Mixtures of trembling aspen (Populus tremuloides Michx.) and white spruce (Picea glauca (Moench) Voss) are a prominent component of the boreal forests in Western Canada, and overyielding indicating better productivity in mixtures than monocultures has been observed in mature stands in this region. In this study we use data collected from 2011 to 2015 from 6 agencies including forestry companies and governments in Alberta and Saskatchewan for the Western Boreal Growth and Yield Association (WESBOGY) Long Term Study to examine whether overyielding occurs in young mixtures of these species. At this site spruce was established at densities of 500 and 1000 stems per hectare, with aspen densities of 0, 200, 500, 1500, 4000 created by thinning, as well as an unthinned control. In addition, pure aspen treatments included unthinned, thinned to 4000 sph, and to 1500 sph. Stem biomass was calculated from diameter at breast height (DBH) and tree height and used as the production variable, and species proportions were calculated based on basal area. Productivity ratio (mixture production / expected production based on pure stands of the two species adjusted according to species proportion) was used to examine overyielding.
Results indicate that 68 out of 102 plots show overyieding (66.7 %). The combination of 1000 spruce and 1500 aspen has the highest frequency of overyielding plots (14 out of 68 plots (20.6 %)). However, only the combination of 1000 spruce with 1500 aspen (P-Ratio=1.25) is significantly different from 1000 spruce with unthinned aspen (P-Ratio=0.96). The mean productivity ratio of three combinations (Sw 1000 sph with Aw 1500 sph or 4000 sph and Sw 500 with Aw 4000 sph) are significantly higher than 1. An examination of the effects of climate and other factors on overyielding is underway.