Revegetation of disturbed land reclaimed with suboptimal topsoil replacement depth and organic amendments

Successful reclamation and revegetation of disturbed sites depends on the availability of sufficient topsoil. Current regulations in western Canada require the replacement of ≥80% of the original topsoil depth (TRD) to restore pre‐disturbance productivity. However, salvaged topsoil at many legacy and other disturbed sites is often insufficient to achieve the 80% TRD (TRD80). This 5‐year study examined vegetation responses to 50% TRD without organic amendment (TRD50) or amended with either peat (PTRD50) or biochar (BTRD50), relative to the TRD80 treatment, following borrow site reclamation in northeastern Alberta, Canada. Amendments were applied once at rates calculated to bring organic carbon in the topsoil to concentrations approximating those in the TRD80 treatment. Tree and shrub seedling mixes were transplanted following amendment application and vegetation attributes were measured annually thereafter. Results showed that PTRD50 performed as well as TRD80 and outperformed BTRD50 with respect to native species' canopy cover and tree seedling height. Canopy covers of forb and non‐native species decreased significantly whereas those of graminoid and native species increased over time for all treatments. Across treatments, native plant species richness increased by 5% per year while non‐native species richness decreased by 19% per year. Overall, peat showed a satisfactory rate of recovery and a trajectory toward the natural boreal forest, indicating its potential to improve reclamation success at disturbed boreal sites where salvaged soil is insufficient to attain equivalent land capability. This finding will provide an additional tool for regulators seeking to refine land reclamation guidelines for such sites in boreal forest regions.

site preparation for natural gas and oil activities require the removal of vegetation and soil, which disturbs soil biogeophysicochemical properties and, therefore reduces soil productivity and disrupts natural ecosystem functioning (Lupardus et al., 2019).Additionally, the use of topsoil from borrow sites for ancillary purposes such as road construction and reclamation of well sites often leaves behind disturbed sites (borrow pits) that must be reclaimed (Alberta Environment & Parks, 2018;ESRD, 2013).
However, the reclamation of disturbed forest sites is complex and requires the availability of appropriate resources to support successful revegetation to equivalent land capability levels (ESRD, 2013;Fridley, 2002;Macdonald et al., 2012).
The amount of topsoil returned to a disturbed site during reclamation is the primary factor influencing successful revegetation, as soil quantity and quality influence plant growth (Yang et al., 2019).
Previous studies have demonstrated reduced plant growth following reclamation using suboptimal topsoil volumes (Larney et al., 2003;Yang et al., 2019).Suboptimal topsoil depths, particularly those attained using stockpiled soil, are often characterized by nutrient deficiencies, low organic carbon content, poor drainage, and limited availability of viable native seeds or propagules, all of which can impede plant establishment and growth (MacKenzie et al., 2012;Mackenzie & Naeth, 2009;Yang et al., 2019).On the other hand, exceeding the optimum topsoil depth may be expensive as it may not translate to an increase in soil productivity (ESRD, 2013;Yang et al., 2019).Published research indicates that determination of topsoil replacement depth (TRD) in the short term should be based on plant establishment and growth requirements for successful restoration (Dhar et al., 2018), but in the long term, emphasis should be on sustainable and stable plant communities to minimize invasion of native boreal forests by nonnative species and to reduce topsoil erosion (Wick et al., 2011).
Revegetation efforts at disturbed sites are hampered by the scarcity of quality topsoil and organic matter needed to sustain plant growth during early establishment (Alberta Environment & Water, 2012;Bowen et al., 2005;Mackenzie & Naeth, 2009).Achieving a complete replacement of the topsoil layer thickness which existed prior to the disturbance is infeasible due to the inevitable loss of topsoil during the salvaging process (ESRD, 2013;Powter et al., 2012).Additionally, during the early 1990s, there was no regulatory requirement for the oil and gas industry to implement predisturbance measures such as topsoil salvaging and stockpiling.
Instead, the topsoil removed during well site development was often used for other purposes (Powter et al., 2012).Consequently, many older borrow-and well sites were left with insufficient or no salvaged topsoil for use during reclamation to promote plant growth during the revegetation process.
Current regulations in Alberta require the replacement of at least 80% of the pre-disturbance topsoil depth in forested lands and grasslands and at least 85% of the original topsoil in cropland for successful reclamation and restoration (ESRD, 2013;Powter et al., 2012).Optimal topsoil volume ensures adequate nutrient supply and water retention, which are critical in early vegetation establishment and survival on reclaimed forest sites (Dhar et al., 2019;Macdonald et al., 2012).
While most of the soil at disturbed sites associated with oil development is typically of low quality, alternative cost-effective reclamation efforts using organic amendments have been implemented to augment insufficient topsoil of disturbed forest sites (Brown & Naeth, 2014;Page-Dumroese et al., 2018;Rowland et al., 2009).
Several organic amendments have been used in the reclamation of disturbed forest sites in Alberta, Canada.Peat is abundant in the oil sands region and is therefore commonly used in upland reclamation and revegetation projects (Calver et al., 2019;Errington & Pinno, 2015;Rowland et al., 2009).Peat supplies soil nutrients, such as nitrogen (N) and phosphorus (P) (Calver et al., 2019;Hemstock et al., 2010) and improves soil water holding capacity (Li et al., 2020;Moskal et al., 2001).While peat is commonly used in boreal forest reclamation, the majority of previous studies focused on the use of peat mineral mix on sites disturbed by oil sands mining (Dhar, Comeau, Naeth, Pinno, & Vassov, 2020a;Hahn & Quideau, 2013;Rowland et al., 2009).Similarly, Petelina et al. (2014) reported greater vegetation cover in soils amended with peat during reclamation of an oilsands site in Alberta.
Recently, biochar has attracted increasing attention as a potential effective organic amendment for the reclamation and revegetation of disturbed boreal sites (Page-Dumroese et al., 2016, 2018).Previous studies have shown increased water-holding capacity (Page-Dumroese et al., 2016, 2018), sustainable nutrient supply and high organic carbon (Mukherjee & Zimmerman, 2013), and increased cation exchange capacity (CEC) (Glaser et al., 2002;Joseph et al., 2010)  While most published research has addressed the use of organic amendments in the revegetation of disturbed forest ecosystems (Dhar et al., 2019;Dhar, Comeau, Naeth, & Vassov, 2020b;Hahn & Quideau, 2013), it remains unclear if organic amendments could augment reclamation undertaken using suboptimal topsoil volume.Therefore, the objective of this research was to determine the effects of TRD with or without organic amendments on early vegetation establishment and whether these responses vary with time elapsed since reclamation.This information is needed to formulate recommendations for reclamation and revegetation of disturbed boreal sites where available topsoil falls short of the volume needed to attain equivalent land capability.We hypothesized that the addition of organic amendments to suboptimal TRD would improve early vegetation establishment and plant community development to levels comparable to those of the required 80% TRD.

| Study site
The boreal forest region in Alberta is home to numerous disturbed sites characterized by varying levels of suboptimal salvaged topsoil volumes available for reclamation of the sites.This research was conducted at a borrow site located near Cold Lake in northeastern Alberta, Canada (54 36 0 22.26 00 N, 110 29 0 28.24 00 W).Salvaged topsoil at the site was sufficient to attain only 50% of the original topsoil depth (i.e., 50% TRD).The 2.7 ha borrow site was established in 2010 to provide the subsurface material needed for the construction of access roads and well pads ancillary to nearby oil developments.The site was decommissioned in 2014 and reclaimed the same year using insufficient topsoil that had been stockpiled at the site.
Reclamation was completed in November when cold temperatures averaging À9.6 C and an average snow depth of 38.5 cm ensured minimal soil structure disruption.
The study area had previously been dominated by tree species that included trembling aspen (P.tremuloides Michx.), white spruce (Picea glauca), and balsam poplar (Populus balsamifera L.), understory by shrub species such as wild red raspberry (Rubus idaeus) and low bush cranberry (Viburnum edule) (Table S1) (Natural Regions Committee, 2006).The soil at the site was an Orthic Gray Luvisol of the Athabasca soil series, with a sandy loam to sandy clay loam surface layer (pre-disturbance thickness = 28 cm) overlying a subsoil dominated by medium-sized gravel with some presence of fine-sized gravel.The soil structure was angular blocky to subangular blocky.

| Experiment layout and treatments
Study plots (20 m Â 20 m with 2 m buffers) were established at the time of reclamation on a 0.5 ha section of the borrow site.The experimental design was a randomized complete block with three blocks representing three replicates per treatment (Figure S1).The treatments were (1) the recommended optimal topsoil depth (TRD80) as the control, (2) the unamended suboptimal topsoil depth (TRD50), (3) the 50% TRD amended with peat (PTRD50), and (4) the 50% TRD amended with biochar (BTRD50).The topsoil layer thickness was 22 cm (that is, 80% of the 28 cm pre-disturbance topsoil thickness) for the TRD80 treatment and 14 cm for the TRD50 treatment.

| Amendment application
Peat and biochar were applied in May 2015 at rates of 20 and 4.75 kg m À2 , respectively, to bring the soil organic carbon (SOC) in the corresponding plots to the same level as measured in the TRD80 plots.The peat had been stockpiled for 1 year at a nearby well-pad site prior to application.Biochar was procured from Biochar Now, CO, USA and had been prepared from raw pine tree logs using slow pyrolysis at temperatures of 550-650 C for 6 to 12 h.Each amendment was uniformly spread on the designated plots using a tractor-(Massey Ferguson 1525) mounted manure spreader (520 New Holland) and a Buhler Farm King 705 disc to incorporate the amendments into the topsoil.Chemical characterization of the amendments was completed prior to application (Table 1).

| Soil sampling and analysis
Soil sampling and analysis were described in detail by Nawu et al. (2023).Briefly, initial soil samples were collected in 2014 for baseline characterization before the start of the experiment (Table 1).Subsequently, soil samples were collected annually from 2015 to 2019 at a depth of 0-15 cm.The soil samples were analyzed by Bureau Veritas Laboratories (Calgary, Alberta).The analysis included measuring SOC using the dry combustion method (Nelson & Sommers, 1996), total Kjeldahl nitrogen (TKN) using the EPA 351.2 method (USEPA, 1993); available P and K using the modified Kelowna method (Qian et al., 1994), and available sulfur following CaCl 2 extraction (Houba et al., 2000).

| Seedling transplanting
Following plot preparation and treatment application, $3125 seedlings ha À1 , equivalent to a total of 125 trees and shrub seedlings, were planted in each plot in May 2015.The seedlings (8-12 months old) were obtained from Boreal Horticulture Ltd., Bonnyville, Alberta.Tree seedlings [balsam poplar (Populus balsamifera), trembling aspen (P.tremuloides), white spruce (Picea glauca), and white birch (Betula papyrifera)] were individually planted and uniformly distributed in each plot while shrub seedlings [saskatoon (Amelachier alnifonia), green alder (Alunus veridis), and red osier dogwood (Cornus sericea)] were planted in clusters in accordance with standard reclamation practice (Table S1).Additional tree and shrub seedlings were planted in buffer zones between the plots and along the perimeter of plot areas.The practice of replacing salvaged topsoil on disturbed sites is widely acknowledged as a strategy to promote the germination and growth of volunteer herb species, as it has been observed that the salvaged topsoil contains seed propagules of various native understory graminoids and forbs (MacKenzie et al., 2012).Therefore, revegetation efforts aim at transplanting woody species while allowing understory herbs to naturally recover (MacKenzie & Naeth, 2007).

| Vegetation measurements
A pre-disturbance site assessment for the species composition and canopy cover was conducted in 2010 as per reclamation planning requirements.This assessment provided baseline vegetation attributes prior to the disturbance of the site (Table S1).Canopy cover and vegetation health were assessed annually starting in August 2015.Plot centers were divided into equal sections equivalent to four 10 m Â 10 m quadrats.Azimuth and distance from the plot center were used for stem mapping of the planted tree/shrub species while observing quadrat boundaries (Man & Yang, 2015).Stem mapping and vegetation assessments of planted species were conducted concurrently to map and tag the seedlings for identification purposes.Vegetation assessments recorded included height (cm) and species type.
Total canopy cover was enumerated for different functional groups [graminoids, forbs, woody (trees and shrubs), native, and non-native species] and exceeded 100% for all but woody species due to overlap when cover estimates of species within a functional group were summed.Species richness, Shannon Diversity Index (H 0 ; Equation 1; Kent & Cooker, 1992), species evenness (E; Equation 2), and percent survival (Equation 3) were calculated using vegetation assessment data collected from the quadrats.
where S = number of species, P ¼ the total sum of species (S), p i =proportion of individual species or abundance of the i th species expressed as a proportion of total cover, and ln = natural log.
Percent survival ¼ number of survived trees total number of trees planted Â 100: ð3Þ

| Statistical analysis
Data were analyzed using the generalized linear mixed model procedure (PROC GLIMMIX) of SAS version 9.4 (SAS Institute, 2013) to determine treatment effects on vegetation (woody, graminoid, forb, native, and non-native) canopy cover, tree height, species richness, Shannon diversity index (H 0 ), evenness (E), and percent survival.Treatment was modeled as a fixed effect and block as a random effect, while year was a repeated measures factor.For tree and shrub survival, species were also modeled as a fixed effect.The first-order autoregressive (AR[1]) covariance structure was selected as the most suitable for repeated measures analysis of vegetation attributes data, based on the corrected Akaike information criterion (AIC; Littell et al., 1996).Data for vegetation (except woody species) cover, H 0 , and seedling height were normally distributed whereas those for woody species cover and E followed a beta distribution.Percent survival followed a binomial distribution, and species richness followed the Poisson distribution.Treatment effects on seedling height were determined using analysis of covariance, with initial height as a covariate.Means were compared using the Tukey multiple comparison procedure at α = 0.05.When year main and interaction effects on species richness, woody plant height, and percent survival were significant, orthogonal polynomial contrasts were tested, and appropriate regressions were fitted and compared where applicable.

| Weather conditions
Annual total precipitation during the study ranged from 403 to 501 mm, of which 304 mm was recorded during the growing season (May to October) (Figure 1).The 30-year (1981-2010) mean cumulative growing season precipitation for Cold Lake is 308 (Environment Canada, 2020).Monthly total precipitation was highest between June and July in all years.The highest monthly mean temperatures ranging from 16.9 to 18.7 C were recorded in July, while the lowest monthly mean temperatures ranging between À11.7 and À22.6 C were recorded in January in all years.

| Canopy cover
Reclamation planning necessitates a pre-disturbance site assessment, which identifies and documents the site's baseline species composition and canopy cover prior to well site disturbance (ESRD, 2013).
The collected data serve as a reference for revegetation efforts subsequent to the restoration of a decommissioned disturbed site.The predisturbance site assessment for vegetation cover conducted in 2010 showed canopy covers of 5%, 29%, and 250% for graminoids, forbs, and woody species, respectively (Table 2).The canopy cover of the site after reclamation showed that total cover and graminoid canopy cover were significantly greater for the TRD80 than the BTRD50 treatment but did not differ significantly between the TRD50 and the PTRD50 treatments (Table 4).Canopy cover of forbs and woody plants did not differ significantly among treatments but varied significantly with time elapsed (year) since the start of the experiment (Table 4).Averaged across treatments, the canopy cover of all species, forbs, non-natives, and bare ground declined over time whereas those for woody, native, and graminoid species increased with increasing time since reclamation.
There was a significant treatment Â year interaction for native and non-native species cover (Table 4).Native species canopy cover was not significantly different between treatments in 2015 whereas in 2016 native canopy cover was greater for the PTRD50 than the BTRD50 treatment (Figure 2a).In 2017, the TRD80 treatment had significantly greater native species cover than the BTRD50 and TRD50 treatments whereas, in 2019, there was no significant difference in native species canopy cover among the three treatments.
Notably, in 2019, native species canopy cover approximately doubled for the TRD50 and increased by 45% for the BTRD50 treatment relative to 2018.Additionally, when comparing the organic-amended plots, the PTRD50 treatment produced a significantly greater native species canopy cover than the BTRD50 treatment in 2018, but there was no significant difference between the treatments in 2019.
The treatment effect on the canopy cover of non-native species was only significant in 2017 and 2018.Non-native canopy cover was significantly lower for PTRD50 than for BTRD50 and TRD50 in 2017 (Figure 2b).By comparison, BTRD50 had a significantly greater nonnative canopy cover than TRD80 in 2018, but this difference was not significant in 2019.Notably, across treatments, non-native species canopy cover declined significantly (to <20%) in 2019 relative to the preceding 3 years, indicating a decrease in the dominance of nonnative species over time.
Bare ground and coarse woody debris cover did not vary significantly with treatment (Table 4).However, there was a significant change in bare ground over time.Bare ground decreased significantly after 2015, averaging 1.5% during 2016 through 2018, and disappearing by 2019.

| Species richness, diversity, and evenness
Trend analysis using orthogonal polynomial contrasts revealed a significant treatment Â year linear interaction for total, graminoid, F I G U R E 1 Monthly cumulative precipitation and mean temperature during 2015-2019 at the borrow pit near Cold Lake, Alberta, Canada.
T A B L E 2 Pre-disturbance species canopy cover.

Functional group
Canopy cover (%) Forbs 29 Graminoids 5 Woody 250 Total 287 native, and non-native species richness (Table 5).However, subsequent Poisson regression analysis showed no significant treatment effects on regression coefficients for total, graminoid, and native species richness.Therefore, a common linear regression was fit for all treatments and indicated a 3%, 19%, and 10% increase in total, graminoid, and native species richness per year, respectively (Table S3).By comparison, forbs and nonnative species richness decreased by 8% and 22% per year, respectively.Woody species richness for TRD50 was significantly greater than that for BTRD50 but was not significantly different from those for TRD80 and PTRD50 (Table 5).Both forb and woody species richness showed significant change over time.Forb species, averaged across treatments, were more abundant during the early years (2015 and 2016) but declined significantly in subsequent years.In contrast, woody species richness increased from 2015 through 2019 (Table 5).
Regression analysis across all treatments showed an 8% per year decrease in forb species richness whereas woody species richness increased by 6% per year (Table S2).
Species evenness and H 0 did not differ significantly among treatments (Table 5).Across all treatments, both indices were significantly higher in 2015 than in 2016 but did not differ significantly from the other years (Table 5).Although orthogonal polynomial contrasts revealed a significant quadratic trend, regression analysis showed no significant temporal change in H 0 across all treatments.Five years after reclamation, H 0 decreased to 2.6 from 2.9 in 2015.

| Seedling height
There was a significant treatment Â year interaction for seedling heights of aspen, birch, poplar, and spruce (Table 6).Orthogonal polynomial contrasts revealed significant quadratic temporal trends in seedling height that varied among treatments, as indicated by quadratic Â treatment interaction for aspen, birch, poplar, and spruce (Table 6).Regression analysis showed that aspen seedling height for the TRD80 and PTRD50 treatments was described by a common regression and increased as a quadratic function of year but increased linearly with year for BTRD50 and TRD50 (Figure 3a).Thus, while aspen heights increased from 10.1 cm in 2015 to 49.5 cm in 2019 for TRD50 and from 25.7 to 75.7 cm for BTRD50, it increased from 35.9 to 144 cm for TRD80 and PTRD50.Aspen heights for PTRD50 and TRD80 increased at a slower rate during the first 2 years but increased exponentially over time, thereafter, reaching a maximum of 169 cm in 2019 (Figure 3a).While aspen height increased by 12 cm year À1 for BTRD50, it increased by only 6.56 cm year À1 for TRD50.
The temporal increase in birch height was described by a common regression for TRD80 and PTRD50 and increased as a quadratic function of the year whereas it increased linearly and jointly for TRD50 and BTRD50 (Figure 3b).Similarly, spruce height increased quadratically with time for TRD80 and PTRD50, whereas it increased linearly for TRD50 and BTRD50 (Figure 3c).
The increase in poplar seedling height was described by two quadratic functions: one for TRD80 and PTRD50 treatments and the other for the BTRD50 treatment, whereas there was no significant trend in seedling height for the TRD50 treatment.Poplar tree heights for the TRD50 treatment were significantly lower than those for the other three treatments in 2017 through 2019 (Figure 3d).
There was a significant treatment effect on the shrub height of dogwood and Saskatoon but not for alder (Table 6).Averaged across years, dogwood height was significantly lower for BTRD50 than for TRD80, TRD50, and PTRD50.Similarly, averaged across years, saskatoon height was significantly lower for BTRD50 than for TRD80 and PTRD50 but did not differ significantly between BTRD50 and TRD50.
There was also a significant year effect on the height of all three shrub species (Table 6).Orthogonal polynomial analysis and subsequent regression analysis revealed that, averaged across all treatments, shrub height increased as a linear function of year for alder

| Tree and shrub survival
There was a significant treatment Â species interaction for tree survival (Table 7).Across all years, spruce maintained a high survival for all treatments while birch showed a significantly lower percent survival for BTRD50 relative to TRD80, TRD50, and PTRD50 (Figure 4).
Birch survival was significantly greater than poplar survival for the TRD80, TRD50, and PTRD50 treatments but the survival of the two species did not differ significantly in the BTRD50 treatment.In comparison to other tree species, aspen had the lowest survival (<50%) across all treatments (Figure 4).
Tree survival, averaged across treatments, differed significantly with year.Orthogonal polynomial contrasts showed a significant linear function for tree survival, with regression analysis indicating a decrease of 28% per year for all treatments.Shrub survival did not differ significantly among treatments and years, but varied significantly among species, with dogwood showing a significantly greater survival than saskatoon and alder (Table 7).

| Canopy cover, species richness, and diversity
The revegetation process is viewed as an important indicator of successful reclamation and restoration of forest ecosystems, with a goal of achieving equivalent land capability (ESRD, 2013).It is widely acknowledged that greater topsoil depth facilitates earlier plant establishment and vegetation cover compared to shallow depths in reclamation (Bowen et al., 2005;Redente et al., 1997).This is because thick topsoil layers provide ample space for root development and expansion, while also retaining sufficient nutrients and water to support prompt revegetation (Redente et al., 1997;Schladweiler et al., 2005)  suboptimal topsoil replacement to levels comparable to the required 80% TRD.Our results demonstrate that, in the absence of optimal soil volume, peat incorporation into the suboptimal topsoil (TRD50) can enhance vegetation canopy cover to levels comparable to the required topsoil depth (TRD80).The higher initial TKN and total nitrogen (TN) concentrations, combined with a low C:N ratio of peat (Table 1), provided favorable conditions for promoting early vegetation growth in peat-amended plots compared to biochar-amended plots.Further, moisture retention properties of peat (PTRD50 treatment) observed in the amended soils across 5 years in our study (Table 3) make peat a better choice than biochar (BTRD50) for enhancing vegetation cover.This is consistent with previous research, which showed that biochar delayed vegetation establishment and growth due to low plant available nutrients like N during restoration of boreal sites (Page-Dumroese et al., 2016).Research has demonstrated that the nitrogen (N) concentration in biochar decreases as the pyrolysis temperature increases (Nguyen et al., 2017;Zimmerman et al., 2011).Biochar derived from woody biomass tends to have low nitrogen content when the pyrolysis temperature is >525 C (Zimmerman et al., 2011).The biochar utilized in our study was produced at a pyrolysis temperature ranging from 550 to 600 C, which likely resulted in low nitrogen content in the biochar treatment.In addition, the presence of woody biochar in the soil may have enhanced N immobilization due to its high C:N ratio (945) (Table 1) and recalcitrant nature, which limits inorganic N availability for plant uptake, likely hindering the early vegetation establishment in our study.In field studies evaluating the impacts of amendments in land reclamation in Lake Athabasca, Alberta, Petelina et al. ( 2014) concluded that the better performance of peat over biochar with respect to vegetation cover was due to increased water and nutrient retention capacity in soil amended with peat.
Understory forbs canopy cover was not influenced by TRD, which shows that insufficient TRD can still support the natural establishment of understory vegetation at levels comparable to those for the required TRD80.On the other hand, the higher graminoid canopy cover in the TRD80 treatment than in the biochar may indicate high graminoid seed propagules that may have been present in the TRD80 topsoil.This implies that stockpiled topsoil can be a source of seeds that may germinate later during the revegetation process (Alberta Environment & Water, 2012;Dhar et al., 2019).Similarly, Redente et al. (1997) and Bowen et al. (2005) observed greater canopy cover of graminoids at greater topsoil depths than at shallow depths (15 cm) in mine reclamation in Wyoming, USA.Importantly, unlike forbs, graminoids are more competitive plant species and can thrive in a variety of soil conditions due to their fibrous root systems, which can effectively compete for nutrients and water.It is therefore not surprising that graminoids were able to thrive in TRD80 treatment with low nutrient (TKN and TOC) and moisture content (Table 3) in our study.
The lower graminoid canopy cover in biochar treatment observed in our study may be related to the biochar preference in supporting forbs more than grasses (Gundale et al., 2016).This corroborates the findings by Schimmelpfennig et al. (2015), who observed a shift from grasses to forbs in biochar-amended soils due to the superior nutrient retention capacity of forb species in soils with low nutrient supply.
Contrary to the typical pattern of early dominance and subsequent decline in graminoid species cover and richness following reclamation of boreal sites, as shown by previous studies (Dhar, Comeau, Naeth, Pinno, & Vassov, 2020a;Pinno & Hawkes, 2015;Rowland et al., 2009), our study revealed an increasing trend in all treatments for graminoid species cover and richness, reaching 87% and 15, respectively, 5 years post-reclamation, which was greater than the pre-disturbance graminoid cover (5%) (Table 2).We suspect that these graminoids were established from the seed bank in the stockpiled topsoil used in this study, and later became more competitive as forb canopy cover and richness decreased.Based on previous research, stockpiling of topsoil may increase graminoid species abundance after reclamation of boreal sites as some of the graminoids can still maintain their viability at greater depths during stockpiling (Alberta Environment & Water, 2012;Dhar et al., 2019).Although reclaimed sites have been reported to have a higher proportion of graminoids than undisturbed natural forests (Rapai et al., 2021;Rowland et al., 2009), we anticipate these graminoids to decline as the T A B L E 3 Soil properties responses to topsoil replacement depth and organic amendments treatments across 5 years.Means within a column followed by the same letter are not significantly different at α = 0.05 according to the Tukey multiple comparison procedure.
However, 16 years later, the authors reported a significant reduction in forb canopy cover and a four-fold increase in tree canopy cover (80%).Thus, during the early years following reclamation, understory forb species typically predominate over slow-growing woody species, taking advantage of nutrients and water at newly disturbed sites (Dhar, Comeau, Naeth, Pinno, & Vassov, 2020a;Dhar, Comeau, Naeth, & Vassov, 2020;Hart & Chen, 2006).However, trees and shrubs eventually outgrow and shade the forb species, thus out-competing them (Lieffers et al., 1999;Messier et al., 1998), thereby limiting competition and enhancing recolonization by desirable perennial forb and woody species (Dhar, Comeau, Naeth, Pinno, & Vassov, 2020a;Zhang et al., 2017).After 5 years of reclamation, the woody cover in all treatments reached 20%, which was 12 times lower than the woody canopy cover observed before the disturbance (250%).Additionally, our results indicated that the forb cover declined to 22% after 5 years of reclamation, which is similar to the forb canopy cover observed in the pre-disturbance assessment (29%).This suggests that the plant composition of our recovered site is gradually shifting toward that of a natural boreal site, where woody species gradually become more dominant than forb species over time.
In our study, the availability of nutrients and moisture in the peat treatment during the first few years following reclamation (Table 3) likely increased the response of the native canopy cover.Biochar, in contrast, has been shown to inhibit initial plant growth due to the immobilization of plant-available nutrients (Chan & Xu, 2009;McElligott, 2011), as indicated by its low TKN concentration in our study (Table 3).Additionally, since the peat used in our study was salvaged from a nearby well pad, this potentially introduced native plant seed propagules that germinated and were established in the PTRD50 plots.Mackenzie and Naeth (2009) found 3300 native plant species seeds and 17 non-native plant species seeds in peat mineral mix seed propagules.This indicates that peat amendments can be a good source of native seed propagules that can help with plant community development on reclaimed disturbed sites.
T A B L E 4 Effects of topsoil replacement depth and organic amendment on canopy cover following borrow site reclamation.The increasing native canopy cover and richness and decreasing non-native canopy cover and richness over time, which have been demonstrated in previous studies (Dhar, Comeau, Naeth, Pinno, & Vassov, 2020a;Dhar, Comeau, Naeth, & Vassov, 2020b), meant that non-native species outcompeted native species during the early postreclamation years, as previously observed in other studies (Dhar et al., 2018;Pinno & Hawkes, 2015).While most non-native species in our study were agronomic forbs (Melilotus alba, Trifolium hybridum, and Trifolium pratense), they are highly competitive for nutrients, moisture, and space (Dhar et al., 2019;Snively, 2014).As a result, they suppress the growth of desirable woody plants and delay the successional progression of ecosystem recovery.On the other hand, their canopy and roots can reduce soil erosion and contribute to soil formation processes (Dhar et al., 2018;Rapai et al., 2021).The decrease in non-native canopy cover and richness and dominance of native species 5 years after reclamation translates to a dissipation in competition of non-native species for available resources as desirable perennial native species became more established (Dhar, Comeau, Naeth, Pinno, & Vassov, 2020a;Pinno & Hawkes, 2015;Rowland et al., 2009).Our results corroborate the findings by Dhar et al. (2019), who observed a decline in non-native species cover from 32% to 10% within 16-24 years following boreal forest site reclamation in the oil sands region of Alberta.Similarly, Pinno and Hawkes (2015) reported a mean non-native species canopy cover of 10%, 20 years after reclamation in the same region.The non-native species in our study were significantly less abundant than perennial native species, indicating that the trajectory for all treatments was toward the desired plant community development of a typical natural boreal forest moving from invasive and annual species to perennial native species (Dhar et al., 2019;Hart & Chen, 2006).Additionally, the indicator species, Rubus idaeus, observed in our study may signify reclamation success and a trend toward the natural boreal forest ecosystem (Dhar, Comeau, Naeth, Pinno, & Vassov, 2020a;Dhar, Comeau, Naeth, & Vassov, 2020b;Hart & Chen, 2006).
The abundant bare ground during the first year was expected since canopy cover was not fully developed, and invasive and other native plants were not fully established.However, consistent with previous research (Groninger et al., 2017), as the canopy cover expanded, the bare ground gradually disappeared.Zvomuya et al. (2011) reported an inverse relationship between bare ground and lichen cover, which agrees with observations in our study.
The observed greater H 0 and E in the early years post-reclamation in our study corroborate the findings of Corns & Roi (1976), who reported high species diversity during the early years following reclamation, after which a decline was observed as overstory canopy cover increased.Overall, H 0 was high and within the range (1.5 < H 0 < 2.5) T A B L E 5 Species richness, species diversity index, and evenness as affected by topsoil replacement depth and organic amendment at a reclaimed borrow site in northeastern Alberta.considered normal for boreal sites (Kent & Cooker, 1992).Species diversity is considered one of the drivers for ecological restoration in reclaimed boreal sites where it plays a critical role in the resilience and productivity of forest ecosystems (Drever et al., 2006;Tilman, 1994).
Additionally, reclaimed sites with high species diversity are regarded as stable and capable of rapid recovery following disturbance (Dhar et al., 2018;Hooper et al., 2005).Thus, the higher species diversity and the presence of characteristic species (Table S4) in our study suggest that the reclaimed site may recover into a stable and resilient plant community (Dhar et al., 2018).

| Tree and shrub heights
Successful revegetation of disturbed sites entails the establishment of native trees and shrubs.Topsoil placement depth is one of the important factors controlling the early establishment of planted woody species during revegetation of disturbed forested sites.The greater tree heights for TRD80 relative to TRD50 confirm the importance of sufficient topsoil replacement, which provides ample space for plant root development and expansion, enabling the roots to scavenge for and access water and nutrients essential for tree growth (Dietrich & MacKenzie, 2018).Notably, our results show that, in the absence of sufficient topsoil, peat application can improve tree growth to levels similar to those for the optimal TRD80.The superior performance of peat relative to biochar may be related to greater nitrogen availability and moisture retention properties of peat-amended relative to biochar-amended soil (Table 3), which likely resulted in faster tree growth for PTRD50 than BTRD50.Nawu et al. (2023) reported high TKN concentration in suboptimal TRD amended with peat (PTRD50) compared to BTRD50 following reclamation in the same study.
Nitrogen availability is among the critical drivers of tree growth and is typically limiting in boreal forest soils (Turkington et al., 1998).
Therefore, organic amendments with a low C:N ratio, such as peat (C: 1) in our study, enhance nutrient availability through mineralization, thereby promoting the early establishment of trees following reclamation.It has been suggested that, due to the high C:N ratio (C:N = 945) (Table 1) and recalcitrant nature of biochar, immobilization of plant available nutrients is likely to occur during early plant establishment stages (Chan & Xu, 2009;McElligott, 2011).However, its positive effect may become evident in later years.Additional research is needed to investigate whether the immobilization and mineralization of biochar is a short-, mid-, or long-term occurrence in forest ecosystem restoration studies (McElligott, 2011).Thomas and Gale (2015) conducted a meta-analysis which revealed that the combination of nitrogen limitation, low levels of phenolics, and other compounds that impede growth resulted in a decrease in tree growth responses in biochar-amended soils.Additionally, previous studies have shown that most biochar, especially hard wood biochar, have low N content (Sackett et al., 2015) and their high sorptive nature has been found to reduce the plant-availability of NH 4 + (Nguyen et al., 2017).The use of fertilizers with biochar may be beneficial during the early years of revegetation.Previous studies have demonstrated that the addition of fertilizers together with biochar enhances nutrient availability and therefore promotes the early establishment of trees during the restoration of forest ecosystems (Hogberg et al., 2020;Pinno & Errington, 2015;Tremblay et al., 2019).
In contrast to our findings, Palviainen et al. (2020) reported a 12% increase in tree height in soil amended with 5 Mg ha À1 biochar relative to the control in a clear-cut boreal zone in southern Finland.This is not surprising since biochar properties differ widely, depending on feedstock type.The pine wood-based biochar used in our study may not support as much vegetation growth as the wood biochar examined by Palviainen et al. (2020).Zimmerman et al. (2011) suggested that using feedstocks such as manure or compost can stimulate mineralization due to their low C:N ratio compared to plant-based biochars with high C:N ratio.This underlies the need for future research to evaluate different biochars alone or in combination with other amendments for their effects on early plant establishment.
Although competition for resources and space from other functional groups such as forbs and graminoids typically has a negative effect on tree and shrub seedling growth (Dhar et al., 2019;Dhar, Comeau, Naeth, & Vassov, 2020b;Dhar, Comeau, Naeth, Pinno, & Vassov, 2020a;Pokharel & Chang, 2016), it is noteworthy that these species can add organic matter from leaf and root litter, which supply nutrients through mineralization for tree growth (Feng et al., 2019).In addition, alder shrub species, which was included in our study, is a nitrogen-fixer that can increase N availability and organic matter in the soil, thereby promoting the growth of other vegetation species, including trees (Balandier et al., 2006;Lefrançois et al., 2010;Pinno & Hawkes, 2015).Additionally, shrubs promote tree growth by retaining moisture or snow, thereby reducing seedling mortality and mitigating drought stress (Rowland et al., 2009).

| Tree and shrub survival
Survival rates of most trees in the first few years following reclamation are determined by the presence of light, moisture, and nutrients (Dietrich & MacKenzie, 2018).While there is a lack of evidence on the effect of biochar alone on the survival rates of specific tree species at disturbed sites, in our study, the low availability of nitrogen in the biochar treatment may have contributed to lower white birch survival rates compared to other treatments (McElligott, 2011).Aspen species had higher mortality rates than all other tree and shrub species in our study, likely due to the presence of a dense forb and graminoid cover (Dhar, Comeau, Naeth, Pinno, & Vassov, 2020a;Holl, 2002;Torbert & Burger, 2000), which can obscure the planted trees and reduce their chances of survival (Torbert, 1995).Aspen trees are shade-intolerant and are negatively affected by low light conditions (Loach, 1970;Ung et al., 2001).The greater dogwood survival rates in our study corroborate the findings by Koropchak et al. (2013) shrub characteristics in degraded soils during early establishment, making it a desirable shrub for reclamation.
In contrast to the low survival rates (<50%) for green alder in our study, Pietrzykowski et al. (2018) reported survival rates of 72%-93% after 5 years of restoration of fly ash disposal sites in central Poland.
Although green alder is regarded as a moderately shade-tolerant species (Krajina et al., 1982), we speculate that the greater grass canopy cover observed in our plots may have partially negatively impacted the early establishment, growth, and survival of green alder.
The decrease in tree survival with time is a typical pattern on disturbed sites.Tree survival is typically initially high and declines over time, $5 years, depending on the species, before peaking as the woody canopy closes over the ruderal understory species (Skousen et al., 2009).Skousen et al. (2009) reported high survival rates for black cherry (82%) and red oak trees (96%), which declined to 36% and 47%, respectively, after 7 years.The authors also noted that it took approximately 5-8 years for the same species to reach a constant population post-reclamation.Although we used different tree species from those in their study, we observed a similar pattern of declining tree species survival with time.
Overall, the survival rates of aspen and alder were less than 50%, indicating a high mortality rate (Skousen et al., 2009).By comparison, spruce, birch, poplar, dogwood, and saskatoon had survival rates greater than 50%, which are considered high.

| CONCLUSION
This study is the first to examine borrow site reclamation in a boreal ecosystem using suboptimal TRD in conjunction with organic amendments.Findings from this study support our hypothesis that organic amendments can improve early vegetation establishment and plant community development at disturbed boreal sites reclaimed with suboptimal topsoil replacement to levels comparable to the required 80% TRD.Our results indicate that peat can be used to improve vegetation establishment and plant community development when topsoil available for reclamation is insufficient (50% in this case) to attain the mandatory 80% TRD.By comparison, biochar showed no significant improvement in the vegetation attributes when applied with insufficient topsoil (50% TRD).Nonetheless, all treatments met the characteristic species (Table S4) thresholds, indicating progression toward the desired moist rich d ecosite.Continued monitoring would be beneficial in determining the long-term effects of TRD and amendments on ecosystem recovery as communities may shift over time in response to treatments.Similarly, future research should examine the efficacy of different organic amendments in augmenting reclamation success at sites with suboptimal topsoil volumes below and above the 50% TRD tested in our study.
in soils amended with biochar during reclamation.Page-Dumroese et al. (2018) reported improved vegetation cover when biochar was applied, with a smaller percentage of bare ground (23%) 2 years after reclamation of a mine site in northeastern Oregon, USA.Dietrich and Mac-Kenzie (2018) reported a significant increase in soil K and trembling aspen (Populus tremuloides) seedling growth on reclaimed peat-mineral mix cover soil mixed with biochar in the Athabasca oil sands region.

F
I G U R E 4 Treatment by species interaction effect on tree percent survival following reclamation of a borrow pit from 2015 to 2019 in northeastern Alberta.Vertical bars represent standard errors of the mean.Bars with different letters are significantly different according to the Turkey mean comparison procedure.
Selected initial chemical properties of the salvaged topsoil, peat, and biochar used in the reclamation.
Archibald et al. (2023)te those ofArchibald et al. (2023), which Dhar et al. (2018) at shallow depths (15 cm).Dhar et al. (2018)suggested that soil replacement depths of ≤20 cm can successfully promote early vegetation establishment and total vegetation cover.Findings from our study support our hypothesis that organic amendments can improve early vegetation establishment and plant community development at disturbed boreal sites reclaimed with Means within a column followed by different letters are significantly different at α = 0.05 according to Tukey's multiple comparison procedure. b T A B L E 6 Tree and shrub height response to topsoil replacement depth and organic amendments at a reclaimed borrow site in northeastern Alberta.
b Means within a column followed by the same letter are not significantly different at α = 0.05 according to the Tukey multiple comparison procedure.
Topsoil replacement depth and organic amendment effects on tree and shrub species survival following reclamation of a borrow pit in northeastern Alberta.Means within a column followed by the same letters are not significantly different at α = 0.05 according to Tukey's multiple comparison procedure.
Zipper et al. (2011)survival rates (>80%) for gray dogwood and green hawthorn on a reclaimed site in West Virginia, USA, after one growing season.Similarly,Zipper et al. (2011)reported dogwood species survival rates of 86% 9 years following reclamation in southwestern Virginia.These studies indicate that dogwood species exhibit desirable T A B L E 7 b Missing elements for non-shrub species.c Missing elements for non-tree species.d