When to Thin Pines
Are my pine trees ready to thin?
There is no definite answer to this question, because every pine plantation may be different. Some plantations may be ready to thin as early as age 12, and some may not be ready for thinning until age 18 or older. Many factors determine when a pine plantation is ready to be thinned: site productivity, planting density, genetics, and weed competition. But the timing of the first thinning is very important. This first thinning may affect the productivity and economics of the pine plantation for the next 20 to 30 years, so it is a must that thinning be done at the right time.
Understanding the biology of pine trees can help clarify why and when pine trees should be thinned. Pine trees do not tolerate shade. This cannot be reiterated enough! They need direct sunlight to survive and grow well. As pine trees grow, they compete with each other for water, nutrients, and sunlight. The green needles in the tree crowns make food for tree growth. The fastest growing trees are the ones with the biggest crowns. These trees take a “dominant” position in the stand, where they receive direct sunlight from above and from the side. Since pines do not tolerate shade, their branches die from the ground up as the trees get taller and the lower branches become more shaded. Known as “natural pruning,” this process results in fewer and smaller branches on the lower stem and a higher quality tree. As a landowner, you want to grow a tall tree with a clean stem and well-developed crown. An expanded, well developed crown can make more food and lets the tree grow quicker. Trees are like other crops in that they don’t grow well if there are too many per acre. The number of trees per acre affects diameter growth of individual trees, thus the yield and growth of the entire stand. As the trees grow larger, the site can support fewer trees per acre. To maintain the vigor and growth rates of the best trees, known as “crop trees,” pine plantations are thinned to a density the site can best support. When pines are thinned at the proper time and in the proper manner, landowners benefit in several ways:
- High quality trees can grow. Lower quality trees are removed to give “crop” trees more growing space. Growth is increased on fewer, higher quality trees. It takes less time for trees to reach the more valuable saw timber size class.
- Landowners receive intermediate income. Trees that become crowded and overtopped die before the final harvest. Thinning lets landowners sell and use these trees that would otherwise be lost in the “natural thinning” process.
- Health and vigor of the stand are maintained. By reducing competition and removing weak trees, the remaining trees are more vigorous and less susceptible to Southern Pine Beetles and other insects and diseases.
- Wildlife habitat is enhanced. Thinning lets sunlight reach the forest floor, resulting in greater production of browse for deer and other wildlife.
While you as a landowner should care about the financial rewards today of thinning your stand, money, by NO MEANS should be the determining factor. Understand, this is the most critical step in insuring the monetary return on your investment in the future. Once thinned, you are married to the quality of worked performed for the next 20-30 years. Thinning pines is akin to building a house, the stronger the foundation, the longer it will stand.
Do it correctly and you will be rewarded for years to come.
It is to your advantage to thin your pine plantation. When is the proper time to conduct your first thinning?
Consider these points before you conduct a pine thinning:
- Tree diameters
- Stand density
- Tree heights
- Natural pruning
- Growth rates
All four are important, but tree diameters and stand density are the most important because they influence growth rates.
We will happily come to your property and assist you in determining how and when your valuable pines should be thinned. Call us at 404-956-2922 and we will set up an appointment at your convenience to view your timber.
Tree Diameter
Diameter at breast height (DBH) is the diameter of the tree stem 41 /2 feet above the ground. You can take this measurement with a diameter measuring tape. DBH is important because trees must average at least 6 inches DBH to be sold for pulpwood. Trees smaller than 5 inches DBH are not “merchantable” and typically will not be cut. Thinning your stand before the average size of the trees is 6 inches DBH or larger may result in “high grading,” where the only trees harvested are the larger, faster growing, “dominant” trees. These are the trees you want to leave as your “crop trees,” not the ones to harvest. When your trees average 6 inches DBH or larger, you can harvest the slower growing, smaller, less vigorous trees and give your “crop trees” more room to grow.
Stand Density
Stand density is determined by both the size (DBH) of the trees and the total number of trees per acre (TPA). As the average size of the tree increases, the number of trees the site can support decreases. For example, at the time of tree planting, a site may easily support 600 or more tree seedlings per acre. But as tree diameters and crowns increase in size, the amount of nutrients, soil moisture, and sunlight required for best growth also increases. Eventually, the stand density (TPA and DBH) becomes too high for good growth rates. The goal of thinning is to reduce stand density by removing the slow growing, lower quality trees, thus maintaining rapid growth on the straight, healthy, vigorous, and evenly spaced crop trees.
Natural Pruning
Since pines do not tolerate shade, their branches die from the ground up as the trees become crowded and overtopped. These dead limbs, over time, shed or fall off the trees. This is known as “natural pruning” and results in a tree of higher value with a clean stem, a well-developed crown, and wood production concentrated in the stem. Natural dying of the lower branches to a minimum height of 18 feet should happen before you thin a pine plantation. If there are live, green limbs less than 18 feet from the ground, thinning the stand could lower tree quality. These green limbs will be exposed to sunlight after thinning and will continue to grow. Thinning too early can result in growing larger lower limbs, which eventually lower the quality of the logs, hurt diameter growth, and reduce the value of the tree. In other words, no matter how large the tree gets, you may only have pulpwood.
Growth Rates
The main objective of thinning a pine stand is to maintain the vigor and growth rates on the best trees, known as “crop trees.” When growth rates decline, it’s time to thin. The ideal situation is for the “crop trees” to continue growing at a steady and vigorous rate. Take a growth increment core from trees to determine growth rates of your dominant and co-dominant trees. Calculate the percent annual stem growth by using increment boring and DBH measurements. This annual stem growth rate is the final criterion you should use to determine if your plantation is ready to thin. For example, your plantation may have the DBH, heights, natural pruning, and density levels to justify a thinning. But if tree stems are still growing at an annual rate of 10 per cent, it may be wise to postpone thinning. Why would you want to harvest half your trees when they are still growing at 10 per cent?
Attracting buyers and harvesting efficiency may be other reasons to postpone thinning. Harvesting efficiency increases dramatically with diameter growth. For example, it takes about 298 trees 5 inches in DBH to make a truckload of pulpwood. It takes only about 221 trees 6 inches in DBH and 183 trees 7 inches in DBH to make a truckload. If your pine plantation is growing at an annual growth rate of 10 per cent, many of the smaller 5- to 6-inch DBH trees may grow into 6- and 7-inch DBH trees in only a few years. The result will be higher harvest volumes per acre, higher stumpage prices, and ultimately a higher per acre income from your first thinning. In some situations you may want to postpone thinning a pine plantation, and in other situations you may not. Growth rate is the final determining factor, but can vary by your own objectives, and soil site quality. Decide what growth rate is acceptable to you. You may decide that as long as the trees are growing at the prime interest rate, you want to let them grow. Or, you may have already set a rate of return you want the trees to produce. An acceptable factor could range between 5 percent and 15 percent, depending on your objectives.