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 diff erent locations in eastern Fennoscandia. In this paper, we analysed the out break history of 13 separate 24-47-year-old stands of Prunus padus. the mai n 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 Russ ian border. We also studied the effect of an insect outbreak on radial grow th 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 i nsect outbreak. Heavily infested tree species sustained a large reduction i n 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. W e suggest that a decline in the area of farmland after the 1950s led to dev elopment of an extraordinarily large amount of high-quality habitats on the Russian side which, together with a period of mild winters in the tarry 19 90s, created the conditions for this novel O. brumata epidemic. (C) 2001 El sevier Science B.V. All rights reserved.