are shed. The shedding of leaves by trees in the autumn decreases water loss due to transpiration, a side effect of photosynthesis. When leaves are shed, water loss is decreased and photosynthesis is in turn decreased. Since new carbohydrates are no longer produced by photosynthesis, trees enter a stage of torpor, or dormancy, in which most physiological functioning ceases. Growth of woody material stops as well.

Leaves experience a change of color in autumn triggered by a change in the allocation of sugars and chemical components and a withdrawal of chlorophyll from the leaves. Trees are able to draw nutrients from the leaves back into woody tissues for storage over winter. For example, nitrogen, an often limiting nutrient in many forests, can be translocated, or transferred, from the leaf back into the tree prior to leaf shedding. Once leaves are shed, trees enter dormancy for the winter, and transpiration and photosynthesis cease for the most part. Thick exterior bark allows trees to withstand cold winter temperatures. Leaves and flowers reemerge in the spring when growing conditions become more favorable for growth, and photosynthesis resumes. In the spring, large conducting elements comprising the tree's vascular system (i.e., phloem and xylem) reestablish as well, allowing for transport of water from the roots to the treetops and transfer of sugars and carbohydrates generated by photosynthesis from the leaves to the rest of the tree for use in growth and storage. These conducting elements comprise the annual growth rings of trees, and thus a new, annual ring is added each year to deciduous species as trees exit dormancy and resume growth and photosynthesis each spring.

The most common deciduous tree species that exist in temperate forests belong to the oak (Quercus), ash (Fraxinus), maple (Acer), beech (Fagus), hickory (Carya), and chestnut (Castanea) families. Biodiversity is high; along with canopy-dominant tree species, understory shrubs, herbs, and flowering plants are also important for these ecosystems. Closed-canopy forests typically contain shade-tolerant tree species reestablishing in the forest understory. When forest stands reach maturity, canopies are referred to as “closed,” signifying a relative closure in leaf space and a decrease, but not total elimination, of the sunlight penetrating the canopy. There are also several different kinds of plants that exist in the shady conditions of the forest floor where only small amounts of sunlight penetrate, such as shrubs, flowers, and mosses.

Global Distribution and Structure

In terms of global distribution, temperate deciduous forests are found in the eastern half of North America in latitudes between 35 and 48 degrees north and in central Europe in latitudes between 45 and 60 degrees north. Deciduous forests reach farther north in Europe than in other regions of the Northern Hemisphere as a result of the Atlantic conveyor belt carrying warmer waters from the mid-Atlantic and Caribbean up around Great Britain, resulting in warmer, temperate climates in higher, northern latitudes. The deciduous forests of Asia are located in southwest Russia, Japan, Korea, and eastern China. In South America, southern Chile and the midcoastal regions of Paraguay contain deciduous forests. Also in the Southern Hemisphere, New Zealand and southeastern Australia have extensive deciduous forests. While all these regions are characterized by four distinct seasons over the year, there are unique characteristics of each region attributed to changes in soil texture, nutrients, topography, and common species of flora and fauna. For example, the deciduous forests of the eastern United States contain interspersed coniferous trees, especially in northern regions and areas of high elevation such as the Appalachian Mountains, with species indigenous only to North America.

Temperate deciduous forests cover large areas in the Northern and, to a lesser degree, Southern Hemispheres. Further categorization of these forest communities can be done by assessing the repeating units of dominant tree species in a stand. For example, some of the most common forest communities in the United States can be described by eight categories: mixed mesophytic, Appalachian oak, hemlock-white pine-northern hardwoods, oak-hickory, maple-basswood, beech-maple, oak-pine, and southern pine. Mesophytic signifies an environment receiving a moderate amount of moisture.

The forest structure in temperate deciduous regions is separated into five zones based on vegetation height. The tree stratum zone comprises the first zone and ranges in height between 60 and 100 feet (10 to 30 meters). The tree stratum zone contains trees such as oak, beech, maple, chestnut hickory, elm, basswood, linden, walnut, and sweet gum trees. Smaller tree and juvenile saplings are in the second zone. The shrub zone, or third zone, consists of short shrubs and flowering plants such as rhododendrons, primrose, bluebells, painted trillium, azaleas, mountain laurel, and huckleberries. The herb zone is the fourth zone, and it contains plants close to the ground such as herbal plants like ferns and perennial forbs that blossom in spring. The ground zone contains low-to-the-ground growth forms such as lichens, true mosses, and club mosses. The canopy in most deciduous temperate forests contains openings and gaps created by fallen trees and low tree densities that allow some light to penetrate to the forest floor, thus allowing for a great diversity in flora, which in turn supports a great diversity in fauna.

Animals found in temperate deciduous forests must adapt to the seasonal cycles, so many hibernate or migrate when freezing temperatures arrive. Some animals are adapted to the harsher winter months and can withstand the high risk of predation and cold temperatures during the winter. For example, black bears are adapted to temperate conditions. They have sharp claws for climbing trees, and as omnivores, they eat other vegetation and prey on other forest creatures. While their thick fur protects them from the cold temperatures of winter, they do hibernate during the time of hardest freeze to avoid food shortage. Other common forest animals are deer, squirrels, opossums, mice, and raccoons. Amphibians such as salamanders and frogs, reptiles such as snakes and turtles, and a multitude of insect species including beetles and mosquitoes also contribute to the diversity of fauna found in deciduous temperate forests. A huge variety of bird species including robins, eagles, cardinals, chickadees, hawks, various owls and woodpeckers, hummingbirds, and wild turkeys also are found in these forests.

Timber and Resource Management

Temperate deciduous forests have acted as important sources of timber historically and continue to provide important resources presently; thus, these areas have often been settled and used for resources. Less than one-third of primary forests, including temperate deciduous forests, still existed globally in 2015, due not only to timber harvesting but also due to land clearing for agriculture. For example, many European colonists in the northeastern United States cleared the deciduous forests for agriculture. Upon finding more arable, or easier to plow, soils toward the western United States, colonists allowed many former agricultural fields in the northeast to return to forested states. Prior to European arrival, forests in the northeastern United States contained higher species diversity and greater natural variation in forest stands, which were influenced by natural and human disturbances such as hurricanes and clearing of small areas for native villages. This led to trees of varying age, density, size, and species across a wide range of forested sites.

With the arrival of Europeans, forests diversity decreased and tree ages and densities became more homogenous, or decreased in variety, due in part to the influences of larger-scale hunting, trapping, and clear-cutting by Europeans beginning around 1820. Through the passage of about 30 years, European agricultural efforts in New England notably declined and many of the fields returned to forested states. Early forest regrowth was primarily represented by white pine, but conifers have since been replaced by the diversity of broadleaf species that currently comprise eastern forests. Forest regrowth in the northeast United States has continued for the most part, and the deciduous forests of New England represent secondary succession forest growth, or rather the forest has regrown after the original forests were cleared for timber or agriculture. In another example, forests in China have been harvested for over 4,000 years, thus most of the forests existing in the country are human-made.

Secondary succession and current forest management of temperate deciduous forests create some ecosystem problems; because most trees are of the same age, or even-aged, stands are denser than historic stands, and species are often selected for their commercial value instead of to maintain historic biodiversity with a wide variety of species. To contrast current temperate deciduous forests, it is important to recognize some of the valuable characteristics of old-growth deciduous temperate forests. Old-growth forests are more complex forest structures characterized by trees representing a variety of ages, large fallen trees and woody debris on the forest floor, and multiple layers in the canopy representing differences in tree size. Forest regrowth typically decreases the complexity of forest communities by decreasing the variety of age, layers, species, and habitats available.

The ecosystem services provided by temperate deciduous forests, or the commercial or aesthetic value that is placed on nature, are not limited to timber products such as wood or paper. Forests provide oxygen as a by-product of photosynthesis, decrease atmospheric carbon dioxide through sequestration, provide food including mushrooms and game, create recreational spaces and natural beauty, and purify the water that many communities around the globe rely upon. The change in the autumn colors of foliage in deciduous trees leads to great beauty. This can be recognized among the wide array of tours and recreational opportunities in the fall in New England. Tourists flock to these regions to view the autumn colors among the maples, and Acadia National Park in Maine is flooded with guests enjoying the reds, yellows, and oranges that flame through the leaves during the fall. Not only does this represent a great aesthetic value, but also an important economic aspect for the region.

Forests are often managed to maintain high densities of valuable tree species such as red oak, and stands are often even-aged because most trees within a stand would have been established at the same time following forest clear-cutting. High densities of trees make stands more vulnerable to the negative impacts of drought as competition between neighboring trees creates more demand for limited water resources. This makes trees more vulnerable to drought-related mortality or secondary stresses such as insect or disease pathogen attacks. An example of this can be observed in the oak-hickory stands of northwest Arkansas. An oak decline event in the early 20th century that resulted in high mortality of red oak species was linked to increased vulnerability of trees to insect infestation following a cycle of droughts. The mortality event was also related to the unnaturally high densities of red oak species that existed in the area. As a commercially valuable species, the even-aged stands were managed for high timber production, and these high densities increased moisture stress during drought. The increased stress may have made trees more vulnerable to insect attacks as more resources had to be allocated to accessing water (i.e., roots) than to defensive chemicals. Decreased diversity of species also poses a threat to forest ecosystems.

For example, eastern U.S. forests were dominated by chestnut trees in the early 1900s until the chestnut blight, an introduced fungal pathogen, mortally impacted this key species and left the canopy in many forest stands open to other species such as maples and sycamores. A pathogen decimating an entire forest species is rare, but as introduced pathogens become more common, it is vital for continued ecosystem function to maintain a wide variety of codominant tree species within forest canopies. The management practices that create even-aged, high-density forest stands with relatively low diversity found in many temperate deciduous forests make these regions more vulnerable to the impacts of disturbances such as hurricanes, windfall events, and disease pathogens.

Environmental Challenges and Forest Conservation

Temperate deciduous forest habitats are also threatened by timber harvesting, acid rain, and invasive plants and animals worldwide. Nonnative, invasive species may compete for food and habitats, potentially threatening the native plant and animal species. Because temperate deciduous forests provide rich areas of flora and fauna diversity along with globally important ecosystem services, conservation of these regions is important. Acid rain impacts leaf photosynthesis and soil nutrient balances and has been recognized as a serious problem in many areas such as central Europe. Evidence of leaf bleaching and slowed growth resulting from acid rain deposition in forest stands has been linked to vehicle fossil-fuel emissions through decades of research. Programs begun in the early 1990s through the US Environmental Protection Agency (EPA) to decrease fossil-fuel emissions of sulfur by-products, which cause sulfuric acid deposition in rain, have helped decrease the impacts of acid rain up to 90 percent of 1990 levels by 2016. Meanwhile, European signatories to the 1999 Gothenburg Protocol to Abate Acidification, Eutrophication and Ground-Level Ozone had cut nitric emissions contributing to acid rain by one-third as of 2010. Another threat to temperate deciduous forests is land-use change. Continued clearing for agriculture and commercial mining threatens forests globally. For example, in Germany, much of the historic deciduous forest land has been strip-mined for metal and minerals, decreasing the range of forests in the region.

The large amount of standing biomass existing in temperate deciduous forests represents a significant contribution to the carbon balance of the earth. The carbon stored in the woody biomass, roots, and soil of these forests is a large component of the global carbon balance, which is a biogeochemical cycle of exchanges among the biological components, soil, rocks, atmosphere, and water cycles of the earth. The carbon cycle is vital for recycling and reuse throughout the biosphere and all organisms globally.

As of the turn of the twenty-first century, forests in general stored up to 31 percent of global carbon in biomass and 69 percent in soil, according to the United Nations' Food and Agriculture Organization. Disturbance of soils along with burning or decay of woody biomass releases this stored carbon back to the atmosphere. Because trees convert carbon dioxide into carbohydrates during photosynthesis, releasing oxygen, they are an important component of global atmospheric carbon dioxide levels. The drawdown of global atmospheric carbon dioxide levels can be observed in deciduous forests during leafing-out in the spring. Given that deciduous forests are most extensive in the Northern Hemisphere due to larger land masses, the Northern Hemisphere spring dominates the drawdown trend, most famously observed by the Keeling curve, which measures global carbon dioxide levels.

Related to changes in global carbon dioxide levels, the recognized increases in global temperatures also threaten forest biomes through a decrease in moisture availability and increases in secondary stress factors such as invasive insects and pathogens. Climate change impacts may also alter the species habitats, composition, and adaptations of forest stands, leading to changes in forest structure or function. For example, the impacts of global climate change have already been noted in hardwood forests in Ohio, where decreases in annual snowfall, important for soil water recharge, and increased temperatures in winters have placed carbohydrate and water constraints on some forest tree species. A 2017 Science Advances study found that 73 percent of eastern American tree populations had moved westward while 62 percent had moved northward since 1980, a period that corresponded not only with changing temperatures but also shifts in regional precipitation; most of those moving westward were deciduous and most moving northward were conifers, suggesting a divergent trend that could affect the survival of those forest ecosystems. With global increases in disturbances and climate impacts, conservation efforts in forests are an ever-increasing concern for many U.S. national organizations such as the Sierra Club and Audubon Society and for international organizations such as the Commonwealth Forestry Association.

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