Finnish Forest Research Institute  Metla

Press release 15.12.2005

Study on long-term effects of thinnings on stand and tree growth conducted by Metla
Scots pine more sensitive to increment loss than Norway spruce and silver birch

The results from thinning experiments initiated at the Finnish Forest Research Institute (Metla) since the 1960s have now been analysed and compiled in a recently completed research project, ”Thinnings in forest management”. The project provides new and more accurate information on the effects of timing and intensity of thinnings on the growth and yield of Scotch pine, Norway spruce and silver birch.

After forest regeneration, thinning is the most important means of directing stand development. It has long been understood that removal of some trees will promote the growth of those remaining. Knowledge of the effects of the currently practiced thinnings in even-aged stands has already been gathered over more than fifty years. However, the life span of a stand is so long that it has taken decades to gather reliable results covering the entire rotation period.

The best time to do precommercial thinning is rather late, when the stand height is 5-8 metres, after which the first commercial thinning can be left until later than usual, with more profitable results. The first commercial thinning forms still part of the silvicultural practices for all tree species. At this phase, the most important objective is to promote the development of the stand. In fertile stands of Scots pine, where differences in stem quality are apparent, the first commercial thinning can be done earlier than normal. The purpose of this “quality thinning” is also to remove the most dominating trees with thick branches.

It is particularly important for Scots pine that a sufficient amount of trees is growing in the stand, because pine is more sensitive to increment loss through intensive thinnings than other tree species. For example, removing large pines in a quality thinning may easily lead to a too intensive thinning treatment. On the other hand, if the thinning takes place too late, it can lead to increment losses if the crowns are already too small.

The growth of Norway spruce increases quickly after thinning. Even for spruce, the total yield per hectare is not increased by thinning, but the increment loss is small and lasts for only a short time, being rapidly compensated for by the value increase of the remaining trees. Since spruce reacts quickly to thinnings and has better tolerance for high stand density than other tree species, rather wide limits can be set for spruce stands especially on fertile soil, both regarding the intensity and timing of the thinning. Without thinning, growing of a spruce stand is not financially profitable, because stems will remain thin and many of the trees will die before the final cutting.

A profitable first commercial thinning for silver birch (Betula pendula Roth.) must be done within a period of few years. Since silver birch is a light-demanding species, its crown length tends to shrink, and thinnings must be made in time. On the other hand, birch trees are more slender than coniferous trees, so thinning the stand too early would produce a meagre amount of timber to sell. However, silver birch is quick to increase its growth after thinning, so even the first commercial thinning can be intensive, up to the density of 700-800 stems/ha. The thinning response of downy birch (Betula pubescens Ehrh.) is lower than that of silver birch, and it does not suffer from high stand density to the same degree as silver birch. Therefore, later and less intensive thinnings are recommended for downy birch than for silver birch. Downy birch is rarely a source of high quality saw timber or plywood, which also makes thinnings of low intensity a profitable alternative.

In later thinnings more attention can be paid to incomes than in the first commercial thinning. It is possible to increase the profitability of the later commercial thinnings by removing larger log-sized trees and by leaving somewhat smaller, healthy and vital trees that will have time to reach more valuable log-size later. This procedure, called “thinning from above”, is suitable for Scots pine and also Norway spruce but in birch stands it leads to increment losses. The thinning method used will not affect the total yield of saw timber, but in the thinning from above, a larger part of the incomes is gained already at the thinning phase. Early and intensive thinnings speed the growth of stems increasing saw timber yield, whereas intensive thinning of mature stands reduces it.

Intensive management of young Scots pine stands was also studied in the project. Through intensive thinning and fertilisation it is possible to shorten the stand rotation by approximately 15 years without a large annual increment loss. The results of systematic line and row thinning experiments will also be published.

The results of these studies have been released in several reports published in international scientific journals. In Finnish, the most important results are available in “Tuottava metsänkasvatus” (Profitable silviculture) published by Metla. As a whole, the book comprises the final report of an extensive research programme called "Alternatives of silvicultural practices in forest management and their effects on forest production", in which research into thinning was of essential importance. The research project also included measurements of new field material needed for the model development for the MOTTI growth simulation software.

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