Forest 'rotation' explained
Wednesday, January 2, 2013 11:20 am
An explanation of forest "rotation" for hardwood timber
David Mercker, Extension Specialist, Forestry
Readers should use this article only as a general guide. Always seek the advice of a professional forester prior to marketing timber or making forest management decisions.
In the simplest sense, forest rotation refers to the time interval between the establishment of a stand of trees and the final cutting. Normally rotation is a term used with even-aged silvicultural systems, such as stands that originate following the establishment of a forest plantation or when a new forest begins after a major disturbance (tornado). This definition seems fairly logical, but it can become complicated depending on the criteria used to determine the end-point. Exactly when is the final cutting? For instance, landowners might select tree size or age as the parameters; foresters might consider financial maturity or biological condition; yet loggers are driven by local sawmill demand. So who's right?
They all are. There are no regulations to control when a tree has "matured" and should be harvested any more than there are laws established to dictate when livestock is ready for slaughter. A tree (or a steer) is ready for market when its owner says it is ready. However, in the case of livestock, ranchers understand that when the law of diminishing returns has been met the steer should go to market (the principle that a continual increase in investment does not lead to a continual increase in profit). This concept is true with trees too, but not often practiced, especially with hardwoods.
Foresters are regularly asked how big a tree should be in order to harvest. Unless other criteria have been established, we tend lean toward the financial maturity criteria, and the answer is, "It depends." Species and condition of tree, as well as the site productivity are three variables to consider. On good sites, with healthy trees that are in demand by the industry, timber reaches financial maturity at a larger diameter than unacceptable trees growing on poor sites. For example, a prime quality cherrybark oak existing on a rich bottomland site might reach financial maturity at 28 inch diameter, while a post oak on a dry, south facing slope may mature at only 18 inches. Thus, to make a blanket recommendation of harvesting all trees above a set diameter, normally is not a good financial decision. This is referred to a diameter limit cutting and should be avoided. Having said that, trees do eventually reach financial maturity, and if the forest is treated as any other financial investment, trees should be liquidated to capture their value. Diameter certainly can be one criteria used to estimate the economic maturity of a trees.
The takeaway message is that forest landowners should allow trees with good investment potential to reach financial maturity; undesirable trees reach financial maturity much earlier in the rotation. Undesirable trees can be either species of traditionally lower value, or desirable species that have developed with very poor form or grade, and their improvement is not likely. Also, readers should understand that site productivity is a determining factor when estimating financial maturity on timber. This requires educated judgment. It's complicated. That's why for most landowners, professional assessment and counsel is recommended.