## 4 Discussion  
A thorough comparison of the changes in the penetration resistance is difficult to make since there was a sufficient variation in the soil physical properties from one testing site to another to provide heterogeneous control datasets. For instance, the moisture content of the soils in the test sites decreased in the order H-S-F, while the clay content decreased in the order H-F-S. This may seriously limit the interpretation of results when comparing between sites; still, it provides the means for interpretation of results for each particular site. Such interpretations may only characterize the local effects brought by a given extraction means on given soil characteristics as found in the natural state. What is clear is that the extracted payload increased in the order H-S-F, which is common sense given the capacity of the studied extraction means. While it approached the maximum for H and F, for S it was half of that allowable as specified by the machine manufacturer, and different results may have occurred when skidding timber from final fellings. Horse logging produced small changes (although statistically significant in some cases) in all the considered soil layers. Taking, for instance, the first 30 cm of soil, the mean penetration resistance increased only marginally, irrespective of the number of passes. At this soil depth, the mean penetration resistance in the control data was 0.71 MPa. It increased by \(-\)0.03, 0.00, 0.06, \(-\)0.02, 0.01, 0.01 and 0.10 MPa after the first, second, third, fourth, fifth, tenth and fifteenth passes, meaning a maximum relative increment of 12.2% after the last pass. Beyond this soil depth the effects were particularly low. This means that at least fifteen passes may be sustained on the same trail without important changes in the penetration resistance, and probably without adverse effects on root development, results which are consistent with the low-impact findings of horse logging [14]. However, this extraction option has one main limitation, namely that of being fully manual, which means a low productivity in loading, transport (low moving speed) and unloading, which is further affected by its limited loading capacity. In other words, what can be extracted with this option at low speed in fifteen work cycles can be extracted with a forwarder, at much higher speed, in two work cycles. In addition, this option can only be used when there is no requirement to extract long logs or tree lengths. Skidders are typically designed to extract long wood, and sometimes they may operate with high payloads [4]. This extraction option is not only important in terms of share in Romania but also around the world [53]. In this study, the machine was tested in the conditions of a thinning operation, which meant a higher number of tree lengths per load and a lower payload volume and mass per turn, which accounted for 2.6 tons. Despite being a four-wheeled machine, which is similar to the H experiment, the mass in the unloaded state seemed to be an important factor in causing changes in the soil, although only a part of the payload discharges directly on the wheels. For the same first 30 cm of soil, the mean penetration resistance in the control data was 1.97 MPa. It increased by 0.01, 0.29, 0.38, 0.61, 0.24, 0.08 and 0.62 MPa after the first, second, third, fourth, fifth, tenth and fifteenth passes, respectively, meaning a maximum relative increment of 31.5% after the last pass. Finally, forwarders are designed to carry loads, which means that, in addition to their unloaded mass, which is high, the mass of the payload is distributed to the soil through the wheels. The main advantage of this option is that it is fully mechanized and may operate at increased speeds [54], typically being characterized by a high capacity. A high capacity combined with a higher operational speed and full mechanization brings important benefits in terms of economics, ergonomics and safety, but it may cause irreversible changes in soil in terms of compaction.