Local colonization-extinction dynamics of a tree-killing bark beetle during a large-scale outbreak

Abstract

Forest pest insects may cause large-scale tree growth reductions and tree mortality during outbreaks. The large-scale development of outbreaks has frequently been studied, while the colonization-extinction dynamics during outbreaks is less known. We study the colonization-extinction dynamics of a severe tree-killing bark beetle, Ips typographus, during an outbreak across a 130 000 ha forest landscape in southern Sweden. We recorded annual colonization and extinction events in 1 ha pixels across the landscape by helicopter surveys during three consecutive years, and modeled colonization and extinction probabilities based on focal pixel quality, local population size, connectivity to surrounding beetle populations, and tree composition in the surrounding landscape. The local populations had a high turnover; 81-93% of all occupied pixels were the result of colonizations in the same year and the annual extinction rates were 84-90%. The colonization probability increased and the extinction probability decreased with increasing spruce (host-tree) volume in focal pixels and the connectivity. The fitted spatial scaling parameter of the connectivity measure suggested that the colonization probability was mainly affected by the number of surrounding occupied pixels within a few hundred meters. The colonization probability also decreased with increasing volume of birch (a nonhost) in the focal pixel, while the extinction probability decreased with increasing local population size. In conclusion, the local population size and quality of the focal pixel explained more of the colonization and extinction probability than the connectivity and composition of surrounding forest. The distribution of tree-killing bark beetles during outbreaks can be patchy and highly dynamic. Two reasons for this are the increased probability of successful attacks when exceeding a critical attack density and the fragmented distribution of large spruce volumes throughout the forest landscape.