Increasing insect abundance by killing deciduous trees: a method of improving the food situation for endangered woodpeckers

A total of 135 trees were girdled or notched in a forest area close to the river Dalälven in central Sweden. The purpose was to see whether good microhabitats for wood-living insects could be created and if so, could be used as food for the endangered white-backed woodpecker Dendrocopos leucotos. The chosen tree species were three of the most frequent used for foraging by the white-backed woodpecker, i.e. aspen Populus tremula , birch Betula alba and common alder Alnus glutinosa.
The process of dying and decay in the manipulated trees was followed for eight to nine years, and the following variables were studied: tree condition, tree structure, wood-condition, consistency of bark and cambium, frequency of polypori, foraging marks made by woodpeckers and abundance of insect species of importance as food for woodpeckers. Notched aspen died faster and were more quickly colonized by wood-living insects than were girdled aspen. Since the notched aspen was also reduced more quickly to low stumps or lost their bark, their value for wood-living insects was short-lived. Notched birch died almost as quickly as girdled birch but were populated by slightly fewer insect species. Notching, in general, seemed to create good foraging trees in the short-term whereas girdling produced trees of a longer-term value. In order to improve the local food situation for the white-backed woodpecker, a combination of the two methods is suggested. Nonetheless, both must be seen as emergency measures and cannot compensate for a large-scale reduction in the abundance of old deciduous trees in the forest.