Page 337 - Environment: The Science Behind the Stories

and to some extent it can mimic natural disturbance events so that they can reseed the logged area. In the shelterwood
such as fires, tornadoes, or windstorms that knock down trees approach (also represented by Figure 12.14b), small numbers
across large areas. However, the ecological impacts of clear- of mature trees are left in place to provide shelter for seed-
cutting are considerable. An entire ecological community is lings as they grow. These three methods all lead to largely
removed, soil erodes away, and sunlight penetrates to ground even-aged stands of trees.
level, changing microclimatic conditions such that new types of In contrast, selection systems (FIGURE 12.14c) allow
plants replace those of the original forest. Clear-cutting essen- uneven-aged stand management, because only some trees are
tially sets in motion a process of succession (pp. 103, 106) in cut at any one time. In single-tree selection, widely spaced
which the resulting climax community may be quite different trees are cut one at a time, whereas in group selection, small
from the original climax community. For example, when we patches of trees are cut. The stand’s overall rotation time may
clear-cut an eastern U.S. oak–hickory forest, the forest that will be the same as in an even-aged approach, because multiple
regrow may be dominated by maples, beeches, and tulip trees. harvests are made, but the stand remains mostly intact between
Concerns about clear-cutting eventually led foresters harvests. Selection systems maintain uneven-aged stands, but
and the timber industry to develop alternative harvesting they still have ecological impacts, because moving trucks and
methods (FIGURE 12.14). Clear-cutting (FIGURE 12.14a) remains machinery over a network of roads and trails to access indi-
the most widely practiced method, but alternative approaches vidual trees compacts the soil and disturbs the forest floor.
involve cutting some trees while leaving others standing. In Selection methods are also unpopular with timber companies
the seed-tree approach (FIGURE 12.14b), small numbers of because they are expensive, and with loggers because they are
mature and vigorous seed-producing trees are left standing more dangerous than clear-cutting.

Original forest

HARVEST

(a) Clear-cutting system (b) Seed-tree or shelterwood system (c) Selection system

REGROWTH FOLLOWING HARVEST

FIGURE 12.14 Foresters have devised several methods to harvest timber. In clear-cutting (a), all trees are
cut, extracting a great deal of timber inexpensively but leaving a vastly altered landscape. In seed-tree systems
and shelterwood systems (b), a few large trees are left in clear-cuts to reseed the area or provide shelter for
336 seedlings. In selection systems (c), a minority of trees is removed at any one time, while most are left standing.

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