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Don’t forget about pre-commercial thinning

Wed, 03/30/2016 - 10:01am
Many aesthetic and habitat objectives of family forest landowners come with older, less dense stands like this stand of about 70 years. It is important to get on this path early.

Brad Withrow-Robinson, Forestry & Natural Resources Extension Agent, Benton, Linn and Polk Counties.

In previous segments I argued that many people have too many trees in their young stands   which may be costly and harmful to the long term growth of the stand. Most importantly, having too many trees at this stage can undermine common landowner objectives of growing attractive, longer rotation diverse forest habitats and can force landowners into shorter rotations than imagined.

While this suggests that people should think about planting fewer trees per acre in the future (a step deserving some careful consideration), it highlights the need for pre-commercial thinning in many existing stands to correct overstocking at an early age. This may include your stand.

But pre-commercial thinning (PCT) seems to have fallen out of common practice lately. It has come to be seen (mistakenly, I think) as an avoidable expense rather than an important investment in the stand. An investment that begins to shape how the stand will look and behave in the future and which adds resilience and options to the landowners’ woodland portfolio.

The idea of PCT is to avoid harmful overcrowding later by removing excess trees early on. PCT lets the remaining “leave trees” grow faster and larger before serious crowding sets in. This means that trees reach a usable size sooner, and hopefully allows the very important first thinning harvest (also called a commercial thinning) to be done “on time” when the stand is in its 20’s.  This first thinning harvest  is costly and the difference between it being another  big expense for the landowner rather than breaking even or even paying some small profit, often comes down to the size of the trees harvested.  PCT is meant to ensure that this very important thinning harvest operation can pay for itself.

Ideally, young Douglas-fir stands in Western Oregon should be thinned when the dominant trees are about 15 feet tall, or about 10 years old around here.  Yikes, that seems early.  Frankly I don’t know many people who are thrilled about thinning trees they just barely got established and free to grow.  People are looking forward to the trees’ shade suppressing hated weeds like blackberries and broom and are inclined to postpone PCT until the weeds decline and the stand begins to “look crowded”.

Why thin so early? Even by the time trees are 15 feet, you can already begin to distinguish the good trees from the bad.  The trees are past browse and should be beating the weeds.  Once that has happened, the sooner you remove the extras and limit competition among trees, the stronger the beneficial effects and the less the costs of the PCT will be.  Yes, it may be possible to delay until trees are 30 feet tall, but waiting until crowns close and competition begins means a loss of some growth that you would rather have on your leave trees.  Also by the time trees look crowded to many people, it is getting very late.

Referring to the illustrations of the previous article, the recommended  timing of a PCT is meant to occur well down in the uncrowded green zone, well before competition gets going in the Goldilocks zone.   The PCT is meant to shift your stand from the right-hand column to the left-hand column, with more room and  new growth potential until it is time to you your first thinning harvest.

An earlier PCT also means it can be done more efficiently and cheaply. Small trees can be felled much more quickly with less slash building up. There is little concern about stand stability, or delayed growth response when trees are thinned early.

My point is that if an area was planted at a 10×10 spacing (440 TPA) and had good survival, the stand will get too crowded before the trees are big enough for a thinning harvest.  If that is the case, it should be PCT’ed down to at least 300 TPA, meaning you may need to remove a quarter to a third of the trees (110 to 145 TPA).  This is a lot of work, even when trees are small, which takes us to some good advice for many family forest landowners: “Thin early and thin often”.

I’ll give some strategies for thinning in a later article.

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