## 4 Discussion
In the first 30 cm of soil, the mean penetration resistance was found to be 1.44 MPa for the control data. It increased consistently during the first ten passes by 0.47, 0.23, 0.41, 0.26, 0.37, and 0.25 after the first, second, third, fourth, fifth and tenth passes, respectively, showing a maximum relative change of 32.6%, right after the first pass, which was close to that of the skidder after the last pass. In terms of relative changes in soils' penetration resistance, it is obvious that horse logging and lighter machines such as the skidder used in the tests cause less soil compaction and may allow several loaded passes until reaching the same effects that heavier machines may cause right after the first loaded passes. As a reference, H, S and F added on average to the control RP 0.07, 0.52 and 0.12 MPa, respectively, after 15 passes in the first 10 cm of the soil. It is difficult, however, to relate the changes found in this study with the real increments (or decrements) in bulk density of the soils and with eventual negative effects that may affect tree root development and growth. In addition, the soils from the test areas H, S and F had different depth profiles in terms of penetration resistance in their natural state, as well as in terms of granulometry and moisture contents. For that reason, to be able to accurately estimate the effects in vegetation decline, if any, studies should be designed and implemented to see how the penetration resistance relates to the loss of growth or other important processes specific to the variability of forest ecosystems [14]. Some studies found that the majority of roots of some tree species are found in soil layers where penetration resistance is up to 3 MPa, while a significant number of those roots are located in those parts of the soil where it is up to 2 MPa [55]. From this point of view, animal logging produced no important changes and the natural soil itself did not show a mean penetration resistance higher than 2 MPa (the value approached only at 65 cm in depth, see for reference Figure 11). In the case of the S site (Figure 11), the mean penetration resistance of natural soil approached a value of 2 MPa starting after the first 5 cm in depth and kept values between 2 and 2.5 MPa up to 40 cm in depth, beyond which it approached a value of 3 MPa. Skidding seemed to produce important changes between 5 and 30 cm in depth, with the mean penetration resistance sometimes exceeding 3 MPa. At the F testing site, the mean penetration resistance of the natural soil increased from 2 to 2.5 MPa from 20 to 65 cm in depth. Only following the third pass did the results indicate a change higher than 2 but less than 3 MPa at a depth of 30 to 40 cm. Although this study was performed in controlled conditions in terms of payload and slope, it is worth mentioning some of the important limitations. First of all, the passes over the testing areas were carried out in a closed loop, which might not resemble the real operations in such cases in which the same trail is used for both empty and loaded turns. However, empty turns are less likely to produce additional stress over the soil, beyond that which the loaded turns may cause, particularly after a given number of passes. While the authors are quite confident in the accuracy of measurements, some caution should be exercised particularly for the last ones since there might be some effects on accuracy induced by increasing the number of measurements taken close together. The study was designed to avoid such errors by distributing the point measurements at a reasonable distance among them so as to limit the effect of loosening of the soil on the next measurement by the previous one. However, it was difficult to predict and control the changes in the soil caused by the measurements themselves. Another concern is that related to the depth of the measurements which in all cases took as a reference the surface of the soil.