Plant ecology

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Plant_ecology

A tropical plant community on Diego Garcia

Rangeland monitoring using Parker 3-step Method, Okanagan Washington 2002

Plant ecology is a subdiscipline of ecology that studies the distribution and abundance of plants, the effects of environmental factors upon the abundance of plants, and the interactions among plants and between plants and other organisms. Examples of these are the distribution of temperate deciduous forests in North America, the effects of drought or flooding upon plant survival, and competition among desert plants for water, or effects of herds of grazing animals upon the composition of grasslands.

A global overview of the Earth's major vegetation types is provided by O.W. Archibold. He recognizes 11 major vegetation types: tropical forests, tropical savannas, arid regions (deserts), Mediterranean ecosystems, temperate forest ecosystems, temperate grasslands, coniferous forests, tundra (both polar and high mountain), terrestrial wetlands, freshwater ecosystems and coastal/marine systems. This breadth of topics shows the complexity of plant ecology, since it includes plants from floating single-celled algae up to large canopy forming trees.

One feature that defines plants is photosynthesis. Photosynthesis is the process of a chemical reactions to create glucose and oxygen, which is vital for plant life. One of the most important aspects of plant ecology is the role plants have played in creating the oxygenated atmosphere of earth, an event that occurred some 2 billion years ago. It can be dated by the deposition of banded iron formations, distinctive sedimentary rocks with large amounts of iron oxide. At the same time, plants began removing carbon dioxide from the atmosphere, thereby initiating the process of controlling Earth's climate. A long term trend of the Earth has been toward increasing oxygen and decreasing carbon dioxide, and many other events in the Earth's history, like the first movement of life onto land, are likely tied to this sequence of events.

One of the early classic books on plant ecology was written by J.E. Weaver and F.E. Clements. It talks broadly about plant communities, and particularly the importance of forces like competition and processes like succession. The term ecology itself was coined by German biologist Ernst Haeckel.

Plant ecology can also be divided by levels of organization including plant ecophysiology, plant population ecology, community ecology, ecosystem ecology, landscape ecology and biosphere ecology.

The study of plants and vegetation is complicated by their form. First, most plants are rooted in the soil, which makes it difficult to observe and measure nutrient uptake and species interactions. Second, plants often reproduce vegetatively, that is asexually, in a way that makes it difficult to distinguish individual plants. Indeed, the very concept of an individual is doubtful, since even a tree may be regarded as a large collection of linked meristems. Hence, plant ecology and animal ecology have different styles of approach to problems that involve processes like reproduction, dispersal and mutualism. Some plant ecologists have placed considerable emphasis upon trying to treat plant populations as if they were animal populations, focusing on population ecology. Many other ecologists believe that while it is useful to draw upon population ecology to solve certain scientific problems, plants demand that ecologists work with multiple perspectives, appropriate to the problem, the scale and the situation.

History

Main article: History of ecology

Alexander von Humboldt's work connecting plant distributions with environmental factors played an important role in the genesis of the discipline of plant ecology.

Plant ecology has its origin in the application of plant physiology to the questions raised by plant geographers. Carl Ludwig Willdenow was one of the first to note that similar climates produced similar types of vegetation, even when they were located in different parts of the world. Willdenow's student, Alexander von Humboldt, used physiognomy to describe vegetation types and observed that the distribution vegetation types was based on environmental factors. Later plant geographers who built upon Humboldt's work included Joakim Frederik Schouw, A.P. de Candolle, August Grisebach and Anton Kerner von Marilaun. Schouw's work, published in 1822, linked plant distributions to environmental factors (especially temperature) and established the practice of naming plant associations by adding the suffix -etum to the name of the dominant species. Working from herbarium collections, De Candolle searched for general rules of plant distribution and settled on using temperature as well. Grisebach's two-volume work, Die Vegetation der Erde nach Ihrer Klimatischen Anordnung, published in 1872, saw plant geography reach its "ultimate form" as a descriptive field.

Starting in the 1870s, Swiss botanist Simon Schwendener, together with his students and colleagues, established the link between plant morphology and physiological adaptations, laying the groundwork for the first ecology textbooks, Eugenius Warming's Plantesamfund (published in 1895) and Andreas Schimper's 1898 Pflanzengeographie auf Physiologischer Grundlage. Warming successfully incorporated plant morphology, physiology taxonomy and biogeography into plant geography to create the field of plant ecology. Although more morphological than physiological, Schimper's has been considered the beginning of plant physiological ecology. Plant ecology was initially built around static ideas of plant distribution; incorporating the concept of succession added an element to change through time to the field. Henry Chandler Cowles' studies of plant succession on the Lake Michigan sand dunes (published in 1899) and Frederic Clements' 1916 monograph on the subject established it as a key element of plant ecology.

Plant ecology developed within the wider discipline of ecology over the twentieth century. Inspired by Warming's Plantesamfund, Arthur Tansley set out to map British plant communities. In 1904 he teamed up with William Gardner Smith and others involved in vegetation mapping to establish the Central Committee for the Survey and Study of British Vegetation, later shortened to British Vegetation Committee. In 1913, the British Vegetation Committee organised the British Ecological Society (BES), the first professional society of ecologists. This was followed in 1917 by the establishment of the Ecological Society of America (ESA); plant ecologists formed the largest subgroup among the inaugural members of the ESA.

Cowles' students played an important role in the development of the field of plant ecology during the first half of the twentieth century, among them William S. Cooper, E. Lucy Braun and Edgar Transeau.

Distribution

Main articles: Phytogeography and Species distribution

World biomes are based upon the type of dominant plant.

Plant distributions is governed by a combination of historical factors, ecophysiology and biotic interactions. The set of species that can be present at a given site is limited by historical contingency. In order to show up, a species must either have evolved in an area or dispersed there (either naturally or through human agency), and must not have gone locally extinct. The set of species present locally is further limited to those that possess the physiological adaptations to survive the environmental conditions that exist. This group is further shaped through interactions with other species.

Plant communities are broadly distributed into biomes based on the form of the dominant plant species. For example, grasslands are dominated by grasses, while forests are dominated by trees. Biomes are determined by regional climates, mostly temperature and precipitation, and follow general latitudinal trends. Within biomes, there may be many ecological communities, which are impacted not only by climate and a variety of smaller-scale features, including soils, hydrology, and disturbance regime. Biomes also change with elevation, high elevations often resembling those found at higher latitudes.

Biological interactions

Further information: Biological interaction

Competition

Further information: Competition (biology)

Plants, like most life forms, require relatively few basic elements: carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur; hence they are known as CHNOPS life forms. There are also lesser elements needed as well, frequently termed micronutrients, such as magnesium and sodium. When plants grow in close proximity, they may deplete supplies of these elements and have a negative impact upon neighbours. Competition for resources vary from complete symmetric (all individuals receive the same amount of resources, irrespective of their size) to perfectly size symmetric (all individuals exploit the same amount of resource per unit biomass) to absolutely size-asymmetric (the largest individuals exploit all the available resource). The degree of size asymmetry has major effects on the structure and diversity of ecological communities. In many cases (perhaps most) the negative effects upon neighbours arise from size asymmetric competition for light. In other cases, there may be competition below ground for water, nitrogen, or phosphorus. To detect and measure competition, experiments are necessary; these experiments require removing neighbours, and measuring responses in the remaining plants. Many such studies are required before useful generalizations can be drawn.

Overall, it appears that light is the most important resource for which plants compete, and the increase in plant height over evolutionary time likely reflects selection for taller plants to better intercept light. Many plant communities are therefore organized into hierarchies based upon the relative competitive abilities for light. In some systems, particularly infertile or arid systems, below ground competition may be more significant. Along natural gradients of soil fertility, it is likely that the ratio of above ground to below ground competition changes, with higher above ground competition in the more fertile soils. Plants that are relatively weak competitors may escape in time (by surviving as buried seeds) or in space (by dispersing to a new location away from strong competitors.)

In principle, it is possible to examine competition at the level of the limiting resources if a detailed knowledge of the physiological processes of the competing plants is available. However, in most terrestrial ecological studies, there is only little information on the uptake and dynamics of the resources that limit the growth of different plant species, and, instead, competition is inferred from observed negative effects of neighbouring plants without knowing precisely which resources the plants were competing for. In certain situations, plants may compete for a single growth-limiting resource, perhaps for light in agricultural systems with sufficient water and nutrients, or in dense stands of marsh vegetation, but in many natural ecosystems plants may be colimited by several resources, e.g. light, phosphorus and nitrogen at the same time.

Therefore, there are many details that remain to be uncovered, particularly the kinds of competition that arise in natural plant communities, the specific resource(s), the relative importance of different resources, and the role of other factors like stress or disturbance in regulating the importance of competition.

Mutualism

Further information: Mutualism (biology)

Mutualism is defined as an interaction "between two species or individuals that is beneficial to both". Probably the most widespread example in plants is the mutual beneficial relationship between plants and fungi, known as mycorrhizae. The plant is assisted with nutrient uptake, while the fungus receives carbohydrates. Some the earliest known fossil plants even have fossil mycorrhizae on their rhizomes.

The flowering plants are a group that have evolved by using two major mutualisms. First, flowers are pollinated by insects. This relationship seems to have its origins in beetles feeding on primitive flowers, eating pollen and also acting (unwittingly) as pollinators. Second, fruits are eaten by animals, and the animals then disperse the seeds. Thus, the flowering plants actually have three major types of mutualism, since most higher plants also have mycorrhizae.

Plants may also have beneficial effects upon one another, but this is less common. Examples might include "nurse plants" whose shade allows young cacti to establish. Most examples of mutualism, however, are largely beneficial to only one of the partners, and may not really be true mutualism. The term used for these more one-sided relationships, which are mostly beneficial to one participant, is facilitation. Facilitation among neighboring plants may act by reducing the negative impacts of a stressful environment. In general, facilitation is more likely to occur in physically stressful environments than in favorable environments, where competition may be the most important interaction among species.

Commensalism is similar to facilitation, in that one plant is mostly exploiting another. A familiar example is the epiphytes which grow on branches of tropical trees, or even mosses which grow on trees in deciduous forests.

It is important to keep track of the benefits received by each species to determine the appropriate term. Although people are often fascinated by unusual examples, it is important to remember that in plants, the main mutualisms are mycorrhizae, pollination, and seed dispersal.

Parasitism

Further information: Parasitic plant

Parasitism in biology refers to an interaction between different species, where the parasite (one species) benefits at the expense of the host (the other species). Parasites depend on another organism (their host) for survival in general, which usually includes both habitat and nutrient requirements at the very minimum. Parasitic plants attach themselves to host plants via a haustoria to the xylem and/or phloem. Many parasitic plants are generalists and are able to attack multiple hosts at the same time, greatly affecting community structures. Host species' growth, reproduction, and metabolism are affected by the parasite due to the nutrients, water, and carbon being taken by the parasite. They are also able to alter competitive interactions among hosts and indirectly affect competition in the community.

Commensalism

Commensalism refers to the biological interaction between two species in which one benefits while the other simply remains unaffected. The species that benefits is referred to as the commensal while the species that is unaffected is referred to as the host. For example, organisms that live attached to plants, known as epiphytes, are referred to as commensals. Algae that grow on the backs of turtles or sloths are considered as commensals, too. Their survival rate is higher when they are attached to their host, however they do not harm nor benefit the host. Nearly 10% of all vascular plant species around the world are epiphytes, and most of them are found in tropical forests. Therefore, they make up a large fraction of the total plant biodiversity in the world, being 10% of all species, and 25% of all vascular plant species in tropical countries. However, commensals have the capability to transform into parasites over time by which results in a decrease in success or an overall population decline.

Herbivory

Main articles: Herbivory and Plant defense against herbivory

Reindeer in front of herbivore exclosures. Excluding different herbivores (here reindeer, or reindeer and rodents) has different effects on the vegetation.

An important ecological function of plants is that they produce organic compounds for herbivores in the bottom of the food web. A large number of plant traits, from thorns to chemical defenses, can be related to the intensity of herbivory. Large herbivores can also have many effects on vegetation. These include removing selected species, creating gaps for regeneration of new individuals, recycling nutrients, and dispersing seeds. Certain ecosystem types, such as grasslands, may be dominated by the effects of large herbivores, although fire is also an equally important factor in this biome. In few cases, herbivores are capable of nearly removing all the vegetation at a site (for example, geese in the Hudson Bay Lowlands of Canada, and nutria in the marshes of Louisiana) but normally herbivores have a more selective impact, particularly when large predators control the abundance of herbivores. The usual method of studying the effects of herbivores is to build exclosures, where they cannot feed, and compare the plant communities in the exclosures to those outside over many years. Often such long term experiments show that herbivores have a significant effect upon the species that make up the plant community.

Other topics

Abundance

Main article: Abundance (ecology)

The ecological success of a plant species in a specific environment may be quantified by its abundance, and depending on the life form of the plant different measures of abundance may be relevant, e.g. density, biomass, or plant cover.

The change in the abundance of a plant species may be due to both abiotic factors, e.g. climate change, or biotic factors, e.g. herbivory or interspecific competition.

Colonisation and local extinction

Main article: Biogeography

See also: Colonisation (biology), Biological dispersal, Seed dispersal, Local extinction, and Soil seed bank

Whether a plant species is present at a local area depends on the processes of colonisation and local extinction. The probability of colonisation decreases with distance to neighboring habitats where the species is present and increases with plant abundance and fecundity in neighboring habitats and the dispersal distance of the species. The probability of local extinction decreases with abundance (both living plants and seeds in the soil seed bank).

Life forms

Main article: Plant life-form

See also: Raunkiær plant life-form

Reproduction

Main article: Plant reproduction

There are a few ways that reproduction occurs within plant life, and one way is through parthenogenesis. Parthenogenesis is defined as "a form of asexual reproduction in which genetically identical offspring (clones) are produced". Another form of reproduction is through cross-fertilization, which is defined as "fertilization in which the egg and sperm are produced by different individuals", and in plants this occurs in the ovule. Once an ovule is fertilized within the plant this becomes what is known as a seed. A seed normally contains the nutritive tissue also known as the endosperm and the embryo. A seedling is a young plant that has recently gone through germination. Another form of reproduction of a plant is self-fertilization; in which both the sperm and the egg are produced from the same individual- this plant is therefore a self-compatible titled plant.

Old-growth forest

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Old-growth_forest

Old-growth European beech forest in Biogradska Gora National Park, Montenegro

Cool temperate rainforest in Tasmania, Australia

First growth or virgin forest near Mount Rainier

An old-growth forest, sometimes synonymous with primary forest, virgin forest, late seral forest, primeval forest, first-growth forest, or mature forest—is a forest that has attained great age without significant disturbance, and thereby exhibits unique ecological features, and might be classified as a climax community. The Food and Agriculture Organization of the United Nations defines primary forests as naturally regenerated forests of native tree species where there are no clearly visible indications of human activity and the ecological processes are not significantly disturbed. Barely one-third (34 percent) of the world's forests are primary forests. Old-growth features include diverse tree-related structures that provide diverse wildlife habitats that increases the biodiversity of the forested ecosystem. Virgin or first-growth forests are old-growth forests that have never been logged. The concept of diverse tree structure includes multi-layered canopies and canopy gaps, greatly varying tree heights and diameters, and diverse tree species and classes and sizes of woody debris.

As of 2020, the world has 1.11 billion ha of primary forest remaining. Combined, three countries (Brazil, Canada, and Russia) host more than half (61 percent) of the world's primary forest. The area of primary forest has decreased by 81 million ha since 1990, but the rate of loss more than halved in 2010–2020 compared with the previous decade.

Old-growth forests are valuable for economic reasons and for the ecosystem services they provide. This can be a point of contention when some in the logging industry desire to harvest valuable timber from the forests, destroying the forests in the process, to generate short-term profits, while environmentalists seek to preserve the forests in their pristine state for benefits such as water purification, flood control, weather stability, maintenance of biodiversity, and nutrient cycling. Moreover, old-growth forests are more efficient at sequestering carbon than newly planted forests and fast-growing timber plantations, thus preserving the forests is important to climate change mitigation.

Characteristics

Antarctic beech old-growth in Lamington National Park, Queensland, Australia

Old-growth forests tend to have large trees and standing dead trees, multilayered canopies with gaps that result from the deaths of individual trees, and coarse woody debris on the forest floor.

A forest regenerated after a severe disturbance, such as wildfire, insect infestation, or harvesting, is often called second-growth or 'regeneration' until enough time passes for the effects of the disturbance to be no longer evident. Depending on the forest, this may take from a century to several millennia. Hardwood forests of the eastern United States can develop old-growth characteristics in 150–500 years. In British Columbia, Canada, old growth is defined as 120 to 140 years of age in the interior of the province where fire is a frequent and natural occurrence. In British Columbia's coastal rainforests, old growth is defined as trees more than 250 years, with some trees reaching more than 1,000 years of age. In Australia, eucalypt trees rarely exceed 350 years of age due to frequent fire disturbance.

Forest types have very different development patterns, natural disturbances and appearances. A Douglas-fir stand may grow for centuries without disturbance while an old-growth ponderosa pine forest requires frequent surface fires to reduce the shade-tolerant species and regenerate the canopy species. In the boreal forest of Canada, catastrophic disturbances like wildfires minimize opportunities for major accumulations of dead and downed woody material and other structural legacies associated with old growth conditions. Typical characteristics of old-growth forest include the presence of older trees, minimal signs of human disturbance, mixed-age stands, presence of canopy openings due to tree falls, pit-and-mound topography, down wood in various stages of decay, standing snags (dead trees), multilayered canopies, intact soils, a healthy fungal ecosystem, and presence of indicator species.

Biodiversity

The northern spotted owl primarily inhabits old-growth forests in the northern part of its range (Canada to southern Oregon) and landscapes with a mix of old and younger forest types in the southern part of its range (the Klamath region and California).

Old-growth forests are often biologically diverse, and home to many rare species, threatened species, and endangered species of plants and animals, such as the northern spotted owl, marbled murrelet and fisher, making them ecologically significant. Levels of biodiversity may be higher or lower in old-growth forests compared to that in second-growth forests, depending on specific circumstances, environmental variables, and geographic variables. Logging in old-growth forests is a contentious issue in many parts of the world. Excessive logging reduces biodiversity, affecting not only the old-growth forest itself, but also indigenous species that rely upon old-growth forest habitat.

Mixed age

Some forests in the old-growth stage have a mix of tree ages, due to a distinct regeneration pattern for this stage. New trees regenerate at different times from each other, because each of them has a different spatial location relative to the main canopy, hence each one receives a different amount of light. The mixed age of the forest is an important criterion in ensuring that the forest is a relatively stable ecosystem in the long term. A climax stand that is uniformly aged becomes senescent and degrades within a relatively short time to result in a new cycle of forest succession. Thus, uniformly aged stands are less stable ecosystems. Boreal forests are more uniformly aged, as they are normally subject to frequent stand-replacing wildfires.

Canopy openings

Forest canopy gaps are essential in creating and maintaining mixed-age stands. Also, some herbaceous plants only become established in canopy openings, but persist beneath an understory. Openings are a result of tree death due to small impact disturbances such as wind, low-intensity fires, and tree diseases.

Old-growth forests are unique, usually having multiple horizontal layers of vegetation representing a variety of tree species, age classes, and sizes, as well as "pit and mound" soil shape with well-established fungal nets. Because old-growth forest is structurally diverse, it provides higher-diversity habitat than forests in other stages. Thus, sometimes higher biological diversity can be sustained in old-growth forests, or at least a biodiversity that is different from other forest stages.

Virgin forest about 2500 m above sea level in Shennongjia Forestry District, Hubei, China

Topography

The characteristic topography of much old-growth forest consists of pits and mounds. Mounds are caused by decaying fallen trees, and pits (tree throws) by the roots pulled out of the ground when trees fall due to natural causes, including being pushed over by animals. Pits expose humus-poor, mineral-rich soil and often collect moisture and fallen leaves, forming a thick organic layer that is able to nurture certain types of organisms. Mounds provide a place free of leaf inundation and saturation, where other types of organisms thrive.

Standing snags

Standing snags provide food sources and habitat for many types of organisms. In particular, many species of dead-wood predators such as woodpeckers must have standing snags available for feeding. In North America, the spotted owl is well known for needing standing snags for nesting habitat.

Decaying ground layer

Downed wood replenishes topsoil as it decays.

Fungus Climacocystis borealis on a tree stump in the Białowieża Forest, one of the last largely intact primeval forests in Central Europe

Fallen timber, or coarse woody debris, contributes carbon-rich organic matter directly to the soil, providing a substrate for mosses, fungi, and seedlings, and creating microhabitats by creating relief on the forest floor. In some ecosystems such as the temperate rain forest of the North American Pacific coast, fallen timber may become nurse logs, providing a substrate for seedling trees.

Soil

Intact soils harbor many life forms that rely on them. Intact soils generally have very well-defined horizons, or soil profiles. Different organisms may need certain well-defined soil horizons to live, while many trees need well-structured soils free of disturbance to thrive. Some herbaceous plants in northern hardwood forests must have thick duff layers (which are part of the soil profile). Fungal ecosystems are essential for efficient in-situ recycling of nutrients back into the entire ecosystem.

Definitions

Ecological definitions

Stand age definition

Stand age can also be used to categorize a forest as old-growth. For any given geographical area, the average time since disturbance until a forest reaches the old growth stage can be determined. This method is useful, because it allows quick and objective determination of forest stage. However, this definition does not provide an explanation of forest function. It just gives a useful number to measure. So, some forests may be excluded from being categorized as old-growth even if they have old-growth attributes just because they are too young. Also, older forests can lack some old-growth attributes and be categorized as old-growth just because they are so old. The idea of using age is also problematic, because human activities can influence the forest in varied ways. For example, after the logging of 30% of the trees, less time is needed for old-growth to come back than after removal of 80% of the trees. Although depending on the species logged, the forest that comes back after a 30% harvest may consist of proportionately fewer hardwood trees than a forest logged at 80% in which the light competition by less important tree species does not inhibit the regrowth of vital hardwoods.

Forest dynamics definition

From a forest dynamics perspective, old-growth forest is in a stage that follows understory reinitiation stage. A review of the stages helps to understand the concept:

1. Stand-replacing: Disturbance hits the forest and kills most of the living trees.
2. Stand-initiation: A population of new trees becomes established.
3. Stem-exclusion: Trees grow higher and enlarge their canopy, thus competing for the light with neighbors; light competition mortality kills slow-growing trees and reduces forest density, which allows surviving trees to increase in size. Eventually, the canopies of neighboring trees touch each other and drastically lower the amount of light that reaches lower layers. Due to that, the understory dies and only very shade-tolerant species survive.
4. Understory reinitiation: Trees die from low-level mortality, such as windthrow and diseases. Individual canopy gaps start to appear and more light can reach the forest floor. Hence, shade-tolerant species can establish in the understory.
5. Old-growth: Main canopy trees become older and more of them die, creating even more gaps. Since the gaps appear at different times, the understory trees are at different growth stages. Furthermore, the amount of light that reaches each understory tree depends on its position relative to the gap. Thus, each understory tree grows at a different rate. The differences in establishment timing and in growth rate create a population of understory trees that is variable in size. Eventually, some understory trees grow to become as tall as the main canopy trees, thereby filling the gap. This perpetuation process is typical for the old-growth stage. This, however, does not mean that the forest will be old-growth forever. Generally, three futures for old-growth stage forest are possible: 1) The forest will be hit by a disturbance and most of the trees will die, 2) Unfavorable conditions for new trees to regenerate will occur. In this case, the old trees will die and smaller plants will create woodland, and 3) The regenerating understory trees are different species from the main canopy trees. In this case, the forest will switch back to stem-exclusion stage, but with shade-tolerant tree species. 4) The forest in an old-growth stage can be stable for centuries, but the length of this stage depends on the forest's tree composition and the climate of the area. For example, frequent natural fires do not allow boreal forests to be as old as coastal forests of western North America.

Of importance is that while the stand switches from one tree community to another, the stand will not necessarily go through old-growth stage between those stages. Some tree species have a relatively open canopy. That allows more shade-tolerant tree species to establish below even before the understory reinitiation stage. The shade-tolerant trees eventually outcompete the main canopy trees in stem-exclusion stage. Therefore, the dominant tree species will change, but the forest will still be in stem-exclusion stage until the shade-tolerant species reach old-growth stage.

Tree species succession may change tree species' composition once the old-growth stage has been achieved. For example, an old boreal forest may contain some large aspen trees, which may die and be replaced by smaller balsam fir or black spruce. Consequently, the forest will switch back to understory reinitiation stage. Using the stand dynamics definition, old-growth can be easily evaluated using structural attributes. However, in some forest ecosystems, this can lead to decisions regarding the preservation of unique stands or attributes that will disappear over the next few decades because of natural succession processes. Consequently, using stand dynamics to define old-growth forests is more accurate in forests where the species that constitute old-growth have long lifespans and succession is slow.

Social and cultural definitions

Redwood tree in northern California redwood forest: According to the National Park Service, "96 percent of the original old-growth coast redwoods have been logged."

Common cultural definitions and common denominators regarding what comprises old-growth forest, and the variables that define, constitute and embody old-growth forests include:

• The forest habitat possesses relatively mature, old trees;
• The tree species present have long continuity on the same site;
• The forest itself is a remnant natural area that has not been subjected to significant disturbance by mankind, altering the appearance of the landscape and its ecosystems, has not been subjected to logging (or other types of development such as road networks or housing), and has inherently progressed per natural tendencies.

Additionally, in mountainous, temperate landscapes (such as Western North America), and specifically in areas of high-quality soil and a moist, relatively mild climate, some old-growth trees have attained notable height and girth (DBH: diameter at breast height), accompanied by notable biodiversity in terms of the species supported. Therefore, for most people, the physical size of the trees is the most recognized hallmark of old-growth forests, even though the ecologically productive areas that support such large trees often comprise only a very small portion of the total area that has been mapped as old-growth forest. (In high-altitude, harsh climates, trees grow very slowly and thus remain at a small size. Such trees also qualify as old growth in terms of how they are mapped, but are rarely recognized by the general public as such.)

The debate over old-growth definitions has been inextricably linked with a complex range of social perceptions about wilderness preservation, biodiversity, aesthetics, and spirituality, as well as economic or industrial values.

Economic definitions

In logging terms, old-growth stands are past the economic optimum for harvesting—usually between 80 and 150 years, depending on the species. Old-growth forests were often given harvesting priority because they had the most commercially valuable timber, they were considered to be at greater risk of deterioration through root rot or insect infestation, and they occupied land that could be used for more productive second-growth stands. In some regions, old growth is not the most commercially viable timber—in British Columbia, Canada, harvesting in the coastal region is moving to younger second-growth stands.

Other definitions

A 2001 scientific symposium in Canada found that defining old growth in a scientifically meaningful, yet policy-relevant, manner presents some basic difficulties, especially if a simple, unambiguous, and rigorous scientific definition is sought. Symposium participants identified some attributes of late-successional, temperate-zone, old-growth forest types that could be considered in developing an index of "old-growthness" and for defining old-growth forests:

Structural features:

Avatar Grove near Port Renfrew, British Columbia: Giant Douglas firs (left) and red cedars (right) fill the grove.

• Uneven or multi-aged stand structure, or several identifiable age cohorts
• Average age of dominant species approaching half the maximum longevity for species (about 150+ years for most shade-tolerant trees)
• Some old trees at close to their maximum longevity (ages of 300+ years)
• Presence of standing dead and dying trees in various stages of decay
• Fallen, coarse woody debris
• Natural regeneration of dominant tree species within canopy gaps or on decaying logs

Compositional features:

• Long-lived, shade-tolerant tree species associations (e.g., sugar maple, American beech, yellow birch, red spruce, eastern hemlock, white pine)

Process features:

• Characterized by small-scale disturbances creating gaps in the forest canopy
• A long natural rotation for catastrophic or stand-replacing disturbance (e.g., a period greater than the maximum longevity of the dominant tree species)
• Minimal evidence of human disturbance
• Final stages of stand development before a relatively steady state is reached

Importance

Eucalyptus regnans forest in Tasmania, Australia

• Old-growth forests often contain rich communities of plants and animals within the habitat due to the long period of forest stability. These varied and sometimes rare species may depend on the unique environmental conditions created by these forests.
• Old-growth forests serve as a reservoir for species, which cannot thrive or easily regenerate in younger forests, so they can be used as a baseline for research.
• Plant species that are native to old-growth forests may someday prove to be invaluable towards curing various human ailments, as has been realized in numerous plants in tropical rainforests.
• Old-growth forests also store large amounts of carbon above and below the ground (either as humus, or in wet soils as peat). They collectively represent a very significant store of carbon. Destruction of these forests releases this carbon as greenhouse gases, and may increase the risk of global climate change. Although old-growth forests serve as a global carbon dioxide sink, they are not protected by international treaties, because it is generally thought that aging forests cease to accumulate carbon. However, in forests between 15 and 800 years of age, net ecosystem productivity (the net carbon balance of the forest including soils) is usually positive; old-growth forests accumulate carbon for centuries and contain large quantities of it.

Ecosystem services

Old-growth forests provide ecosystem services that may be far more important to society than their use as a source of raw materials. These services include making breathable air, making pure water, carbon storage, regeneration of nutrients, maintenance of soils, pest control by insectivorous bats and insects, micro- and macro-climate control, and the storage of a wide variety of genes.

Climatic impacts

The effects of old-growth forests in relation to global warming have been addressed in various studies and journals.

The Intergovernmental Panel on Climate Change said in its 2007 report: "In the long term, a sustainable forest management strategy aimed at maintaining or increasing forest carbon stocks, while producing an annual sustained yield of timber, fibre, or energy from the forest, will generate the largest sustained mitigation benefit."

Old-growth forests are often perceived to be in equilibrium or in a state of decay. However, evidence from analysis of carbon stored above ground and in the soil has shown old-growth forests are more productive at storing carbon than younger forests. Forest harvesting has little or no effect on the amount of carbon stored in the soil, but other research suggests older forests that have trees of many ages, multiple layers, and little disturbance have the highest capacities for carbon storage. As trees grow, they remove carbon from the atmosphere, and protecting these pools of carbon prevents emissions into the atmosphere. Proponents of harvesting the forest argue the carbon stored in wood is available for use as biomass energy (displacing fossil fuel use), although using biomass as a fuel produces air pollution in the form of carbon monoxide, nitrogen oxides, volatile organic compounds, particulates, and other pollutants, in some cases at levels above those from traditional fuel sources such as coal or natural gas.

Each forest has a different potential to store carbon. For example, this potential is particularly high in the Pacific Northwest where forests are relatively productive, trees live a long time, decomposition is relatively slow, and fires are infrequent. The differences between forests must, therefore, be taken into consideration when determining how they should be managed to store carbon.

Old-growth forests have the potential to impact climate change, but climate change is also impacting old-growth forests. As the effects of global warming grow more substantial, the ability of old-growth forests to sequester carbon is affected. Climate change showed an impact on the mortality of some dominant tree species, as observed in the Korean pine. Climate change also showed an effect on the composition of species when forests were surveyed over a 10- and 20-year period, which may disrupt the overall productivity of the forest.

Logging

Further information: Deforestation

See also: Deforestation § Historical causes

Old-growth red cedar stump near Port Renfrew, British Columbia

According to the World Resources Institute, as of January 2009, only 21% of the original old-growth forests that once existed on Earth are remaining. An estimated one-half of Western Europe's forests were cleared before the Middle Ages, and 90% of the old-growth forests that existed in the contiguous United States in the 1600s have been cleared.

The large trees in old-growth forests are economically valuable, and have been subject to aggressive logging throughout the world. This has led to many conflicts between logging companies and environmental groups. From certain forestry perspectives, fully maintaining an old-growth forest is seen as extremely economically unproductive, as timber can only be collected from falling trees, and also potentially damaging to nearby managed groves by creating environments conducive to root rot. It may be more productive to cut the old growth down and replace the forest with a younger one.

The island of Tasmania, just off the southeast coast of Australia, has the largest amount of temperate old-growth rainforest reserves in Australia with around 1,239,000 hectares in total. While the local Regional Forest Agreement (RFA) was originally designed to protect much of this natural wealth, many of the RFA old-growth forests protected in Tasmania consist of trees of little use to the timber industry. RFA old-growth and high conservation value forests that contain species highly desirable to the forestry industry have been poorly preserved. Only 22% of Tasmania's original tall-eucalypt forests managed by Forestry Tasmania have been reserved. Ten thousand hectares of tall-eucalypt RFA old-growth forest have been lost since 1996, predominantly as a result of industrial logging operations. In 2006, about 61,000 hectares of tall-eucalypt RFA old-growth forests remained unprotected. Recent logging attempts in the Upper Florentine Valley have sparked a series of protests and media attention over the arrests that have taken place in this area. Additionally, Gunns Limited, the primary forestry contractor in Tasmania, has been under recent criticism by political and environmental groups over its practice of woodchipping timber harvested from old-growth forests.

Management

Old-growth forest in the Opal Creek Wilderness, a wilderness area located in the Willamette National Forest in the U.S. state of Oregon, on the border of Mount Hood National Forest. It has the largest uncut watershed in Oregon.

Increased understanding of forest dynamics in the late 20th century led the scientific community to identify a need to inventory, understand, manage, and conserve representative examples of old-growth forests with their associated characteristics and values. Literature around old growth and its management is inconclusive about the best way to characterize the true essence of an old-growth stand.

A better understanding of natural systems has resulted in new ideas about forest management, such as managed natural disturbances should be designed to achieve the landscape patterns and habitat conditions that are normally maintained in nature. This coarse filter approach to biodiversity conservation recognizes ecological processes and provides for a dynamic distribution of old growth across the landscape. And all seral stages—young, medium and, old—support forest biodiversity. Plants and animals rely on different forest ecosystem stages to meet their habitat needs.

In Australia, the Regional Forest Agreement (RFA) attempted to prevent the clearfelling of defined "old-growth forests". This led to struggles over what constitutes "old growth". For example, in Western Australia, the timber industry tried to limit the area of old growth in the karri forests of the Southern Forests Region; this led to the creation of the Western Australian Forests Alliance, the splitting of the Liberal Government of Western Australia and the election of the Gallop Labor Government. Old-growth forests in this region have now been placed inside national parks. A small proportion of old-growth forest also exists in South-West Australia, and is protected by federal laws from logging, which has not occurred there for more than 20 years.

In British Columbia, Canada, old-growth forests must be maintained in each of the province's ecological units to meet biodiversity needs.

Locations of remaining tracts

Main article: List of old-growth forests

In 2006, Greenpeace identified that the world's remaining intact forest landscapes are distributed among the continents as follows:

• 35% in South America: The Amazon rainforest is mainly located in Brazil, which clears a larger area of forest annually than any other country in the world.
• 28% in North America, which harvests 10,000 km² of ancient forests every year. Many of the fragmented forests of southern Canada and the United States lack adequate animal travel corridors and functioning ecosystems for large mammals. Most of the remaining old-growth forests in the contiguous United States and Alaska are on public land.
• 19% in northern Asia, home to the largest boreal forest in the world.
• 8% in Africa, which has lost most of its intact forest landscapes in the last 30 years. The timber industry and local governments are responsible for destroying huge areas of intact forest landscapes and continue to be the single largest threat to these areas.
• 7% in South Asia Pacific, where the Paradise Forests are being destroyed faster than any other forest on Earth. Much of the large, intact forest landscapes have already been cut down, 72% in Indonesia, and 60% in Papua New Guinea.
• Less than 3% in Europe, where more than 150 km² of intact forest landscapes are cleared every year and the last areas of the region's intact forest landscapes in European Russia are shrinking rapidly. In the United Kingdom, they are known as ancient woodlands.

Person-centred planning

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Person-centred_planning

Person-centred planning (PCP) is a set of approaches designed to assist an individual to plan their life and supports. It is most often used for life planning with people with learning and developmental disabilities, though recently it has been advocated as a method of planning personalised support with many other sections of society who find themselves disempowered by traditional methods of service delivery, including children, people with physical disabilities, people with mental health issues and older people. PCP is accepted as evidence based practice in many countries throughout the world.

Person-centred planning was adopted as government social policy in the United Kingdom through the 'Valuing People' white paper in 2001, and as part of 'Valuing People Now', a 3-year plan, in 2009. It is promoted as a key method for delivering the personalisation objectives of the UK government's 'Putting People First' programme for social care. The coalition government continued this commitment through 'Capable Communities and Active Citizens' (2010), and in 2011 over 30 health and social care organisations set up a sector-wide agreement 'Think Local, Act Personal' (2011) to transform adult social care.

Background

Person Centred Planning discovers and acts on what is important to a person. It is a process for continual listening and learning, focussing on what are important to someone now and in the future, and acting on this in alliance with their family and their friends.

Person-centred planning was created in response to some specific problems with the way in which society responds to people with disabilities. Those who first described the processes were responding to the effects that 'services' can have on people's lives. In this context 'services' refers to the organisations which are set up to help people in relation to their disability (or at least in relation to how other people have responded to that disability). It would include health and social care services funded by government or local authorities, but also privately funded or voluntary sector projects of many kinds.

Person-centered planning has similarities to other processes and ideas, but was first named and described more definitely by a group of people in the US, including the Center on Human Policy's Rehabilitation Research and Training Center (RRTC) on Community Integration e.g., Julie Ann Racino, Zana Lutfiyya, Steve Taylor, John O'Brien, Beth Mount, Connie Lyle O'Brien, technical assistance "partners" of the RRTC (e.g., Michael Smull, Wade Hitzing, Karen Green-McGowen, Nick Arambarri) and person-centred planning in Canada by Jack Pearpoint, Judith Snow and Marsha Forest. Whilst it was developed because of the social and service response to disability, it was quickly recognised to be as useful for many other individuals and groups of people.

Disabled people in the UK and USA developed the social model of disability, arguing for a shift in the balance of power between people and the services on which they rely. Person centred planning is based in the social model of disability because it places the emphasis on transforming the options available to the person, rather than on 'fixing' or changing the person. Specifically person-centred planning was based diversely on principles of community integration/inclusion/ normalisation/social role valorization. Prior to its inception, these principles were crystallised by John O'Brien and Connie Lyle O'Brien in the 'Framework for Accomplishment' which listed five key areas important in shaping people's quality of life, and asserting that services should be judged by the extent to which they enable people to:

• Share ordinary places
• Make choices
• Develop abilities
• Be treated with respect and have a valued social role
• Grow in relationships

The title 'person-centred' is used because those who developed it and used it initially shared a belief that services tend to work in a 'service-centred' way. This 'service-centred' behaviour appears in many forms, but an example is that a person who is isolated would be offered different groups to attend (each run by a service specifically for people sharing a specific label), rather than being helped to make friends in ordinary society.

The person-centered concept grew out of the critique of the "facility-based services" approach in the US (and worldwide) that was central to the development of "support approaches" in the US. The nationwide technical assistance funded by the National Institute on Disability Research and Rehabilitation (NIDRR), which included the person-centered approaches, is reported in the "Journal of Vocational Rehabilitation".

A central idea behind person-centred planning, is that services which are set up to respond to problems of social exclusion, disempowerment, and devaluation, can unintentionally make the situation of individual people worse (i.e. further disempower, devalue and exclude people). Person-centred planning is designed specifically to 'empower' people, to directly support their social inclusion, and to directly challenge their devaluation. One of the benefits of person-centered planning is that it can address the perennial "service problems" of ethnicity, gender, culture and age by starting with planning by or with the "whole person".

Person-centred planning is not one clearly defined process, but a range of processes sharing a general philosophical background, and aiming at similar outcomes. As it has become more well known further processes and procedures have also been given the title 'person-centred planning'. Some of these have little in common with person-centred planning as originally envisaged. Person-centered planning through the Rehabilitation Research and Training Center on Community Integration in the US was, in part, an agency and systems change process as opposed to only an "individual planning" process moving to an "individual budgeting process".

Person-centred planning involves the individual receiving the service, with family members, neighbors, employers, community members, and friends, and professionals (such as physician/ doctors, psychiatrists, nurses, support workers, care managers, therapists, and social workers) developing a plan on community participation and quality of life with the individual. In contrast, traditional models of planning have focussed on the person's deficits and negative behaviours, labelling the person and creating a disempowering mindset from the start.

Person-centred planning offers an alternative to traditional models, striving to place the individual at the centre of decision-making, treating family members as partners. The process focusses on discovering the person's gifts, skills and capacities, and on listening for what is really important to the person. It is based on the values of human rights, interdependence, choice and social inclusion, and can be designed to enable people to direct their own services and supports, in a personalised way.

Methods

Person-centered planning utilises a number of techniques, with the central premise that any methods used must be reflective of the individual's personal communication mechanisms and assist them to outline their needs, wishes and goals. There is no differentiation between the process used and the output and outcomes of the PCP; instead, it pursues social inclusion through means such as community participation, employment and recreation.

Beth Mount characterised the key similarities or 'family resemblances' of the different person centred methods and approaches into four themes:

• seeing people first, rather than diagnostic labels
• using ordinary language and images, rather than professional jargon
• actively searching for a person's gifts and capacities in the context of community life
• strengthening the voice of the person, and those who know the person best in accounting for their history, evaluating their present conditions in terms of valued experiences and defining desirable changes in their life^[16]

Person centred thinking skills, total communication techniques, graphic facilitation of meetings and problem solving skills are some methods commonly used in the development of a person centred plan, as are PATH (Planning Alternative Tomorrows With Hope), circles of support (Canada), MAPS (Canada), personal futures planning (O'Brien & Mount, US), Essential Lifestyle Planning (Maryland, US), person centred reviews, Getting to Know You (Wisconsin, US), and most recently the use of Person centred thinking tools to build from one page profiles into person centred descriptions/collections of person centred Information and on into full scale plans.

The resultant plan may be in any format that is accessible to the individual, such as a document, a drawing or an oral plan recorded onto a tape or compact disc. Multimedia techniques are becoming more popular for this type of planning as development costs decrease and the technology used becomes more readily available. Plans are updated as and when the individual wishes to make changes, or when a goal or aspiration is achieved. If part of a regular planning process in the US, regular plan updates are usually required by regulatory agencies (e.g., state offices in the US through local agencies).

Person-centred planning can have many effects that go beyond the making of plans. It can create a space during which someone who is not usually listened to has central stage. It can insist that discussion is centred on what the person is telling us is important to them, with their words and behaviours, as well as what others feel is important for the person. It can engage participants personally by allowing them to hear of deeply felt hopes and fears. It can assist people in a circle of support to re-frame their views of the person it is focused on. It can help a group to solve difficult problems. In the US, person-centered planning can help to create new lifestyles, new homes and jobs, diverse kinds of support (informal and formal) and new social relationships.

Limitations

Many of the limitations discussed below reflect challenges and limitations in the implementation of person-centered planning approaches in the context of formal human service systems.

Another approach to this question is to envision person-centered planning as an approach that is anchored in the person's natural community and personal relationship network. In this view, the Person-Centered Plan (PCP) offers a platform for the person and their trusted allies to identify and express their vision and commitments without limiting that expression to what can or will be provided by the service system.

Some time later, the formal system can develop a plan for service delivery that may be based on and consistent with the person's plan, that recognizes and supports the contributions of the person, family and community, and that clearly acknowledges the limitations of what the system is prepared to provide.

John O'Brien sums up the problem of trying to deliver person centredness through formal service systems that have a very different culture thus:

Many human service settings are zones of compliance in which relationships are subordinated to and constrained by complex and detailed rules. In those environments, unless staff commit themselves to be people's allies and treat the rules and boundaries and structures as constraints to be creatively engaged as opposed to simply conforming, person centred work will be limited to improving the conditions of people's confinement in services.

He calls for leadership to challenge these boundaries:

Most service organisations have the social function of putting people to sleep, keeping them from seeing the social reality that faces people with disabilities ... People go to sleep when the slogan that "we are doing the best that is possible for 'them'" distracts from noticing and taking responsibility for the uncountable losses imposed by service activities that keep people idle, disconnected and alienated from their own purposes in life. One way to understand leadership is to see it as waking up to people's capacities and the organisational and systemic practices that devalue and demean those capacities.

A key obstacle to people achieving better lives has been the risk averse culture that has been prevalent in human services for a variety of reasons. Advocates of person centred thinking argue that applying person centred thinking tools to the risk decision-making process, and finding strategies that are based on who the person is, can enable a more positive approach to risk that doesn't use risk as an excuse to trap people in boring and unproductive lives.

The key advocates of PCP and associated person centered approaches warn of the danger of adopting the model in a bureaucratic way – adopting the 'form' of PCP, without the philosophical content. By changing it to fit existing practices rather than using it in its original form, most or all of its effects are lost. The hope of funding it in the USA was to influence the processes, such as planning through the Medicaid home and community-based waiver services for people moving from institutions to the community.

The philosophical content expects services to be responsive to the needs of people that use the service, rather than prescriptive in the types of services offered. These principles are reliant on mechanisms such as individualised funding packages and the organisational capacity to design and deliver "support" services. It is essential that organisations and agencies providing services make a commitment to strive for person-centredness in all of their activities, which can result in major changes in areas of practice such as recruitment, staff training, and business planning and management.

While secondary users may debate the use of person-centered approaches to achieve the myriad goals it attempts to achieve, i.e., increased inclusion (Schwartz, Jacobson and Holburn, 2000) and "defining person-centeredness", others point to recent research such as "The Impact of Person Centred Planning", which suggests that person centred planning can make a considerable difference to people's quality of life and explores the optimum conditions for person centred approaches. 'Valuing People Now' says

Person centred planning has been shown to work. The world's largest study into person centred planning described how it helps people get improvements in important parts of their lives and indicated that this was at no additional cost.

However it continues:

too few people have access to proper person centred planning... In too many local authorities, person centred planning is not at the centre of how things are done. The challenge of the next three years is to take all this innovative work and make sure that more – and eventually all – people have real choice and control over their lives and services.

Person-centered planning in the USA has continued to be investigated at the secondary research level and validated for more general use (e.g., Claes et al. 2010).

Local authorities in Britain are now being challenged by government to change their model to one that is founded on person centred approaches: "This move is from the model of care, where an individual receives the care determined by a professional, to one that has person centred planning at its heart, with the individual firmly at the centre in identifying what is personally important to deliver his or her outcomes." The government recognises that this will require a fundamental change in the way services are organised and think: "Personalisation is about whole system change."

In New York State (USA), the Office for People with Developmental Disabilities (OPWDD), has mandated the use of person-centered planning in all new service development for people with intellectual disabilities. Person-centered planning is central to the new approaches to person-directed supports with are based on stronger self-determination than traditional person-centered approaches.

Outcomes

Person centred thinking and planning is founded on the premise that genuine listening contains an implied promise to take action. Unless what is learned about how the person wishes to live, and where they wish to go in their lives is recorded and acted upon, any planning will have been a waste of time, and more importantly a betrayal of the person and the trust they have placed in those who have planned with them.

In the UK initiatives such as individual budgets and self-directed supports using models like In Control mean that person centred planning can now be used to directly influence a person's Support Planning, giving them direct control over who delivers their support, and how it is delivered.

Detonation

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Detonation

Detonation of TNT, and shock wave

Detonation (from Latin detonare 'to thunder down/forth') is a type of combustion involving a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it. Detonations propagate supersonically through shock waves with speeds in the range of 1 km/sec and differ from deflagrations which have subsonic flame speeds in the range of 1 m/sec.

Detonations occur in both conventional solid and liquid explosives, as well as in reactive gases. The velocity of detonation in solid and liquid explosives is much higher than that in gaseous ones, which allows the wave system to be observed with greater detail (higher resolution).

A very wide variety of fuels may occur as gases (e.g. hydrogen), droplet fogs, or dust suspensions. In addition to dioxygen, oxidants can include halogen compounds, ozone, hydrogen peroxide, and oxides of nitrogen. Gaseous detonations are often associated with a mixture of fuel and oxidant in a composition somewhat below conventional flammability ratios. They happen most often in confined systems, but they sometimes occur in large vapor clouds. Other materials, such as acetylene, ozone, and hydrogen peroxide are detonable in the absence of an oxidant (or reductant). In these cases the energy released results from the rearrangement of the molecular constituents of the material.

Detonation was discovered in 1881 by four French scientists Marcellin Berthelot and Paul Marie Eugène Vieille and Ernest-François Mallard and Henry Louis Le Chatelier. The mathematical predictions of propagation were carried out first by David Chapman in 1899 and by Émile Jouguet in 1905, 1906 and 1917. The next advance in understanding detonation was made by John von Neumann and Werner Döring in the early 1940s and Yakov B. Zel'dovich and Aleksandr Solomonovich Kompaneets in the 1960s.

Theories

The simplest theory to predict the behaviour of detonations in gases is known as Chapman-Jouguet (CJ) theory, developed around the turn of the 20th century. This theory, described by a relatively simple set of algebraic equations, models the detonation as a propagating shock wave accompanied by exothermic heat release. Such a theory describes the chemistry and diffusive transport processes as occurring abruptly as the shock passes.

A more complex theory was advanced during World War II independently by Zel'dovich, von Neumann, and Döring. This theory, now known as ZND theory, admits finite-rate chemical reactions and thus describes a detonation as an infinitesimally thin shock wave, followed by a zone of exothermic chemical reaction. With a reference frame of a stationary shock, the following flow is subsonic, so that an acoustic reaction zone follows immediately behind the lead front, the Chapman-Jouguet condition.

There is also some evidence that the reaction zone is semi-metallic in some explosives.

Both theories describe one-dimensional and steady wavefronts. However, in the 1960s, experiments revealed that gas-phase detonations were most often characterized by unsteady, three-dimensional structures, which can only, in an averaged sense, be predicted by one-dimensional steady theories. Indeed, such waves are quenched as their structure is destroyed. The Wood-Kirkwood detonation theory can correct some of these limitations.

Experimental studies have revealed some of the conditions needed for the propagation of such fronts. In confinement, the range of composition of mixes of fuel and oxidant and self-decomposing substances with inerts are slightly below the flammability limits and, for spherically expanding fronts, well below them. The influence of increasing the concentration of diluent on expanding individual detonation cells has been elegantly demonstrated. Similarly, their size grows as the initial pressure falls. Since cell widths must be matched with minimum dimension of containment, any wave overdriven by the initiator will be quenched.

Mathematical modeling has steadily advanced to predicting the complex flow fields behind shocks inducing reactions. To date, none has adequately described how the structure is formed and sustained behind unconfined waves.

Applications

When used in explosive devices, the main cause of damage from a detonation is the supersonic blast front (a powerful shock wave) in the surrounding area. This is a significant distinction from deflagrations where the exothermic wave is subsonic and maximum pressures for non-metal specks of dust are approximately 7 - 10 times atmospheric pressure. Therefore, detonation is a feature for destructive purposes while deflagration is favored for the acceleration of firearms' projectiles. However, detonation waves may also be used for less destructive purposes, including deposition of coatings to a surface or cleaning of equipment (e.g. slag removal) and even explosively welding together metals that would otherwise fail to fuse. Pulse detonation engines use the detonation wave for aerospace propulsion. The first flight of an aircraft powered by a pulse detonation engine took place at the Mojave Air & Space Port on January 31, 2008.

In engines and firearms

Unintentional detonation when deflagration is desired is a problem in some devices. In Otto cycle, or gasoline engines it is called engine knocking or pinging, and it causes a loss of power, excessive heating, and harsh mechanical shock that can result in eventual engine failure. In firearms, it may cause catastrophic and potentially lethal failure.

Pulse detonation engines are a form of pulsed jet engine that has been experimented with on several occasions as this offers the potential for good fuel efficiency.