Climate change and impacts of boreal forest insects

The circum-polar boreal forest has played an important role in the wealth of northern nations since the 15th century. Its natural resources spurred strategic geopolitical developments beginning in the 16th century but intense development of the boreal forest is largely limited to the 20th century. Insects cause considerable loss of wood that has an adverse effect on the balance of carbon sequestered by forests. Current understanding of processes that lead to stand-replacing outbreaks in three insect species is reviewed in this paper. Many of these processes depend on climate either directly, such as reduced survival with extreme weather events, or indirectly, mainly through effects on the host trees. In the boreal zone of Canada, pest-caused timber losses may be as much as 1.3-2.0 times the mean annual depletions due to fires. Pests are thus major, but consistently overlooked forest ecosystem components that have manifold consequences to the structure and functions of future forests. Global change will have demonstrable changes in the frequency and intensity of pest outbreaks, particularly at the margins of host ranges. The consequent shunting of carbon back to the atmosphere rather than to sequestration in forests as biomass is thought to have positive feedback to global warming. Whereas significant progress has been made in developing carbon budget models for the boreal forests of Canada, enormous problems remain in incorporating pest effects in these models. These problems have their origins in the nature of interactions among pests with forest productivity, and problems with scaling. The common problems of verification and validation of model results are particularly troublesome in projecting future forest productivity. The interaction of insects with fires must be accounted for if realistic carbon sequestration forecasts in a warming climate are to be made, These problems make assessments of mitigation and adaptation of pest management alternatives difficult to evaluate at present. Nevertheless, the impacts of stand-replacing insect population outbreaks is important in formulating future resource management policy. Crown Copyright (C) 2000 Published by Elsevier Science B.V. All rights reserved.