Thinning is a key forest management practice which aims to create better growth conditions, health, vitality and quality by reducing the density of trees in a stand. In other words, it aims to create more revenues in a rotation time through more volume and value. Some would go even so far that they say that thinning is the best forest management there is. Thinning can be applied in even-aged harvesting and in a form of selective cutting in uneven-aged harvesting as well as in continuous-cover forestry. Certainly, thinning is a valuable tool of climate-smart forestry and faster growing trees absorb more carbon dioxide from the atmosphere.
Why should we consider thinning?
Sustainable forest management is aiming to produce wood and services while maintaining the economic, environmental, cultural and social values of forests today and in the future. In Finland we used to think simply that we should always grow more than we harvest. The basic playbook was to regenerate after clear-cutting and do thinnings in time. Thinning has proven to be an effective forest management practice. With thinnings you can produce more wood and achieve forest stands with more volume and better quality of wood.
Not only for productive reasons
In British Columbia thinning is introduced for generating more opportunities to harvest pulpwood and for prevention of severe forest fires. Thinning reduces excess biomass from the stand and thus can reduce the intensity of forest fires. Thinning has to be light enough that it won´t expose the stand for too much sunlight. Too much exposure can dry up the forest floor and make it easier to catch fire. After generations of clear-cuttings, recent infestations of bark beetles and massive forest fires, the seemingly abundant logging possibilities have decreased and there is in fact shortage of pulpwood.
Mitigation of the climate change
After thinning the remaining trees grow faster due to the increased amount of space, sunlight, water and nutrients. Faster growth equals faster carbon sequestering. Forests are also more resilient to diseases and insect attacks and withstand better the effects of the climate change. It means more of volume, more valuable wood and more revenues in a rotation time.
As stated earlier thinning is a standard procedure in Finnish forest management. We usually have 1-3 thinnings per rotation time. For example in Japan there can be as much as 3-5 thinnings before the clear-cut. The need and timing for thinning is depending on the species and growing conditions. Aggressively growing species in fertile conditions needs to be thinned earlier and more often. Usually, the guidelines for thinnings are based on growth or harvesting models which are a product of years of research of forest management.
The most used form of thinning is called low thinning. Sick and damaged trees are removed first and after that smaller trees are removed so that the density of trees is optimal for the growth.
High thinning is the opposite and concentrates on removing the tallest trees. Same as with low thinning the sick and damaged trees are removed. High thinning produces more sawlogs that the low thinning. This is also a viable way to prolong the rotation time.
Quality thinning is a method which aims to leave the best quality trees to grow. Overshadowed smaller trees will be removed as well as trees with some quality defects.
Selective cutting is a method of harvesting trees of a certain size, species or quality. This method was actually banned in Finland for a long time because it led into degradation of forests. Selective cutting can be harmful if it is used time after time to harvest the most valuable trees and the low-value trees are left to grow. When applied correctly and the future of the stand is considered, the selective cutting can be a great tool for forest management.
Corridor thinning is a new method. Trees are removed only from the strips and corridors of certain angle and no selection of trees is made. The plus side of this method is that it is much more simpler to perform than usual thinning and cheaper. It does not include any selection of removed trees and operators without experience can manage it quite well.
Here is a great article for further reading.
Suitable equipment for thinning
There is no doubt about that single grip harvester-forwarder combo using the cut-to-length method is universally the best for thinning. We could spend the rest of our lives arguing which brand of forest machines is the best but let´s not go into that.
Generally, there is no need to have the biggest and the most powerful forest machines if you are doing thinnings, because the volume of the trees is considerably smaller than in clear-cuttings. One can be productive with smaller and cheaper machinery. It is all about finding the balance between raw power, reach and weight. Sure, it is nice to be able to easily move the felled trees and reach far but all that means more weight and therefore more surface pressure. Heavier machinery causes more soil compacting, soil displacement and finally rutting. There is no point in thinning if we make deep ruts and break all the roots while going. Of course, the unwanted effects to the soil are dependent of the properties of the soil, slope, weather conditions and number of passages among the other things.
More powerful machines can be advantageous in some cases. For example, in continuous cover forestry or late stage selective cuttings in British Columbia. In continuous cover forestry it is important to be able to cut some of the largest trees and move them to be processed in an optimal location without damaging the neighboring trees or seedlings. Same need for careful handling applies to selective cutting and really the only difference is that you don´t need to pay so much attention for the smaller trees or undergrowth.
Tractor or excavator?
We prefer wheel-based harvesters in the Nordics because the productivity is considered to be higher, our sites are usually quite flat, and off-road capability is appreciated. Excavators are basically used only working on peatlands. This is because lower surface pressure can be achieved with correctly equipped excavators and peatlands are easier to move.
Modern excavator-based harvesters can be quite good in terms of productivity, and they excel in steeper slopes. Excavators equipped with special cable-assisting systems can operate safely above 60 % slope depending on the soil, ground roughness and the skill level of operator. Mechanized thinning on the steepest slopes is probably not the best idea and the costs per cubic meter can be unacceptable.