Thinning is the practice of removing trees from a stand to promote the growth and health of the remaining trees. When carried out correctly, this regulation of stand density helps maintain a healthy forest, improves wildlife habitat, and enhances the field of vision and visual appeal. Depending on landowner goals and available markets, a southern pine stand’s management regime may call for thinning multiple times throughout its life cycle.
Here in the Carolinas, loblolly pine is the predominant species of pine planted. Initial planting densities typically range from 400 to 650 trees per acre. On a properly prepared site, a newly planted stand will grow relatively unencumbered for the first 10 to 15 years, with diameters increasing at a healthy rate. Density will eventually reach a point where competition for nutrients, sunlight, and water results in a slowing of diameter growth, increased susceptibility to pests and disease, and diminished wildlife habitat.
A thinning harvest solves the problem of overcrowding by removing the inferior quality trees that would otherwise die or rob valuable resources from the higher quality “crop” trees. Not only will the removal of these trees open up growing space, but their harvest will also generate income. Additionally, thinning opens the canopy, allowing sunlight to reach the ground, which stimulates the growth of grasses and other herbaceous plants that provide wildlife food, cover, and nesting habitat.
Timing of the First Thinning
To maximize growth response and ensure the stand stays healthy, the timing of the thinning harvest is crucial. The age of the first thinning depends on several factors including site productivity, initial planting density, and local markets and logging contractors. A thinning harvest may occur as early as ten years of age on highly productive sites, but age fourteen to eighteen years is typical.
I look at two factors when determining thinning needs. The first is basal area per acre (see Figure 1), which is a measure of stand density. Stand health and growth rate begins to decline when a stand’s basal area approaches 120 ft²/acre.
Secondly, I look at the crown ratio, which is the percentage of total tree height that contains live crown (See Figure 2). It is advantageous to maintain stand density at a level where the crown ratios for crop trees will average between 40% and 50%. This ratio ensures that trees have enough leaf area to produce adequate carbohydrates for steady diameter growth while allowing sufficient natural pruning of lower limbs for quality timber production. If stand density is allowed to get too high, crowding causes lower limbs to be shaded out, resulting in their death and diminished diameter growth. This limb shedding is desirable to a point—we want clear, knot-free wood for sawtimber logs. The key is to strike the right balance, keeping crown ratios in the 40% to 50% range. Within this live crown range, the upper stem may have a few more limbs and knots, but the quality loss will more than be made up for in the maximized diameter growth in the lower logs. When the live crown is allowed to drop below 30%, diameter growth is severely affected, and the trees will show little or no growth response after thinning. Additionally, a well-formed, hardy crown is an indication of healthy roots.
As a rule of thumb, when a pine stand’s crown ratio reduces to 50% or basal area rises above 100ft²/acre planning should begin for the first thinning. At these levels, the stand may not be quite ready for thinning, but by initiating the assessment and planning at this stage, you can ensure that the harvest timing is optimal. There is an exception to this rule–in stands that were planted densely (greater than 550 trees per acre) the basal area and crown ratio may reach undesirable levels several years before it is economically feasible to perform a thinning harvest due to small diameters and short tree heights.
The Thinning Process
For first thinnings, the most practical approach is a combination of row thinning and low thinning. In a row thinning harvest, every third, fourth, or fifth row of trees (the “access corridors”) is harvested completely. In a low thinning harvest (often referred to as “a thin from below”) the smallest, poorest quality trees are harvested, leaving the best quality “crop” trees.
The first step in the process is to harvest the access corridors completely–creating room for equipment travel and maneuverability. The spacing between the access corridors is determined by terrain, management goals, and the equipment used for the harvest. Following access corridor removal, the operator selectively harvests the smaller, poorer quality trees from the remaining rows until the desired stand density is reached. In natural stands or planted stands where the rows are not easily discernible, a designated spacing (usually 35 to 50 feet on center) is set for row removal. See Figure 3 for a “view from above” perspective of the thinning process.
In subsequent thinnings, the low thinning harvest method is utilized.
Figure 3 – Harvest Tree Selection
Video from a First Thinning Harvest
Harvest Tree Selection
Operator select method. In this method, the harvest trees are selected by the cutter operator. The competence of cutter operators varies widely–some understand basal area and crown ratio, but many thin based purely on tree spacing and visual estimation (“Does it look about like the last one I did?”). Even with a high-quality operator, the harvest should be monitored closely, with basal area and tree selection checked frequently. Many times due to cost, terrain, or understory conditions, operator select is the only option. Operator select harvests are inherently more risky for the landowner.
Marked tree method. If conditions are right, with moderate terrain and relatively open understory conditions, the trees can be marked with high-visibility forestry-grade paint. For first thinnings, I typically mark the leave trees (the larger trees are easier to mark and for the cutter operator to see, plus there are fewer of them). Access corridors are marked with one color (usually orange or pink) to indicate that they are to be removed completely and leave trees are marked with a different color (usually blue–marked high and on two sides for visibility). When marking, I inspect each leave tree for size, health, form, and crown structure before marking. Basal area readings are taken frequently to make sure I’m hitting the right density level. Also, it is important to keep in mind the limited maneuverability of the cutting machine in mind when marking. Marking is the only way to ensure that you are leaving the best-growing stock and getting the prescribed post-harvest basal area. You will pay a little extra to get your stand marked but will come out ahead in the long run due to better health and volume growth. To the uninitiated, a 10ft²/acre difference in basal area is not noticeable but equates to 6 to 9 tons (depending on tree height) of growing stock in a first thinning. Leaving too high a density will reduce thinning revenues and result in slower post-harvest diameter growth. Taking too much volume out may reduce your growing stock to a level that will hurt your future stand value. Missing the density mark combined with poor tree selection can add up to substantial future financial losses.
For second thinnings I typically mark the harvest trees if conditions allow, due to the fact that, if the first thinning was done properly, there are fewer trees to mark, and the loggers I work with seem to prefer having the “cut” trees marked rather than the “leave” trees.
Harvest tree selection is even more crucial at this stage because the trees are of higher value. The trees are taller, making the upper stems more difficult for the operator to see. Just a few mistakenly cut trees per acre can diminish the “savings” gained by choosing the operator select method over having the stand marked. Cutter operators tell me they prefer marked thinnings due to the fact that it improves their production—they don’t have to take the time to consider each tree, they just cut.
The density to which your stand will be thinned is determined by management objective(s), terrain, and site productivity. For example, I have clients whose primary goal is creating habitat for rabbit, deer, turkey, and quail. For those clients, their pine stands will be first thinned to between 40ft² and 60ft² per acre, which leaves as much as half the volume on the ground than if we were strictly managing for timber production. This level of stand density reduction can have a negative impact on the rate of return on the timber investment, but the lower density will allow for the development of cover and food sources for wildlife.
When I first entered the timber business in the mid-1990s, pine chip-n-saw and sawtimber prices were high, and pulpwood prices were comparatively low. The standard management regime for maximizing the internal rate of return on a timber investment was to manage for sawtimber–first thin age 14-18, second thin age 22-27, then grow to 35 to 40 years old to produce large, high-quality sawtimber and peeler logs. These days (since The Great Recession) chip-n-saw and sawtimber prices are considerably lower, and pulp prices are substantially higher, making the price differential between the pulp and sawn product classes less dramatic. From a strictly an economic standpoint, it no longer makes sense to grow a stand for 35 to 40 years. A one-thinning (as early as feasible), final harvest at age 25 to 30 will give a better rate of return on the investment in many market areas. In some areas, where pine pulp prices can spike to extremely high stumpage price levels (sometimes surpassing chip-n-saw prices) there is little financial incentive to thin–just grow the stand 15 to 20 years, cut it, start again. This is closer to farming than forestry, but it is the direction the industry is going in some areas. Use of engineered wood products (oriented strand board, I-joist) and so-called “green” products (pellets, biofuels, etc.–products that are unprofitable unless backed/incentivized by government handouts and mandates) are driving markets toward greater fiber consumption and more of a “plant them thick, cut them quick” mentality. In my opinion, this type of management will result in a loss of native forest types, wildlife habitat, and biodiversity in the long-run. Sometimes “green” isn’t really all that green.
Since no two sites (or clients) are alike, I evaluate each one, writing a custom prescription for thinning based on the landowner’s goals and site conditions. My inventory software helps me estimate thinning volume and value yields at various densities. Using growth and yield software, I can evaluate the timing and yield of the subsequent thinnings and the final harvest.
When Not To Thin
There are a few situations when it may be prudent to consider not thinning.
- In areas where pulp prices are extremely high, you may want to forgo thinning and grow short rotations for fiber if your goals are purely financial.
- Hilly, rough terrain can be difficult to thin effectively, possibly doing more damage than good. Consider clearcutting when financially mature and planting the hilly areas back at a wider spacing (12 feet by 12 feet or 303 trees per acre) if you are planning to grow chip-n-saw and sawtimber.
- In older (20+ years old), densely planted pine plantations that weren’t previously thinned. These stands have a low potential for growth response due to poor crown and root development and a high likelihood of being damaged by storms (ice and wind) as a result of weak, brittle tops, low diameter to height ratio, and small root balls. See pictures below of a +/-25-year-old first thinning. I advised against this thinning, but the landowner chose to thin, losing approximately 20% of his remaining stand the following year.
Each of these conditions should be evaluated by a qualified (a green truck, uniform, and badge do not necessarily make you qualified) professional forester on a case by case basis to determine whether thinning is advisable.
Thinning should be performed periodically (usually every 5 to 10 years after the first thinning) throughout the life of a pine forest. Determination of harvest frequency and intensity depends on several factors including management objectives, age, soil productivity, crown ratio and closure, initial planting density, markets, access, terrain, and economic feasibility. Timing and tree selection are crucial components of a thinning harvest. A thinning done incorrectly can result in a less than optimal growth rate, reduced future income, and increased risk of damage from insects, disease, and storms.
I can help you determine the proper method and thinning density for your pine timber and sell your timber to a buyer who is equipped to do the job properly. I will oversee operations to make sure the tree selection and density are right for your management objectives.