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| The long answer |
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There has been a great deal of controversy over the right definition for old-growth forests, but most definitions include the following (Dunwiddie and Leverett 1996):
A. AREA: (i.e. one really old tree doesn't count.) The 1996 old growth survey in Massachusetts set 3 ha as a minimum.
B. AGE: Some trees that are old for their species. The Massachusetts survey used 50% of maximum age, meaning at least 175-225 years old, depending on the species.
C. UNDISTURBED: Minimal signs of human disturbances such as burning, logging or plowing
Different people allow different levels of disturbance. Some people might rule out a forest that has been selectively logged (i.e. had some trees removed but not clear cut) 150 years ago, while others would not.
Q: By the definition above, if a large area (for example, 200 ha) was cleared for pasture 400 years ago and has since grown up as a forest, would it qualify as old- growth?
A: No. Since it was once cleared for pasture, it would not qualify
as old-growth. Why? Because even though a 400 year old forest
is special, it may be different in some ways from forests untouched by
people.
While the example above would probably never happen in North America,
where native agriculture was small scale and shifting, it would disqualify
many old forests in Europe. In Europe, old-growth usually refers
to a forest that has been relatively undisturbed for the last 200-300 years,
regardless of its previous history (Peterken 1996).
| While most North American old-growth was probably never cleared, some of these stands may have been burned by Native Americans. This was a common practice to increase populations of game animals such as deer and to make traveling through the forest easier (Cronon 1983). |
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When people say old-growth, they often mean forest that has never been disturbed by people, regardless of the age of its trees. This is a larger category which is usually referred to as virgin forest. Virgin forest may not have any old trees if it has suffered a big natural disturbance recently. An example of a virgin forest is Harvard Forest's Pisgah Tract in Pisgah, NH, which was devastated by the Hurricane of 1938. Click here to see a picture of it after the hurricane.
Recently, people have suggested that definitions of old-growth should
vary from region to region (Hunter and White 1997). Some say that
we should call the oldest forests in any region old-growth, even if they
don't meet the requirements above. Their reasoning is that forests
don't suddenly take on new characteristics after they pass some magic age.
Therefore, there is no reason to pick a particular age as a threshold
for old-growth-ness. Instead, we should recognize that the oldest
5% or so of forests in any region are special for that region and should
be protected as old growth.
| The drawback to this approach is that supporters of logging or other development may feel that old-growth forests are not valuable if there is no strict definition for them. |
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