Understanding how reclamation practices influence plant community assembly and succession is an important step in developing realistic indicators and targets for reclamation of oil sands mine sites to upland forest ecosystems. We currently have a poor understanding of factors affecting plant community assembly and succession in reclaimed oil sands sites. Through synthesis of research completed over the last 24 years, we identify four key findings: (i) use of surface soil and forest floor material salvaged from mined areas increases plant species cover, richness, and diversity relative to the use of various other cover soil materials (such as clay subsoil); (ii) stockpiling of salvaged surface soils decreases the abundance of native plant propagules and delays early vegetation community development; (iii) differences in plant community composition between reclaimed and adjacent mature forests remain two decades after placing cover soils; however, differences are smaller with use of forest floor–mineral mix than peat–mineral mix; and (iv) plant community assembly is in progress but communities remain different to those found in natural undisturbed conditions. Our review identified critical knowledge gaps for further research to improve understanding of: (i) long-term (60 to 100 years) plant community composition in reclaimed oil sands sites; (ii) how residual forest patches near disturbed oil sands sites act as seed and propagule sources in newly reclaimed sites; (iii) plant community assembly processes in reclamation sites; (iv) the effect of micro-topographic heterogeneity on plant community development; and (v) how soil nutrient availability in different substrates influences plant community development over the long term. Ongoing support for selected existing studies and establishment of new studies focusing on plant community development through long-term monitoring are highly recommended.

Stockpiling of cover soil can influence vegetation development following reclamation. Cover soil, comprising the upper 15-30 cm of the surface material on sites scheduled for mining, is commonly salvaged prior to mining and used directly or stockpiled for various lengths of time until it is needed. Salvaging and stockpiling causes physical, chemical, and biological changes in cover soils. In particular, stockpiling reduces the availability and vigor of vegetative propagules and seed, and can lead to increases in the abundance of some weedy species. This study uses data from monitoring plots to assess how stockpiling of cover soil impacts plant community development on reclaimed oil sands mine sites in northern Alberta. Development of plant communities differed distinctly between directly placed and stockpiled cover soil treatments even 18 years after reclamation. Direct placement of cover soil resulted in higher percent cover, species richness, and diversity. Nonmetric multidimensional scaling and multiresponse permutation procedure revealed compositional differentiation between the treatments. Indicator species analysis showed that direct placement treatment was dominated by perennial species while grasses and annual forb species dominated sites where stockpiled soil was used. Results indicate that stockpiling leads to slower vegetation recovery while direct placement of cover soil supports more rapid succession (from ruderal and annual communities to perennial communities). In addition, direct placement may be less costly than stockpiling. However, scheduling of salvage and placement remains a challenge. Implications for Practice• Direct placement of cover soil during reclamation can facilitate establishment of a plant community with many desirable native forest understory species. • Although the trajectory of community development on the direct placement treatment follows the typical early successional progress of boreal forests, the community assembly process is unstructured and requires more time to form a stable plant community. • It is very important to consider alternative options in stockpile management in order to maintain the viability of native seed and vegetative propagules.

Understanding the effects of reclamation treatments on plant community development is an important step in setting realistic indicators and targets for reclamation of upland oil sands sites to forest ecosystems. We examine trends in cover, richness, evenness, and community composition for four cover soil types (clay over overburden, clay over tailings sand, peat-mineral mix over overburden, and peat-mineral mix over tailings sand) and natural boreal forests over a 20 year period in the mineable oil sands region of northern Alberta, Canada. Tree, shrub, and nonvascular plant species cover showed similar increases over time for all reclamation treatments, with corresponding declines in forb and graminoid cover with time. These trends resemble those in the natural boreal forests of the region and the trajectory of community development for the reclamation treatments appears to follow typical early successional trends for boreal forests. Species richness and diversity of natural forest differed significantly from reclamation treatments. Nonmetric multidimensional scaling ordination and multi-response permutation procedure revealed that species composition was not affected by reclamation treatment but clearly differed from natural forest. Analysis of species co-occurrence indicated random plant community assembly following reclamation, in contrast to a higher proportion of nonrandom plant community assembly in natural forests. Thus, reclaimed plant communities appear to be unstructured through year 20 and assembly is still in progress on these reclaimed sites. Implications for Practice• The temporal changes in understory plant communities reveal that species composition in the reclaimed ecosystems would be much different from those naturally disturbed ecosystems. • Although the trajectory of plant community development shows the typical early successional progress of boreal forests, random plant community assembly suggests that the communities remain unstructured 20 years after reclamation. • The significant temporal changes in reclaimed ecosystems suggest that studies of ecosystem establishment and development on reclaimed areas should be conducted over the long term as these monitoring data will be useful in developing adaptive management strategies for ecosystem recovery.

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