Results

Heights

Microtopography Enhances Tree Growth on a Southern Site

Overall, microtopographic variation (hilled and ridged) positively influenced tree growth at the south site, leading to taller trees after 4 years. These findings illustrate that differences between treatments became more pronounced over time and across all species (see Figure 22). The hilled treatment consistently promoted the greatest growth across species, especially compared to the control treatment (p-values were all <0.001). These results show, for example, how the height average was approximately 16 cm taller than the control for Pinus banksiana and approximately 50 cm taller for Populus tremuloides (95% confidence intervals). The ridged treatment was similarly successful to the hilled treatment, except in Pinus banksiana, which showed no statistical difference to Picea glauca but did perform better than the control, with Pinus banksiana and Populus tremuloides in ridged plots growing 11 cm and 34 cm taller, respectively. These results highlight the significant role of microtopographic variability—particularly hilled treatments—in enhancing growth across species over time as trees mature.

Figure 22: Average heights throughout the study for the south site based on species, treatment and year. The columns divide the species: Picea glauca, Pinus banksiana and Populus tremuloides. Each color corresponds to a different treatment: yellow for control, green for hilled, and blue for ridged. Error bars represent standard errors.

Contrasting Responses Between East and South Sites

While microtopography enhanced growth on the southern slope, the results differed significantly on the east-facing slope (see Figure 11). A possible reason for these results is the difference in climatic conditions that influence either slope. Southern sites have more light, higher temperatures, and reduced humidity. These harsher conditions make soil variations essential for sapling survival, especially in reclamation, where there is minimal protective vegetation. A hilled and ridged treatment provides more areas where moisture is retained, similar to what would occur in a natural environment, and thus benefits the dry south-facing slope. On the other hand, the increased water availability and lower temperatures on an east-facing slope may reduce the need for microtopography for successful establishment.

Figure 11: Average heights throughout the study for the south (top) and east (bottom) sites based on species, treatment and year. The columns divide the species: Picea glauca, Pinus banksiana and Populus tremuloides. Each color corresponds to a different treatment: yellow for control, green for hilled, and blue for ridged. Error bars represent standard errors.

The evident positive effects from the hilled treatment observed on the southern slope were absent on the east-facing slope, where growth was unaffected by treatments or lower for control (see Populus tremuloides in Figure 23).

Figure 23: Average heights throughout the study for the east site based on species, treatment and year. The columns divide the species: Picea glauca, Pinus banksiana and Populus tremuloides. Each color corresponds to a different treatment: yellow for control, green for hilled, and blue for ridged. Error bars represent standard errors.

For Picea glauca and Pinus banksiana, both visual assessments and statistical analyses confirmed no significant differences in growth between treatments. This suggests that microtopographic variations are less critical for these species under milder conditions. In contrast, the results for Populus tremuloides revealed some differences. While the original mean height plot suggested that control plots performed best, followed by ridged and hilled treatments, the statistical analysis showed a more nuanced picture (Figure 18). Control and ridged treatment showed higher results (p.values <0.001 and 0.04, respectively), with at least a 6cm increase for the control and a 2cm overlap with the ridged effect. The ridged and control treatments, however, were not statistically different.

These findings stress the variability among species and how site conditions influence the effectiveness of microtopography. The one-year delay between site preparation and planting on the east slope likely played a role in these results, as it allowed natural seed sprouting, particularly for the fast-growing Populus tremuloides. However, these results suggest that the eastern site, with increased moisture and cooler temperatures, reduces the reliance on microtopography for plant establishment.

Figure 18: Interaction plot showing the combined effects of treatment and site on the mean height (cm) of Populus tremuloides at the end of the experiment. The x-axis represents the two sites (East and South) and the colors represent the different treatments(control - yellow, hilled - green, and ridged - blue) Error bars indicate the 95% confidence intervals for the mean volume estimates.

Overall effects of microtopographic variation on height

While microtopography enhanced growth on the southern slope, using a technique other than business-as-usual (control treatment) was not statistically better for either species on the eastern slope, where moisture is more abundant. On the southern slope, however, using a hilled treatment consistently displayed more growth across all species, especially Populus tremuloides. Considering that these differences are due to only four years of growth, we can expect greater variability in topography over time, similar to a natural environment that enhances growth under drier conditions. For mining companies, using a hilled treatment that is easier to prepare than a leveled soil could benefit restoration when a site is influenced by drier and warmer conditions such as those in souther-facing slopes. Based on these results, those cases where conditions are moist, such as those on an eastern-facing slope, including topographic variations, seem to not provide any benefits to plant growth.

Species-Specific Responses to Microtopography

The timing and extent of growth improvements varied among species on the southern slope (see Figure 22). Populus tremuloides , a fast-growing pioneer, responded early to the hilled treatment, while Pinus banksiana showed intermediate growth rates and benefited sooner than Picea glauca. The latter, which establishes later in succession, only exhibited significant treatment effects towards the end of the study, indicating treatment preferences in later successional stages (Grime, 1974).

Volumes

Larger volumes are not always influenced by treatment

When comparing volumes across treatments, the patterns observed in tree heights were relatively consistent, though there were differences when analyzing log-transformed data and the ANOVA results.

On the southern slope, the hilled treatment always showed larger volumes across all species, both in the log-transformed bar charts and the ANOVA (p-value <0.005). While some smaller variations between treatments were less clear in the bar charts (Figure 14), the statistical analysis revealed that for all species, the control treatment consistently had smaller volumes compared to the more heterogeneous treatments (ridged and hilled).

Figure 14: Bar chart of average log-transformed volumes (cm3) for each species and treatment on the south site. Each bar chart section provides the increase over 1 year (from 2017 to 2018) and compares the different treatments (control - yellow, hilled - green, and ridged - blue) for Picea glauca, Pinus banksiana and Populus tremuloides. Error bars represent standard error.

As mentioned, when comparing the most contrasting treatments, hilled always outperformed the control. Analyzing the 95% confidence interval results showed that for Picea glauca, the volume difference was at least 41 cm³; for Pinus banksiana, hilled volumes exceeded the control by at least 65 cm³, and for Populus tremuloides, the control volumes were at least 237 cm³ smaller than those in the hilled treatment (all pairwise-comparisons had a p-value <0.001). These findings suggest that the hilled treatments enhance volume, likely due to increased moisture retention and improved growing conditions in the harsher southern slope environment.

In contrast, the volume results from the eastern slope were more variable between bar charts and ANOVA, with overall non-significant treatment effects. Although visual trends indicated higher volumes for business-as-usual for Picea glauca and Populus tremuloides (Figure 15), statistical analysis showed no significant differences across all species and treatments. Even for Populus tremuloides, where the difference in volumes seems high, the interaction plots from the ANOVA show no differences (Figure 21). This contrast suggests that the more favorable growing conditions on the east slope, such as increased moisture and lower temperatures, may reduce the need for microtopographic variation to influence volume growth. Overall, these results highlight that the effectiveness of microtopography in influencing tree growth, particularly volume, is strongly site-dependent, with more pronounced effects observed in the harsher conditions of the southern slope.

Figure 21: Interaction plot showing the combined effects of treatment and site on the mean volume (cm³) of Populus tremuloides at the end of the experiment. The x-axis represents the two sites (East and South) and the colors represent the different treatments(control - yellow, hilled - green, and ridged - blue) Error bars indicate the 95% confidence intervals for the mean volume estimates

Figure 15: Bar chart of average log-transformed volumes (cm3) for each species and treatment through time on the east slope. Each bar chart section provides the increase over 1 year (from 2017 to 2018) and compares the different treatments (control - yellow, hilled - green, and ridged - blue) for Picea glauca, Pinus banksiana and Populus tremuloides. Error bars represent standard error.

Conclusion

The results of this study emphasize that the effectiveness of microtopographic variation on tree growth is strongly site-dependent. On the southern slope, with drier and warmer conditions, treatments that introduced more microtopographic variation—particularly the hilled treatment—significantly enhanced tree height and volume across all species. They likely promote better moisture retention and, thus, growing conditions in an otherwise harsh environment. In contrast, on the east slope, where moisture is more abundant and temperatures are cooler, the benefits of microtopography were not as evident. For heights and volumes and across all species, the business-as-usual treatment was not significantly different from at least one of the other treatments. This suggests that, in milder environments, the advantages of microtopography are less critical for successful establishment.

For mining companies, these findings offer new insights for reclamation. On southern-facing slopes, where conditions are harsher, implementing a hilled treatment would provide significant benefits and restoration success by increasing growth. This treatment could also be easier to implement compared to traditional leveling methods, as it simply involves creating mounds, which could reduce preparation time and costs. However, on cooler, moister slopes like those facing east, microtopography may not be necessary to provide additional benefits, and a more standard approach could suffice.

Page updated

Report abuse