Even-Aged versus Uneven-Aged
As agriculture is the cultivation of fields (agra),
silviculture is the cultivation of forests (silva). Silviculture is the domain of foresters who are trained in the systems used to maximize the volume and value growth of forests. Silviculture
also has to do with sustained yield and keeping the forest productive
through multiple rotations (lifespans of trees) for wildlife habitats, clean water and recreational uses as well as forest products.
Within the life of a forest stand, different silvicultural
techniques may be applied. A very simple example would be the silviculture of a plantation where you plant the stand, then thin it periodically to give the better trees room to grow. Stands will
be thinned every 5 to 20 years, depending upon access, markets, and intensity of management. When the stand reaches maturity, you make plans to regenerate it in one or more cuts, and finally to harvest
In this example we have intermediate thinnings, regeneration cuts, and a final harvest. It's quite straighforward. But with stands of mixed species and ages, treatment prescriptions become more complicated. The purpose is generally to concentrate growth on the more valuable
for future value growth and/or seed production. Trees to be cut may be mature, of poor quality, or of species with low value. Or they may be good quality trees that are in direct competition with slightly better quality trees.
Most early silvicultural treatments pay for themselves. Later treatments are generally quite profitable. But sometimes precommercial thinnings may be called for in young stands where trees to
be cut have no market value. The same may apply to older stands that have been severely high-graded and need to have unmarketable culls removed.
Commercial thinnings are feasible when stands have reached sufficient size that trees to be removed have value as fuelwood/pulp or sawtimber. These operations are often essentially weedings
where the non-grade species are removed so that the better crop trees can grow. A more intensive variant is called crop tree management, and it involves identifying the best trees in a stand and fully
releasing them from competition.
Improvement harvests are called for where there is significant poor quality timber competing with smaller, good quality growing stock. In practice, the distinction
between improvement harvests and commercial thinnings is often not so clear.
Selection harvests are typically done in somewhat older stands where most of the trees have reached
timber size and are of good quality--either as a result of past management or by accident. Such cuttings are designed to realize some of the present value, while leaving good quality stands free to
Regeneration cuttings occur in older stands that are mature and ready for harvest, but that don't have adequate regeneration of
desirable species. Regeneration cuttings may also occur in younger stands of poor quality that are not worth growing to maturity. Regeneration cuttings are usually done by what's called the
shelterwood method where the overstory is selectively removed in two or three operations separated by 10-20 year intervals.
A variant of the shelterwood system is called the seed tree method.
Both methods leave large, vigorous trees for seed production--more with the shelterwood method, less with the seed tree method. Both methods can be quite successful, particularly if they are timed to
coincide with a good seed year of the species that are to be regenerated.
Older stands now cover large areas in Massachusetts because there hasn't been much pasture abandonment or heavy cutting since the
1950s. With these older stands, opportunities for intermediate thinnings have been missed and it's time to start thinking about regeneration strategies. This typically involves cutting the low
grade trees and leaving most or all of the high grade trees for seed production and value growth in a shelterwood operation.
Even-Aged versus Uneven-Aged
While most stands that developed following pasture abandonment or very heavy cutting are even-aged, there are other many other stands that have been periodically high-graded and therefore have two or
more age classes. Depending upon the quality of these stands, some sort of
uneven-aged management may be called for, particularly if there are good trees in both the older and younger age classes.
is always some overstory in uneven-aged stands, regeneration will tend more towards the shade-tolerant species. Therefore uneven-aged management is usually called for with shade-tolerant species such
as sugar maple, beech and hemlock. But uneven-aged management will also work for shade-intolerant species if thinnings and harvests are done frequently enough.
If the objective is to grow
species that have intermediate requirements for light and heat for seed germination--such as oaks, cherry and ash--then the shelterwood system is usually most appropriate. Of course this kind of
silviculture requires eventual removal of the overstory. This means those big old seed trees won't be around to look at any more.
Another silvicultural system that is occasionally used is called group selection which removes small groups of trees, leaving the rest of the stand undisturbed. Patch clearcuts remove larger
groups of trees and leave the rest of the stand undisturbed. Regeneration comes from the surrounding residual stands. These practices create many patches of even-aged groups or stands, and create
an overall uneven-aged structure for the whole forest.
Diameter limit cutting is a common practice among loggers and lumber companies. Typically, such cutting calls for removal of
everything over 14" DBH. This tends to result in
where only the more suppressed and less vigorous trees are left. But staggered diameter limit cuts can qualify as reasonable silviculture if, for example, all the grade hardwoods and pine over 18" DBH are cut, all hemlock over 14" DBH, and all the low grade hardwoods over 10" DBH are cut.
Another method which is seldom used now is the so-called coppice with standards method which leaves widely spaced overstory trees of very good quality (standards). This encourages the seedlings and
sprouts (coppice) to grow rapidly. This method was formerly used with species which sprout vigorously from the stumps, particularly American chestnut and red oak. Modern crop tree management is
similar to coppice with standards.
Clearcutting is virtually unheard of in this region for several reasons. First, there are restrictions of size of clearcuts imposed by the cutting practices
law. Second, markets for the small trees are not very good. Third, the high aesthetic and recreational values of forest land reduce the incentive for clearcutting. Landowners that want to
maximize short-term gain usually resort to high-grading followed by sale of the land.
Row thinnings are often used in plantations of red pine and other conifers. Additional suppressed and poor
quality trees may also be removed from the residual rows. Strip thinnings are a wider variant of this method and are sometimes used in northern hardwoods and mixed stands where the primary objective is
regeneration from the residual trees.