- February 24, 2008 at 4:13 pm #39490Carl RussellModerator
I realized after responding to the thread on maple sugaring that I made reference to “stocking” and the C-line, which are main stream forestry terms that have to do with managing the density of a forested stand.
USDA Forest Experimental Stations in association with Universities, and funded by Industry have developed these guides for pretty much every commercially valuable species and cover-type in NA. They are based for the most part on even-aged management of trees, but for Un-even-aged management there are also guides to maximize tree growth.
They are presented as graphs that show Basal Area/ac ( BA is the cumulative measure of the cross-sectional area at 4.5 feet (Breast Height) of the trees on a site. Quantifying the area occupied by tree stems.) on the X axis, and Number of Stems/ac on the Y axis.
Through experimentation these graphs have been overlaid with lines that represent the point at which the relationship between the size of trees and the number of trees/ac.(density) affects tree growth. There is the A-line that represents a condition above which the stand is considered overstocked, where cumulative tree growth is reduced by density.
The B-line is a curve that represents a lower level of stocking, from which in ten years, at normal expected growth rate, the stand will regain its overstocked condition. The area between the A & B lines is considered to be “adequate” stocking.
The aforementioned C-line is the lowest acceptable stocking, below which the stand will have a very slow rate of accumulated growth as it establishes seedling and saplings, which requires a lot of energy.
Although most foresters will insist that they don’t manage exclusively based on these tools, that they make judgment calls and modifications in the field, the truth of the matter is every educational institution promotes forest management and silviculture based on this foundation, and State Use-value Appraisal programs insist on the use of these standards.
There are many features of this system that have led to the ecological degradation of our forests. Right off the bat it creates the sense that the forest is just a two dimensional numerical calculation. Secondly there is the assumption that Even-aged or controlled Uneven-aged conditions are appropriate for forest growth.
My biggest pet peeve has always been the A-line. With this line comes the negative appraisal of the overstocked forest. Above this line the forest disappears from view.
Why is that line there anyway? Because the natural forest strives for the overstocked condition.
It is in the overstocked condition that vigorous trees are selected to survive against environmental pressures. It is in this condition that the ecological function of the forest is at its highest viability. A forest will establish an overstocked condition from the sapling stage, and even through a series of natural disturbances, and mortality, will continue to strive for the maximum density that can be attained on a site.
This is the truth about tree growth, and it is the truth about forest ecology. When we implement management strategies to maintain maximum timber growth on individual trees across an entire stand, we degrade the forest ecosystem, and we disregard the absolute function of trees in that process.
This is why systems like “worst first” practiced by HHFF, and the crop tree release and forest improvement that I champion are considered on the fringe of acceptable management. However, these practices, in combination with low impact, sustainable power such as animals, are in fact absolutely in line with the long term ecological health of our forests.
By selecting individual trees to receive maximum growth, and improving the growing stock with regular low-moderate volume harvests to maintain high over-all stocking, the cost-effectiveness of harvest increases, the long term financial return increases, AND the ecological integrity is restored.
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