New technical and alternative silvicultural approaches to pre-commercial thinning

In the last decade relative pre-commercial thinning costs have increased, as a proportion of total silvicultural costs, and the annual area treated with pre-commercial thinning has decreased, partly because stands are denser and the development of tools has been slow compared to advances in tools for other forestry measures, calling for new methods and techniques to be developed. Reducing competition by topping secondary stems might be an attractive alternative to traditional pre-commercial thinning for biological, technical and financial reasons. However, the topped secondary stems must not overtop the main stems and should not present obstacles at the time of the first commercial thinning. Topping, i.e. top-cutting or top-breaking of secondary stems, was tested in birch (Betula sp.) stands with the background that increased competition from topped secondary stems may promote higher quality in the main stems and that topped secondary stems might die and disappear by the time of the first commercial thinning as a result of treatment. Furthermore, growth of the topped birch stems was studied after treatment in different seasons. Motor-manual equipment for topping and a mechanised prototype were tested in an experimental rig and the mechanised prototype was also tested in a field experiment. All tools were compared with a conventional brush saw, regarding both time requirements and quality of work. Results indicated that topping, especially at a higher level above ground leading to a smaller height lead for the main stems, gave a significant increase in main stem quality of birch, compared to traditional pre-commercial thinning. Secondary stems showed higher survival after topping compared to traditional pre-commercial thinning, but topping at a lower level above ground gave lower survival than topping at a higher level. No differences in growth or survival were detected between top-cut and top-broken stems over three years and survival and height growth was lower for stems treated in a growing condition compared to stems treated earlier in the year. Despite having a significantly less powerful engine, a motor-manual pole saw prototype designed to be used for topping was a competitive alternative to the brush saw in terms of both time consumption and damage to the residual stand. The mechanised prototype seemed to be a competitive alternative in high diameter and dense stands. Although the quality of work obtained with the mechanised prototype was equally high to the quality obtained with the brush saw, the results regarding time requirements for the mechanised prototype from the experimental study could not be verified in field experiments, resulting in a faster operation under field conditions with the brush saw irrespective of type of stand. It was also concluded that current standards for time requirements for brush saws might need to be revised, and that the height/diameter ratio might have an important influence on the time requirements for both motor-manual and mechanised pre-commercial thinning tools. Topping seemed to be an attractive alternative to traditional pre-commercial thinning. However, further studies with varying initial and remaining stand density, stand height, species composition and tests of other possible advantages, e. g. reduced browsing pressure on main stems and reduced time requirements for mechanised tools when the cutting height can be raised above obstacles, should be performed. Tools for topping could be developed that would give equal or better results with respect to both quality and costs compared to the traditional brush saw. Mechanised equipment for pre-commercial thinning that can give acceptable results in terms of quality of work are available, but further development is needed in order to lower time requirements.