The entire assemblage of organisms (trees, shrubs, herbs, bacteria, fungi, and animals, including people) together with their environmental substrate (the surrounding air, soil, water, organic debris, and rocks), interacting inside a defined boundary. Forests and woodlands occupy about 38% of the Earth's surface, and they are more productive and have greater biodiversity than other types of terrestrial vegetation. Forests grow in a wide variety of climates, from steamy tropical rainforests to frigid arctic mountain slopes, and from arid interior mountains to windy rain-drenched coastlines. The type of forest in a given place results from a complex of factors, including frequency and type of disturbances, seed sources, soils, slope and aspect, climate, seasonal patterns of rainfall, insects and pathogens, and history of human influence.
Often forest ecosystems are studied in watersheds draining to a monitored stream: the structure is then defined in vertical and horizontal dimensions. Usually the canopy of the tallest trees forms the upper ecosystem boundary, and plants with the deepest roots form the lower boundary. The horizontal structure is usually described by how individual trees, shrubs, herbs, and openings or gaps are distributed. Wildlife ecologists study the relation of stand and landscape patterns to habitat conditions for animals.
Woody trees and shrubs are unique in their ability to extend their branches and foliage skyward and to capture carbon dioxide and most of the incoming photosynthetically active solar radiation. Some light is reflected back to the atmosphere and some passes through leaves to the ground (infrared light). High rates of photosynthesis require lots of water, and many woody plants have deep and extensive root systems that tap stored ground water between rain storms. Root systems of most plants are greatly extended through a relation between plants and fungi, called mycorrhizal symbiosis. See Photosynthesis, Root (botany)
The biomass of a forest is defined here as the mass of living plants, normally expressed as dry weight per unit area. Biomass production is the rate at which biomass is accrued per unit area over a fixed interval, usually one year. If the forest is used to grow timber crops, production measures focus on the biomass or volume of commercial trees. Likewise, if wildlife populations are the focus of management, managers may choose to measure biomass or numbers of individual animals. Ecologists interested in the general responses of forest ecosystems, however, try to measure net primary production (Npp), usually expressed as gross primary production (Gpp) minus the respiration of autotrophs (Ra).
Another response commonly of interest is net ecosystem production (NEP), usually expressed as where Rh is respiration of heterotrophs. See Biomass
Productivity is the change in production over multiple years. Monitoring productivity is especially important in managed forests. Changes in forest productivity can be detected only over very long periods. See Biological productivity
Forested ecosystems have great effect on the cycling of carbon, water, and nutrients, and these effects are important in understanding long-term productivity. Cycling of carbon, oxygen, and hydrogen are dominated by photosynthesis, respiration, and decomposition, but they are also affected by other processes. Forests control the hydrologic cycle in important ways. Photosynthesis requires much more water than is required in its products. Water is lost back to the atmosphere (transpiration), and water on leaf and branch surfaces also evaporates under warm and windy conditions. Water not taken up or evaporated flows into the soil and eventually appears in streams, rivers, and oceans where it can be reevaporated and moved back over land, completing the cycle.
Forest plants and animals alter soil characteristics, for example, by adding organic matter, which generally increases the rate at which water infiltrates and is retained. Nutrient elements cycle differently from water and from each other.
Elements such as phosphorus, calcium, and magnesium are released from primary minerals in rocks through chemical weathering. Elements incorporated into biomass are returned to the soil with litterfall and root death; these elements become part of soil organic matter and are mineralized by decomposers or become a component of secondary minerals. All elements can leave ecosystems through erosion of particles and then be transported to the oceans and deposited as sediment. Deeply buried sediments undergo intense pressure and heat that reforms primary minerals. Volcanoes and plate tectonic movements eventually distribute these new minerals back to land.
Nitrogen is the most common gas in the atmosphere. Only certain bacteria can form a special enzyme (nitrogenase) which breaks apart N2 and combines with photosynthates to form amino acids and proteins. In nature, free-living N2-fixing microbes and a few plants that can harbor N2-fixing bacteria in root nodules play important roles controlling the long-term productivity of forests limited by nitrogen supply. Bacteria that convert ammonium ion (NH+4) to NO-3 (nitrifiers) and bacteria that convert NO-3 back to N2 (denitrifiers) are important in nitrogen cycling as well. See Nitrogen cycle
Changes in the plant species of a forest over 10 to 100 years or more are referred to as succession. Changes in forest structure are called stand development; changes in composition, structure, and function are called ecosystem development. Simplified models of succession and development have been created and largely abandoned because the inherent complexity of the interacting forces makes model predictions inaccurate. See Ecological succession, Forest fire
One of the products of an undisturbed forest is water of high quality flowing in streams. The ecological integrity of the stream is a reflection of the forested watershed that it drains. When the forest is disturbed (for example, by cutting or fire), the stream ecosystem will also be altered. Forest streams are altered by any practices or chemical input that alter forest vegetation, by the introduction of exotic species, and by the construction of roads that increase sediment delivery to streams.
Forest animals are the consumers in forest ecosystems. They influence the flow of energy and cycling of nutrients through systems, as well as the structure and composition of forests, through their feeding behavior and the disturbances that they create. In turn, their abundance and diversity is influenced by the structure and composition of the forest and the intensity, frequency, size, and pattern of disturbances that occur in forests. Forest vertebrates make up less than 1% of the biomass in most forests, yet they can play important functional roles in forest systems.
Invertebrates are major components of forest ecosystems, affecting virtually all forest processes and uses. Many species are recognized as important pollinators and seed dispersers that ensure plant reproduction. Even so-called pests may be instrumental in maintaining ecosystem processes critical to soil fertility, plant productivity, and forest health.
Invertebrates affect forests primarily through the processes of herbivory and decomposition. They are also involved in the regulation of plant growth, survival, and reproduction; forest diversity; and nutrient cycling. Typically, invertebrate effects on ecosystem structure and function are modest compared to the more conspicuous effects of plants and fungi. However, invertebrates can have effects disproportionate to their numbers or biomass.
Changes in population size also affect the ecological roles of invertebrates. For example, small populations of invertebrates that feed on plants may maintain low rates of foliage turnover and nutrient cycling, with little effect on plant growth or survival, whereas large populations can defoliate entire trees, alter forest structure, and contribute a large amount of plant material and nutrients to the forest floor. Different life stages also may represent different roles. Immature butterflies and moths are defoliators, whereas the adults often are important pollinators.
Microorganisms, including bacteria, fungi, and protists, are the most numerous and the most diverse of the life forms that make up any forest ecosystem. The structure and functioning of forests are dependent on microbial interactions. Four processes are particularly important: nitrogen fixation, decomposition and nutrient cycling, pathogenesis, and mutualistic symbiosis.
Nitrogen fixation is crucial to forest function. While atmospheric nitrogen is abundant, it is unavailable to trees or other plants unless fixed, that is, converted to ammonia (NH4), by either symbiotic or free-living soil bacteria.
Most microorganisms are saprophytic decomposers, gaining carbon from the dead remains of other plants or animals. In the process of their growth and death, they release nutrients from the forest litter, making them available once again for the growth of plants. Their roles in carbon, nitrogen, and phosphorus cycling are particularly important. Fungi are generally most important in acid soils beneath conifer forests, while bacteria are more important in soils with a higher pH. Bacteria often are the last scavengers in the food web and in turn serve as food to a host of microarthropods.
Microorganisms reduce the mass of forest litter and, in the process, contribute significantly to the structure and fertility of soils as the organic residue is incorporated.
Some bacteria and many fungi are plant pathogens, obtaining their nutrients from living plants. Some are opportunists, successful as saprophytes, but capable of killing weakened or wounded plant tissues. Others require a living host, often preferring the most vigorous trees in the forest. Pathogenic fungi usually specialize on roots or stems or leaves, on one species or genus of trees.
Pathogenic fungi are important parts of all natural forest ecosystems. The forest trees evolved with the fungi, and have effective means of defense and escape, reducing the frequency of infection and slowing the rates of tissue death and tree mortality. However, trees are killed, and the composition and structure of the forest is shaped in large part by pathogens.
Pathogens remove weak or poorly adapted organisms from the forest, thus maintaining the fitness of the population. Decay fungi that kill parts of trees or rot the heartwood of living trees create an essential habitat for cavity-nesting birds and the other animals dependent on hollow trees.
By killing trees, pathogens create light gaps in the forest canopy. The size and rate of light gap formation and the relative susceptibility of the tree species present on the site determine the ecological consequences of mortality. Forest succession is often advanced as shade-tolerant trees are released in small gaps. Gaps allow the growth of herbaceous plants in the island of light, creating habitat and food diversity for animals within the forest. In many forests, pathogens are the most important gap formers and the principal determinants of structure and succession in the long intervals between stand-replacing disturbances such as wildfires or hurricanes. See Ecological succession, Plant pathology
The fungus roots of trees, and indeed most plants, represent an intimate physical and physiological association of particular fungi and their hosts. Mycorrhizae are the products of long coevolution between fungus and plant, resulting in mutual dependency. Mycorrhizae are particularly important to trees because they enhance the uptake of phosphorus from soils. Mycorrhizal fungi greatly extend the absorptive surface of roots through the network of external hyphae. See Ecosystem, Forest and forestry, Mycorrhizae