Tree Mortality by the Bark Beetle Ips typographus (L.) in storm-disturbed stands

Abstract

To maintain biodiversity in forests more wind-felled trees must be left. However, there is concern among forest owners that this may result in higher tree mortality caused by the spruce bark beetle (SBB). Data from six studies on number of trees killed by the SBB in storm-disturbed stands was analysed. The studies include 16 areas, 2–346 ha in size, where wind-felled spruce trees were left. In one of the studies also stands where all wind-felled trees were removed directly after the storm were included. Few trees were killed in the first summer following the storm disturbance. In most stands the tree mortality peaked already in the second or third summer following the storm. But in five of the stands tree mortality peaked later. The number of trees killed per ha by the SBB varies much between the studied stands. There was a trend that the larger areas experienced on average higher tree mortality per ha than the smaller areas. One of the studies, including six stands, demonstrated an almost perfect correlation between the number of colonised wind-felled trees and the number of killed trees in a 4-year period following the storm disturbance. In the same study the removal of wind-felled trees resulted in a 50% reduction in number of trees killed per ha in the 4-years following the storm.