An old growth forest is a forest which has been allowed to grow with minimal interference for at least 100 years. The precise definition of old growth forest varies, depending on the region of the world and the type of trees under discussion. Several characteristics remain common to all old growth forests, regardless as to what definition is being used. The identification and protection of old growth forest is an issue for many environmental activists and scientific researchers.
One of the most important defining characteristics of old growth forest is the presence of multiple generations of trees and undergrowth, representing a wide variety of ages. The forest includes living trees, standing dead trees known as snags, fallen trees, and extensive layers of undergrowth, marked by occasional openings in the forest canopy caused by fallen trees. Typically, the topography in an old growth forest is very irregular, with huge holes where the roots of trees have been ripped out, along with mounds of soil. A thick layer of decaying organic material covers the forest floor, creating a layer of dense, rich earth which supports a wide variety of plants and animals.
Since an old growth forest has been subjected to less intervention, it is often more biodiverse than other forests. It usually contains a wide assortment of plants and animals, and if a large area of old growth forest is left intact, it can support very large animals which require space to roam. Since some trees take several centuries to mature, while others grow quickly, a forest can feel like an old growth forest after as few as 150 years, or it may be thousands of years old.
Many visitors to old growth forests say that the forest feels very dense and peaceful. Some of the alternative names for old growth forests reflect its uniqueness; in varying parts of the world, old growth forest may be called primary, ancient, virgin, or primeval. Some people also call old growth “first growth,” to distinguish it from “second growth,” a forest which has burned or been logged once within recent human memory.
Some people mistakenly believe that in order to be considered old growth forest, a stretch of trees must never have been impacted by humans. This is not the case; many generations of early humans, for example, shaped large stretches of the world's forests, even those which are thought of as virginal. In addition, humans have contributed to a rise in greenhouse gases and a general shrinkage of animal and plant habitats which has impacted old growth forests. We are constantly impacting the forests around us, even if we do not do so directly through logging.
Many people consider old growth forest to be ecologically valuable because it can shelter species of plants and animals which will not thrive in second growth or on more frequently logged land. Old growth forests also contribute to carbon sequestration, helping to control the world's climate. They may also contain previously unidentified plant and animal species, some of which could provide new sources of food, medicine, or general scientific interest. Because the value of old growth forest is difficult to fully calculate, some nations have taken steps to protect their old growth forest. However, protection is only truly effective for large forests, since isolated stands of trees are too heavily impacted by neighboring human activity to behave as true old growth forests do.