Pine Stand Thinning

Forest management practices are recommended for various reasons to meet landowner management objectives, including generating income, improving habitat for wildlife species, and, most importantly, maintaining a healthy, productive forest.  Properly timed thinnings in natural and planted pine stands will provide all these benefits. Thinnings are “intermediate cuttings that are aimed primarily at controlling the growth of stands by adjusting stand density.” The objective of thinning is to favor the healthiest and best-formed dominant trees in the stand, removing suppressed, forked, crooked, and diseased stems.  This allows the residual stems to take advantage of the additional available site resources (water, light, and soil nutrients), increasing their growth rate and, thus, their value when harvested. Properly timed thinnings throughout the life of a pine stand can more than double the diameter of trees at the final harvest compared to an unthinned stand. This article will discuss why pine tree physiology determines the spacing and number of seedlings needed for successful pine stand establishment, how a thinning schedule is selected, different thinning methods, and the benefits of thinning to the residual stand.

Pine Tree Growth Characteristics

Trees, like humans, can grow in two directions at the same time. They elongate at their shoots and roots to increase their height (primary growth) while at the same time growing in girth to increase the diameter of stems and branches (secondary growth). The cells responsible for the development of shoots and roots are apical meristems. Growth habitat varies between species and will determine the tree’s shape, which can then dictate the value of that species for wood products.  The southern pine species have a shoot growth pattern classified as monopodial growth, where a terminal bud elongates to initiate height growth. Pines can flush multiple times in a single growing season, with the terminal bud growing more than the lateral buds. The number of flushes is related to site quality, weather, and genetics.  Pine trees also exhibit strong (excurrent) apical dominance, with the elongation of lateral buds below the terminal (apical) bud suppressed by plant hormones called auxins (figure 1). These factors create a relatively straight main stem or trunk, with a crown composed of outward-growing branches in mature trees.

Seedling spacing at the time of establishment, whether through artificial or natural regeneration, is essential in utilizing the southern pine species’ growth habits to maximize the future stand’s growth potential while meeting landowner objectives. While natural regeneration is a viable option for reforestation, it requires planning for adequate seed fall and implementation of site preparation to control competing vegetation before harvest for natural regeneration to be successful. Bumper crops of seed for loblolly pine (Pinus taeda) commonly occur every 3-to-5 to years, while longleaf pine (Pinus palustris) can be as infrequent as every ten years. Depending on when a stand is harvested, this can often lead to a “feast or famine” situation, with either too few natural seedlings established per acre or thousands of seedlings per acre requiring pre-commercial thinning (figure 2).

Artificial regeneration, or the planting of pine seedlings, began in the 1920s by the US Forest Service to reforest cutover timberland and abandoned farmland, expanding over the next few decades as pulp and paper manufacturing grew across the south. In the early 1950s, there were less than 2 million acres of pine plantations in the US south, but this number grew to over 7 million acres by 1962 due to the Soil Bank Conservation Reserve Program (CRP).  The development of a range of forestry herbicides in the 1970s and 1980s improved the control of competition on harvested sites, reducing the use of mechanical operations, which can negatively impact seedling growth by causing uneven distribution of organic matter and topsoil across a site.  Additionally, the advent of pine tree improvement programs in the 1950s improved tree growth and stem quality. It increased the resistance of trees to disease, resulting in rootstock, which was more desirable than relying on natural regeneration. These factors combined to increase the number of acres of pine plantations to over 30 million acres in the US south by the beginning of the 21^st Century.

The required number of seedlings to successfully establish the next stand of pine trees will depend on landowner objectives and site quality.   Planting recommendations usually range from 400 to 700 trees per acre (table 1), with the lower stocking (total trees per acre) favoring wildlife habitats.  Wider spacing allows additional sunlight into the stand, increasing the variety, quality, and quantity of forages and creating a greater diversity of cover types.  The more open the stand, the more lateral branches the pine seedlings will produce in response to increased sunlight until canopy closure, often growing limbs along the entire stem length.   The resources sent into the growth of lateral branches reduce the resources expended for height growth. More closely spaced trees encourage increased height growth, slower radial growth, and smaller lower limbs until crown closure.  Widely spaced seedlings have greater radial growth than closely spaced stems, increasing juvenile wood, which can lower their value for dimensional lumber at harvest.

Table 1. Tree planting spacing.

Determining a Thinning Schedule

The natural development of any forest results in the healthiest, best adapted trees expressing dominance through rapid height growth, overtopping and suppressing those that are diseased or unable to successfully compete for available site resources. Smith describes this process as a “race for the sky”, noting that tree height is the main factor in determining how well a tree will survive competition from other stems within a stand.  Canopy closure is the point in stand development when crowns of adjacent trees begin to touch. This further increases the competition for light between the trees and shades out lower limbs, increasing natural pruning and beginning the process where the stand will naturally thin itself.   Depending on the site quality, initial planting density, and quality of seedling rootstock, a first thinning can be timed following canopy closure, which typically occurs between 10 and 15 years in loblolly pine plantations and slightly later in longleaf pine stands due to their time spent in the grass stage.

Although stand age can provide a range of years for determining thinning schedules, it should not be the only consideration.  The criteria for determining when a stand should be thinned and how many trees should be removed, in addition to canopy closure, are basal area, live crown ratio, and stand volume. Each reflects the site quality, the stand’s stocking, and the vigor of the trees in the stand. A combination of 2 or more of these factors will usually be used to determine the right timing for a thinning.

Basal Area: Trees per acre (TPA) is an adequate measure of the density of trees in young stands. However, as a stand ages and the size of individual stems begin to vary, trees per acre will no longer adequately reflect the stand density since trees of different diameters take up a different amount of growing space on a unit of land. Therefore, the basal area is used.  Basal area is “the area in square feet taken up by an individual tree trunk at DBH (diameter at breast height or 4.5 feet above the ground)”. It can be determined by measuring the tree diameter and using a formula (BA=0.005454xDBH²). For forest management purposes, basal area is expressed as a total of the growing space occupied by stems on one acre (ft²/acre). If the DBH of all trees on one acre is measured, the sum of the basal area of all trees would be the basal area expressed in ft2/acre that a forest manager would use to make management decisions. Measuring all trees in a stand can be cumbersome and costly, and more often, forest managers strategically sample the trees in a stand using a wedge prism.

A wedge prism is a piece of glass cut at a specific angle that allows trees within a certain distance from the prism to be counted as “in” or “out” trees. Prisms can be cut at different angles to represent a different basal area that each “in” tree represents per acre. Common prism angles are for basal area factors of 10 ft²/acre, 20 ft²/acre, or 30 ft²/acre, meaning each tree represents this much basal area per acre. Pohlman describes how a homemade measuring device can also be used to get a rough estimate.   An example of how much basal area can vary based on tree diameter is that it takes almost 230 trees per acre for 8” DBH trees to equal 80 ft²/acre of basal area, but only 57 TPA if the trees are 16” DBH.  Southern pine stands, either natural or planted, that reach 100-120 ft²/acre of basal area need to be thinned, with a target residual basal area of 60-80 ft²/acre.

Live Crown Ratio: Smith describes live crown ratio (LCR) as the “percentage of the length of stem [occupied] with living branches.”  The formula that can be used to calculate the live crown ratio is LCR = (Total Height – Height to First Live Branch) \ Total Height x 100. Although this may seem simplistic for use in making a forest management decision, it reflects that even dominant trees in a stand that is too dense will eventually have their crowns become too small to support their size. When the average live crown ratio in a stand drops below 40%, the stand needs to be thinned.  Allowing the live crown ratio in the stand to drop too low can result in even dominant trees not being able to recover enough to reach their full volume potential.

Stand volume: Stand volume is often used to describe the merchantable timber volume, or weight, within a stand. While it is useful to know the total stand volume, the expected harvestable volume or weight is of more concern. Many logging businesses require at least one truckload of timber per acre, and a weeks-worth of timber volume, to make a harvest profitable for them. A truckload of timber is often considered 25 tons (50,000 lbs), but it can be more. We have a wide range of logging businesses in South Carolina that can produce anywhere from 20 to over 120 truckloads per week. While logging businesses with a small production can handle small acreages with lower harvestable volumes, logging businesses with a larger production need more acres or harvestable volume per acre, or both, to be profitable. Stand volume is typically calculated using tree volume equations specific to individual species, diameters, and heights. If a stand does not have enough merchantable volume for a commercial harvest, it may be wise to wait another year or two to grow enough volume for a harvest.

Thinning Operations

Pine thinnings are typically done by conventional harvesting equipment consisting of a feller-buncher, grapple skidder, and knuckle-boom loader with a pull-through delimber. For a detailed description of this equipment, consult Hiesl and Steele’s publication. In a first thinning, the feller-buncher will typically remove one entire row of the pine plantation to create a skid trail that the feller-buncher and grapple skidder can drive down. Thinnings can happen at different intensities, but a fifth row thinning, where every fifth row of pine trees is removed, is very common in South Carolina. While the entire row is removed, the feller-buncher will also selectively remove individual trees from the residual rows between the skid trails. This is to remove undesired trees of low vigor or bad form and unlikely to grow into valuable timber products. The feller-buncher will create bunches, or bundles, of cut trees in the middle of a skid trail.  A specific number of trees is piled so that the grapple skidder can carry a full load of trees to the landing on each trip. The landing, or log deck, is where the harvested trees will be delimbed by a trailer-mounted loader and the pull-through delimber, and it is also where individual products will be sorted. While first thinnings often only produce pulpwood and possibly biomass chips, subsequent thinnings may also have other products such as chip-n-saw, saw logs, ply logs, poles, and more. The skid trail is already established for subsequent thinnings, and the equipment function is the same.

Stand Damage

Following a thinning, trees can grow into higher valued products if they were not damaged during harvesting. Damage to the residual trees in the form of removed bark and exposed wood fiber can lead to rot and insect infestations, both of which can reduce the value of the stem. Most logging business owners and logging equipment operators strive to avoid damaging trees while performing a thinning. However, given the size of the harvesting equipment, this is not always possible, and some trees will get damaged. Loggers often select trees along a skid trail as bumper trees used at turns in the trail to help pivot the load behind the skidder. The bumper trees can get damaged but will protect the trees behind them from any damage. Seeing a bumper tree during a thinning operation can shock the landowner. However, these trees are typically harvested at the end of the thinning. Despite any protective measures, a few trees will always have minimal damage left after a thinning. This is normal and not a cause for concern. Often up to 20% of the residual trees have some form of damage, ranging from broken branches to a bark scuff and small areas of exposed wood fiber. If more trees than that are damaged and noted during the thinning, it is time to talk to the timber buyer or logger to minimize any further damage to the stand.

Improving Forest Health through Thinning

Another critical consideration for conducting thinning operations when needed is that it will maintain a healthy forest with a reduced risk of insect attack, primarily from southern pine beetles (SPB). Pine stands that are overstocked and have surpassed the target age for a thinning have less resistance to beetle attack, particularly those located on drier soils and during periods of extended drought.  A report from Georgia found that the SPB hazard rating was high on a low-quality site when basal area was 120 to 145 ft²/acre in loblolly pine stands 18-21 years of age and increased to very high in older stands with higher basal area.  While thinning is not a guarantee that a pine stand will not be attacked during a major SPB outbreak, it can lessen the impacts since the stand will be growing more hardy trees with less competition for site resources. Finally, thinned stands will have great air movement, which dilutes SPB pheromones and reduces the attraction of additional insects to the trees.

Conclusion

Properly timed thinnings will allow a forest landowner to improve the growth rate and health of the residual stand while earning income from removing suppressed or diseased trees that would die naturally if left unharvested or continue to grow so slowly that they are of little value at the end of the stand rotation.  The desired response in the trees that remain following a thinning will be increased radial growth and the ability of the crop trees to support an adequate live crown ratio until the subsequent thinning or a final harvest.  Generally, a thinning cycle of every 6 to 8 years will improve tree quality in the stand and allow the landowner to focus on growing trees into the desired product class.

References

Baker, JB, Langdon, OG. Pinus taeda L. Loblolly pine, In: Burns, RM,  Honkala, BH, editors.

Silvics of North America, Volume 1, Conifers.  Washington, D.C. (US):  U.S. Department of Agriculture, Forest Service; 1990.  [accessed 2022 August 4]: Agriculture Handbook 654. p. 497-512.

Clark, AI, Saucier, JR, Baldwin, VC, & Bower, DR. Effect of initial spacing and thinning on lumber grade, yield, and strength of loblolly pine. Forest Products Journal. 1994 [accessed 2022 July 28]; 44(11, 12): 14-21. https://www.proquest.com/docview/214631563?pq-origsite=gscholar&fromopenview=true.

Conrad, JL, Greene WD, and Hiesl P. The evolution of logging businesses in Georgia 1987-2017 and South Carolina 2012-2017. Forest Science. 2018 [accessed 2022 August 30]; 64 (6): 671–81. https://doi.org/10.1093/forsci/fxy020.

Cunningham K, Barry J, Walkingstick T. Managing loblolly pine stands…from A to Z. Fayetteville (AR): University of Arkansas Cooperative Extension Service; 2008 [accessed 2022 July 8].FSA5023. https://www.uaex.uada.edu/environment-nature/forestry/managing-woodlands/FSA-5023.pdf.

Demers C, Andreu M, McGowan B, Long A, Nowak, J. Thinning Southern pines- a key greater returns. Gainesville (FL): University of Florida Extension; 2013 [accessed 2022 July 8]. SS FOR24. https://edis.ifas.ufl.edu/pdf/FR/FR159/FR159-Dkcmuj4wmy.pdf.

Dickens W, Morehead D. A guide to thinning pine plantations. Athens (GA): Warnell School of Forestry and Natural Resources; 2015 [accessed 2021 July 16]. Publication Number 0010. https://bugwoodcloud.org/bugwood/productivity/pdfs/Guide_thinning_pine_plantations.pdf.

Franklin J, Mercker D. Tree growth characteristics. Knoxville (TN): UT Extension; 2009 [accessed 2022 August 8]. W227. https://extension.tennessee.edu/publications/Documents/W227.pdf.

Fox T, Jokela E, Allen H. The development of pine plantation silviculture in the Southern United States. Journal of Forestry. 2007  [accessed 2022 August 4]; 105(7), 337-347. http://rothforestry.com/Resources/Fox%20et%20al.%202007.pdf.

Helms J. Brief history of the USDA Soil Bank Program. Washington (DC): United States Department of Agriculture Natural Resources Conservation Service; 1985 [accessed 2022 August 9]. Historical Insights 1. https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1045666.pdf.

Hiesl, P. A brief summary of the 2017 South Carolina logging business survey. Clemson (SC): Clemson Cooperative Extension Service; 2020 [accessed 2022 August 30]. LGP 1048. https://lgpress.clemson.edu/publication/a-brief-summary-of-the-2017-south-carolina-logging-business-survey/.

Hiesl P, Steel J. Timber harvesting equipment in South Carolina. Clemson (SC): Clemson Cooperative Extension; 2020  [accessed 2022 August 30]. LGP 1056. https://lgpress.clemson.edu/publication/timber-harvesting-equipment-in-south-carolina/.

Kramer P, Kozlowski T. Physiology of Woody Plants. Orlando (FL): Academic Pres, Inc.; 1979.

Moldenhauer MC, Bolding MC. Parcelization of South Carolina’s private forestland: Loggers’ reactions to a growing threat.  Forest Products Journal. 2009; 59 (6): 37–43.

Moorman C, Hamilton R. Developing wildlife-friendly pine plantations. Raleigh (NC): NC State Extension; 2019 [accessed 2022 August 4]. WON-38. https://content.ces.ncsu.edu/developing-wildlife-friendly-pine-plantations.

Murray, J. Longleaf pine restoration in the South Carolina Sandhills wiregrass gap. Clemson (SC): Clemson University; 2021. https://tigerprints.clemson.edu/all_theses/3707/.

Nowak J, Meeker J, Coyle  D, Steiner C, Brownie C. Southern pine beetle infestations in relation to forest stand conditions, previous thinning, and prescribed burning: Evaluation of the southern pine beetle prevention program. Journal of Forestry.  2015 [accessed 2022 July 8]; 113(5), 454-462. https://doi.org/10.5849/jof.15-002.

Pohlman S. Homemade devices to determine basal area. Clemson (SC): Clemson University Cooperative Extension; 2020 [accessed 2020 November 23]. CU in the Woods Fall 2020 Edition. https://www.clemson.edu/extension/forestry/newsletter/newsletter_files/2020-fall-article-homemade-basal-area-tools-spohlman.

Radford A, Ahles H, Bell C. Manual of the Vascular Flora of the Carolinas. Chapel Hill (NC): The University of North Carolina Press; 1968.

Smith D, Hawley R. The Practice of Silviculture. New York, NY: John Wiley & Sons; 1986.

Stokes, BJ, Watson WF. Plantation thinning systems in the Southern United States,  In: Mitchell P, Stevens E, Angus-Hankin C, Kofman P, editors. Problems and prospects for managing cost effective early thinnings; a report from the concerted action: cost effective early thinnings. Hoersholm (Denmark): Danish Forest and Landscape Research Institute; 1996. [accessed 2022 July 8]: AIR2-CT93-1538. p. 107-121. https://www.srs.fs.usda.gov/pubs/ja/ja_stokes019.pdf.

Siry J, Hood H, Baldwin S, Smith J,  Gaw M, editors. Biomass, logging rates, and Species detail. Athens (GA): Frank W. Norris Foundation; 2022. Timber Mart South 2nd Quarter 2022.

Tonn N., Greb T. Radial plant growth. Current Biology. 2017 Sep [accessed 2022 August 4]. 27: R878-R882. https://www.sciencedirect.com/science/article/pii/S096098221730355X.

Vasiliauskas R. Damage to trees due to forestry operations and its pathological significance in temperate forests: A literature review. Forestry. 2001 [accessed 2022 August 30]; 74 (4): 319–36. https://doi.org/10.1093/forestry/74.4.319. 2001.

Walker M. The Southern pine region, In: Barrett JW, editor. Regional silviculture of the United States. New York (US): John Wiley & Sons; 1980. p. 231-276.

Author(s)

Patrick Hiesl, Clemson University, Associate Professor of Forest Operations

Janet Steele, Cooperative Extension, Forestry and Wildlife Agent

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