Science, Tech, Math › Animals & Nature Introduction to Old Growth and Virgin Forests Share Flipboard Email Print USDA Forest Service / OSU Animals & Nature Forestry Tree Identification Basics Arboriculture Tree Structure & Physiology The Science Of Growing Trees Conifer Species Individual Hardwood Species Pests, Diseases, and Wildfires Tree Planting and Reforestation Amphibians Birds Habitat Profiles Mammals Reptiles Wildlife Conservation Insects Marine Life Dinosaurs Evolution View More By Steve Nix Forestry Expert B.S., Forest Resource Management, University of Georgia Steve Nix is a natural resources consultant and a former forest resources analyst for the state of Alabama. He is a member of the Society of American Foresters. our editorial process Steve Nix Updated October 24, 2019 An old-growth forest, late serial forest, primary forest or ancient forest is a woods of great age that exhibits unique biological features. Depending on tree species and forest type, the age can be from 150 to 500 years. Old-growth forests typically contain a mixture of large live and dead trees or "snags". Unharvested fallen tree logs in various states of decay litter the forest floor. Some environmentalists blame the dramatic loss of U.S. old-growth forests to exploitation and disruption by Euro-Americans. It is true that old-growth stands need a century or more to grow. How Will You Know You are in an Old Growth Forest? Foresters and botanists use certain criteria to determine old growth. Sufficient age and minimal disturbance are necessary to be classified as old-growth. Characteristics of the old-growth forest will include the presence of older trees, minimal signs of human disturbance, mixed-age stands, canopy openings due to tree falls, pit-and-mound topography, downed and decaying wood, standing snags, multi-layered canopies, intact soils, a healthy fungal ecosystem, and presence of indicator species. What is a Second Growth Forest? Forests regenerated after harvests or severe disruptions like fire, storms or insects are often referred to as a second-growth forest or regeneration until a long enough period has passed that the effects of the disturbance are no longer evident. Depending on the forest, to become an old-growth forest again may take anywhere from one to several centuries. Hardwood forests of the eastern United States can develop old-growth characteristics with several generations of trees existing in the same forest ecosystem, or 150-500 years. Why are Old Growth Forests Important? Old growth forests are often rich, biodiverse communities harboring wide varieties of plants and animals. These species must live under stable conditions free from severe disturbance. Some of these arboreal creatures are rare. The age of the oldest trees in an ancient forest indicates that destructive events over a long period were of moderate-intensity and did not kill all the vegetation. Some suggest that old-growth forests are carbon "sinks" that lock up carbon and help prevent global warming.