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Research into different thinning systems in Sitka and Norway spruce crops

Teagasc is conducting research on the impact of different thinning systems on the growth and development of productive Sitka and Norway spruce crops. Dr Niall Farrelly, Research Officer, Teagasc Forestry Development Department has more information on the ongoing research taking place.

The research will focus on how different thinning systems which remove different trees may affect the structure, development, timber quality and the economics of the forestry operation.

The outputs of the research hope to inform recommendations on thinning practice regarding best management practice for productive crops of Sitka and Norway spruce.  

The thinning operation presents an opportunity for growers to influence the production of higher value products from their crop. Often the best results are achieved where the crop is of variable quality. The operation removes those stems which have less desirable characteristics such as crookedness and other defects. This allows resources to be focused on those desirable trees capable of producing higher value sawlog.

A variety of thinning systems are used for different purposes or with different species. Traditionally low thinning has been favoured for use in spruce crops. It focuses on retaining the best stems in a prominent position in the crown and removes the supressed trees. In general, it favours the removal of smaller trees at first thinning (Picture 1) as these trees are incapable of accumulating significant volume over a medium rotation period. Rougher trees are also removed because of their undesirable timber properties (e.g. increased branch size, rapid ring development). Second thinning focuses on removing any remaining pulpwood and poor quality stems.  Third and subsequent thinnings concentrate on reducing stem numbers.

Picture 1: A low thinning in Norway spruce focused on removing smaller stems and poor quality wolf trees (i.e. rougher trees) and equal distribution of remaining stems.

Other thinning systems exist such as the crown thinning system. This method favours the removal of defective dominant trees while focusing production on trees with superior form and retaining smaller trees. Other thinning systems, such as selection thinning focuses on the removal of trees based on a target diameter size. This may increase thinning revenue. Thinning systems may also focus on conversion to continuous cover forestry (CCF). The objective is to provide structural diversity in the stand in order to develop a continuous supply of sawlog over a longer rotation while maintaining an overstorey (continuous cover) (Picture 2). Ultimately all thinning systems aim to produce quality timber. 

Picture 2: A continuous cover thinning in Norway spruce. Smaller stems are selected to fill the growing space between larger stems to create an uneven structure.

However, caution needs to be exercised in thinning so as not to remove too many stems during the thinning operation. The crop’s capacity to respond to increased growing space can be more limited if too many open spaces are introduced or where the canopy is opened up as this will affect total volume production. Therefore, thinning operations need to be rigorously monitored.

The objective of thinning is to develop a crop capable of producing straight logs providing the highest revenue returns or a desired management outcome. A uniform crop of straight trees can be achieved with minimal intervention if the stand is of sufficient quality. This may present the best balance between production, economics and quality timber production (Picture 3).

Picture 3: A uniform crop of Norway spruce achieved with minimal intervention owing to superior quality stems showing a high proportion of valuable sawlog.

The research will utilise a number of thinning trials in Sitka and Norway spruce in counties Roscommon, Cork and Offaly, to examine the long-term crop development in response to the thinning systems.

Specifically this research will:

  • Examine the effect of different thinning systems (i.e. low, crown and selection thinning) and thinning intensities on the growth and development of Sitka and Norway spruce crops
  • Examine the effect of thinning systems on timber quality, product assortments, profitability and rotation length
  • Provide scientifically validated data to inform best practice development in the management of spruce crops in Ireland

For more information on thinning or for details on the thinning research being conducted by Teagasc, click here