Thinning your forest is one of the most important silvicultural operations you can conduct on your land. It is an intermediate treatment that focuses on selectively removing trees to reduce competition and allow residual trees to flourish. However, the process is more complicated than simply cutting out a few trees. The stand dynamics and silvics of the species must be understood to successfully meet desired outcomes, and so over the years, multiple types of thinning regimes have been developed by silviculturists and foresters to meet diverse needs of forests around the world.
While the development of thinning as a science and art has been a boon to forest management practices, it has created a complicated field intimidating to beginners and especially amateur forest landowners looking to increase the value and vigor of their land. In light of that, we are here to help you demystify the process and understand the many nuances of thinning your forest so you can unlock the benefits of silviculture for your own land.
The Objectives of Thinning
The primary goal of thinning your forest is increasing the growth rate of individual trees and reducing the time the stand will take to reach maturity, but that is not the sole objective. Thinning also achieves a wide array of benefits that are crucial for successfully improving the future value of the forest, including improving the mean stand diameter, improving the economics of later harvests, and improving the health and vigor of individual trees by preventing the recession of crowns caused by dense forest environments. Because a certain percentage of the stand is removed, thinning is also an effective way of improving species composition and average stem quality if lower-value species and poorer quality stems are targeted for removal.
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The Silvics of Thinning Your Forest
When thinning your forest, one must have a deep understanding of the silvics of the species to implement the best suited prescription. While specifics may differ, the general dynamics of a species go like this:
When a stand is young, there are thousands of stems per acre–too much to grow in one area. The mortality is inevitable, so trees allocate most of their energy to grow upwards to get sunlight before being out-competed and over-shadowed by neighbors.
Over time, trees are outcompeted and drop out of the race, but then trees face a rising risk of wind, snow, and ice, as their skinny stems must increasing stand unsupported by neighbors. At this point, the tree allocates its energy to growing outward, expanding the stem diameter to ensure structural stability against these threats.
The Different types of Forest Thinning
Thinning can be broken down into two primary categories: pre-commercial and commercial thinning. Of the two, commercial thinning is the most complex, with numerous subtypes and methods.
Pre-Commercial Thinning (PCT)
Pre-commercial thinning (PCT for short) is a type of thinning that involves removing smaller, unmerchantable trees in order to allow more desirable species. There are two primary benefits of targeting stands at a young age: a higher availability of crop trees and faster overall growth. When a stand is young, the total count of trees per acre is high, and as those trees grow, trees naturally die out as they are outcompeted. You can see this relationship in the graph below.
By thinning the stand at an unmerchantable age before this mass-mortality happens (known as the stem exclusion phase), the forester (or landowner) has more options to choose from in terms of species and quality. Being able to select from a greater number of trees results in superior outcomes.
Moreover, when trees are thinned at a young age, it prevents a great deal of crown recession, which ensures growth remains robust by keeping total foliar area high. We will discuss these dynamics more in-depth later.
Pre-commercial thinning is usually conducted with crews equipped with brush saws. Trees are cut manually and left on the forest floor to decompose.
What Types of Forests are Suitable for PCT?
Pre-commercial thinning is usually reserved for young, even-aged softwood stands between 8 and 15 feet tall. Particularly in the northern US, stands of spruce, fir, and pine are very frequently pre-commercially thinned, and in the south, PCT is often used to clean plantations of yellow pine. While hardwood can be pre-commercially thinned, it usually is not due to the lower ROI and dependence of hardwood value on quality, which can be negatively affected by lower density. To learn more about this dynamic, check out our free guide to DIY forestry, which covers the difference between quantitative and qualitative management.
The Proper Spacing for PCT
Crucial to a successful PCT operation is determining the proper density to space trees. If trees are left too dense, the benefits of the PCT will be short-lived, and the stand won’t reach its full potential. However, if trees are too sparse, growing space will be wasted or the stand will begin to regenerate, leading to a messy, uneven-aged structure.
While PCT spacing is expressed in literature as a count of trees per acre (TPA), it is usually expressed as a dimensional spacing regime for the purposes of practical application. Below is a conversion table for typical PCT spacing regimes and TPA.
The best spacing for your PCT will be dependent on species, region, and site quality, but you can research more about the proper PCT for various species of commercial timber here:
If you can’t find what you are looking for, a good rule of thumb is 8-10ft spacing for softwoods, and 10-12ft spacing for hardwoods. However, hardwoods should be space when trees are taller at 20-30 feet in height.
In contrast, to PCT, commercial thinning is a full-scale harvest in which harvested stems are sold as a usable product. The goal of the harvest is similar to other types of thinning: the removal of competition to accelerate the growth of residual timber. But because trees are released from competition at a later stage in life, there is arguably more nuance in silvicultural decisions that need to be made, leading to a variety of sub-types of commercial thinning, including low thinning, crown thinning, free thinning, and systematic thinning.
Low Thinning (Thinning From Below)
Low thinning (Also called thinning from below) is a type of thinning that involves the targeted removal of trees that are below the average canopy height. This is one of the most common types of thinning, as it favors fast-growing individuals that are presumably already growing under ideal conditions. It removes individuals with little prospects that will only interfere with ideal crop trees.
Typically, low-thinning is used in stands of fast-growing softwoods grown to be sold as sawlogs. Because the goal is to grow logs to sawlog size, it is important to favor trees most likely to reach that size threshold.
Crown thinning (Thinning From Above)
Crown thinning (Also called thinning from above) is a type of thinning that removes trees from the upper canopy. Typically, this occurs in stands with either uniform height or where there is a pronounced difference in quality in older, larger trees. In my experience, crown thinning is usually used when there is a component of low-density, older trees, where the canopy has a wide range of species or in “even-aged” stands that had asymmetrical regeneration patterns, resulting in worse quality for some older trees. Typically, crown thinning is used in stands in hardwood with the goal of improving quality, though not always.
Free Thinning (Crop Tree Release)
Free thinning (also called a crop tree release) is a type of thinning in which trees are removed from all size and age classes. Typically, free thinning is used for the promotion of quality in stands of high-value hardwoods. Individuals are chosen based on their own individual attributes (usually quality and vigor) in order to improve the average quality of growing stock and improve the long-term value production of the stand. In this sense, the system is similar to a selection cut, but it is used as part of an even-aged silvicultural system.
Systematic thinning is a type of thinning in which trees are removed on the basis of a pre-determined rule, such as every third tree or every other row, sometimes called strip thinning. This is commonly used in plantations where trees are planted in orderly rows, making systematic removal more tenable. Systematic spacing could also be based on strict spacing requirements such as spacing trees 10 feet apart. In such cases, it can be called geometric spacing. However, because removal does nothing to tip the balance of growth in favor of any parameters such as quality or growth rate, systematic thinning is best used in situations where the quality and size of individual trees is largely irrelevant and focus must be placed on maximizing volume per acre. Such is the case with trees grown for pulpwood.
What Type of Thinning Is Suitable for Your Forest?
With so many different types of thinning, it can be confusing about which is best for you and your forest type and condition. While not to be used as a definitive decision maker, we made this decision tree to help you decode what type of thinning you should look into.
Effects of Thinning on Tree Growth
Thinning is used to increase the growth rate of a tree, but that doesn’t mean that thinning will always increase growth or that responses will always be equal. Understanding how trees respond to thinning and how it differs by age and stem form is crucial for implementing a successful thinning regime.
Live Crown Ratio
When trees grow densely, the individual tree’s imperative is to grow up toward the sun to avoid their own mortality from being suppression. As their lower branches are shaded out, these branches are discarded to conserve energy and focus resources toward limbs that are photosynthetically productive. Thus, A dense stand will have many trees, but the crown on each tree will be smaller. The size of these crowns is expressed as the Live Crown Ratio or LCR. While the total growth of a dense stand with low individual LCRs may still be adequate, the growth per tree will likely be miniscule as the trees have limited foliar resources to grow.
Thinning in a stand with an inadequate LCR may fail to respond desirably. Because the trees per acre has been reduced through thinning, the overall stand growth has been reduced, but the growth per tree will still remain stagnant as each tree does not have enough foliar area to respond to the increase in light availability.
Optimizing LCRs can be tricky business: Once a crown recedes, the only way for the LCR to recover is for the tree to grow taller. Leaves and branches don’t grow back from the base, so it is critical that efforts are focused on preventing crown recession in the first place. Timing and consistency is crucial to ensuring a thinning has as much efficacy as possible.
The age of trees is crucial to the effect thinning will have on tree growth. The growth of a tree will resemble a sigmoid shaped curve over time. When a tree is young, It grows at a slow rate. As it ages, its growth rate accelerates until it reaches maturity, at which point growth once again slows. The growth pattern can be seen in the chart below.
When a forest is thinned, volume per acre is reduced as a bet that the volume per tree will increase enough to make up for the loss. If trees are thinned when they are mature and no longer grow quickly, a thinning treatment may only result in wasted and unutilized growing space.
Economics of Thinning
Thinning operations can be expensive endeavors. Timber harvesting involves big machinery, and if money can’t be made, it can get expensive quick. Whether a thinning operation is conducted commercially or pre-commercially, the economics must be considered.
One of the prime considerations to be made as to whether or not a stand can be thinned is whether the logs will be large enough to sell to a mill. We call this the merchantability of timber. If wood can be sold as pulpwood, a stand can be thinned at a younger age, but for sawtimber, the size must be larger, sometimes much larger. This can be thing single most limiting factor to successfully thinning your land.
If trees cannot be merchandized, the stand can still be pre-commercially thinned, but generally this becomes less viable as trees grow larger. Larger trees are harder and more expensive to cut, and the benefit is reduced. Moreover, one must ask whether it makes since to leave trees to die on the forest floor when they may be large enough to be sold in only 5 years. Often, then, there does exist a window between PCT viability and commercial thinning viability when a stand of timber simply cannot be thinned.
Even if a stand is merchantable, larger average stem sizes will still be better from a harvest return standpoint. Machines are more productive when piece size is larger, so thinning your forest is more economic when trees are larger, even if they can still be sold at smaller sizes.
However, such productivity can often be a Faustian bargain. The longer one waits to thin, the more crowns recede and the lower the LCR, limiting the response and benefit of the thinning.
Consider the Economic Objective.
This is why it is often wise to PCT To lower densities. pre-commercially thinning to lower densities allows individual stems to grow larger without the associated loss of LCR. However, lower PCT densities may result in lower overall volume per acre. Even so, lower volume per acre can result in higher overall returns, as larger piece sizes can create higher stumpage value per unit, as seen conceptually in the graph below.
Carefully consider the economic objectives of your harvest to ensure the best outcomes from thinning your forest. This includes planning what timber products will be sold from harvests and how to balance operational viability with silvicultural viability. The timing of a harvest is critical, so do not neglect to plan.
Ensuring Stand Stability
One of the principles behind thinning is to prevent competition, but for trees, competition can resemble cooperation. When trees grow up tight and dense, they have skinny stems and tall heights, allocating energy to break the canopy to get light. However, at this point, they also depend on the presence on their neighbors to stay upright. without them, such a tree would be like a single corn stalk in a storm, vulnerable to wind, snow, and ice.
Thus, there is an increased risk of instability when thinning these stands of tall, skinny trees due to the high height to diameter ratio (H/D ratio). However, some species are more vulnerable than others. Fore example, balsam fir and red spruce are more vulnerable while white pine is less so. Thoroughly research the silvics of the species you intend to fully understand risk factors.
That said, stands with higher H/D ratios almost always have a lower LCR, making them less than ideal candidates for thinning regardless.
Implementing a Thinning
If you are ready to thin your forest, you can use the information provided here to give you a good understanding of the process and science behind the silviculture. While giving yourself a forestry education is crucial to successful long-term management, it is often still best to hire a forester, particularly if you plan on having someone else log your land. They can provide critical feedback on your desired objectives and outcomes and build upon the information provided here. That said, don’t be afraid to go out (with proper training) and try thinning yourself after doing your due silvicultural dilligence. Nothing will give you a better education than real experience.
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