Thinning Pine For Profit, Health, and Wildlife

Thinning is the practice of harvesting 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 aesthetics. Depending on landowner goals and available markets, a southern pine stand’s management regime may call for thinning multiple times throughout its life cycle.

This stand was thinned, treated with herbicide to control woody vegetation, then understory burned.

Why Thin?

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 at which competition for nutrients, sunlight, and water slows diameter growth, increases susceptibility to pests and disease, and diminishes 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, local market specifications, 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 factor is basal area per acre (see Figure 1), which is a measure of stand density. Stand health and growth rate generally begin to decline when a stand’s basal area approaches 120 ft²/acre.

Figure 1 – Basal Area

Secondly, I look at the crown ratio, which is the percentage of total tree height that contains live crown (See Figure 2). Maintaining stand density at a level where the crown ratios of crop trees average 40% to 50% is advantageous. This ratio ensures trees have enough leaf area to produce adequate carbohydrates through photosynthesis for steady diameter growth while allowing sufficient natural pruning of lower limbs for quality timber production. If stand density becomes too high, crowding shades lower limbs, leading to their death and reduced 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 between 40% and 50%. 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 indicates healthy roots.

Figure 2 – Crown Ratio

As a rule of thumb, planning should begin for the first thinning when a pine stand’s crown ratio reduces to 50%, or the basal area rises above 100ft²/acre. 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 optimal harvest timing. There is an exception to this rule—in stands that were planted densely (greater than 500 trees per acre), the basal area and crown ratio may reach undesirable levels a few years before it is economically feasible to perform a thinning harvest due to small diameters and inadequate tree heights.

18-year-old pine stump. Note how the growth rings have narrowed over the past 5 years. Had the stand been planted at a wider spacing or thinned at age 13 or 14, this rapid decline in diameter growth rate would not have occurred.

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 used.

Figure 3 – Harvest Tree Selection

Access Corridor

Thinning Harvest Video

Harvest Tree Selection

Operator select method. In this method, the cutter operator selects the harvest trees. 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 selection is the only option. Operator-select harvests are inherently more risky for the landowner.

Cutter Operator’s View of Trees to Be Thinned.

Cutter Operator’s View of Tree Crowns

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. The decline in pulpwood market (both in number and price) has made the marking of first thinning economically unfeasible, so these days marking a thinning is generally reserved for second thinnings where a mixture of pulpwood and sawtimber is harvested (see "The Pine Pulpwood Problem" here).

Harvest tree selection is even more crucial for second thinnings 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 because it improves their production—they don’t have to take the time to consider each tree, they just cut.

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 cutting machine's limited maneuverability 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 you will come out ahead in the long run thanks to better health and increased volume. To the uninitiated, a 10ft²/acre difference in basal area is not noticeable but equates to around 10 tons/acre (depending on tree height) of growing stock in a second or third 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.

Wildlife Thinning. This mature loblolly pine stand was thinned to a low density (40 to 50 sqft/ac basal area) for quail habitat. Within the next two years, an understory burn will be performed to control woody vegetation and help establish grasses and forbes.

Density Considerations

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 to create habitat for rabbits, deer, turkeys, and quail. For those clients, their pine stands will be thinned to between 40ft² and 60ft² per acre, which leaves as much as half the volume standing 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.

Density Considerations. The density you’ll thin your stand to is determined by your objective(s), terrain, and site productivity.

As crowns close, wildlife forage declines. Graphic adapted from "The North American Forests" by Laurence C. Walker.

For many clients, commercial timber production is a goal, but wildlife habitat is also important. For these clients, I prescribe hybrid thinning: most of the stand is thinned to a commercial timber production density, while edges along fields and roadsides are tapered to wildlife density levels. There may also be small patches (usually one to two acres) within the interior of the stand where density is reduced to wildlife habitat levels.

When I first entered the timber business in the mid-1990s, competition and demand for pine products were high. The standard management regime for maximizing the internal rate of return on a timber investment was to manage for sawtimber—first thin, aged 14 to18, second thin, aged 22 to27, then grow to 35 to 40 years old to produce large, high-quality sawtimber and peeler logs.

These days (since The Great Recession of 2008), pine markets are fewer, pine supply is higher, competition is less, and timber prices are down across the board, meaning more scrutiny is needed for initial planting density, thinning harvest timing, and final harvest age. Each site needs a custom prescription based on the landowner’s goals, site conditions, and available markets.

When Not To Thin

There are situations in which it may be prudent not to thin.

• 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 the 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.

Storm Damage. This stand was over 25 years old when 1st thinned, leaving it particularly susceptible to storm damage.

Each of these conditions should be evaluated by a qualified (a green government truck, uniform, and badge do not necessarily make you qualified) professional forester on a case-by-case basis to determine whether thinning is advisable.

Summary

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.

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