Studies on Growth Trends of Forests in Sweden and Norway
In the Swedish National Forest Inventory (NFI), a steady increase in the estimated productivity of forest land has been noticed since inventory was begun in 1923. Young stands generally indicate higher site indices than old stands at equal site conditions. For spruce, this rise of site index has been estimated at 0.05–0.11 m.year−1, with the highest value in the south. In a study based on sample tree data from the NFI, a highly significant annual increase of both height and basal area growth of the magnitude 0.5–0.8% was found for the period 1953–1992. At the same time, top height seems to develop according to the site curves on permanent plots both in Sweden and Norway.
There are two alternative explanations to these observations:
Altered silviculture has changed the population structure.
A long-term rise of natural site productivity is taking place.
In the first instance, an altered way of cutting, from selective cutting to clearfelling and thinning from below, has probably had the biggest impact so far. Genetic improvement, better species-site adaption and different site treatments performed during the last decades will enhance this trend.
In the second case, the increasing atmospheric deposition of nitrogen is suspected to have the biggest influence. A long-term trend of this type may have been built into our prediction models. Comparison of consecutive generations on the same site should then reveal the trend. A spruce study in southern Sweden indicated a rise, while comparison of pine plantations established in the same way on similar sites during different decades gave a more dubious result.
KeywordsEnglish Summary Site Index Growth Trend Permanent Plot National Forest Inventory
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