Factors affecting interannual variation in growth of western Canadian aspen forests during 1951-2000

TTrembling aspen ( Populus tremuloides Michx.) is the most important deciduous tree in the North American boreal forest and is also the dominant tree in the aspen parkland zone along the northern edge of the Canadian prairies. Since the 1990s, observations of dieback and reduced growth of aspen forests have led to concerns about the potential impacts of climate change. To address these concerns, a regional- scale study ( CIPHA) was established in 2000 that includes annual monitoring of forest health and productivity of 72 aspen stands across the western Canadian interior. Tree- ring analysis was conducted to determine the magnitude and cause of temporal variation in stand growth of aspen at the scale ( 1800 km x 500 km area) encompassed by this study. The results showed that during 1951 - 2000 the region's aspen forests underwent several cycles of reduced growth, notably between 1976 and 1981, when mean stand basal area increment decreased by about 50%. Most of the growth variation was explained by interannual variation in a climate moisture index in combination with insect defoliation. The results of the analysis indicate that a major collapse in aspen productivity likely occurred during the severe drought that affected much of the region during 2001 - 2003.