Climate and the autumnal moth (Epirrita autumnata) at Mountain Birch (Betula pubecens ssp. czerepanovii) Treelines in northern Sweden.

Abstract

The main objectives of this investigation were to determine the impact of climate on mountain birch (Betula pubecens ssp. czerepanovii (Orlova)) growth and to develop a regional chronology of autumnal moth outbreaks. To accomplish the objective, cores of mountain birch were taken from 21 sites in Norrbotten, Sweden. Tree-ring chronologies were developed for each site. Climatic influences were determined by correlating ring widths to climatic variables (average monthly temperature, average monthly precipitation and NAO). Outbreaks were recovered from the ring width indices using the non-host method with Scots pine (Pinus sylvestris (L.)) as the non-host. This method removes the climatic influence on growth to enhance other factors. Patterns of synchrony and regional outbreaks were detected using regression and cluster analysis techniques. The primary climatic influences on the tree ring growth of mountain birch are June and July temperatures; precipitation during October is of secondary importance. Climate explained 46% of yearly tree ring width variation. Outbreaks of the autumnal moth occur at varying time intervals depending on the scale of study. Intervals between outbreaks on the tree level are twice as long as at the plot level. On the regional scale plots within the same valley had more similar outbreak intervals and magnitudes of outbreaks. Elevation is a driver in determining the length of outbreaks and length between outbreaks. The percent monocormicity of a plot is also a determining factor of the length between outbreaks. This study is the first regional scale study on climate and outbreaks of the autumnal moth on mountain birch. The results complement research being conducted on autumnal moth larval densities and will help in modeling and assessing the effects of outbreaks with increasing climatic change.