Effects of broadleaf density on soil and ecosystem carbon in the western Canadian boreal forest

Canada’s forests store large carbon (C) stocks, with most C stored below-ground. Mixtures have been proposed as a management alternative that balances timber yield and long-term C storage better than monocultures. In western Canadian boreal forests, trembling aspen (Populus tremuloides) and white spruce (Picea glauca) commonly occur in mixed and pure stands, though their influences on ecosystem C vary. Uncertainty remains regarding which mixture proportions and densities maximize C storage under current and future climate conditions. To address this, I examined the effects of white spruce and trembling aspen on soil and ecosystem C dynamics in two Western Boreal Growth and Yield Long-Term Study (WESBOGY) installations, established in Saskatchewan, Canada. The first chapter evaluated total soil organic C (SOC), total nitrogen (N), C:N ratios, and understory vegetation across pure spruce and mixtures of varying spruce-aspen densities at age 34. In the second chapter, I used aboveground measurements from WESBOGY to run the Mixedwood Growth Model (MGM) to project the stands to age 80 under a no climate change (NCC) scenario and with future climate projections using ClimateNA7.6 and MGM-Climate. Resulting volume-age curves were input into the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3) to simulate total ecosystem C (TEC), biomass, SOC, litter, and C fluxes under NCC, SSP245, and SSP370 scenarios to 2050 and 2070. Results from the first chapter showed no significant differences in total SOC, N, or C:N ratios among treatments overall, although site and depth-specific differences occurred in mixtures. Understory grass and herb cover increased significantly with aspen presence and reduced spruce density. In the second chapter, TEC, biomass, SOC, and litter pools decreased under warming scenarios, partly due to increasing heterotrophic respiration increasing decomposition, though mixtures with 500 aspen stems ha-1 had the highest TEC as net C sinks across all climate scenarios, whereas natural aspen mixtures and pure spruce stands transitioned to net C sources under warming. Overall, the findings from my thesis suggest that mixtures with 500 stems ha-1 of aspen may enhance long-term ecosystem C storage under future climates relative to pure spruce stands or mixtures with natural aspen densities.