Population density estimates and temperature records in relation to topography will, together with other factors, be used to predict the likelihood of defoliating outbreaks of Epirrita autumnata in the mountain birch forest.Severe severe defoliation by E. autumnata larvae may lead to a die-back of old trees and initiate a regrowth of from basal sprouts and seedlings and thus changing the age-structure of the forest. The change to a younger successional stage may have implications for reindeer husbandary. The interactions between insect-herbivory and reindeer grazing is studied in other parts of the project.