Surface and subsurface material selections influence the early outcomes of boreal upland forest restoration

Abstract

Soil is an essential component supporting the growth and maintenance of forests, providing anchorage, water, and nutrients for a diversity of plants. In Canada's boreal forest, surface soils differ widely in their chemical and physical conditions, ranging from coarse to fine textured mineral soils in the uplands to organic soils in the lowlands. Industrial disturbances in the boreal region require the selective salvage of surface and sub-soils from low- and upland areas during open pit mine operations, which can be used to reconstruct different soil profiles by arranging the materials in variable layers and thicknesses. The type of materials and their configuration in the reconstructed soil profile can have a profound effect on early forest establishment in reclaimed areas. We compared the growth of trembling aspen (Populus tremuloides Michx.), jack pine (Pinus banksiana Lamb.), and white spruce (Picea glauca Moench.) on sites that had different reconstructed soils caps using varying surface soil materials (salvaged lowland peat and upland forest floor material (FFM)), placement depths (10 or 30 cm for peat, 10 or 20 cm for FFM), and subsoil material types determined by salvage depth (Bm, BC, and C). Early seedling establishment and growth and soil and climatic parameters were monitored over a five-year period. Generally, seedling growth was greatest on FFM and appeared to be related to phosphorous availability, while peat as a surface soil reduced growth, likely due to delayed soil warming in the spring and cooler soil conditions that potentially limited resource availability. However, the greater water holding capacity of the organic matter in peat also provided a benefit for seedling growth that was apparent during water limiting climatic conditions. The underlying subsoil material also influenced growth, particularly later in establishment when roots occupied the deeper subsoils. Aspen growth was greatest when the subsoil was shallow salvaged and represented a weathered subsoil (Bm) compared to the more deeply salvaged, less weathered subsoils BC and C. Aspen and pine seedlings, with their larger roots systems, may have benefited from small increases in the silt fraction of the subsoils, likely due to the greater water holding capacity of these otherwise coarse textured sandy soils. Spruce regeneration responded marginally to soil treatments because of its overall slow growth-strategy and tolerance to resource limitations. Our results indicate that different soil capping strategies affect the performance of forest regeneration during post-mining site reconstruction, and boreal forest species may vary in responses according to their ecological life strategies.