To Grow Trees, Kill Them: A Primer On Thinning

If you have spent any length of time exploring Maine’s forests, you have probably encountered a dense thicket of young trees. These are the kind of areas that can induce tears on a camping trip when you have no choice but to push through them carrying forty pounds of equipment and a canoe. They also take the form of dark evergreen stands with an impenetrable canopy: the type of forest kids flock to thanks to the abundant opportunities they supply in playing hide-and-go-seek, or the endless hours of fun provided by pushing down dead trees while yelling, “Timber!” Tears and joy aside, it’s an exceedingly common sight in our woods, and it creates a paradox for foresters.

Forestry has the unique problem of growing products that actively try to outcompete and ultimately kill each other. The dense patches you see in the woods are, for the trees, a stagnant environment devoid of light necessary for expedient growth. Left unhindered, such thickets ultimately give way to dark stands full of dead stems, the losers in the battle for survival. Ultimately, stronger trees will survive and the resulting stand will be vigorous and ready to supply Maine with the forest products that enrich us all; but it takes a while for nature to take its course. It takes a long while. Luckily, foresters have a tool to hasten growth and bypass the natural selection process, and we call it thinning. Thinning takes two forms: pre-commercial and commercial.

Pre-Commercial Thinning

Pre-commercial thinning operations (known as PCT) aim to remove the competition of desirable species and individuals and to reduce overall density before the trees reach a size at which the wood can be milled into a final product. In Maine, these operations are usually conducted in young softwood stands, both natural and planted. Unlike commercial operations, which utilize large logging equipment, these thinnings are done by crews equipped with brushsaws, which resemble weed-whackers. These workers enter the stand and cut the trees based on desired objectives. The trees, once cut, are left on the ground to decompose. But the actual labor of cutting is only a small piece of the work involved.

The workers can’t just go in and start cutting. Extensive research takes place and considerations are made to determine just how much to cut. If too many trees are removed the remaining trees will grow to become too branchy, which will reduce wood quality, as well as making the stem excessively tapered, which could reduce merchantable volume. Of course, cutting too much also reduces final volume, which lowers the revenues when the stand is harvested. Conversely, if not enough is cut, the remaining trees may have suboptimal growth rates; and, still stressed from competition, will grow upward and not produce enough diameter growth. The resulting volume per tree will be lower, and revenues lower at the time of harvest.

Along with density, the residual species may also be considered in this process. Fir, for instance – while an important species for lumber – is highly vulnerable to spruce budworm and susceptible to rot, so foresters may decide to favor other species, such as spruce. Or a stand may be naturally interspersed with maple and birch; and a forester may choose to leave these for the sake of biodiversity, though they are not the species targeted to be grown.

Commercial Thinning

The aftermath of a commercial thinning operation in northern Maine. The dead branches are an indication of a life lived in close proximity with other trees
The aftermath of a commercial thinning operation in northern Maine. Dead branches are indication of a life lived in close proximity to other trees.

In more mature stands with wood at merchantable sizes, commercial thinnings are conducted. These thinnings operate under the same principles as pre-commercial operations. They aim to increase the growth of residual trees and reduce competition by removing a significant portion of the trees. Most of the same considerations are made with these operations as with PCT, so the key difference is the scale. Commercial thinning is a full logging operation with harvested wood brought to the road to be sold to mills. In Maine, these operations are commonly conducted in both softwood and hardwood stands.

Like PCT, there are a multitude of systems by which the trees can be thinned, but commonly these operations seek to remove lower grade trees – crooked trees, slow-growing trees, etc. – and leave only high-grade, genetically superior trees as residual growing stock. The economic rationale for such a system is simple. If a tree in a hardwood stand is crooked or forked, it is considered to be pulpwood, and no matter how large it becomes it will always be regarded as low-grade material that will fetch a low-grade price. Trees that are straight and free of defect can one day grow into veneer logs, which could fetch prices of around $1,000 per log. Leaving higher-grade trees assures a higher revenue for every additional cubic foot of wood grown.

The Art and Science

Every market is different, every species is different, and every landowner is different. How, when, and where thinnings are done depend entirely on the objectives, values, and even the research of the landowner. It’s as much an art as a science, but no matter how it is done, it is always an investment. Large sums of money are spent by Maine landowners every year to administer these operations. In the case of commercial thinnings such operations can certainly be profitable, but often they are break-even propositions at best. Nonetheless, they are conducted; conducted with the hope that it will grow better trees, that it will grow trees faster, and that it will make Maine’s forests healthier and more robust for the next generation.

Zachary Lowry

About Zachary Lowry

I am a 22 year old forestry graduate living in Fort Kent, and I am starting my new career and life in forestry working as an operations forester.