Primary forest encyclopedia topics

Definitions

The concept of old growth forest is somewhat subjective because there are varying global and cultural definitions regarding what comprises old growth forest, and of the variables that define, constitute and embody to serve as common denominators to provide a singular definition of this term.

It is generally agreed that old growth forests share these traits in common:

Possess relatively mature, old trees
Have long continuity on the same site
Are land areas that have not been subjected to logging and other significant activities by humans

"Ancient Woodland" is a term used in the United Kingdom to refer specifically to woodland dating back to 1600 or before (in England and Wales), or 1750 (in Scotland). Before this, planting of new woodland was uncommon, so a wood present at these dates was likely to have developed naturally. By this definition Ancient Woodland may have been affected by human management, and may have no very ancient trees: the important characteristic is long continuity of woodland on the land.

In the United States, 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 (including megafauna),climate change, and regional modifications of ecological disturbance regimes (e.g., fire suppression).

Characteristics

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.

Mixed age

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 is a less stable ecosystem, because it becomes senescent and degrades within a relatively short time to result in a new cycle of forest succession.

Canopy openings

Openings in the forest canopy 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 created by natural disturbance events such as wind, ice and mixed-severity fire retain much structural enrichment from dead trees, unlike openings created by logging.

Topography

The characteristic topography of much old growth forest consists of pits and mounds. Mounds are caused by decaying fallen trees, and pits by the roots pulled out of the ground when trees fall due to natural causes, or are pushed over by animals. Pits expose humus-poor, mineral-rich soil and often collect moisture and fallen leaves soon form a thick organic layer and so able to nurture certain types of organisms, while mounds provide a place free of leaf inundation and saturation, where other types of organisms thrive.

Standing snags

Standing snags provide food sources and habitat for many types of organisms. In particular, many species of dead-wood predators such as woodpeckers must have standing snags available for feeding. In North America the spotted owl is well-known for needing standing snags for nesting habitat.

Decaying ground layer

Fallen timber contributes carbon-rich organic matter directly to the soil, thus providing a substrate for mosses, fungi and for seedlings, and in creating microhabitats by creating relief on the forest floor. In some ecosystems, such as the temperate rain forest of the North American Pacific coast, fallen timber may become nurse logs, providing a substrate for seedling trees.

Soil

Intact soils harbor many life-forms and usually have well-defined soil profiles. Different organisms may need different soil profiles, while many trees need well-structured soils free of disturbance. Some herbaceous plants in northern hardwood forests need 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.

Importance

Old growth forests often contain rich communities of plants and animals that settle there thanks 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.

Logging

The large trees in old growth forests are often economically valuable, so these forests have been subjected to aggressive logging around the world. Due to extensive deforestation, this has led to much controversy between logging companies and environmental groups. An example of this was that over Spotted Owls in the 1980s and 1990s. It has been well-documented that corporate and government interests, specifically the timber industry working in unison with the United States federal government and state governments, have long relied upon the use of propaganda techniques to encourage humans to devalue the merits of old growth forests, in favor of exploiting these lands to realize short-term profits, at the direct expense of long-term biodiversity.

In Australia, the regional forest agreement (RFA) attempted to prevent the clearfelling of defined "Old Growth Forests". This led to struggles over what constitutes "Old Growth". For example in Western Australia, the timber industry tried to limit the area of Old Growth in the karri forests of the Southern Forests Region; this led to the creation of the Western Australian Forests Alliance, the splitting of the Liberal Government of Western Australia and the election of the Gallop Labor Government. Old Growth Forests in this region have now been placed inside National Parks. A small proportion of Old Growth Forest also exists in South-West Australia, and is protected by a Federal laws from logging, which hasn't occurred there for more than twenty years.

Locations of remaining intact forests

In 2006 Greenpeace identified that the world's remaining intact old growth forest area was distributed among the continents as follows:

35% in Latin America. The Amazon rainforest is mainly located in Brazil, which clears a larger area of forest annually than any other country in the world.
28% in North America. North America harvests 10,000 square kilometres of ancient forests every year. Many of the fragmented forests of southern Canada and the US lack adequate animal travel corridors and functioning ecosystems for large mammals.
19% in Northern Asia. Northern Asia is home to the second largest boreal forest in the world. The Siberian tiger once roamed across huge areas of Northern Asia but today can only be found in a small area of intact forest near the Sea of Japan. Only about 400 remain in the wild and 800 in zoos.
8% in Africa. Africa has lost most of its intact forest landscapes in the last 30 years. The timber industry is responsible for destroying huge areas of intact forest landscapes and continues to be the single largest threat to these areas.
7% in South Asia Pacific. The Paradise Forests of Asia Pacific are being destroyed faster than any other forest on Earth. Much of the large intact forest landscapes have already been cut down, 72% in Indonesia and 60% in Papua New Guinea.
Less than 3% in Europe. In Europe, more than 150 square kilometres of intact forest landscapes are cleared every year and the last areas of the region’s intact forest landscapes in European Russia are shrinking rapidly.

Effect on global warming

Old growth forests store large amounts of carbon, which is stored in wood, soil humus and peat. When forests are cut, the trees' wood, soil humus and peat all decay, releasing the carbon as carbon dioxide or methane. Logging practices often include burning of the logged area, releasing further CO₂.

While old growth forests are often perceived to be in equilibrium — releasing as much carbon dioxide as they capture; or even in a state of decay, studies of soils in undisturbed tropical rain forests, Siberian woods and in German national parks have found that soils contain enormous amounts of carbon derived from fallen leaves, twigs and buried roots that can bind to soil particles and remain for 1,000 years or more. Replacing old growth forests with plantations is counter-productive from a carbon-storage view, as the new forest may take centuries to recapture the carbon lost. Further, the loss of biodiversity in a plantation monoculture lessens the performance of ecosystems regarding biomass production, nutrient retention and carbon dioxide absorption.