Part two: Thinning of high-yielding Sitka spruce - Results from second and subsequent thinning

Dr. Niall Farrelly's two-part series on thinning high-yielding Sitka spruce in Frenchpark, Co. Roscommon, highlights the benefits of timely and progressive thinning operations, with research showing that managed thinning significantly improves tree growth, quality, and financial returns.

In part one of this two part series, Dr Niall Farrelly gave an overview of Teagasc research into thinning high yielding Sitka spruce in Frenchpark, Co. Roscommon and the outcomes from the first thinning operation. Part one emphasises the importance of timely commencement of fast growing Sitka spruce and the outcomes from first thinning. In his second article, Niall focuses on the results of second and subsequent thinning operations.

Second thinning

Second thinning in Frenchpark commenced 5 years after first thinning, when the crop had reached 20 years of age. The average diameter (dbh) of the crop before second thinning was between 22 and 24 cm. At second thinning, approximately 1 in 5 stems was removed. Stems were removed by walking through the forests and selecting the lowest quality tree from a group of five trees. Stocking was reduced after second thinning to between 825 and 1,180 trees per hectare, depending on the different thinning types (Table 1).

Table 1: Stocking table for Sitka spruce for each thinning type based on results from Frenchpark experiment after 2^nd thinning.

A general point to note about second thinning is that the volume removed can be lower than that in first thinning. This is largely due to the increased volume removed as a result of a complete line of trees being removed at first thinning.  In the Frenchpark experiment, the volume removed at first thinning for the light, medium and heavy thinning was 50, 62 and 73 m³ per hectare respectively. The volume removed at second thinning was less at 40, 60 and 6262 m³ per hectare for the light, medium and heavy thinning treatments respectively (Table 1).

The volume assortments at second thinning are highly dependent on the type of first thinning carried out. Where the first thinning was light, the percentage palletwood removed is much lower at 25% of the total volume. Where the first thinning was medium or heavy, a higher proportion of palletwood is available to be removed at second thinning, with 31 m³ and 35 m³ per hectare of palletwood being produced (Figure 1). The increased palletwood volumes illustrate that the value of the second thinning is highly dependent on the quality and intensity of the first thinning.

Figure 1: The volume of pulpwood and palletwood per hectare (m³ / ha) produced at second thinning (in net realisable volume) in Frenchpark for the three different thinning intensities.

Third thinning

The third thinning operation took place at 25 years. The volume removed for the light, medium and heavy thinning was 53, 61 and 68 m³ per hectare respectively. At third thinning, timber was processed into six categories. These were decided by the harvesting driver based on an algorithm which produced the most lucrative assortment for a particular tree. At this thinning, lengths classed as small sawlog (3.7 m lengths) and commercial sawlog (4.9 m) were achieved from all the three thinning types (Figure 2).  A notable feature of thinned plots was that the volume of the average tree left after thinning was between 0.6 and 0.7 m³ after 27 years. This is a merchantable size for most sawmills. In contrast, the unthinned plots has a considerably smaller average volume of 0.4 m³ per tree.

Figure 2: The volume assortments produced per hectare (m³ / ha) at third thinning in Frenchpark according to three thinning grades, light (grade b), medium (grade C) and heavier (Grade D).

Outlook for final harvest

At the recent field day in Frenchpark in September, participants were brought through the plots and had the opportunity to see examples of each of the thinning treatments carried out. The first plot visited was the unmanaged plot of spruce which was unthinned (Picture 1).The standing volume was estimated to be 780 m³/ha, mean dbh was 24 cm and mean tree volume of 0.44 m³. Overall timber quality, straightness and branchiness was poorer in these plots.

Picture 1: An unthinned plot in Frenchpark experiment, with little opportunity to improve the crop beyond its current quality.

Crop development in the light, medium and heavily thinned plots was discussed as each example also visited in turn (Picture 2). After 29 growing seasons, the impact of thinning is obvious, from the volume of the average tree of 0.44 m³ in the unthinned plot to 0.85 m³ per tree in the heaviest thinned plots. Crop details after 29 years for each of the thinning treatments are presented in Table 2.

Table 2: Main crop details,  thinning outputs per hectare (m³ / ha) and estimated rotation age for Sitka spruce, based on results from Frenchpark experiment after 29 growing seasons in 2024.

*Time to reach mean tree volume of 0.7 m³

The field event served to provide growers with an opportunity to view the culmination of nearly 15 years of research into thinning research conducted by Teagasc. The large attendance at the Frenchpark even, were left in no doubt as to the benefits of thinning (Picture 2) on suitable and stable forests.

Picture 3: A well-managed Sitka spruce plantation with a higher proportion of straight trees capable of yielding a high proportion of sawlog after 29 years.

The research conducted at Frenchpark showed returns from thinning ranging from €2,700 to €4,500 per ha over the three thinning operations. For growers with 10 hectares of Sitka spruce, returns from thinning of between €20,000 to €40,000 are achievable over three thinning operations, while also increasing the value of the final crop. For example, the heavy thinning type, as demonstrated in Frenchpark, resulted in the production of higher quantities of merchantable sawlog, with volumes of 0.7 to 0.8 m³ per tree, achieved after 27 years.

There is considerable potential to harvest productive crops of Sitka spruce at rotations of 28 to 30 years of age by following the example of Frenchpark. While returns will vary according to a range of factors, Sitka spruce continues to be a potentially very lucrative tree species based on its versatile timber, wide range of end uses.  On good quality, productive forests with suitable access, highly attractive returns of can be readily achieved by following these recommendations and can serve as a guide towards best practice in management of such crops.

As each forest is different, it is strongly recommended that forests are fully assessed as they approach each thinning stage in terms of its suitability and timing. Teagasc forestry staff are available to provide independent and objective advice to forest owners on all aspects of forestry including suitability for forest thinning and best practice in its implementation.

Further information on thinning and management of productive Sitka spruce

Also read: Part one: Thinning of high-yielding Sitka spruce - an overview and results