The Influence of Selection Silviculture Biomass Harvesting on Soil Carbon, Nutrients, and Respiration in a Northern Mixed-Deciduous Forest

Concerns have been raised about soil fertility following increased recovery of biomass in northern mixed deciduous forests. Several studies suggest that biomass extraction and enhanced decomposer activity can lead to losses of soil C and nutrients, increased nitrification, and reduced soil CO2 efflux (FCO2) from compaction and root senescence. However, results are inconsistent; differences may be linked to insufficient investigation and characterisation of variability in forest structure, silvicultural methods, and intensity and heterogeneity of disturbances. Without a comprehensive understanding of how soil biogeochemical processes respond to different types and intensities of harvesting disturbances, we cannot accurately predict losses and ecological sustainability of harvesting. My thesis tests this by evaluating changes to forest structure and various soil edaphic and physical properties following tree-length (TL) and more intensive biomass selection-harvests for 3 years and on primary and tertiary skid trails. Several metrics of forest harvesting intensity were correlated to higher post-harvest FCO2 rates and losses of soil and dissolved soil organic C (SOC/DOC), K+, and NH4+. Increased canopy openness and soil temperatures accounted for 34-41% of elevated FCO2. Woody debris inputs and SOC/DOC losses correlated to higher autumn FCO2 rates and NH4+ losses, while harvested tree biomass correlated to lower summer rates and higher nitrate concentrations. Primary skid trails had low SOC/nutrient concentrations and FCO2 from compaction and inhibited regrowth, potentially functioning as C sources. Biomass harvesting halved woody debris inputs relative to TL harvesting; otherwise, short-term treatment differences were nominal. However, estimated base cation losses through additional biomass recovery may produce net long-term soil depletion not observed in conventional harvesting. These results support my hypotheses that harvesting increased decomposer and nitrification activity, losses of C and nutrients, and that harvesting and trail use intensity can potentially predict these changes. Ecosystem-scale projections of forest C and nutrient storage that do not distinguish between spatiotemporally variable source/sink components (e.g. skid trails, harvested forest) may not effectively estimate changes. Combined with a detailed mensuration of pre/post-harvest forest structure and environmental covariates, future studies that representatively sample these different areas may improve our ability to predict system responses to disturbances.Ph.D