Thinning hardwood is a different ballgame
We can grow more high-quality sawlogs, if we do it right

by Gaetan Pelletier

We are all familiar with thinning of softwoods, in which the goal is to focus the site’s growth potential on the crop trees by removing individuals of less desirable species, inferior quality, and poor growth potential. Thinning is a family of intermediate treatments implemented in young, even-aged stands – to optimize site productivity, control species composition, and improve overall stand quality. The most common treatments are pre-commercial thinning, commercial thinning, and the crop tree release variant (Figure 1).

  Pre-commercial thinning (PCT), carried out in well-stocked stands with an adequate number of well-distributed crop trees, is intended to reduce stand density to a certain target (i.e., 2,500 stems/ha) at the sapling stage. The treatment is most commonly conducted when the trees have a DBH (diameter at breast height) of less than 10 centimetres, and it is usually carried out using brush saws. Commercial thinning is conducted in older stands – once the trees have reached pole size (10-20 cm), and can generate some products such as pulpwood. The treatment is well suited for small, wheeled harvesters. 

The crop tree release variant can be used in either case. Rather than space trees uniformly, the focus is solely on releasing the crowns of selected crop trees (200-400/ha), leaving the matrix largely untouched.

But can density management offer the same benefits in hardwoods? In New Brunswick, we anticipate a reduction in future growing stock of quality tolerant hardwood trees. The proportion of Sugar maple is expected to decline, as well as the number of larger trees that can contain sawlogs. Increasing the amount of pre-commercial thinning in mixed and hardwood stands is believed to be a potential solution. Density regulation treatments in young, even-age stands can be part of the answer, but only if we pay attention to some key concepts. 

One thing we know for certain is that conducting thinning operations in the same manner as for softwood stands does not work. Studies of pre-commercial thinning operations from more than 20 years ago reveal that the systematic spacing of crop trees, without attention to species and quality, can have negative consequences. Such an approach is not likely to improve species composition and quality, which are the main objectives in applying the treatment. Silviculturists need to adjust their thinning prescriptions in mixed and hardwood stands.

The first consideration is the quality of crop trees. Give priority to desired species, such as Yellow birch, Sugar maple, and oaks – and avoid treating stands that have high proportions of intolerant hardwoods and Balsam fir. Focus attention on co-dominant trees with symmetrical and oval crowns (avoid vase shape), and a crown ratio of greater than 50 percent. Boles should be straight, and free of defects and large branches or forks. Trees that are already in a position of dominance respond poorly to any type of thinning, as they are already free to grow (see Figure 2), in addition to the fact that they are usually of poorer quality, with large limbs and crowns. Whenever possible, avoid leaving trees that regenerated from stump sprouts – and if the proportion of them is too high, stratify your stand, in order to take those areas out of your thinning plan.

When considering pre-commercial thinning, the height of the saplings is of particular importance. For hardwoods, we recommend treating a stand only when it has reached a height of six metres. This will ensure that some self-pruning has occurred, minimizing the potential for large branches to develop. It also allows for natural differentiation among potential crop trees, making selection easier. 

More importantly, we recommend looking at density regulation holistically, not just in terms of identifying stands to justify a specific operation. In other words, when stand parameters are marginal, it is perfectly acceptable to skip PCT and wait for the stand to become eligible for commercial thinning with small harvesters. Again, the goal is to focus growth on a few crop trees. That will happen on its own if the treatment is properly conducted. Similarly, a stand with high potential to grow quality tolerant hardwoods might receive both a pre-commercial and a commercial thinning treatment.

CROP TREE RELEASE
The crop tree release (CTR) variant applies to both types of thinning. It offers the advantages of reducing costs, focusing on future crop trees by performing positive selection, and adjusting the composition in stands with some variability. We recommend choosing between 200 and 400 crop trees per hectare; releasing their crowns on at least three sides; and leaving the rest of the stand largely untouched. It is desirable to leave small companion trees that do not interfere with the crop tree, to provide shade along its stem and reduce the likelihood that it will grow large limbs on the bole. (See Figure 4.)

Studies at the Northern Hardwoods Research Institute have demonstrated that implementing this variant costs half as much as a traditional thinning operation, and the productivity of PCT workers is largely unaffected by the number of crop trees left. Seasoned thinners have relatively few problems adjusting to this method, but managers and landowners must revise their quality-control protocols. 

Density management can be a powerful strategy for mitigating reduced future growing stock of quality tolerant hardwood trees, if the practice is tailored for hardwood stands. The silviculturist, forest manager, and landowner must be oriented toward the objective of improving the quality of stands and trees. Density management must be conducted with much latitude – with due consideration for the timing and the combination of treatments. Waiting until the stand reaches operability targets, such as minimum stand height, is perfectly acceptable. Whenever possible, adopt the crop tree release variant. And most importantly, spend enough time to properly stratify your stand; it will be beneficial in the long run, reducing cost and focusing resources where they are needed.

(Gaetan Pelletier is executive director of the Northern Hardwoods Research Institute, in Edmundston, N.B.)