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An old-growth forest (also termed primary forest, virgin forest, primeval forest, late seral forest, or in Britain, ancient woodland) is a forest that has attained great age without significant disturbance, and thereby exhibits unique ecological features and in some cases may be classified as a climax community. Old-growth features include diversity of tree-related structures that serve as diversified wildlife habitat that leads to higher bio-diversity of the forested ecosystem. Diversified tree structure includes multi-layered canopies and canopy gaps, high variance of tree heights and diameters, diversity of decaying classes and sizes of woody debris, and diversity of tree species.
Old-growth forests tend to have more large trees and standing dead trees, multi-layered canopies with gaps resulting from the deaths of individual trees, and coarse woody debris on the forest floor.
Forest regenerated after a severe disturbance, such as wildfire, insect infestations 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 anywhere from a century to several millennia. Hardwood forests of the eastern United States can develop old-growth characteristics in one or two generations of trees, or 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, large eucalypt trees have been radio carbon dated at around 600 years old but the understory species can be much older at around 1000 years old.
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, catastrophic disturbances like wildfires minimize opportunities for major accumulations of dead and downed woody material and other structural legacies associated with old growth conditions.
Many botanists specifically define old growth in terms of meeting several criteria, under which system forests with sufficient age and minimal disturbance are considered old growth. Typical characteristics of old-growth forest include 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), multi-layered canopies, intact soils, a healthy fungal ecosystem, and presence of indicator species.
Old-growth forests are often biologically diverse, and home to 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.
A forest in old-growth stage has 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 one of them has different spatial location relative to the main canopy and 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-period to result in a new cycle of forest succession. Thus, uniformly aged stands are a less stable ecosystem.
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 forest, or at least a biodiversity that is different from other forest stages.
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 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.
Fallen timber, or coarse woody debris, contributes carbon-rich organic matter directly to the soil, thus providing a substrate for mosses, fungi, and seedlings, and in 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.
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 in order to live, while many trees need well-structured soils free of disturbance in order 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.
Stand age can also be used to categorize forest as old-growth. For each geographical area, there is an average time since disturbance when the forest will reach old-growth stage. This method is useful, because it allows quick and objective determination of forest stage. However, this definition does not provide explanation about forest function. It just gives a useful number to measure. Due to that fact, 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 logging of 30% of the trees, we can wait less time for old-growth to come back than after removal of 80% of the trees.
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 relatively open canopy. That allows more shade-tolerant tree species to establish below even before Understory Reinitiation stage. The shade-tolerant trees will eventually out-compete 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.
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. If old growth stage is seen as an end point of stand development, it can be easily evaluated using structural or static 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 forest is more useful in cases where the species that constitute old-growth forest can have long life span or in ecosystem where succession is very slow.
Common cultural definitions and common denominators regarding what comprises old-growth forest, and of the variables that define, constitute and embody old-growth forests include:
The debate over old growth definitions has been inextricably linked with a complex range of social perceptions about wilderness preservation, aesthetics and spirituality, as well as economic or industrial values.
Old-growth forests were often given harvesting priority because they have the most commercially valuable timber, they are considered to be at greater risk of deterioration through root rot or insect infestation, and they occupy 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.
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:
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 breathable air, 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.
The effects of old-growth forests in relation to Global Warming has been contested in various studies and journals.
While some studies that old-growth forests are an important part of carbon sequestration and its impacts on climate change and climate change mitigation. 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.”
Critics note that at old-growth forests are often perceived to be in equilibrium, but could be releasing as much carbon dioxide as they capture, or are currently in a state of decay. Another scientific studie concluded that forest harvesting has little or no effect on the amount of carbon stored in the soil. As trees grow, they remove carbon from the atmosphere. As they reach maturity, growth slows and ultimately stops as mortality catches up to growth. Harvesting also removes carbon from the forest but some of it is stored in wood products (preventing its immediate release to the atmosphere) and some 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, NOx (nitrogen oxides), VOCs (volatile organic compounds), particulates and other pollutants, in some cases at levels above those from traditional fuel sources such as coal or natural gas. In most North American forests, this drop happens when a tree is between 60 and 150 years old, depending on the species and environmental factors.
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.
The increased understanding of forest dynamics in the late 20th Century has 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. The literature around old growth and its management is inconclusive about the best way to capture 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 (DeLong 1998; Wong and Iverson 2004). 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 a Federal laws from logging, which hasn't occurred there for more than twenty years.
In British Columbia, Canada, old-growth forests must be maintained in each of the province’s ecological units to meet biodiversity needs.
The large trees in old growth forests are economically valuable, and have been subjected to aggressive logging around the world. This has led to much controversy between logging companies and environmental groups.
The island of Tasmania, just off the south east coast of Australia has the largest amount of temperate old-growth rainforest reserves in Australia with approximately 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 reserved. 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, approximately 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.
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