What is Pre-Commercial Thinning (PCT)?


Sometimes, there are just too many trees on a given acre of forest. Excessive competition can choke out individuals and hamper timber growth for decades. Foresters and woodlot owners use pre-commercial thinning, or PCT, as a way to limit competition. Pre-commercial thinning is the process of mechanically spacing out young, unmerchantable trees to improve species composition and allow residual trees to grow healthier and faster. It is a powerful (perhaps THE most powerful) tool landowners have to manage their forests and ensure healthy, profitable forest growth for years to come.

Pre-commercial thinning in Maine
PCT in a stand of red spruce.

Why Pre-Commercially Thin Your Forest?

Trees, like any other organism, are hard-wired to survive. When trees are in grown in dense competition with other individuals, the limiting factor for survival is light. Trees put all of their resources into growing as tall as possible to pierce the dense canopy and reach the sunlight. As a result, the tree will put on very little diameter growth and instead grow tall, creating a stand of spindly trees with little commercial value. These trees will then drop their lower branches, as there is no need to waste energy maintaining leaves or needles that are too shaded to photosynthesize. The reduced foliage (also known as a lower live crown ratio) permanently affects the trees’ ability to grow, hampering it for its life and limiting the amount of wood it can produce for you to later harvest.

Live Crown Ratio
A Full-crowned tree and a tree with a low live-crown-ratio from growing in dense conditions.

Trees with low live crown ratios and grown in dense conditions have three main problems for the landowner seeking a healthy return from their forests:

  • Smaller diameter trees are expensive to harvest and are generally less valuable at the mill.
  • Increased height to diameter ratios make the trees unstable and fragile to wind, ice, and other stressors.
  • Lower foliar area reduces growth rates and the ability for the trees to respond to future commercial thinning treatments.

By pre-commercially thinning a stand, landowners can have a substantial amount of impact on how the trees grow. Thinning a stand at a young age, prior to crown recession, prevents the growth rate of the tree from slowing down due to competition and allows the stem to grow much larger in diameter than it otherwise would. Moreover, it can reach these sizes in a much shorter period of time. For the landowner, this not only means a higher net return on the wood, but a shorter time to wait to see a financial return.

In addition to the improved growth, PCT offers another crucial advantage: improved species composition. By going in and manually selecting which trees to cut and which to leave, the landowner can select species that fit their management objectives best. For example, in Maine, PCT is often done in stands of spruce and fir, and the longer-lived and more valuable spruce is selected over the rot-prone and fragile balsam fir. Usually, landowners are at the whim of the forest when it comes to what they grow, but PCT offers a chance to have near-complete control over growth, second only to planting.

How is Pre-Commercial Thinning Done?

While pre-commercial thinning can be done with larger machines like feller-bunchers, it is most commonly done manually with brushcutters equipped with a saw blade. Forest workers enter a stand and meticulously analyze the details of every tree. Species, crown, and health are all considered. When workers find the best tree, all other surrounding trees are cut away within the set pre-determined spacing, such as 8 or 9 feet. This process is done hundreds of times on each acre until the project is completed.

What Species Can Benefit From Pre-Commercial Thinning?

While thinning is most common in softwood species like spruce, fir, and pine, any species can be pre-commercially thinned, including hardwoods like sugar maple. However, keep in mind that different species have different attributes that make them marketable. Softwood management is primarily focused on producing a large quantity of wood. The goal of PCT in softwood, then, should be to maximize volume production. In stands of hardwood, value is mostly determined by the quality of the lumber. so stem form should be a top consideration, and crop tree selection should take a priority over a systematic spacing regime.

The timing of treatment will also vary depending on which species you intend on thinning. For softwoods, earlier is generally better. The goal is to thin stands before growth has begun to slow, but also when the trees are growing at a fast-enough pace to outgrow any ingrowth that may occur in the canopy gaps. Hardwoods should be treated entirely differently. The value of the hardwood tree is primarily found in the lowest part of the stem, so thinning should only be done when the first 16 feet of the stem or so is free of live branches. This will allow for the production of knot-free lumber and produce high-value timber. Another option is to combine PCT with pruning to achieve the same goal.

A stand of PCT’d sugar maple.

How Wide Should Trees Be Spaced?

The spacing of trees will vary considerably based on what species you are thinning, how old your stand is, and what your specific objectives are. Foresters use a tool known as a stand density management diagram to make decisions regarding residual stand density, but it is a limited tool that doesn’t necessarily account for all management objectives. The best option when deciding what spacing to use is to contact a local forester and ask what is best for your situation. However, from personal experience, I am happy to recommend a spacing regime of 630 trees per acre for northeastern spruce/fir stands.

Can Pre-Commercial Thinning Be Beneficial to Wildlife?

Of course! Mature softwoods can be crucial habitat for species like deer who need the closed, tight canopies to create a comfortable habitat to move around in thick snowfall. These areas are known as deer yards. By promoting the developmentof large, sturdy softwoods and drastically reducing the time to maturity, PCT can increase deer yards and help maintain a healthy herd.

Moreover, the brush created by leaving small trees on the ground creates excellent cover for rodents and other small mammals for years to come. Increased populations of prey animals can support greater populations of predators, greatly increasing wildlife on your property. In fact, in the wake of the budworm outbreak of the 70s, Maine sports a large population of Canada Lynx, owing largely to the massive stands of softwood that functioned as perfect habitat for snowshoe hare, the preferred prey of Lynx.

Combined with other stand and cover types, pre-commercial thinning is an excellent way to enhance habitat diversity for wildlife.

What Are the Risks of Pre-Commercial Thinning?

The primary risk of PCT is financial risk. Pre-commercial thinning can be an expensive operation, and fluctuating lumber prices and harvest costs can affect your net return at the end of a rotation. The best to mitigate financial risk is to only do PCT on the best sites on your property. Nutrient-rich, well-drained soils will produce faster growth rates, and a favorable species composition on a given acreage will further support fast and profitable growth. Generally speaking, PCT will be more profitable in softwoods than hardwoods owing to the higher growth rates, but its up to you to analyze your own situation and determine the best decisions for your property. Consulting with a forester is the best way to ensure you get the highest return from your land.

Another large risk of PCT is thinning to a density that doesn’t support management objectives. During my time as a forester, we often dealt with stands that were pre-commercially thinned as far back as 1993. Unfortunately, these stands were not originally thinned to a low-enough density. While they produced better timber than if the stand had been left in a natural state, there was still too much competition. Live crown ratios were too small, and height to diameter ratios were too high. When we attempted to enter the stands for a commercial thinning, the spindly trees could not support themselves on there own, and the stand collapsed as soon as a windstorm passed through. Sadly, we had to clearcut many of these stands. If you are planning a commercial thinning later, you must pay closer attention to PCT densities and, later on, the timing of harvest.

Poor-quality PCT can lead to future failures if care is not taken.

Lastly, if you live in a drier climate, you should be aware of fire dangers. Pre-commercial thinning can can leave dry woody debris that can act as fire fuel on the ground for years after treatment. However, lower-density stands of wood can lower fire risk in the long-run. Understand the ecology of your forest and your climate prior to commencing any PCT treatment. Your local forest service can give recommendations on how to mitigate fire risks.

How Much Does Pre-Commercial Thinning Cost?

While there is, of course, much variability, it is reasonable to expect PCT to cost between $200-$300 per acre. However, this is only if you are dealing with larger tracts of property and the stands are dense. If you are only looking to thin a few acres or clean up lower-density stands that aren’t necessarily choked-out from competition, it may be cheaper and easier to do it yourself.

How Long Does Pre-Commercial Thinning Take to Complete?

A skilled worker can complete around one acre of PCT a day. Commercial crews usually work in crews of ten, allowing them to cut around ten acres a day. If you are brand new to thinning and working on your own property, however, you can expect it to take much longer until you get the hang of it.

Should You Pre-Commercially Thin Your Forest?

PCT can be one of the best and highest-paying forest management activities you can do for your land. It greatly increases growth, allows you more control over the health, vigor, and quality of your growing stock, and it can even benefit wildlife. As a forester, PCT was always my favorite management activity. Intervening in a forest’s growth trajectory when a stand is still young gives you immense satisfaction. You can personally impact the forest in a positive way, and you will leave your mark on your land and the forest landscape for decades to come. Pre-commercial thinning is at the heart of modern forest management, and if it fits your property, I recommend you give it a try.

Zachary Lowry

Working as a professional forester in northern Maine, I quickly saw the opportunities within the forest industry for small-time investors and woodlot owners. I started The TImberland Investor to bring these insights and opportunities to you.

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