In the Swedish National Forest Inventory (NFI) a steady increase in estimated productivity of forest land has been noticed since start of inventory 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 to 0.050.11 m year1, with highest value in the south. In a study based on sample tree data from NFI
a highly significant annual increase of both height and basal area growth of the magnitude 0.50.8 % was found for the period 19531992. 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:
1. Altered silviculture have changed the population structure.
2. A long-term rise of natural site productivity is taking place.
In the former case an altered way of cutting, from selective cutting to clearfelling and thinning from below, have probably had the biggest impact so far. Genetic improvement, better species site adaptation and different site treatments performed the last decades will enhance the trend.
In the latter 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 did not confirm the trend.
Key words: growth trend, site index, productivity.
Correspondence: Björn Elfving, Swedish University of Agriculture, Department of silviculture,
S-901 83 Umeå, Sweden
Telefax: +46-90-167669
E-mail: bjorn.elfving@ssko.slu.se