The influence of windthrow area and timber characteristics on colonization of wind-felled spruces by Ips typographus (L.)

Abstract

Leaving wind-felled Norway spruces in forests is one way to increase the amount of dead wood. Many forest species would benefit because of these actions but unfortunately also forest pests such as the spruce bark beetle ( Ips typographus) reproduce in newly wind-felled trees. To minimize the risk of subsequent attacks on living trees, the decisions of removing trees out of the forest in high risk areas should be done as early as possible. If fallen trees are not colonized or if there are only a few colonized trees the risk of subsequent attacks on living trees are also minor. For this reason, it is essential to estimate the windthrow area and timber characteristics influencing the colonization of wind-felled trees. We studied the colonization of Norway spruce dominated windthrow areas by I. typographus (L.) in 65 stands in southern Finland on two consecutive summers after the storms in November 2001. The studied areas ranged from single tree gaps to large gaps with over 3000 wind-felled spruces. Using generalized linear modelling, we evaluated the significance of windthrow area and timber characteristics for the probability of the windthrow area to be colonized by I. typographus, the percentage of colonized wind-felled spruces, the number of egg galleries, and the colonization density. In the first summer (2002) after the wind-felling 71% of studied windthrow areas were colonized by I. typographus. Only 1 of the 19 areas not colonized in 2002 was colonized in 2003. The number and mean diameter of wind-felled spruces and the basal area of recently dead standing spruces were the most important windthrow characteristics influencing colonization. The models showed that an increase in these characteristics also increased the values of the response variables. This study shows that local population levels of I. typographus varied greatly within the study area (appox. 200 km × 200 km) and that this variation should be included in models predicting the colonization risks of windthrown areas.