Climate warming and insect outbreaks in sub-arctic birch forest

Population outbreaks in geometrid moths constitute the most pronounced disturbance factor in sub-arctic birch forest that sometimes leads to extensive forest death. We have recently made two discoveries that suggest that distinct, climate-change related s hifts in geometrid outbreak ranges are now taking place in northern Fennoscandia. Firstly, the winter moth Operopthera brumata, which previously had population outbreaks restricted to low-altitude, coast-near forest in NW Norway, has expanded its outbreak range to reach the altitudinal and latitudinal forest limit in NE Finnmark. There the winter moth now overlaps with the normal outbreak range of the autumnal moth Epirrita autumnata as to cause intense and long-lasting defoliation leading to extensive fo rest death. Secondly, during the last 3 years, a novel geometrid for northern Norway, Agriopis aurantiaria, has rapidly become abundant as to inflict forest damage locally in the coastal birch forest in Troms County. A. aurantiaria is known as a forest pe st species further south in Europe and it may thus represent a putative new pest in the sub-arctic birch forest. These changing distributions of geometrids have coincided with a period of increasingly earlier springs. We hypothesize that the earlier sprin gs have provided a spreading phenological match between the geometrids and their food resource (i.e. birch) or a mismatch between the geometrids and their enemies leading to an escape from regulatory mechanisms. Here we propose to test this match-mismatch hypothesis in a food web perspective, combining small-scale process studies with large-scale, pattern oriented approaches. Through these studies, we aim to provide means of forecasting the further developments of the geometrid outbreak dynamics in northe rn mountain birch forest under various scenarios of climate change.