Abstract

I used data from 149 repeated aerial photographs taken in (1950s), middle (1970s or 1980s) and recent (1990s) times, together with forest inventory maps prepared by the Alberta Environmental Protection Agency in the year 1999 known as "Phase 3 inventory maps" to analyze changes in spruce crown closure (percentage of canopy cover) and height in mixed forest stands dominated by aspen ( Populus tremuloides Michx) and white spruce ( Picea glauca (Moench) Voss) in a boreal forest in Alberta, Canada. A total of 81 stands located in good (N = 11), medium (N = 60), and fair (N = 10) quality sites were studied for spruce emergence. Site quality and the time since the last fire accounted for about 70% of the variance in spruce canopy cover. A significant correlation between spruce crown closure and age was detected in good (r 2 = 0.71), medium (r 2 = 0.81) and fair (r 2 = 0.54) quality sites. There was no significant correlation between the rate of change in spruce cover (% cover/year) and the time since the last fire. The white spruce reached the main forest canopy height and overtopped the aspen in about 80-100 years since the fire. Results of the present study suggest that a longer time is required for the emergence of spruce through aspen canopy than previous estimates.