Mature white spruce stands in the Mid-Boreal Upland Ecoregion of Saskatchewan
Creators: name: Hope, Sharon Margaret
Publisher: name: University of British Columbia
Year: 2009

description: A three-stage approach was used to investigate mature white spruce (Picea glauca (Moench) Voss) stand types in Saskatchewan. The approach involved: 1) identification of potential stand types in a conceptual model, 2) description and quantification of stand structure and forest floor processes in selected representative stand types and 3) simulation of two stand types using the ecosystem management model, FORECAST. The computer model explored the role of nitrogen fixation in succession patterns under different disturbance regimes. White spruce stands with high-density aspen were hypothesized to have greater levels of forest floor nitrogen and greater stemwood biomass than stands with low-density aspen. Analyses of field data on stand structure (stem densities, basal area, snag distribution, and coarse woody debris volumes) provided recognition of four distinct white spruce stand types similar to those depicted in the conceptual model. Summer season measures of nitrogenase activity and potential mineralizable nitrogen rates were greater in the spruce-aspen (Populus tremuloides Michx.) stand type (wS/tA), than in the spruce-balsam-fir (Abies balsamea (L.) Mill.) stand type (wS (bF)). Rates of microbial activity were greater in litter than in well-decayed logs or mineral soil. Greater nitrogen concentrations and nitrogen content occurred in aspen logs than in white spruce logs that had been decomposing under wS/tA stands for up to 45 years. Nitrogen fixation rates measured in spruce litter and decaying logs were used as inputs for the FORECAST computer simulations that evaluated white spruce succession over 240 years. The simulations predicted that time between disturbances, disturbance type and disturbance severity were key determinants of stand composition over the 240-year simulation period. Computer simulated white spruce stemwood biomass was greatest in aspen-dominated stands when multiple partial harvests were introduced compared to simulations with multiple light fires. The simulated harvest disturbances deposited moderate amounts of stemwood on the forest floor over the 240-year period and did not remove forest floor matter. In contrast, multiple stand replacement fires and light fires consumed forest floor organic matter and left some stemwood on the forest floor. The increase in white spruce biomass in simulations with harvesting in comparison to simulations with fire was attributed to increased available nitrogen from the accumulations of litter and stemwood on the forest floor and to reduced nitrogen loss due to fire. Results show that there are different nitrogen legacies associated with wS/tA stands, compared to wS (bF) stands. These legacies may respond differently to various forest management practices and to a range of disturbance intensities. However, the inferences from my research are limited by a single season of microbial activity measurement, by the small number of sites used to test the conceptual model and by the fact that the simulations run with FORECAST have yet to be validated.