Emilson, Erik

Mining and smelting degraded landscapes are characterised by heavily eroded soils that are acidic, contaminated with toxic metals, and depleted of essential nutrients. Regreening degraded landscapes has been proposed to support global carbon (C) mitigation measures and protect biodiversity. One of the world's largest regreening programs in the City of Greater Sudbury, Canada has been ongoing since 1978 and involves liming and fertilizing selected areas followed by planting primarily jack pine (Pinus banksiana Lamb.) and red pine (Pinus resinosa Ait.) trees. The main objective of this thesis was to improve our understanding of biogeochemistry in the City of Greater Sudbury regreened forests, and to determine how nutrient pools and cycling change as stands age. I established a chronosequence of forested sites between 15–40 years-old and to account for the effects of erosion, each site was categorized as "stable" (<10% bedrock cover) or "eroded" (>30% bedrock cover). Individual tree growth and nutrient accumulation in aboveground biomass (AGB) did not differ between stable and eroded sites and were comparable to rates reported from pine plantations in similar ecozones. Aboveground nitrogen (N) pools were six times larger than N applied in fertilizer, suggesting N limitation is most likely not a concern. Rates of C cycling were generally similar to those measured at unimpacted jack and red pine plantations. The exception being a decrease in mineral soil and aggregate C concentrations. However, at the ecosystem-scale the loss of soil C is trivial in comparison to increases in AGB C pools, leading to an overall increase in total ecosystem C following regreening (550,547 Mg in aboveground C across the 19,649 ha regreening landscape). Litter decomposition rates were higher at the regreening sites using a site-specific litter compared to a general common litter, indicating a home-field advantage for local decomposers. Soil temperature varied at the regreening sites and higher soil temperatures were related to higher rates of soil respiration. The regreening sites are rich in calcium (Ca) and magnesium (Mg); and while soils were generally poor in phosphorous (P) and potassium (K), foliar concentrations of P and K were comparable to those of "healthy" red pines. Overall, the regreening program appears to have increased tree growth and produced jack and red pine plantations that are biogeochemically similar to conifer plantations unimpacted by over a century of mining and smelting impacts.

Author Keywords: biogeochemistry, degraded landscape, forests, nutrient cycling, regreening, soil carbon

Many communities in Canada rely on water sourced from boreal forest headwaters for their drinking water. The Boreal Shield Ecozone is highly susceptible to climate change which threatens to exacerbate the effects of natural and human-driven disturbances such as wildfire, insect infestation and harvesting on water quality. Therefore, examining source water quality in headwater catchments within the Boreal Shield Ecozone is crucial to elucidating the potential implications of these disturbances to water treatment processes in the context of a changing climate. A synoptic water sampling investigation was conducted to evaluate how dissolved organic carbon (DOC) quantity and quality and disinfection by-product formation-potential (DBP-FP) quantity varied across space and time in the Algoma region of central Ontario. Over a five-month timeframe (June 2021 - October 2021), 168 streamflow estimates and 176 water samples were collected across 30 catchments (catchment areas from 0.2 - 106.8 km2) which varied in their forest disturbance histories. DOC concentration ([DOC]) ranged from 2.4 - 38.2 mg L-1 and tended to be higher in harvest-dominated sites, while no discernible differences in SUVA254 were observed between catchment types. DOC export estimates ranged from 1.0 - 63.2 g C m-2 over a 141-day period (June 5th - Oct. 23nd, 2021). Fluorescence indices for quantifying DOC composition suggested that all catchments were dominated by humified and terrestrially sourced carbon. DBP-FP values were positively correlated to UV-254 (r = 0.76 - 0.78) and [DOC] (r = 0.85 - 0.88), such that DBP-FP spatiotemporal patterns were strongly coupled to DOC dynamics. Multiple linear regression analysis identified that open water was negatively related to [DOC] and SUVA254 and explained the most variability in their spatiotemporal patterns. In addition, catchment area, which was negatively related to [DOC] and SUVA254, and legacy insect infestation and harvesting disturbance helped improve model explanatory power. Other predictor variables, such as slope, wetland cover, coniferous forest cover and recent forest disturbance (i.e., 5-year harvesting and 5-year insect infestation), showed relatively poor explanatory power. Variability in DOC export estimates may be explained by harvesting disturbance (adjusted r2 = 0.68 - 0.82). The results of this study emphasise that complex processes across the terrestrial-aquatic continuum, which are influenced by several factors, such as runoff, forest disturbance and landscape heterogeneity, govern the spatiotemporal patterns in water quality across boreal headwaters within the Algoma region.