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Thinning as a Forest Practice

Thinning is one of the most common forestry practices in most parts of the world to improve not only wood quality of the forest, but also its growth rate and its general stability and health.  It has less been viewed as a tool for transformation, as often after reaching harvesting age large parts of or the entire forest are harvested and a new cycle begins.

Around the Beijing area and in many parts of Eastern China, however, producing wood is not the primary function of the forests. Largely afforested during the 70s until today, these forests were planted to prevent erosion, even out water flows and generally restore the functions the natural forests provided there before they were largely cut down. While many of the primary concerns were sufficiently addressed through the planting of monocultures of Oriental Thuja (侧柏; Platycladus orientalis), Chinese Pine (油松; Pinus tabuliformis) or different species of poplar, the full restoration of the ecosystem services has not been possible. Species diversity, general forest health and resilience, habitat value, improvement of water quality and opportunities for locals to harvest forest products have all only achieved partial success at most.

While it’s economically speaking most efficient to wait until the trees reach harvesting age, this is not necessarily the relevant measurement in these areas, especially as the “Natural Forest Protection Program”, applying to many afforested areas, currently prevents the removal of large volumes of wood, which consequently means full harvests. The thinning approach therefore can not only short-circuit the succession cycle in the afforested areas, but also complies with local laws.

Considerations for Thinning Operations

Thinning should be not only to introduce the native species into the forest, but also improve its general resilience. It is important therefor to not just randomly select trees to be removed. A forest management plan for the respective stand with concrete goals should be developed, defining the criteria for the removal of trees and the definition of target trees (trees to be kept and promoted in their growth).

Possible criteria suggested for much of the Beijing area could be:

  • the healthiest trees with good form and a h/d ratio >33% are kept
  • removal of trees until <70% crown coverage
  • direct competitors in crown to target trees are to be removed
  • native species that have naturally established in the understory are favored, competitor trees to those also removed
  • if supplemental planting is planned, depending on the light requirements of the species an uneven thinning approach, leaving space for small clearings, might be considered

 

Recommendations
  1. Supplemental Planting

The ability of native species to regenerate naturally through seed production strongly depends on the presence of a seed source in form of mature trees. If desired native species lack such a seed source supplemental planting is necessary. Particularly with non-commercial local species this can be a problem. It is important to find a nursery willing to grow those species early on. Furthermore, a few years later additional thinnings might be necessary to ensure survival and competitiveness of the planted trees.

  1. Compliance with Harvest Quotas

Our experience indicates that planning ahead for the approval of harvesting quotas, especially in areas that have quite restricted quotas is crucial for a timely execution of the thinning operations. Often approval can take several months and the proposed thinning volume could, depending on scale, also compete with other proposals. Thus an early submission of the intended volume removed is of the essence. This, in turn requires early visits to the site, where standing volume, stocking, DBH and other relevant data for the planning of the thinning operation will be compiled.

  1. Training of Personnel for Marking Trees

It is important that, when funds for training people in the marking of trees for thinning become available, the training efforts are focused on people being able to do this kind of work long-term. Therefore, it should be inquired beforehand if the course takers are seeing this as a long-term work, or a temporary job. Even better, a staff member at the own organization should receive advanced training, so he could teach others later, if necessary. If the treated areas are small enough, one permanent marker might also be sufficient in general and prevent a drop in quality when people with little experience are marking and selecting trees.

Discussion

Thinning is a forest management and improvement method used all over the world. The biggest divergence in the BFS approach lies in the amount of wood that can be harvested and the goals the thinning is to satisfy. In other countries, like in the US, a forest is usually thinned to improve not only the forest health and ensure regeneration, but mainly to improve the quality of the wood that is to be harvested in the future. As such, thinnings are not necessarily intended to promote regeneration (that would be the harvest itself, for example a shelterwood cut). The reason BFS has opted for thinnings as the primary method for forest regeneration is that the harvest quotas in the area are due to the Natural Forest Protection Law, that often applies to important watersheds in China, the harvestable amounts of wood are extremely low, therefore cuts with a primary regeneration goal like shelterwood cuts or seed tree systems are hard to execute on the ground. Effectively the thinning could also be regarded as a selection harvest system, with the important difference that the trees “harvested” have not reached their full value yet, meaning they have not reached the harvesting age and are pre-maturely harvested. As the 2nd primary goal is to improve forest quality, this hybrid has still been classified has a “thinning”.

Possible complications such an approach can bring is the amount of light available to the regeneration. Many species like oaks and other mid-successionals need higher amounts of light than available under this approach. This means that either in areas where supplemental planting is used, more trees have to be thinned at the expense of other areas, or later successional species with a lower need for light need to be promoted. A third option, if available, is to have very frequent thinnings every 3-5 years and suppress competition of the target regeneration.

References

Annex Sino-German Project – Guideline Forest Management Practices

How to write a cutting prescription Info Sheet

Smith , D., et al. (1997). The Practice of Silviculture: Applied Forest Ecology, Ch. 10, 13, 21.

Wang, Y., et al. (2015). “A Water Yield-Oriented Practical Approach for Multifunctional Forest Management and its Application in Dryland Regions of China.” JAWRA Journal of the American Water Resources Association 51(3): 689-703.

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