The standard deviation values highlight the influence of climate variation on maximum LAI for the two forests. Among all climatic components, drought has the strongest effect on LAI (Alavi 2002). The annual precipitation in this study area ranged from 242 mm in 1998 to 680 mm in 1972 with an average of 443 mm over a 60-year period. In addition, severe low temperatures in winter also restrict LAI (Gholz
1982). Trembling aspen, as a deciduous forest, is more sensitive to climatic variation than jack pine