Everything about Old Growth totally explained
Old growth forest, also called
primary forest,
ancient forest,
virgin forest,
primeval forest,
frontier forest or (in the
UK)
Ancient Woodland, is an area of
forest that has attained great age and so exhibits unique
biological features. Old growth forests typically contain large live
trees, large dead trees (sometimes called "
snags"), and large
logs. Old growth forests usually have multiple vertical layers of
vegetation representing a variety of tree
species and age classes.
Forest regenerated after severe disruptions, such as
clear-cut or
fire is often called
second-growth or
regeneration until a long 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.
Old growth forests may be home to rare species which depend on this now-rare habitat, making them
ecologically significant.
Biodiversity may be higher or lower in old growth forests than in second-growth forests depending on specific circumstances. Logging in old growth forests is a contentious issue in many parts of the world.
Definitions
Concepts of “old growth” vary widely and are not always easily applied or reconcilable with one another.
“
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 had considerable artificial interference, the important characteristic being continuity of woodland on the land.
By contrast, in the US, “old growth” is often used to imply a forest has experienced little direct disruption during contemporary historical epochs and looks about as it would had Europeans not come to America. This criterion is difficult to apply, since it's often impossible to determine the history of human management (Euro-American or Native American). And, since landscapes are naturally dynamic, there can be no certainty what forests would look like now had pre-Columbian regimes been uninterrupted. While it's generally agreed that old forests defined as “old growth” have not been subject to
logging, the role of natural disturbances in defining old growth is more ambiguous. Some definitions, for example, exclude recently burned forests, even where
fire has been part of the natural dynamics for millennia; in other cases, such natural disturbance is incorporated in the old growth concept. However, it's often difficult to distinguish the ecological effects of natural disruption from human-caused disruption. Finally, even forests that have never experienced direct manipulation by Euro-Americans have been subjected to indirect effects in the form of
invasive species, climate change, and regional modifications of ecological
disturbance regimes (for example, 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, down wood 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's 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 in a darker 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 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. 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, several species of
woodpecker must have standing snags available for feeding. The
spotted owl is well-known for needing standing snags for nesting habitat.
Decaying ground layer
Down wood 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
Pacific coast, down wood is significant for providing a seedling substrate, and is called
nurse logs.
Soil
Intact soils harbor many life-forms that rely on them and usually have very well-defined
soil profiles. Different organisms need certain well-defined 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 pseudo-stability. These varied and sometimes species may depend on the unique environmental conditions created by these forests.
Old growth forest serves as a reservoir for species which can't thrive or easily regenerate in younger forest, and so can be used as a baseline for research.
Old growth forests also store large amounts of
carbon above and below the ground. They collectively represent a significant pool of climate gases such as
greenhouse gases. Continued liquidation of these forests may increase the risk of global
climate change.
Logging
The large trees in old growth forests are economically valuable, so these forests have been subjected to aggressive
logging around the world. This has led to much controversy between logging companies and environmental groups. A great example of this was the controversy around
Spotted Owls in the 1980s and 1990s.
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.
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:
