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Controlling Disturbances



Logging, though one of the most important silvicultural tools available, causes stress in many ways. Three are worth highlighting: (1) by opening the canopy, changing light, and temperature levels; (2) by disturbing and compacting the forest soils when heavy equipment is used for extraction; and the most serious, (3) by causing wounds in the remaining trees, disrupting important physiological processes, and leaving them susceptible to decay and disease-causing organisms.

Other potentially negative effects from practices such as high-grading (take the best and leave the rest) reflect poor silviculture but often do not cause any more stress than logging to improve the stand.  Stands with closed canopies (highly shaded conditions) that are opened by logging may experience a phenomenon known as thinning shock. Although it is not exactly clear why this happens, it may be that trees divert energy into crown growth to take advantage of openings at the expense of providing adequate nutrition to other parts of the tree. 

Increased temperatures, light, and wind speeds are factors as well. Thinning shock by itself is not bad. Usually a stand recovers after a growing season or two. However, couple thinning shock with other stresses, and die-back and mortality will occur. Closed stands that have been defoliated recently, or are suffering from drought, should be opened only very gradually or not at all. The way to avoid thinning shock is to thin lightly, but often. Doing so, however, leads to the potential of increasing stress from other factors, namely soil compaction and wounds. By eliminating air spaces in the soil, and decreasing the rate of water percolation, soil compaction is a very serious side effect of harvesting.

Planning Can Reduce Stress Usually, 10 to 16 percent of a harvest area is devoted to roads, landings, and skid trails. Experts say that, with proper planning and good felling techniques, these areas could be reduced by 40 percent. Not only does good planning lessen the ecological impacts of logging, it makes timber extraction considerably easier and cheaper. For example, the best way to prevent soil compaction and root disturbances is to schedule harvesting during frozen-ground conditions. In any harvesting prescription - short of clear-cutting - wounding of the remaining trees is inevitable.

Even the most careful of felling techniques will cause some crown breakage, and stem wounding on remaining trees used as bumpers is difficult to avoid. Of greatest concern, however, is wounding of feeder roots. As indicated earlier, up to 80 percent of the total length of a tree's root system is made up of feeder (or fine) roots. They are mostly near the soil surface and are extremely susceptible to damage, especially during the growing season.

Careful with that Equipment!

Root and stem wounding has increased dramatically with the use of skidders and other mechanized harvesting equipment. Unfortunately, this equipment can compensate for a lack of user skill with power and there is always a tendency to oversize equipment at the expense of maneuverability. Any time the outside protective layer of a tree is broken, it creates an opening called an infection court. In response to wounding, trees have evolved a method of walling-off or isolating the injury. Badly scarred trees with full, healthy crowns standing next to a skid trail are not an uncommon site. These trees, however, will never recover the grade and wood loss due to the injury. Timber management is a process of weighing the benefits of a prescription against the negative effects of harvesting. In the epilogue to Principles of Silviculture (Daniels et al., 1979), the authors cite a German forester, Henrich Cotta, who wrote in 1816 his Advice on Silviculture:

"The good physician lets people die; the poor one kills them. With the same right one can say the good forester [landowner, logger, or others who make decisions about forest use] allows the most perfect forest to become less so; the poor one spoils them."

Logging is a double-edged sword. Regardless of the objective - be it for wildlife habitat, recreation, timber production, or a combination of uses - logging is the principal means of affecting and controlling change in the forest. It also has the potential to do great damage, some of which may be irreversible. Fortunately, most of the practices that lessen the negative impacts of logging are common sense.


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Mail: NIFA, 2303 West Cording Road, Galena, IL  61036

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