SITE-SPECIFIC SILVICULTURE | Silviculture in Mountain Forests
W. Schönenberger, P. Brang, in Encyclopedia of Forest Sciences, 2004
Clear-Cutting
Clear-cutting is a silvicultural system that removes an entire stand of trees from an area of 1 ha or more, and greater than two tree heights in width, in a single harvesting operation (Figure 5). It can be highly profitable. However, its application in mountain forests often involves unacceptable risks, or impairs landscape values.
Clear-cutting mountain forests can initiate erosion processes which may result in a complete loss of the soil. On a regional scale, higher altitudes in mountain areas usually receive higher precipitation. Steep slopes are prone to surface erosion (gullying, rill erosion), nutrient leaching, landslides, and debris flows. Clear-cutting often contributes to reductions in root strength and soil water-holding capacity, due to soil compaction and reduced transpiration. Moreover, the removal of the forest cover exposes the soil surface to heavy precipitation and large variations in temperature. If natural hazards are to be prevented, the size of clear-cut areas in protection forests must be kept small. Thus, clear-cutting is often not an option.
Unstocked, even slopes steeper than about 30° at high altitudes are prone to avalanche release. If a slope exceeds 45°, snow avalanches can start in canopy gaps exceeding 30 m perpendicular to the contour line. Any rough surface structure, such as a rock, trunk, or tree, reduces the risk of snow movement by creating heterogeneity in the snow layer and ‘nailing’ the snow to the ground. While forests can rarely stop flowing snow avalanches, they are highly effective in preventing avalanche release. Surface roughness is also important for impeding rockfalls. However, in this case, forests serve not to prevent rockfall starting, but rather stop falling rocks.
If clear-cutting is not properly applied as a silvicultural system and is the first step to permanent deforestation, it usually has a negative impact on the fresh water supply. More than half of the world's population relies on clean water from mountains. While the demand is increasing, the supply is endangered. Mountains are the sources of most rivers, and mountain forests help to ensure that the water supply is seasonally balanced and that the water is of high quality. Clear-cutting large mountain forests without restoration cannot, therefore, be considered at all sustainable.
The impact of clear-cutting will obviously depend on the size of the clear-cut area. Large clear cuts in environments with pronounced climatic extremes, where tree regeneration depends on the beneficial effects of adult trees, must be avoided. This means that clear-cutting is not appropriate on very dry, very cold, or very wet sites, as it can lead to failures in stand renewal, even with repeated plantings. A system of small patch cuts is similar to the selection system, whereas leaving seed-dispersing trees to facilitate natural regeneration (the seed tree system) is comparable to the shelterwood system.
Not all damage attributed to clear-cutting is caused by the unwanted side-effects of the silvicultural system itself. The damage may actually be the result of inadequate road construction, of inappropriate site preparation treatments such as burning, or of careless logging practices, which damage the advance regeneration. However, even careful clear-cutting should not be used in those mountain forests where protection from natural hazards is needed, where erosion is a matter of major concern, and where the sites do not restock easily.