Metla Project 3372

Population dynamics of forest pests,their influences on different tree species, population estimates and forest protection

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Duration: 2003-2006   Keywords: consequential damage, diversity, estimation of pest population sizes, forest pest insects, forest protection, herbivory , restoration
Research project group: Distinct projects 1 - Structure and function of forest ecosystems


Risk for the consequential damage will be evaluated. Special purpose will be study how much fallen trees can be left into the forests so that the risk for the consequential forest pests is not too high. The diversity of the future forest depends on the dead wood provided. Leaving certain amount of fallen trees into the forest could be an economic way to manage this problem. Especially, since in certain forest patches the removing the fallen trees is very uneconomic. Additional risk for pest problems in protected areas will be studied. Methods for estimation of population and protection of forest pests will be developed.


Both in storm damaged areas and in the restoration experiments the number of spruce bark beetles in damaged spruces increased along with increasing number and mean size of felled spruce trees. The large mean size of damaged trees was beneficial for the reproduction of bark beetles. Small trees (< 20 cm diameter at breast height) had only small numbers of spruce bark beetles. The average densities of Ips egg galleries in damaged spruces were largest in study plots with less than 50 damaged spruce trees. The densities of bark beetles were slightly higher in cut trees than in those with root connection. However, the way of felling did not affect the reproductive success of the spruce bark beetle, but burning decreased the breeding success.

The consequential mortality of trees in storm damaged areas and in restoration plots was low. In the storm damage areas no consequential tree mortality was observed in areas with less than 20 storm felled trees. In about half of the areas with 20 or more storm felled trees, standing spruce trees killed by Ips were found. The highest relative spruce mortality (new deaths in relation to the number of damaged/felled trees) was in areas with 10 to 100 damaged trees. Even in these cases the maximum relative mortality was about 30 % of the number of felled trees. The number and mean size of felled trees were the main factors explaining spruce mortality. Also the number of dead standing trees before the storm was a predictor of consequential mortality in storm damage areas.

The results show, that leaving groups of less than 20 felled spruce trees did not increase the risk of consequential damages in the weather conditions during the studies. Even leaving slightly larger groups of trees seems not to cause great consequential tree mortality – especially in managed forests where the natural mortality of trees is low. One has to remember, however, that the weather during the study period was not always optimal for Ips typographus, but there were also some quite cold and rainy summers. Tree mortality could have been higher, if the study period would have had several successive warm and dry summers. The risk of consequential damages by the spruce bark beetle increases probably also when there are storm damages with only one or two year interval in the same area. The predicted warming of climate may also increase the risk of consequential damages, because warm and long summers benefit the reproduction of spruce bark beetles and allow the development of two generations during the summer.

Project leader: Neuvonen, Seppo
The Finnish Forest Research Institute, Joensuu Office, PL 68, FI-80101 JOENSUU, FINLAND
Phone: +358 29 532 3272

Other researchers: Eriksson, Miikka (2003-06), Mikkola, Nella, JO (2004), Nikula, Ari, RO (2005-06), Pohjola, Pekka (2005-06), Pouttu, Antti, VA (2003), Pyörre, Emmi (2003), Roininen, Heikki (2003-04)

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Updated 12.06.2012