## 2 Materials and Methods  
### Study Sites and Soil Characteristics  
Three study sites (Figure 1) were selected in the central part of Romania to capture as much as possible the variability in soil types for hilly areas, which host more than 30% of the Romanian forests [38]. In addition, a requirement of the study was that of running the experiments on soils located on flat ground so as to remove the effects on soil compaction caused by the slope (e.g., Ref. [39]). A third important criterion was that of having readily available harvesting contractors willing to support the study by following a protocol designed to keep the same payload at each pass. Three contractors were willing to support the study by running the experiment on horse logging by carts of short wood, forwarding of logs by a specialized machine and skidding of tree lengths by a specialized machine (Figure 1). A fourth criterion was to time the measurements with those periods of the year in which the operations are commonly carried out, and a fifth and last criterion was that of having some control over the moisture of the soil. The sites were selected based on a prior documentation based on the local forest management plans and the aim was to have in the experiment similar soils in terms of physical characteristics. On this last requirement, however, there was little control when aiming to have fulfilled some of the above-described criteria. For simplicity, these three sites are called hereafter H for horse logging, F for forwarding and S for skidding. H is located at about N 46\({}^{\circ}\)11\({}^{\prime}\)1.20\({}^{\prime\prime}\)-E 24\({}^{\circ}\)43\({}^{\prime}\)40.44\({}^{\prime\prime}\), F is located at N 46\({}^{\circ}\)08\({}^{\prime}\)4.98\({}^{\prime\prime}\)-E 24\({}^{\circ}\)59\({}^{\prime}\)35.04\({}^{\prime\prime}\) and S is located at N 46\({}^{\circ}\) 2\({}^{\prime}\)32.34\({}^{\prime\prime}\)-E 25\({}^{\circ}\)06\({}^{\prime}\)45.66\({}^{\prime\prime}\). The area of study is characterized by the dominance of broadleaved mixed forests. The harvesting equipment was based on what is typical for real-world operations. For instance, horse logging by carts (H) is typically used to deliver firewood from several types of extractions, assuming that part of the harvested wood is fully processed in the stand or at the trail into very short logs, which are then manually loaded into the cart, transported to the forest road or directly to a customer and then unloaded. This option has a low transportation capacity and it is fully manual. Forwarding by a specialized forwarder (F) is in fact a fully mechanized high-capacity option, in which the wood is loaded with the machine's carne, transported to the landing and unloaded. Skidding by a cable skidder (S) is a partly mechanized, medium- to high-capacity option, in which tree lengths are usually extracted to the landing, where they are unhooked. As such, the expected order of magnitude in soil compaction was from H to F, whereas for the same magnitude in soil compaction more passes with low-capacity options were expected. Physical soil samples were taken from small pits which were dug at all of the sites to be able to identify and characterize the soils (Figure 1e). The samples were taken from each soil horizon and transported laboratory for tests on pH, moisture content (MC, %), hygroscopicity (Hy, %) and carbonate content (CaCO\({}_{3}\), %) and to run a granulometric analysis (Table 1). The preparation and analysis of the soil samples in the laboratory were carried out by the Office of Pedological and Agrochemical Studies Cluj Napoca. The soil samples were taken to the laboratory and conditioned for analysis by drying in well-ventilated rooms or in rooms equipped with heating systems up to a temperature of 40 \({}^{\circ}\)C, after which they were sieved to a particle size of less than 2 mm.