Information on plant community assembly mechanisms is limited on forest reclamation sites after mining in the Canadian boreal forest. We assessed the change in plant community composition after Year 2 and Year 5 on species-rich forest floor mineral mix (FFMM) and species-poor peat mineral mix (PMM) reclamation soils by examining assembly mechanisms, i.e., seed bank, seed rain, biotic dispersal, vegetative expansion, and competition. Initial plant cover and diversity were greater on FFMM due to non-native species originating from the seed bank, which had 5× more seeds in the FFMM. By Year 5, both soil types had approximately 40% cover and 80 species richness due to the addition of wind and biotic-dispersed species and were characterized by a shift towards native species. Native forbs using vegetative reproduction expanded up to 2 m from FFMM into PMM. At Year 5 competition does not seem to have a large role in the structuring of the vegetation community. Overall, multiple factors were involved in structuring plant communities on reclamation sites, but we observed a general convergence between plant communities on different soil types in a relatively short period of time.

The majority of plant diversity in the boreal forest of northern Alberta, Canada is comprised of native understory plant species that are continuously facing competition from other species, including both undesirable native and weedy species. In oil sands mine reclamation, cover soils rich in organic matter are used to cap overburden materials. The aim of this study is to understand the role of weeds on different reclamation cover soils (forest floor-mineral mix and peat-mineral mix) and determine if they hinder the establishment of the native plant community. This study was conducted four growing seasons after site establishment in June 2019. At that time, both soil types had approximately 45% total cover, had 21 species per plot, and were composed of mainly native vegetation. Competition from non-native forbs (11% average cover, mainly Sonchus arvensis and Melilotus alba) did not seem to impact the development of the native vegetation community on either soil type given the high cover and richness of native forbs. However, native graminoids (predominantly Calamagrostis canadensis) were associated with reduced native forb cover and richness at graminoid cover greater than 17%. Overall, non-native forbs appeared to have little impact on the native forb community on either soil type while native graminoids had a negative influence. We suggest that the classification of what is considered an undesirable weedy species should be evaluated in the context of ecosystem management goals rather than simply the presence of non-native species.

Buried wood is an important but understudied component of reclamation soils. We examined the impacts of buried wood amounts and species on the growth of the common reclamation tree species trembling aspen (Populus tremuloides). In a greenhouse study, aspen seedlings were planted into four soil types, upland derived fine forest floor-mineral mix (fFFMM), coarse forest floor-mineral mix (cFFMM), and lowland derived peat and peat-mineral mix (PMM), that were mixed with either aspen or pine wood shavings at four concentrations (0%, 10%, 20% and 50% of total volume). Height and diameter growth, chlorophyll concentration, and leaf and stem biomass were measured. Soil nutrients and chemical properties were obtained from a parallel study. Buried wood primarily represents an input of carbon to the soil, increasing the C:N ratio, reducing the soil available nitrogen and potentially reducing plant growth. Soil type had the largest impact on aspen growth with fFFMM = peat > PMM > cFFMM. Buried wood type, i.e., aspen or pine, did not have an impact on aspen development, but the amount of buried wood did. In particular, there was an interaction between wood amount and soil type with a large reduction in aspen growth with wood additions of 10% and above on the more productive soils, but no reduction on the less productive soils.

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Surface oil sand mining and extraction in northern Alberta’s Athabasca oil sands region produces large volumes of oil sand process–affected waters (OSPW). OSPW is a complex mixture containing major contaminant classes including trace metals, polycyclic aromatic hydrocarbons, and naphthenic acid fraction compounds (NAFCs). Naphthenic acids (NAs) are the primary organic toxicants in OSPW and reducing their concentrations is a priority for all oil sands companies. Previous evidence has shown that constructed wetland treatment systems (CWTS) are capable of reducing the concentration of NAs and the toxicity of OSPW through bioremediation. In this study, we constructed greenhouse mesocosms with OSPW or lab process water (LPW, i.e., water designed to mimic OSPW minus the NAFC content) with three treatments: (1) OSPW planted with Carex aquatilis, (2) OSPW – no plants, and (3) LPW – no plants. The OSPW – C. aquatilis treatment saw a significant reduction in NAFC concentrations in comparison to OSPW- no plants treatments, but both changed the distribution of the NAFCs in similar ways. Upon completion of the study, treatments with OSPW saw fewer high molecular weight NAs and an increase in the abundance of O3– and O4–containing formulae. Results from this study provide invaluable information on how constructed wetlands can be used in future remediation of OSPW in a way that previous studies were unable to achieve due to uncontrollable environmental factors in field experiments and the active, high-energy processes used in CWTS pilot studies.