Thinnings are cuttings or intermediate harvests made in immature stands to stimulate the growth of the remaining trees and improve the health, yield, and profitability of the stand.
Young pine stands in East Texas should be thinned periodically. Trees compete for light, moisture, and nutrients. If trees become too crowded, growth slows and they become susceptible to insects, diseases and wildfires, and may eventually die.
Why thin?
One site can only support so many trees per acre of a given diameter. More trees than can mature on an acre should be planted however with the expectation that periodic thinning be conducted when the trees begin to compete for nutrients, sunlight, water, and other resources. Thinning redistributes the growth potential of the stand to the trees of highest quality and favors their rapid growth.
Pines in overly-dense stands (when basal area exceeds about 120 square feet per acre) grow slower and become increasingly susceptible to insects like bark beetles.
Basal area is a measure of stand density and represents the cross-sectional area of all trees at breast height (4.5 feet above ground level) on an acre of land. Average basal area of a stand can be determined using a 10-factor prism (Figure 1), available from forestry supply stores, or with 1/20-acre (circular plot with 26.3-foot radius) sample plots distributed randomly throughout the plantation.
Thinning removes trees most likely to die from competition, insects, or diseases and reduces the potential for losses from wildfires. In addition, thinning is also economically beneficial. By promoting vigorous growth, thinning reduces the time required to grow trees from low value pulpwood to higher value poles or chip-n-saw material, and ultimately to valuable sawlogs.
Are thinned stands more prone to storm damage?
Commercial thinning provides an intermediate return on investment as trees most likely to die before maturing are harvested and sold. For first thinnings, you may qualify for federal cost shares under our Southern Pine Beetle Prevention Program.
Growing trees and conducting thinning operations are not without risk. Experience has shown that widely spaced trees and heavily thinned stands are more prone to wind and ice damage than non-thinned stands. Nevertheless, the benefits of thinning far outweigh the risks.
To monitor the impact of Hurricane Ike on thinned stands in southeast Texas,
we conducted a survey of landowners within the hurricane impact zone who had thinned their pine stands since 2005 as part of the SPB Prevention Program. Results from 130 landowners showed that only 7% of the surveyed plantations had suffered heavy losses from Hurricane Ike while 55% were unaffected. Typically, the most severe damage occurred when the thinned stand was located adjacent to an open field or clearcut. Most stands with light to moderate damage are expected to recover.
Pre-commercial versus commercial thinning
Thinning conducted in young pine stands before the trees are merchantable is termed “precommercial thinning.” Most pine plantations planted at 622 trees per acre (7 x 10-foot spacing) or less do not require thinning before the trees are 6 inches in diameter, considered the minimum size for commercial thinning.
A common exception is a natural stand that has regenerated with over 700 trees per acre or a plantation that has experienced heavy in-growth of additional natural pine seedlings. Unlike a commercial thinning, a pre-commercial thinning costs the landowner directly.
Pre-commercial thinning in natural stands is often done by cutting parallel rows every 20 or 30 feet in the stand to release the leave trees, using a mulcher, weed eater with saw head, chainsaw, or bull dozer.
When to thin
The first thinning of pine plantations should be scheduled when one or more indicators suggest that competition among trees is becoming a problem. Severe competition leads to stagnation and predisposes the stand to bark beetle infestations.
Useful indicators that a thinning is needed are:
- an average stand basal area that exceeds 100-120 square feet per acre,
- average live crown ratio (ratio of the portion of the tree bole with green branches to the total tree height) of less than 35%, or
- radial growth, determined using an increment borer, begins to markedly slow down (Figure 2).
The first thinning should be dictated by forest health considerations and be conducted to maintain vigorous growth, and not be delayed by poor market conditions. In most cases, the increased growth in response to timely thinning will yield profits that far outweigh the loss of returns due to a poor pulpwood or chip market. In addition, if the stand qualifies, federal cost share incentives are currently available for first thinning of dense pine stands under the Southern Pine Beetle Prevention Program.
Dr. Kronrad, of Stephen F. Austin State University, research confirms the timing of the first thinning and the number of thinnings to schedule prior to harvest will depend on site quality, more than any other factor. Plantations on good sites (productive soils) will grow faster and be ready to thin at an earlier age than plantations on poor sites with slow growth. Similarly, the stand on a good site will produce sawtimber and be ready for harvest at a younger age, following two, three, or four periodic thinnings. Stands on poorer sites may need thinning only once or twice and may have to be carried for longer rotations if sawtimber is the management objective. A professional forester can determine the site quality (index) of your land.
The first commercial thinning should not be scheduled before the trees average at least six inches in diameter at breast height (dbh) and are at least 40 feet tall. This is the minimum size that is operable for most loggers. Most pines in East Texas will be 12-15 years of age when they attain this size. Again, the age at first thinning will vary with site quality.
How to thin
To prepare for thinning, a forester can mark every tree to retain in the plantation, leaving the poorly formed, suppressed, or diseased trees unmarked to be removed. Thinning “from below” is most recommended, to favor the dominant and co-dominant trees as eventual crop trees. In plantation situations, foresters will most often recommend a row thinning, in which a logger removes every third, fourth, or fifth row (Figure 3). This releases the trees adjacent to the downed row and provides access for equipment to thin within the leave rows. Undesirable (crooked, forked, suppressed, diseased, etc.) trees are then removed from the leave rows to reduce the stand density to optimal levels of 70 to 90 square feet of basal area per acre.
In row thinning operations, the forester can mark the trees to remove or leave in the remaining rows or choose the operator select method. In this approach, a trained logger is told the final basal area to be achieved and selects the trees to remove as the thinning progresses to meet the basal area goal. To the extent possible, an experienced thinning operator will leave the best trees more-or-less uniformly spaced with room to grow throughout the plantation. However, it is best to have a professional forester oversee the thinning operation to ensure that the job is done correctly and to meet the landowner’s goals.
Loggers should strive to skin up or damage as few trees as possible during the thinning operation. Trees with large basal wounds (missing bark) should be removed during the thinning operation. If left in the stand, these wounded trees are targets for bark beetles, particularly the black turpentine beetle.
Every thinning method has its advantages and disadvantages. Let’s review the major pros and cons for thinning every third, fourth, or fifth row in pine plantations.
Third row thinning
Removing every third row in thinning operations releases every leave tree on one side and causes the least amount of damage to the residual trees. It allows easy access to the leave rows and is the least costly thinning method.
But the disadvantages outweigh the advantages. This approach directly reduces basal area by 33% and a third of the potential crop trees are eliminated in the downed rows. If the pre-thinning basal area is not more than 120 square feet per acre, then no trees can be removed from the leave rows without reducing basal area below 80 square feet per acre. Such a thinned stand may still be susceptible to southern pine beetle infestations which may expand along unthinned rows.
Fourth row thinning
Removing every fourth row directly removes 25% of the basal area and a quarter of the potential crop trees, allowing the operator to select and remove a greater number of undesirable trees from the leave rows. The operator has more flexibility to adjust spacing and favor crop trees compared to a third row thinning.
On the downside, the equipment must be able to extend into the interior row to thin around chosen crop trees and the potential for damaging leave trees during extraction is increased. Cost of thinning may increase as a result.
Fifth row thinning
Many foresters recommend fifth row thinning because only 20% of the potential crop trees are eliminated with removal of every fifth row. A greater volume of undesirable trees can thus be removed from the leave rows to release crop trees.
But the operator will need to thin two rows deep from each side of every access row to achieve a uniformly spaced thinning operation. Thus, the likelihood of damage to leave trees increases and the cost may be somewhat greater. If done correctly and with care, however, plantations thinned by the fifth row method are probably least vulnerable to the spread of southern pine beetle infestations, due to the more uniform spacing among trees (Figure 3).
Thinning as a preventive measure for southern pine beetle
Pine stands in East Texas are prone to infestation by the southern pine beetle when the stand basal area is greater than 120 square feet per acre and tree height is 50 feet or more. Loblolly pine stands on poorly-drained, bottomland soils are particularly susceptible. The same overcrowded stand conditions that cause trees to stagnate also favor beetle infestations. In turn, the most recommended method to prevent the occurrence and spread of southern pine beetle is to thin pine stands periodically to maintain basal areas in the range of 80-100 square feet per acre. The same stand density is optimal for vigorous tree growth. Definitely, for the landowner, it is a thin, win, win situation.
Landowners have a variety of reasons to thin their pine stands. Although market conditions for wood harvested from thinned stands may be poor, don’t let this be the only factor in the decision of when to thin.
The first thinning, in particular, should be timed to avoid prolonged reduction in tree growth and for forest health reasons, regardless of market conditions or the threat of an occasional wind or ice storm. With the second and third thinning, landowners have more “wiggle room” for timing the thinning to coincide with good market conditions. If in doubt on when and how to thin, consult a professional forester.