## 2 Materials and Methods
### Experimental Design and Data Collection  
Figure 2 shows the layout of the testing areas, with two types of sampling points. In each testing area, control points (hereafter CPs) were conceptually placed outside the guiding marks, and sampling points on the wheels' footprints (hereafter sampling points on the skid trail, SPs) were set up inside the guiding marks. Control points were conceptually located 2 m away of the ground guiding marks, outside the testing area, in a direction that was perpendicular on the trail (Figure 2). In a straight line, on each side of the testing area, about 5 m was kept between the CPs and SPs. The distance between the wheels' footprints (DT) was specific to the type of extraction means in each testing area (H, F and S). The location and number of sampling point were kept the same in each testing area, indicating the means used for extraction (m) and the pass after which a sample of penetration resistance was taken (\(p\), \(p\) = 1, 2, 3, 4, 5, 10, 15). For each CP and m, where m = H, F or S, a measurement on the soil's penetration resistance was taken, resulting in three control (reference) datasets taken from the undisturbed soils, each containing 10 observations. Then, for each SP, m and p, a measurement was taken on the wheel footprint in a range of \(\pm\)1 m so as to avoid measuring in a previously sampled location. By this, for each means of extraction and pass, a dataset of exactly 10 measurements on penetration resistance (PR, MPa) was taken. In each testing area, CPs were used first for sampling by approaching them from outside the boundaries of the testing area, then, after each pass, SPs were used for sampling by approaching them from outside the ground guiding marks with the aim to avoid disturbance of the experimental ground. Measurements on penetration resistance (RP, MPa) were taken by means of a Royal Eijkelkamp penetrologger equipped with an internal GPS [45]; the instrument features a weight of 3.4 kg and capability to take measurements up to 80 cm in depth. The manufacturer specifies a depth and force resolution of 1 cm and 1 N, respectively. For measurement, a cone with an area of 1 cm\({}^{2}\) and a top angle of 60\({}^{\circ}\) was used, based on a project planned and uploaded into the instrument by the use of Eijkelkamp Penetroviewer software (version 6.08). The payloads at H, F and S sites were controlled. The companies were asked about a typical payload in the given operational conditions and they were asked to load the cart and the machines accordingly. These were also checked against the permitted payloads as specified by the manufacturers. Typically, for horse logging the cart is loaded to an extent that permits the extraction without dropping the logs on the trail during the process. In our experiment, it was loaded with beech logs of about one meter in length each, accounting for a volume estimated at about 1.90 m\({}^{3}\). The forwarder was loaded with 19 logs with a dominant length of about 5 m, equally shared between hornbeam and sessile oak and accounting for a volume of 13.77 m\({}^{3}\). However, the hornbeam logs accounted only for a volume of about 3 m\({}^{3}\). Finally, the skidder was loaded mostly with hornbeam tree lengths,  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Study**} & \multirow{2}{*}{**Soil Type 1**} & \multirow{2}{*}{**Soil**} & \multicolumn{4}{c}{**Main Properties**} & \multicolumn{4}{c}{**Granulometry (\%)**} \\ \cline{4-13}  & & & **Hiorzons** & & **MP** & **MC** & **Hy** & **CaCO\({}_{3}\)** & **\%0.2 mm** & **0.2-0.02 mm** & **0.02-0.01 mm** & **0.01-0.0002 mm**  
accounting for a volume of 2.59 m\({}^{3}\). All of these estimates are based on detailed manual measurements that were taken according to the procedure described in [46]. Then, we used the densities of fresh wood from [47] to estimate the weight of the payloads. Accordingly, the payloads of the H, F and S experiments were estimated at 1.8, 13.9 and 2.6 tons.