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The annual growth of the forests of Finland has more than doubled in less than a century. While the increased growing stock and more efficient silviculture have contributed to the observed growth increase, there are also solid grounds for assuming that environmental changes have played a role. The aim of our study was to analyze and quantify the magnitude of changes of volume, basal area and height increment not attributable to changes in growing stock, forest structure and silvicultural practices.

We used the extensive data from National Forest Inventories during 1971-2010 to develop models for volume, basal area and height increment of individual trees on mineral soils without ditching or paludification with tree and stand characteristics as predictor variables. Differences between the measured and predicted increment were used to detect environment-induced increment changes. Then, we estimated the average changes of volume increment per hectare and totals in millions of m(3). Using this approach it was also possible to approximate the enhancing effects of volume increment change in growing stock volume.

From 1971-1975 to 2006-2010, the environment-induced volume increment increase was estimated to be 8.98 million m(3) a(-1) (0.69 m(3) ha(-1) a(-1)), which equals to 37% of the total observed volume increment increase. In relative terms the environment-induced increment increase was larger in the northern regions (up to 45% of volume increment change). During 1971-1990, the difference between the observed and predicted change was small. A large shift was observed after the mid-1990s in all regions. While the environment-induced increment change was substantial, a considerably larger increase representing 63% of the change was attributed to growing stock volume and forest structure, which both changed due to differences in forest management.

A comparison between the environment-induced increment changes and growing season temperature sums revealed similarities. In the southern Finland, April-May seemed to be influential, while in the north the temperature sum of May-September showed similar variation. As climate change is predicted to increase growing season temperatures, the trend can be expected to continue in the boreal conditions of Finland. (C) 2016 Elsevier B.V. All rights reserved.