Geometrid Moth Outbreaks and their Climatic Relations in Northern Sweden

Abstract The alpine treeline in northern Fennoscandia is composed primarily of mountain birch ( Betula pubescensssp. czerepanovii), a deciduous tree that experiences episodic defoliationdue to outbreaksof the autumnal moth( Epirritaautumnata) and winter moth( Operophterabrumata). Here, we use an extensive dendroecological data set to reconstruct historic defoliating outbreaksand relate them to climatic conditions. Our data are from 25 sites in eight valleys in northern Sweden. We used the computer program OUTBREAKto reconstruct moth outbreaks. The reconstructed outbreakrecord matches the historical record well. There is a significant, but weak relationship between the outbreakseverity and temperatures in February, April, July, and August of the outbreakyear. Temperatures in the previous May and November were also positively correlated with outbreakseverity. For seasonally aggregated temperatures, only autumn temperatures are correlated with outbreakseverity. There was no significant correlation...