Minimum Size of Old Growth Forest   Edward Frank
  Nov 16, 2005 19:55 PST 

Scott Wade recently posed a question of whether smaller areas of 10 acres or less should be considered or could be certified as old growth. He cited several smaller patches that contained the typical characteristics of an old growth forest. Actually he asked: "Does this group have anyone that could certify old growth forests? Are they interested in smaller area sites? We have three spots that I think may have made it through, but they are just a few acres. The largest being 11 acres or so."

What do you as a group of individuals consider to be the smallest size for a patch of ground to be considered old growth/ We all know that the eastern forests are chopped up into fragments, some of which have escaped being timbered, or are primary forests, other small patches have been timbered but sufficiently long ago that old growth characteristics are again becoming apparent. In an older article, Bob Leverett listed some of the characteristics of an old-growth forest: "the physical characteristics of a candidate old-growth site, including species composition and tree ages, can take us far in judging the degree of "naturalness" of a forest. A multi-aged structure is the byproduct of many random disturbance events. The resulting site contains a broad spectrum of tree ages, abundant coarse woody debris, some standing snags, pit and mound micro-topography, single and multiple tree fall gaps, a well developed herb layer, undisturbed soils, and the absence of alien species and signs of human use. However, in assessing the individual characteristics, there are no absolutes. Some ecologists consider a forest to be old-growth even if it loses most of its old trees to a wind event." These characteristics can be found in patches of forest regardless of their size.

The concern I would like to hear comments about is the effects of small size. How are small sized patches different from larger areas of old growth. There are considerations of the edge effects. The ecology at the transition between two different ecological regimes, such as old field to forest, represents unique biome. The species of plants, animals, microclimate effects extend past the immediate area of the boundary some distance into the forest itself. Other external factors also can effect areas well within the forest itself. There are consideration of fragmentation. Populations of animals and plants can not easily travel across intervening areas between isolated patches of forest, if they can make the transition at all, whereas they could essentially freely migrate within a contiguous patch of forest from one end to the other. If old-growth forest is considered to be an ecological niche, is there some limit to how small a patch can be before these edge and fragmentation effects transform the old-growth into another type of forest ecosystem? Do these marginal changes make any difference in considering what should be classified as old growth given the limited amount of old-growth remaining within the east?

Ed Frank
RE: Minimum Size of Old Growth Forest   Robert Leverett
  Nov 17, 2005 05:32 PST 


   I would defer to Lee Frelich on this one because of his years of
research into neighborhood effects that influence what tree species
persist in an area. The topic of minimums is interesting and worth
discussing, but there are no absolutes. I've seen small patches as small
as an acre of hemlocks that possessed all the visible old growth
characteristics of much larger areas of hemlock-dominated old growth.

   Minimum sizes for identifying old growth can be imposed for
administrative purposes, but not ecological ones. But, I'll say no more
until hearing from Lee and others. Lee?

RE: Minimum Size of Old Growth Forest
  Nov 17, 2005 10:19 PST 

I have been thinking about this a while now. I would definitely distinguish between Primordial forest and old growth. It is similar to the relation between Rhododendrons and Azaleas. "All Azaleas are Rhododendrons, but not all Rhododendrons are azaleas" Primordial forest would be old growth, but not the other way necessarily. I usually look at the ephemeral plants to judge if an area was farmed or disturbed. They are the most difficult to re-introduce. Trout lily, spring beauty, blood root, and others don't travel quickly, and once removed may never return. Their existence in a forest tells me the forest has been there a long time, undisturbed. A place like Cook, with 400+ year old trees is Primordial. The trees near me are probably 200-280? They could have very well been cut early on, but never farmed due to the rock outcroppings or incline. These trees are old growth, just not primordial. Before Bob's post, I was thinking that four square acres might be a good size for the smallest old growth.

Re: Minimum Size of Old Growth Forest   Edward Frank
  Nov 17, 2005 17:29 PST 
Scott, ENTS,

I would think the nature of the surrounding area would dramatically affect how intact ecologically a small patch of old growth would be. If surrounded by forest even relatively young secondary forest would allow a small patch of old growth to retain more of its ecological characteristics than would a small patch surrounded by open fields or industrial and housing developments.

Re: Minimum Size of Old Growth Forest
  Nov 18, 2005 06:49 PST 

Hello ENTS...

I'll go out on a limb here (ha ha) - TWO TREES. What? Has Tom been eatin' them
kidney stone pain pills again? No, seriously. Ecology is all about scale, but
there is no a priori reason that WE need to put an arbitrary size limit on "old
growth", even as we do legitimately grapple with its defining characteristics.
Obviously, a tiny fragmented remnant cannot possess the same ecological function
as a contiguous tract of thousands of hectares. However, highly prized
ecological and aesthetic virtues of old growth can and do persist even at a very
small scale. Certainly a small grove provides a human-valued aesthetic,
especially if it is located in a readily accessible site such as a park or
housing development. This is of no small concern, considering that conservation
decisions are often based more on human impressions than on ecological
principles. Even the smallest old growth remnant may include specific habitats
such as cavity trees and large downed logs, providing a last vestige for owls,
pileated woodpeckers, martens, etc., that would otherwise be gone from the area.
Also, old growth remnants in developed regions are often associated with steep
ravines, providing streams with abundant debris dams and heavy canopy shading.
Lastly, an old growth grove of even just a few trees may retain critical species
and genetic diversity, and can serve as a biological refugium within its
disturbed surroundings.

Yep... TWO TREES. That's my vote. As long as we don't fool ourselves into
thinking we can reintroduce lynx into a half acre old growth stand in a county

Re: Minimum Size of Old Growth Forest
  Nov 18, 2005 11:15 PST 

Good points, there. I always like to be in a spot where I can't see civilization. Roads, Houses etc. I really like it when I can't hear anything too, but Cook is the only place I have ever been where all I could hear was nature. It is getting harder to find these days. Thanks for your definition of old growth.

RE: Minimum Size of Old Growth Forest   Ernie Ostuno
  Nov 20, 2005 10:20 PST 

I can only offer the perspective of a "tramper" who has wandered through
a bunch of old growth areas of various size, disturbance history, and
health. Bear Run Natural Area in central Pennsylvania, which is just a
2.5 acre wedge of ancient hemlocks spared by a surveying error, has
almost all the old growth indicators of the nearby Snyder-Middleswarth
Natural Area, which may be about 50 times the size. There are nurse
logs, standing snags, pits and mounds, etc, but there may be only one or
two of each, compared to dozens at the larger site. The main difference
is, no matter where you stand in the smaller site, you can see the
"light at the edge of the forest".

Then there is the analogy to islands, which may be applicable. There are
big islands and little islands. The little ones are more susceptible to
"erosion", which may eventually reduce them to "sand bars".