old-growth forest

old-growth forest, a climax forest in the late stages of stand development containing large, old trees and a complex stand structure that has been generally undisturbed by human activities. The definition of an old-growth forest varies from country to country, but most definitions share an understanding that old-growth forests provide important ecosystem services and have social, cultural, economic, and spiritual value that forests in earlier stages of development do not provide to the same extent. Old-growth forests, like other types of forests, are under threat from anthropogenic climate change and human activities, particularly logging and clear-cutting for agricultural purposes.

The Food and Agriculture Organization (FAO) of the United Nations defines old-growth forests, which it calls primary forests, as “naturally regenerated forest[s] of native tree species, where there are no clearly visible indications of human activities and the ecological processes are not significantly disturbed.” Forests cover approximately 31 percent, or 4.06 billion hectares (10 billion acres), of the total global land area. Of the total land area covered by forests, roughly one-third is old-growth forest. More than half of the world’s old-growth forests are found in Brazil, Canada, and Russia.

Characteristics

In the popular imagination, old-growth forests comprised very tall trees with trunks that have large diameters. In actuality, old-growth forests are made up of trees of differing sizes and shapes. The characteristics of different tree species—such as longevity and maximum height and width—that make up a given forest determine whether that particular forest is defined as old-growth.

Old-growth forests have characteristics that promote resiliency and biodiversity, having been relatively free from human disturbance for centuries. Old-growth forests have a complex stand structure with trees of different ages and sizes. The majority of the trees are mature and long-lived, creating multiple layers of canopy, and there are dead and dying trees in various stages of collapse. The forest floor hosts native understory plants and is covered with deadwood and woody debris. There are few invasive species. Old-growth forests are the sites of complex interactions between many different species, which find varied habitat there. For example, older trees can develop cavities in which animals can build nests. Fallen trees provide habitat for a number of organisms, including wide varieties of fungi, bacteria, plants, insects, and amphibians, and create openings in the canopy for saplings to receive sunlight. Streams, lakes, and other bodies of water in old-growth forests provide habitat for fish and other aquatic fauna.

Old-growth forests provide important ecosystem services, including filtering water and sequestering carbon. Like all green plants, the trees in old-growth forests take in carbon dioxide for use in photosynthesis and produce oxygen from the same process. However, given their maturity and significant biomass, the trees of old-growth forests have a greater capacity for carbon storage than many other plants. The forests can also mitigate the impacts of fires and floods. Humans find recreational activities, beauty, and a sense of spirituality in old-growth forests, which are inextricably intertwined with the history, culture, religions, and economy of the people who live near them.

Threats

Old-growth forests face multiple threats. Anthropogenic climate change has increased the number and severity of disturbances to old-growth forests, such as wildfires, droughts, and storms, including hurricanes and tornadoes. Climate change also worsens the potential impacts of threats such as insect pests, diseases, and invasive species.

Data collection varies from country to country and is thus incomplete, but the available data show that since 1990 the land area covered by old-growth forest has decreased by 81 million hectares (200 million acres). This deforestation and forest degradation are mainly due to logging and the clearing of land for agriculture, especially cattle ranching and the cultivation of soy and oil palm. Ecologists stress the need to protect old-growth forests through conservation, sustainable management, and forest restoration. To decrease the rate of conversion of forested land to agricultural land, degraded agricultural land can be enriched so that it is once again useful for growing crops, and existing arable land can be carefully maintained to preserve its soil productivity.