Old growth forest, sometimes called late seral forest or ancient forest is an area of forest that has attained great age and exhibits unique biological features. Old growth forests typically contain large live trees, large dead trees (sometimes called "snags"), and large logs. Old growth forests usually have multiple vertical layers of vegetation representing a variety of tree species and a variety of different age classes.
Old growth forests typically have not been subject to logging or other significant disturbance long enough to reach an ecological state that is free of the effects of disturbance. Forest regenerated after clear-cut or fire is referred to as second-growth or regeneration until a long enough period has passed that the effects of the disturbance are no longer significant. Depending on the forest, this may take anywhere from a century to several millennia. Most hardwood forests of the eastern United States can reach a state definable as old-growth after about a century and a half.
Because old growth forests have not been disturbed they exhibit great biodiversity and ecological importance. Logging in old growth forests is a contentious issue in many parts of the world.
Characteristics of old growth forest
Some workers argue that the term "old growth" should only refer to virgin forest, or forest that has never experienced human disturbance, but most botanists and ecologists today agree with the definition of lack of evidence of disturbance. Many workers completely avoid the term virgin forest today, since the history of human disturbance of forest lands is so universal around the globe that it's virtually impossible to describe any forest as truly virgin. Even in deepest tropical rainforest, there is usually a history of human modification of the forest ecosystem.
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 relative stability of most slopes and streamways.
The mixed age of the forest is an important criterion in ensuring that the forest is a stable ecosystem in the long term. A climax forest that is a uniformly-aged stand becomes senescent and degrades within a relatively short time-period to result in a new cycle of forest succession. Thus, it is not a stable ecosystem, but one very much in flux.
Canopy openings are essential in creating and maintaining the mixed-age stands. In addition, some herbaceous plants only become established in canopy openings although they are able to persist thereafter in the darker understory.
Pit-and-mound topography is the characteristic lay of the land after trees that have fallen due to natural causes create pits where roots have pulled out and mounds where the root mass decays (with the soil clinging to the roots). These places provide, in the pit, fresh exposure of humus-poor, mineral-rich soil, often a place where moisture may collect and in which fallen leaves soon form a thick organic layer and so able to nurture certain types of organisms, while the mound provides a place free from leaf inundation and saturation where other types of organisms may thrive.
Decaying ground layer
Down wood directly contributes carbon-rich organic material directly to the soil, in providing a substrate for mosses and fungi and for seedlings, and in creating microhabitats by creating relief on the forest floor. Down wood which is significant in some ecosystems, such as the temperate rain forest of the Pacific coast, for providing a seedling substrate is termed nurse logs.
Standing snags provide food sources and habitat for many types of organisms. Several species of woodpeckers, in particular, must have standing snags available for feeding. The spotted owl is well-known for needing standing snags for nesting habitat.
Intact soils are 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.
Importance of old growth forests
Due to the lack of disturbance, old growth is often associated with rich communities of plants and animals that may be dependent upon the unique environmental conditions created by these forests. The age of the oldest trees indicates that disturbance events during the long period of development were of moderate intensity at most and did not kill all vegetation. This long period of pseudostability allows the old growth forest to become occupied over time by a wide variety of species, some of them rare.
Old growth forest serves as a reservoir for species which cannot thrive or easily regenerate in younger forest, and as such can be used a baseline for research.
Old growth forests also store large amounts of carbon, both above and below ground. These forests collectively represent a significant pool of climate gases. Continued liquidation of these forests may increase the risk of global climate change.
Logging in old growth forests
Despite the ecological importance of old growth forests they are logged in many parts of the world. The large trees in old growth forests are economically valuable, so these forests have been subjected to aggressive logging almost everywhere they occur.
It is a major issue in Tasmania and thanks to Greenpeace in other countries (such as Papua New Guinea). It is also a major issue in South America, where logging companies provide the local people with a pittance to log the trees that have stood for (in some cases) thousands of years.