Gremmeniella abietina-epidemin 2001 : har skogen återhämtat sig?

Abstract: The fungus Gremmeniella abietina causes damage to conifers that can lead to tree death. In 1999 and 2001 the pathogen caused severe attacks in Sweden, where about 500 000 ha of pine forest were affected. Trees of all ages were attacked, but mainly forests between 30-60 years that were ready for their first commercial thinning. After recommendations from the forest agency, sanitary fellings were made to trees that had more needle loss than 85%, a total of about 50 000 ha. Economic losses have been estimated to more than one billion SEK. In connection with the outbreaks two experiments were put out with the aim to investigate the long-term effects in affected populations. The experiments were laid out in Dalarna on Stora Enso land where the epidemic was most severe. In both experiments trees were selected and monitored yearly until 2004, as well as in 2010. All monitored trees were divided into different groups depending on loss of needle biomass. In experiment A, which started in 2000, thinning, fertilization and control was compared. This experiment was hit heavily by the outbreaks in both 1999 and 2001. In experiment B, which started in 2001, the development was monitored without any further treatments. This experiment was hit mainly by the outbreak in 2001. During the inventory, parameters that were used were: Defoliation, diameter, mortality, and drill samples.The survival in different groups was found to be dependent on needle biomass loss in both experiments. Among the groups that had the least needle loss most of the trees are alive today, while mortality in the groups with greater needle loss is higher. To minimize the risk of mortality after a G. abietina infection, sanitary thinning should be executed to reduce economic losses. Trees with needle loss higher than 60%, have a high risk to die and should be considered to be included in the thinning procedure. In stands that have been exposed to repeated attacks the risk of mortality increases even with minor needle loss. In such stands all trees have an increased risk of dying and forest owners should consider final felling followed by replanting. The annual growth between 1990 and 2010 was analyzed and the results show that groups with a small needle loss increased its growth during and after the attack. The G. abietina infection caused a natural thinning, thus favoring the growth of these groups. The groups with heavy needle loss decreased their growth by at least 60% compared to before the outbreak (growth in 1998 was about 7 cm2 and 2005 about 2,5 cm2). Today, the groups that were affected by one outbreak have recovered. The stands hit by several attacks presented a worse recovery. With the exception of the group with a small needle loss, the trees today grow slower than before. Thinned areas suffered as badly as the control plots in terms of growth, mortality and defoliation. This indicates that it is difficult to prevent an epidemic of G. abietina during the outbreak with thinning. In the experiment with fertilization no statistical differences were found between control and fertilization, probably because the amount of sample was too small. Preventive measures are probably the easiest method to reduce injuries from G. abietina infections.