*2.1. Sites*

We are reporting changes in bulk density over 7 years for 9 sites that were operationally trafficked in western–central Alberta [6]. The primary objective of the study was to assess

the effects that differences in soil wetness and trafficking intensity had on soil bulk density, air-filled porosity, and other changes in soil physical properties. Changes in bulk density, air-filled porosity, infiltration rate, and pore-size distribution immediately after trafficking have been reported for the original 14 sites [11,15,16]. Changes in soil aeration and soil morphology up to 4 years after harvesting have also been reported [14].

Although 14 sites were originally established, funding was only available to measure 9 of the first 10 sites over 7 years (Figure 1). One large cutblock contained two research installations, but only one was instrumented with a weather station for this phase of the project. The locations of the study sites are shown in Figure 1.

**Figure 1.** West-central Alberta location of nine sample sites where soil bulk density was measured six times over 7 years. The distance between sites 7 and 8 is about 330 km.

Prior to skidding, four replicated plots, 40 m on a side, were established about 50 m from the temporary harvest road (Figure 2). This location allowed controlled skidding of each replication and space for the skidder to realign the trees for decking with butts at roadside. Skidding cycles of 3, 7, and 12 empty and loaded passes were assigned to each plot. The same skid trail layout was used for all replications and sites. Control areas were skidded from the sides of the plot or down the center of the 12-cycle plot.

Except for site 2, the sites were harvested with a tracked feller-buncher operating parallel to a temporary harvest road. The feller-buncher made one pass across the site at a spacing of about 12 m. Bunches of whole trees were skidded to the roadside and decked with the butts of the trees at the edge of the road. Skidders were two-axle grapple skidders that weighed 16–17 mg, the width of tires ranged were between 0.8 and 1.2 man, and wider tires were on the rear of skidders (Table 1). Trees at site 2 were felled and processed at the stump with a harvester and log-transported with a small three-axle forwarder (Valmet 540). The compacted slash mat was thin, and the resulting soil compaction was not different from skidders [11].

**Figure 2.** Illustration of location and location of each replication at each site.

**Table 1.** Soil organic matter and particle size distribution (*n* = 4), soil classification, and soil water content and tensiometer water potential (*n* = 64) at the time of skidding [11].


## *2.2. Bulk Density*

We are reporting changes in soil bulk density through the first 7 years (1994–2001) of the study. Bulk density was measured using stainless steel rings (7.6 cm outside diameter and 3 cm height); the wall thickness of the rings was 0.16 cm. Rings were pushed into the soil with a T-bar handle and a driving head [11]. Soil cores were collected from the midpoint of the 5, 10, and 20 cm depths and at random preselected locations in each skidding corridor and control. For each remeasurement period, the sample point was shifted 0.5 m along the corridor in the direction of machine travel from the previous measurement point and always in the same direction; the relative position across the corridor remained the same. Bulk density was measured within 2–3 days after each site was felled and skidded (year 0), and 1, 2, 3, 4 and 7 years later in late summer and early fall. A maximum of 192 samples were collected from each site and time (3 soil depths per point, 4 points per treatment, 4 treatments per replication, and 4 replications per site). Only a few samples were either lost, labels unidentifiable, and bags broken or removed from the database because a value

was an outlier (4 + standard errors). The database contains approximately 10,000 values of bulk density.

Site 10 was not measured in year 4, and the last sampling date was year 6 because of a lack of funds in other years. When the data for Site 10 was excluded, nearly 80% of the resultant mean bulk densities changed by less than 0.01 Mg m<sup>−</sup>3. Thus, the effect that these two missing sample periods had on our analyses is assumed to be minor. We use values of bulk density collected immediately after skidding as the reference bulk density to assess whether there were significant changes in bulk density from 0, 3, 7, and 12 cycles of soil trafficking in subsequent years.

## *2.3. Soil Climate*

The most representative replicate at each site, based on soil and terrain, was instrumented with a multiplexed CR10 data logger (Campbell Scientific Inc., Logan, UT, USA) to measure air and soil temperature in the control soil and adjacent 7-cycle skid trail. The distance between soil measurement points in these two treatments was between 6 and 8 m. Thermocouples were inserted horizontally at the interface between the LFH layer and mineral soil (0 cm depth) and at 5, 10, 20, and 50 cm depths. Soil and air temperatures were recorded every second hour. Snow depth was measured over the harvest-only treatment with a sonic range sensor (Campbell Scientific Inc.).
