Helsetilstanden i norske skoger. Resultater fra den landsrepresentative skogovervåkingen i 2011 (original) (raw)

In 2010, an increase in crown density for all the assessed tree species, Norway spruce, Scots pine and birch, was observed. This was the third consecutive year that an increase in crown density has been observed for all monitored tree species. In contrast, discolouration increased in all assessed tree species, especially for birch. Few conifers with biotic or abiotic damage symptoms were observed, while more than 20 % of the birch trees had signs of damage, most of these caused by Epirrita spp or birch leaf rust. This report presents the results of the national survey of forest vitality conducted by the Norwegian Forest and Landscape Institute in 2010. The results are presented as descriptive statistics for parameters of crown conditions, such as crown density, crown colour and biotic and abiotic damage for Norway spruce, Scots pine and birch. 9730 trees were assessed in total in 2010, hereof 4412 spruce trees, 2967 pine trees and 2351 birches, located on 1711 plots that are distributed throughout the forested area of Norway. Average crown density in 2010 was 85.7 % for Norway spruce, 85.3 % for Scots pine, and 79.4 % for birch. This represents an increase in crown density of 1.1, 0.4 and 1.4 %-points for Norway spruce, Scots pine and birch, respectively, compared to the year before. Crown density of pine and spruce had declined gradually from 1989 to 1997/98, increased in the period 1998-2004, and decreased again after 2004. This year's results show for the third consecutive time since 2007 an increase in crown density for all monitored tree species. Crown density of birch had had a positive development from 1994-2001, and a decrease in crown density thereafter until 2007. In spite of the increase from 2008 till 2010, birch still has low crown density. The proportion of trees with full crown density was 58 % for Norway spruce, 40.7 % for Scots pine and 29.7 % for birch. This represents an increase for spruce and birch, while the proportion of pine trees with full crowns remained more or less unchanged compared to the previous year. Older trees (>60 years) tend to have a lower crown density than younger trees (<60 years). This is particularly true for Norway spruce where the older trees have approx. 20 % lower crown density than the younger ones. Crown density increased in both age classes for all monitored tree species in 2010. The percentage of Norway spruce with fresh green coloured crowns was 79.6 % in 2010, which is a decrease of 1.2 %-points compared to the year before. The proportion of pine trees with fresh green colour decreased with 1.5 %-points to 85 % in 2010. Birch had the largest decrease in the proportion of trees with fresh green colour with 8.2 %-points to 78.1 % in 2010. Vitality classes are a combination of defoliation and discolouration. Vitality increased for all tree species from 2009-2010, although the proportion of severely damaged pine and birch trees increased. 57.9 % of the spruce trees and 40.6 % of the pine trees were classified as "not damaged", an increase from the year before of 3.3 and 0.3 %-points, respectively. 5.2 % of the spruce trees and 1.2 % of the pine trees were "severely damaged". Conifer trees older than 60 years had considerably more damage than trees younger than 60.In spite of an increase of 3.5 %-points, still only 29.5 % of all birch trees were classified as "not damaged", while 5.6 % were "severely damaged". Thus, birch still has more damage in terms of defoliation and discolouration than the conifers, even if most of the damaged birches fell into the vitality class "slightly damaged". The difference between younger and older birch trees with damage is less pronounced than for the conifers. Only 2 % of the spruces and 4 % of the pines had symptoms of known biotic or abiotic damage in 2010. Only few spruce trees were damaged by fungi or insects, while 1.2 % were damaged by abiotic factors. 1.1 % of the pine trees showed signs of insect damage, 0.6 % of fungal damage and 1.4 % of abiotic damage. For birch, insect damage decreased notably, while fungal damage increased compared to the previous years. In 2010, 9.4 % of the birches were attacked by Epirrita spp. or other insects, 9.5 % were damaged by birch leaf rust and 3 % by abiotic factors. In addition to harvested trees, 43 trees had died of natural causes since the last survey.