Old growth forest, (also termed primary forest, ancient forest, virgin forest, primeval forest, frontier forest or in the UK, Ancient Woodland), is a type of forest that has attained great age and so exhibits unique biological features. Old growth forests typically contain large live trees, large dead trees (sometimes called "snags"), and large logs, as well as many other common characteristics representative of forests in general. Due to the great age and height of many trees within old growth forests, they are often more shaded than other types of forests. Old growth forests are unique, and usually have multiple horizontal layers of vegetation representing a variety of tree species and age-classes. Many old growth forest stands are threatened by habitat destruction due to excessive logging and clear-cut logging, activity, which reduces biodiversity, both in terms of decreasing the amount of old growth forest and habitat that remain on the planet, and in terms of the destruction, and therefore reduction in the level of remaining native, or indigenous species that rely upon and thrive within old growth forest habitat.
Forest regenerated after severe disruptions, such as clear-cut or fire is often called second-growth or regeneration until enough time passes that the effects of the disturbance are 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.
Old growth forests are often home to rare species, threatened species and endangered species of plants and animals, making them ecologically significant. One example of a rare species reliant upon old growth forest is the Northern Spotted Owl. Levels of biodiversity, the variation of life forms within unique old growth forest ecosystems, 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 (where the forest is located). Logging in old growth forests is a contentious issue in many parts of the world.
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:
"Ancient Woodland" is a term used in the United Kingdom to refer specifically to woodland dating back to 1600 or before (in England and Wales), or 1750 (in Scotland). Before this, planting of new woodland was uncommon, so a wood present at these dates was likely to have developed naturally. By this definition Ancient Woodland may have been affected by human management, and may have no very ancient trees: the important characteristic is long continuity of woodland on the land.
In the United States, the term "old growth" is often, (but not always), used to characterize a forest that has experienced little direct disruption or disturbance by humans during contemporary historical epochs, although sometimes determining the long-term history of human land management can be difficult. Additionally, because landscapes are naturally dynamic and continue to change as time progresses, it is difficult to ascertain hypothetical old growth forest characteristics that may have come into fruition had humans not destroyed such a great deal of old growth forests.
The role of natural disturbances in defining old growth is more ambiguous. For example some definitions exclude recently burned forests, even where fire has been part of the natural forest dynamics for millennia. In other cases such natural disturbance is incorporated in the old-growth concept. However, it is sometimes difficult to distinguish the ecological effects of natural disruption from human-caused disruption. Furthermore, many forests that have never experienced direct manipulation by humans have been subjected to indirect effects in the form of invasive species, removal of native species (including megafauna),climate change, and regional modifications of ecological disturbance regimes (e.g., fire suppression).
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, fallen timber in various stages of decay, standing snags (dead trees), multi-layered canopies, intact soils, a healthy fungal ecosystem, and presence of indicator species.
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 soon form a thick organic layer and so able to nurture certain types of organisms, while mounds provide a place free of leaf inundation and saturation, where other types of organisms thrive.
The large trees in old growth forests are often economically valuable, so these forests have been subjected to aggressive logging around the world. This has led to much controversy between logging companies and environmental groups. An example of this was that over Spotted Owls in the 1980s and 1990s.
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 2006 Greenpeace identified that the world's remaining intact old growth forest area was distributed among the continents as follows:
Old growth forests store large amounts of carbon, which is stored in wood, soil humus and peat. When forests are cut, the trees' wood, soil humus and peat all decay, releasing the carbon as carbon dioxide or methane. Logging practices often include burning of the logged area, releasing further CO2.
While old growth forests are often perceived to be in equilibrium — releasing as much carbon dioxide as they capture; or even in a state of decay, studies of soils in undisturbed tropical rain forests, Siberian woods and in German national parks have found that soils contain enormous amounts of carbon derived from fallen leaves, twigs and buried roots that can bind to soil particles and remain for 1,000 years or more. Replacing old growth forests with plantations is counter-productive from a carbon-storage view, as the new forest may take centuries to recapture the carbon lost. Further, the loss of biodiversity in a plantation monoculture lessens the performance of ecosystems regarding biomass production, nutrient retention and carbon dioxide absorption.