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The experiment was conducted at the Canadian Natural Resources Limited Albian Sands open-pit oil sands mine, 70 km north of Fort McMurray, Alberta, Canada (see Figure 2).
Figure 2: Map of study area close to Fort McMurry, showing the two sites (south and east) in blue at the Canadian Natural Resources Limited Albian Sands open-pit oil sands mine. Each site has a slope of around 36º facing the direction of each arrow (south and east).
Site preparation
Site preparation
Two 5-ha sites were established on an overburden dump within the mining lease, each on a different slope aspect (i.e. a south-facing and an east-facing site) with an approximate gradient of 36%. Each site was divided into five adjacent 1ha blocks, following a randomized block design, with 5m buffers between them. Each block was split into three treatment areas, also randomized, that correspond to the three microtopographic variations in the study: control (i.e. leveled), ridged, and hilled (see Figure 3 for more details). Site preparation was finalized by early 2015 and consisted of layering an adequate soil mix of forest floor material (FFM) and lowland peat mineral mix (PMM) on top of the overburden material (this soil mix naturally contains native seeds). The variations between treatments were as follows: the control was a leveled soil; for ridged, parallel ridges were dug perpendicular to the slope, and in hilled, loose mounds were randomly arranged in off-set rows (see Figure 4 for more details).
Figure 3: Diagram of study site design. Each 5.2ha site (south and east) was divided into 5 blocks (A to E for south, and F to J for East) with a 1ha area separated by a 0.05ha buffer. Each Block was then split into 3 areas of 0.33ha based on the treatment (CTRL - control, RDGE - ridged, and HILL - hilled), which was assigned randomly throughout the blocks.
Figure 4: Diagram of the three microtopographic treatments (control, ridged, and hilled) achieved by modifying and shaping the soil mix of FFM (forest floor material) and PMM (Peat mineral mix).
Seedling planting
Seedling planting
Once site preparation was complete, one-year-old nursery seedlings of Picea abies, Pinus banksiana, and Populus tremuloides were planted. For the south-facing site, this was done in early 2015, and for the east-facing site in late spring of 2016 due to logistic difficulties. The three native species were randomly planted with an initial density of 3200 trees/ha and a ratio of 2:2:6 respectively to emulate the natural distribution.
Data collection
Data collection
To evaluate seedling response, two measurements were taken. First, tree height (in cm) was measured yearly for each site, from 2015 to 2018 for the south-facing slope and from 2016 to 2018 for the east-facing slope. To do so, on each block and site, a 90m by 2m belt transect was laid across all treatments with a 5 m buffer along the blocks to minimize the edge effects (see more details in Figure 5). Second, starting in 2017 and again in 2018, root collar diameter (RCD in mm) was gathered for all the trees in the transect belts to evaluate volume.
Figure 5: Diagram of the transect belts set on each block treatment to collect height and root collar diameter. Each block had three transect belts, one for each treatment and these were approximately the same for each measurement year.
Statistical analysis
Statistical analysis
Two-way multi-factor analyses of variance (ANOVA) for height and volume were conducted to assess the effects of treatment and slope on tree growth. The analyses included treatment (control, hilled, and ridged) and slope (south and east) as fixed factors, and their interaction was evaluated to identify potential combined effects. Because the main goal is to estimate treatment and slope effects on the overall growth, only data from the last year of the experiment (i.e. 2018) was used. This way, tree behavior can be compared in each treatment and slope after multiple years of growth.
Two-way multi-factor analyses of variance (ANOVA) for height and volume were conducted to assess the effects of treatment and slope on tree growth. The analyses included treatment (control, hilled, and ridged) and slope (south and east) as fixed factors, and their interaction was evaluated to identify potential combined effects. Because the main goal is to estimate treatment and slope effects on the overall growth, only data from the last year of the experiment (i.e. 2018) was used. This way, tree behavior can be compared in each treatment and slope after multiple years of growth.
Blocks were included as a random factor in the models to account for the variation within each site due to spatial heterogeneity. This approach allowed us to address the experimental design.
Blocks were included as a random factor in the models to account for the variation within each site due to spatial heterogeneity. This approach allowed us to address the experimental design.
Post hoc pairwise comparisons were conducted using estimated marginal means (EMMs) to identify differences between treatments within each slope. EMMs were also used to generate 95% confidence intervals for mean estimates to evaluate the difference between treatments.
Post hoc pairwise comparisons were conducted using estimated marginal means (EMMs) to identify differences between treatments within each slope. EMMs were also used to generate 95% confidence intervals for mean estimates to evaluate the difference between treatments.
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