Primary forest

An old-growth forest — also termed primary forest, virgin forest, primeval forest, late seral forest, or (in Great Britain) ancient woodland — 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. Old-growth features include diverse tree-related structures that provide diverse wildlife habitat that increases the biodiversity of the forested ecosystem. 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.

Old-growth forests are economically valuable, and logging of these forests has been a point of contention between the logging industry (which cuts down forests that have been growing for hundreds of years) and environmentalists (who seek to protect and preserve the forests).

Old-growth forests tend to have large trees and standing dead trees, multi-layered canopies with gaps that result 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, 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-West Forest Region, 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 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.

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