Spatial pattern of outbreaks of Operophtera brumata in eastern Fennoscandia and their effects on radial growth of trees

During the mid 1990s, Operophtera brumata outbreaks were reported from different locations in eastern Fennoscandia. In this paper, we analysed the outbreak history of 13 separate 24-47-year-old stands of Prunus padus. the main host of this herbivore in eastern Fennoscandia, based on width of annual growth rings. The stands were located on both sides of the Finnish and Russian border. We also studied the effect of an insect outbreak on radial growth of several dominant tree species in mixed forests. It has been suggested that less-preferred tree species in the mixed stands may benefit from an insect outbreak. Heavily infested tree species sustained a large reduction in radial growth. However. we did not find that less-preferred tree species could directly benefit from an the outbreak, to a degree that is measurable in radial growth. There were no signs of previous outbreaks of O. brumata, and the recent epidemic was almost totally confined to the Russian side. We suggest that a decline in the area of farmland after the 1950s led to development of an extraordinarily large amount of high-quality habitats on the Russian side which, together with a period of mild winters in the tarry 1990s, created the conditions for this novel O. brumata epidemic. (C) 2001 Elsevier Science B.V. All rights reserved.