Effects of tree retention on cavity-nesting birds in northern Sweden

Abstract: Tree cavities are of high ecological value because they influence the abundance, diversity, and conservation of many terrestrial animals in forests worldwide. However, due to forest management practices, cavity-users are under a serious threat in many forest ecosystems. With the intention of reducing the impacts of timber harvest, since the mid-1990’s variable amounts of green and dead trees are retained at the clear-cuts in Fennoscandia as a conservation measure supposed to improve conditions for biodiversity. I studied the availability and use of natural tree cavities in retention. It was conducted on a random selection of 100 clear-cuts up to 5 years old within a 20 km radius in the vicinity of Umeå, northern Sweden. A survey of those clear-cuts was performed to study tree retention and to investigate the availability and the potential use of cavities by cavity nesting birds on clear-cuts in boreal Sweden. All cavities and nests found were monitored during two months to study cavity-use. Previous studies have found that the majority of all tree cavities in Europe are natural decay cavities. In my study I found 133 cavity trees that held 241 cavities. Ninety-eight percent of the cavities found were excavated and just 2% were natural cavities produced by decay. This novel finding suggests that also in European boreal forest the majority of cavities available for secondary cavity nesters are excavated by woodpeckers. Cavity trees were 1.5-2% of the retention trees; Living broadleaved trees, especially aspen, were preferred by birds for breeding. A significantly larger proportion of the excavated entrance holes were facing a direction from east to south, possibly to maximize the heat from the sun during cold mornings. Estimated cavity tree density ranged from 0.21-0.31 cavity trees ha-1 and cavity density was 0.41 cavities ha-1. All breedings found were in solitary retention trees showing that successful breedings can take place on clear-cuts in boreal forests. However, in order to compare cavity densities, their origin and their use by cavity nesting species, a similar study should be performed in old forest. I suggest the conduction of a survey simultaneously in forests and on clear-cuts to provide a basis for comparison between these two different environments.