Egg cold hardiness and topoclimatic limitations to outbreaks of Epirrita autumnata in northern Fennoscandia.
Forests of mountain birch (Betula pubescens subsp. tortuosa [B. tortuosa]) in the lower parts of the Abisko valley, northern Sweden, and in the Finnmarksvidda highland plain, northern Norway, escaped defoliation during outbreaks of larvae of Epirrita autumnata in 1955 and 1964-65, while forests in surrounding areas were severely damaged. Studies of these damage patterns suggested that low temperatures in a cold air 'lake' and a cold air 'dome', respectively, which temporarily built up during the winter, killed hibernating eggs and thus prevented defoliation of the undamaged forests. The spatial distribution of absolute minimum temperatures in the Abisko valley, and across the Finnmarksvidda and the coastal lowland to the north was deduced for the winters preceding the outbreaks. In a complementary study, the supercooling points of the hibernating eggs of E. autumnata were determined. These data, together with the profiles of minimum temperatures, were used to define the distribution of egg-killing cold air accumulations in the Abisko valley and on the Finnmarksvidda. The patterns of cold air accumulations explained the horizontal and vertical extent of the undamaged forests, caused by the combined effects of defoliation and delimiting topoclimate.