Methods

Data collection

STUDY AREA

The study area (Fig 3.) comprises two Natural Subregions in central Alberta: Central Mixedwood and Lower Foothills.  These areas are forested by aspen (Populus tremuloides), white spruce (Picea glauca), and coniferous/deciduous mixedwood (aspen and white spruce) on uplands, along with jack pine (Pinus banksiana) and lodgepole pine (Pinus contorta) stands. 


Well sites were chosen based on the following criteria:

The study area is an active region for human and natural disturbance with frequent harvesting, oil and gas exploration, and wildfire. 

Fig 3. Map from Lupardus et al., 2019 showing the study area. There were 30 study locations each with a well pad and adjacent reference site in Central Mixedwood Natural Subregion (n = 15) and the Lower Foothills Natural Subregion (n = 15). 

SAMPLING DESIGN

This study used a nested design (Fig 5.) to collect environmental and species data in order to account for multiple levels of variability, reduce error variance, and control for confounding variables. After arriving at a wellsite, the center was first identified using a hand-held GPS. Then four transects could be established and oriented to the four corners of the wellsite. This allowed for quadrant setup using 100 m tape (Fig 4.). 


Soil data

Soil cores were sampled in each 10 x 10 m plot (Fig 6.). In the field, a soil core sampler was used to collect undisturbed soil sample that was then kept cool and transported for laboratory analysis. Five cores from each plot were composited to create a sample for a given depth(0-15 and 15- 30 cm). Bulk density measurements were collected 0 to 15 cm. For pH, total organic carbon, and total nitrogen were collected15 to 30 cm. 

Organic matter (LFH layer of the soil horizon) depth was measured by assessing soil characteristics (dark color, coarse and fibrous).The thickness of the organic layer was measured at each of the five sampling points within each 10 x10 m square plot where the soil core is collected from.


Vegetation data

Cover data: There were eight 5 m x 5 m vegetation plots at each well site – four on the well pad itself and four distributed within the nearby, undisturbed (i.e., natural) forest referred to as the reference condition(Fig 5.). If undisturbed vegetation plots contained or were close to anthropogenic disturbance (e.g., a road), they were relocated. At these 5 x 5 m plots, two-dimensional herb/forb and shrub vegetation cover was estimated at three height strata: < 0.5 m, 0.5 m to 2 m, and 2 m to 5 m. Percent cover was visually estimated.

Basal area: Diameter at breast height (dbh) was measured for trees, snags, and stumps using dbh tape for later calculation of basal area. 

Fig 4. Photos from Lupardus et al., 2019 showing example of site setup and well pads at different ages: A) ∼30 years post reclamation B) ∼20 years post reclamation, and C) ∼10 years post reclamation.

Nested plots were used to collect plant cover and basal area data (Figs 5., 7.). In the smallest plot (5 x 5 m) all trees, snags and stumps were measured (dbh, height). In the 10 x 10 m plot, all trees, snags, and stumps > 7 cm diameter were measured (this is excluding the nested 5 x 5 m plot). Finally, in the largest plot (25 x 25 m) All trees, snags and stumps with a diamter >25 cm were measured. In summary the 5 x 5 m plots are squares, whereas the 10 x 10 m and 25 x 25 m plots can be thought of as L-shaped. 


Vascular plant cover data was assessed visually in 5 x 5 m plots at the level of vegetation groups (e.g., grasses, forbs) by recorded percent cover (Fig 7.). At the start of the field season, training to practice calibration for ocular estimation was provided to team members. 

Fig 5. Study design within the well pad and in reference area from McIntosh et al., 2019. This nested design was used for collecting species cover data, soil physical, chemical, and biological properities, and basal area.

Fig 6. Sampling layout for repeat soil sampling within each 10 × 10 m plot on the wellsite and reference sites so that destructive sampling can take place. Colors represent 10 sampling intervals. (McIntosh et al., 2019) 

Fig 7. Layout for vascular plant surveys to quantify plant cover within the wellsite and reference areas. (McIntosh et al., 2019)