Topic 12.2: Thinning

Where future crop trees are crowded by neighbors, thinning can result in substantial increases in growth due to release of soil resources and increased access to light.  Thinning treatments can be applied to entire stands or just in the near vicinities of selected future crop trees.  Both commercial thinning, in which the thinned trees are extracted and sold, and pre-commercial thinning are reasonable options in some stands.  But before discussing some of the many types of thinning, a few of the silvicultural benefits of stand crowding need to be considered.

While diameter or volume increments of selected future crop trees can be improved by removing neighbors, heavy stand thinning can lead to retention and growth of lower branches, formation of epicormic branches, increased stem taper, barkscald, abrupt changes in wood properties, and other changes that lead to reductions in stem or wood quality.  Thinning stands can also make them susceptible to wind throw and weed encroachment.  Finally, thinning does not invariably result in the desired growth response.  For example, after long periods of suppression, trees of many species do not respond well to thinning; some previously suppressed trees may even die if they are too rapidly exposed to high light intensities, high temperatures, and the consequent water deficits.  Where exposure is less rapid and less extreme, formerly suppressed trees that are released from competition may adjust to the new conditions by replacing their shade-adapted leaves with thicker leaves, with thicker cuticles, and other characteristics of “sun” leaves.  Released individuals also adjust their root:shoot ratios so as to increase their water uptake capacities in the more water-demanding conditions of thinned stands.

In silviculturally managed natural forests in the tropics, perhaps the most common thinning operation is the release of selected future crop trees from competition from immediate neighbors.  This treatment, often referred to as “liberation thinning” has many silvicultural, financial, and environmental advantages in the poorly-stocked stands in which tropical foresters generally work.  By restricting thinning operations to the near vicinities of future crop trees, portions of most stands remain untreated, which often makes silvicultural sense, saves money, and avoids needless environmental disruption.

Liberation thinning prescriptions generally call for cutting, frill-girdling, or arboriciding trees with crowns above or within some lateral distance (e.g., 2 or 4 m) of the crowns of future crop trees.  The appropriate extent of lateral opening varies with the species and size of the tree to be released.  For example, tree species that typically develop broad spreading crowns may require large openings for maximum growth, at least after the selected individual has developed the desired length of branch-free bole.

To maximize the likelihood of increased timber volume increments, the future crop trees selected should not have been heavily suppressed for long periods of time.  Because stand records are virtually never available, the silviculturalist must rely on visible characteristics of trees themselves to determine their histories of suppression.  Crown form is generally the best indicator of the conditions under which a tree has been growing.  Trees with small, sparse, or poorly formed crowns are likely to have been suppressed for a long time and may not respond well to release operations.  Heavily vine-laden trees may also not be good candidates for liberation treatments.  Due to the complexity of liberation thinning operations, tree marking should be carried out by trained staff and the silvicultural responses should be monitored in permanent research plots.  Repeated liberation may be required for maximum stand production if the benefits of liberation do not persist for the duration of stand retention.

The primary thinning treatment that most natural forests receive is timber harvesting.  All too often logging is not considered to be the silvicultural treatment that it actually represents.  In stands with substantial advanced regeneration of commercial species and where some trees have been marked for harvesting and others for retention, timber harvesting is equivalent to heavy thinning and results in similar growth responses of future crop trees.  The importance of protecting this advanced regeneration during harvesting is obvious, as discussed elsewhere.

In forests where advanced regeneration of canopy trees is scarce in the understory and the most commercially important species require open conditions to regenerate, even-aged stands often develop after large-scale natural disturbances (e.g., hurricanes), in abandoned agricultural clearings, and after clearcutting.  In some cases these stands are densely stocked with commercially acceptable trees that may respond to stand-wide treatments such as row thinning.  In older stands that are very well stocked, thinnings can be sold and the distinction between stand management and timber harvesting is completely blurred.  Profits at this intermediate stage of stand development can be used to offset the costs of silvicultural treatments and are welcomed by most forest owners.