Old Growth Forests

Old-growth forest (also termed primary forest, ancient forest, virgin forest, primeval forest, frontier forest or in Britain, ancient woodland) is a forest which contains trees which have attained great age (and associated structural features) and so exhibits unique ecological features.

Old-growth forest typically contains large and old live trees, large dead trees (sometimes called "snags"), and large logs. Individual tree mortality creates gaps in the main canopy layer, allowing light to penetrate the main canopy and create favorable photosynthetic conditions for the understory (which is why old-growth understory is more developed than in immature stands).

Forest that is regenerated after severe disruptions, such as clear-cutting 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.

Many old-growth forest stands are threatened by habitat destruction through excessive logging. The resulting destruction reduces biodiversity, affecting not only the old-growth forest itself, but also indigenous species that rely upon old-growth forest habitat.

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 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.

One of the few Old Growth Forests left in the United States - the Coast Redwoods in Muir Woods National Monument, Marin County, California.

Old-growth forest is a forest in a stage that follows the Understory Reinitiation stage.

A review of the stages helps to understand the concept:

1. Stand-replacing disturbance hits the forest and kills most of the living trees.
2. Stand-initiation: population of new trees becomes established.
3. Stem-exclusion: trees grow higher and enlarge their canopy, thus competing for the light with neighbors. Light competition mortality kills slowly growing trees and reduces forest density. This allows surviving trees to increase in size. Eventually the canopies of neighboring trees touch each other and drastically lowers amount of light that reaches lower layers. Due to that, the understory dies and only very shade-tolerant species survive.
4. Understory reinitiation: trees die from low level mortality, such as windthrow and diseases. Individual canopy gaps start to appear and more light can reach forest floor. Hence, shade-tolerant species can establish in the understory.
5. Old-growth: Main canopy trees become older and more of them die. That creates even more gaps. Since the gaps appear in different timing, the understory trees establish in different timing from one another. Furthermore, the amount of light that reaches each understory tree depends on its position relative to the gap. Thus, each understory tree grows at a different speed. The difference in establishment timing and in growth speed create a population of understory trees that are variable in size. Eventually, some understory trees grow to become as tall as the main canopy trees. Hence, the gap created by the old dead tree is closed by a younger one that eventually will also die and will be replaced by another tree. This perpetuation process is typical for the old-growth stage. This, however, does not mean that the forest will be old-growth forever. Generally there are three possible futures for old-growth stage forest:
   1. The forest will be hit by a new stand-replacing disturbance and most of the trees will die.
   2. The tree community will eventually create unfavorable conditions for new trees to regenerate. In this case, the old trees will die and smaller plants will create woodland.
   3. The regenerating understory trees are different species than the main canopy trees. In this case, the forest will switch back to Stem-Exclusion stage, but with different tree species. The forest in old-growth stage can be stable for centuries and even a thousand years, but it all depends on its tree composition and climate of the area. For example, frequent natural fires do not allow boreal forests to be as old as coastal forests of western North America.

It is important to note that while the stand switches from one tree community to another, it is not necessarily that the stand will go through old-growth stage in between. Some tree species have relatively open canopy. That allows more shade-tolerant tree species to establish below even before Understory Reinitiation stage. The shade-tolerant trees will eventually out-compete the main canopy trees in stem-exclusion stage. Therefore, the dominant tree species will change, but the forest will still be in Stem-Exclusion stage.

Stand age can also be used to categorize forest as old-growth. For each geographical area, there is an average time since disturbance when the forest will reach old-growth stage. This method is useful, because it allows quick and objective determination of forest stage. However, this definition does not provide explanation about forest function. It just gives a useful number to measure. Due to that fact, some forests may be excluded from being categorized as old-growth even if they have old-growth attributes just because they are too young. Also, older forests can lack some old-growth attributes and be categorized as old-growth just because they are so old. The idea of using age is also problematic, because human activities can influence the forest in varied ways. For example, after logging of 30% of the trees, we can wait less time for old-growth to come back than after removal of 80% of the trees.

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:

• The forest habitat possesses relatively mature, old trees;
• The old-growth trees have long continuity on the same site;
• The forest itself has not been subjected to significant inhabitation by mankind that has altered the appearance of the landscape and its ecosystems, has not been subjected to logging, and has inherently progressed per natural tendencies.

In North America, 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, 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.

A forest in old-growth stage has 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 one of them has different spatial location relative to the main canopy and hence each one receives a different amount of light. This regeneration pattern is different from the regeneration of trees after a major disturbance, when trees regenerate on the site in relatively similar time. In younger forests trees have similar ages, because they all started to grow at the same time, after the old forest stand was killed.

Forest canopy 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.[9] Because 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 forest, or at least a biodiversity that is different from other forest stages.

But What Is The Importance Of Old Growth Forests?

• Old-growth forests often contain rich communities of plants and animals within the habitat due to the long period of forest stability. These varied and sometimes rare species may depend on the unique environmental conditions created by these forests.
• Old-growth forest serves as a reservoir for species which cannot thrive or easily regenerate in younger forest, and so can be used as a baseline for research.
• Plant species that are native to old-growth forests may someday prove to be invaluable towards curing various human ailments, as has been realized in numerous plants in tropical rainforests.
• Old-growth forests also store large amounts of carbon above and below the ground (either as humus, or in wet soils as peat). They collectively represent a very significant store of carbon. Destruction of these forests releases this carbon as greenhouse gases, and may increase the risk of global climate change.

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