Forest management practices emphasizing stand structural complexity are of interest across the northern forest region of the United States because of their potential to enhance carbon storage. Our research is nested within a long-term study evaluating how silvicultural treatments promoting late-successional forest characteristics affect aboveground biomass development in northern hardwood forests. We are testing the hypothesis that biomass development (carbon storage) will be greater in structural complexity enhancement (SCE) treatments when compared to conventional uneven-aged treatments. SCE treatments were compared against selection systems (single-tree and group) modified to retain elevated structure. Manipulations and controls were replicated across 2-hectare treatment units at two study areas in Vermont, USA. Data on aboveground biomass pools (live trees and coarse woody material, standing dead and downed wood) were collected pre- and post-harvest then again a decade later in 2013. Species group-specific allometric equations were used to estimate live and standing dead biomass and downed log biomass was estimated volumetrically. We used Forest Vegetation Simulator to project "no-treatment" baselines specific to treatment units, allowing measured carbon responses to be normalized relative to differences in site-specific characteristics and pre-treatment conditions. Results indicate that 10 years post-harvest biomass development and carbon storage were greatest in SCE treatments compared to conventional treatments, with the greatest increases in coarse woody material (CWM) pools. Structural complexity enhancement treatments contained 12.67 Mg ha-1 carbon in CWM compared to 6.62 Mg ha-1 in conventional treatments and 8.84 Mg ha-1 in areas with no treatment. Percentage differences between post-harvest carbon and baseline values indicate that carbon pool values in SCE treatments returned closest to pre-harvest or untreated levels over conventional treatments. Total carbon storage in SCE aboveground pools was 15.90% below baseline conditions compared to 44.94% less in conventionally treated areas (P = 0.006). Results from CART models indicated treatment as the strongest predictor of aboveground C storage followed by site-specific variables, suggesting a strong influence of both on carbon pools. Structural enhancement treatments have potential to increase carbon storage in managed northern hardwoods based on these results. They offer an alternative for sustainable management integrating carbon, associated climate change mitigation benefits, and late-successional forest structure.

Bringing together field-based empirical studies, a simulation modelling experiment, and a critical analysis of the natural disturbance emulation paradigm, this thesis evaluates partial harvesting in the black spruce boreal forest. Forest management in Ontario is required to emulate natural disturbances, but in regions of the boreal forest where fire cycles are long, regulated even-aged management by clearcutting has truncated forest age-class distributions. Partial harvesting has been proposed as a means to maintain the structural complexity and biodiversity associated with old forests while allowing continued timber production. Despite the potentially important role of partial harvesting in a strategy for sustainable boreal forest management, little research has examined post-harvest stand development, a critical determinant both of habitat and timber supplies.I used a chronosequence approach in combination with dendroecological techniques, a neighbourhood modelling framework, and maximum likelihood statistical methods to quantify stand dynamics over the first decade after partial harvest in the black spruce (Picea mariana) boreal forest of northeastern Ontario, Canada. Residual trees displayed large but time-lagged growth responses to partial harvest. The largest responses were found in young trees, while old trees were largely unable to react to improved post-harvest resource availability. Growth responses were offset by elevated rates of residual-tree mortality, which peaked in the first year after harvest at nearly 13 times the pre-harvest level. Proximity to harvest machinery trails severely escalated the risk of mortality for residual trees. Considering growth and mortality responses together in a forest simulator model, I found that stand development proceeded most rapidly where skidding intensity was reduced and retention areas were aggregated. Given appropriate prescriptions, my results indicate that partial harvesting can be a viable silvicultural option for black spruce boreal forests. However, the ability of partially harvested stands to emulate natural disturbance is questionable, particularly given the strong influence of harvest machinery impacts on post-harvest stand development. I argue that the natural disturbance emulation framework has important flaws and falls short of a justifiable approach for forest management in Ontario.

Bringing together field-based empirical studies, a simulation modelling experiment, and a critical analysis of the natural disturbance emulation paradigm, this thesis evaluates partial harvesting in the black spruce boreal forest. Forest management in Ontario is required to emulate natural disturbances, but in regions of the boreal forest where fire cycles are long, regulated even-aged management by clearcutting has truncated forest age-class distributions. Partial harvesting has been proposed as a means to maintain the structural complexity and biodiversity associated with old forests while allowing continued timber production. Despite the potentially important role of partial harvesting in a strategy for sustainable boreal forest management, little research has examined post-harvest stand development, a critical determinant both of habitat and timber supplies.I used a chronosequence approach in combination with dendroecological techniques, a neighbourhood modelling framework, and maximum likelihood statistical methods to quantify stand dynamics over the first decade after partial harvest in the black spruce (Picea mariana) boreal forest of northeastern Ontario, Canada. Residual trees displayed large but time-lagged growth responses to partial harvest. The largest responses were found in young trees, while old trees were largely unable to react to improved post-harvest resource availability. Growth responses were offset by elevated rates of residual-tree mortality, which peaked in the first year after harvest at nearly 13 times the pre-harvest level. Proximity to harvest machinery trails severely escalated the risk of mortality for residual trees. Considering growth and mortality responses together in a forest simulator model, I found that stand development proceeded most rapidly where skidding intensity was reduced and retention areas were aggregated. Given appropriate prescriptions, my results indicate that partial harvesting can be a viable silvicultural option for black spruce boreal forests. However, the ability of partially harvested stands to emulate natural disturbance is questionable, particularly given the strong influence of harvest machinery impacts on post-harvest stand development. I argue that the natural disturbance emulation framework has important flaws and falls short of a justifiable approach for forest management in Ontario.