Old growth
Old growth describes trees or forest which 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. It can be used in the sense of an old growth tree, but more often is used to describe an undisturbed forest. 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.
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.
Old growth forest is important for several reasons, one being as a reservoir for species which cannot thrive or easily regenerate in younger forest, another being as a baseline for research, and yet another simply by providing visitors with a sense of the forest primaeval as a portrayal of the truly natural world.
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.
Down wood is important in directly contributing 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 are important in the ecosystem by providing 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 critical for harboring 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.
