Effect of species composition on the production rate and efficiency of young Picea glauca–Populus tremuloides forests

• Production rate and efficiency were examined on 41 fully occupied plots.
• Leaf area index and light interception increased with softwood proportion.
• Species proportion did not affect the production rate of young, mixed stands.
• Efficiency based on light interception decreased slightly with softwood proportion.

Stand stem volume growth, aboveground tree biomass growth, and site occupancy were measured in 20-year-old mixed stands dominated by white spruce (Picea glauca [Moench] Voss) and trembling aspen (Populus tremuloides Michx.) to examine the effects of species composition on the rate and efficiency of forest production. Measures of site occupancy derived from Plant Canopy Analyzer data indicated that sample plots were fully occupied, and that leaf area index and light interception increased with increasing softwood (SW) proportion. Relationships between site occupancy and SW proportion differed among occupancy metrics, suggesting that basal area, stand density index, and crown area index do not adequately represent use of the light resource. No significant effect of species proportion on the rate of volume or biomass production was detected, contradicting the hypothesis that competitive reduction and facilitation might increase the production of mixed stands. Production efficiency based on light interception for both stand components combined varied weakly with SW proportion. Efficiency metrics based on surrogates for light interception showed differing patterns with SW proportion, underscoring the need for caution in interpreting these metrics in relation to resource use.