Old-growth forest
An old-growth forest is a forest that has developed over a long period of time without disturbance. Due to this, old-growth forests exhibit unique ecological features. The Food and Agriculture Organization of the United Nations defines primary forests as naturally regenerated forests of native tree species where there are no clearly visible indications of human activity and the ecological processes are not significantly disturbed. One-third of the world's forests are primary forests. Old-growth features include diverse tree-related structures that provide diverse wildlife habitats that increases the biodiversity of the forested ecosystem. Virgin or first-growth forests are old-growth forests that have never been logged. 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.File:CSIRO ScienceImage 541 Cool Temperate Rainforest Tasmania.jpg|thumb|upright|Cool temperate rainforest in Tasmania, Australia, the world has 11.1 million square kilometers of primary forest remaining. Brazil, Canada, and Russia have 61% of the world's primary forest. The area of primary forest has decreased by since 1990, but the rate of loss more than halved in 2010–2020 compared with the previous decade.
Old-growth forests are valuable for economic reasons and for the ecosystem services they provide. This can be a point of contention when some in the logging industry desire to harvest valuable timber from the forests, destroying the forests in the process, to generate short-term profits, while environmentalists seek to preserve the forests in their pristine state for benefits such as water purification, flood control, weather stability, maintenance of biodiversity, and nutrient cycling. Moreover, old-growth forests are more efficient at sequestering carbon than newly planted forests and fast-growing timber plantations, thus preserving these forests is vital to climate change mitigation.
Characteristics
Old-growth forests tend to have large trees and standing dead trees, multilayered canopies with gaps that result from the deaths of individual trees, and coarse woody debris on the forest floor. The trees of old-growth forests develop distinctive attributes not seen in younger trees, such as more complex structures and deeply fissured bark that can harbor rare lichens and mosses.A forest regenerated after a severe disturbance, such as wildfire, insect infestation, 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 from a century to several millennia. Hardwood forests of the eastern United States can develop old-growth characteristics in 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 forest of Canada, 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 the 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, multilayered canopies, intact soils, a healthy fungal ecosystem, and presence of indicator species.
Biodiversity
Old-growth forests are often biologically diverse, and home to many 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 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.Studies in British Columbia's cedar-hemlock forests have shown that certain lichen species, particularly cyanolichens, are almost exclusively found in old-growth forests, being absent from even-aged stands of the same age. This suggests that true old-growth characteristics, like diverse forest structure and microclimate conditions, are necessary for some specialist species beyond just forest age.
Inland temperate rainforests of British Columbia illustrate how old-growth structure can shape entire food webs. Deep winter snowpacks lift woodland caribou into the lower tree canopy, where their survival depends on heavy accumulations of fruticose "hair" lichens such as Bryoria and Alectoria. These lichens reach stand-level "hyperabundance" only in forests older than about 120–150 years, where long-lived canopy architecture and stable microclimates allow sustained growth. Industrial logging and short-rotation silviculture fail to reproduce these conditions, so the old-growth canopy itself functions as a keystone habitat, underpinning both specialist lichens and the endangered deep-snow mountain caribou that rely on them.
Mixed age
Some forests in the old-growth stage have 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 of them has a different spatial location relative to the main canopy, 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 to result in a new cycle of forest succession. Thus, uniformly aged stands are less stable ecosystems. Boreal forests are more uniformly aged, as they are normally subject to frequent stand-replacing wildfires.Canopy openings
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. As 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 forests, or at least a biodiversity that is different from other forest stages.