Latterini Francesco, PhD

The careful planning of the extraction routes is one of the most important best management practices to limit soil disturbance related to ground-based forest operations. Over the recent years, this task has been commonly addressed in the framework of boreal forestry, by developing soil trafficability maps based on the depth-to-water (DTW) topographic index. The basic concept of trafficability maps developed with the DTW index is that soils at low DTW index, namely <1, could be more prone to soil compaction and rutting as they tend to have higher moisture content. However, previous studies that tried to assess the reliability of these maps reported contrasting results. Therefore, the present meta-analysis was developed to evaluate if soils at low DTW index (≤1) are actually more sensitive to soil compaction and rutting than soils at higher DTW index (>1). A database was created containing all the studies that assessed soil compaction and rutting in soils at low DTW index (experimental treatment) and high DTW index (control treatment), and a multivariate meta-analysis was used to check the presence of statistically significant effect size. Then the influence on the effect size of variables like soil texture, number of machine passage and weight of the machine, was checked by applying sub-group meta-analysis and meta-regression. Finally, a sensitivity analysis was performed by removing possible outliers from the database and repeating the analyses. No statistical differences were found in soil compaction and rutting severity in areas at low DTW index in comparison to the control areas (DTW index ≥1). The results showed that soil texture, number of machine passage and weight of the machine did not have a significant influence on the effect size. The sensitivity analysis developed after removing outliers from the database fully confirmed the obtained results. Thus our meta-analysis showed that the DTW index in its current form is not a fully reliable predictor of soil areas that could be particularly sensitive to machinery-induced disturbance. It is therefore recommended to use the DTW index to create trafficability maps, always taking into account that the results of the algorithms should be validated in the field before starting harvesting operations.

Ground-based forest operations can produce an intense level of soil disturbance and hamper the growth of seedlings. However, previous studies on the topic of seedling growth performance on the skid trail network showed high variability and non-uniform responses. Therefore, a multivariate meta-analysis was applied to investigate the effects of soil disturbance after ground-based forest operations on the aboveground and belowground growth of seedlings. We further assessed the effects of the following moderators: type of regeneration, type of stand, number of years since the forest operations, mass of the machine used, number of machine passes in the investigated skid trails, and the increased soil compaction in the skid trails. The results supported our hypothesis that ground-based forest operations had a greater effect on the growth of broadleaf species and natural regeneration. This was, however, mostly true for belowground growth, that was typically more affected than aboveground growth. We discovered that there is a recovery in seedling biomass and lenght following harvesting, but this is a long process that requires about ten to twenty years to begin. We found that the number of machine passes and the machine mass did not directly correspond to a higher level of impact, indicating that the actual disturbance drivers are more complicated interactions that occur among the machine, the way in which forest operations are carried out, and the soil properties. It was finally demonstrated that soil compaction was the primary cause of disturbance to seedling growth, primarily affecting belowground biomass and lenght. In particular, we identified a minimum threshold of a 30% increase in soil bulk density to observe statistically significant negative effects on seedling belowground growth. Soil compaction proved to be the main factor that can jeopardize the development of forest regeneration in the skid trails. This suggests that the same best management practices that are used to reduce soil compaction caused by machinery should also be used to reduce the detrimental effects of ground-based forest operations on seedling development. As future research directions, long-term studies are recommended to assess the recovery process dynamics. Moreover, more research on broadleaf, natural regeneration, and Cut-to-Length machinery is strongly suggested.

Ground-based forest operations can produce an intense level of soil disturbance and hamper the growth of seedlings. However, previous studies on the topic of seedling growth performance on the skid trail network showed high variability and non-uniform responses. Therefore, a multivariate meta-analysis was applied to investigate the effects of soil disturbance after ground-based forest operations on the aboveground and belowground growth of seedlings. We further assessed the effects of the following moderators: type of regeneration, type of stand, number of years since the forest operations, mass of the machine used, number of machine passes in the investigated skid trails, and the increased soil compaction in the skid trails. The results supported our hypothesis that ground-based forest operations had a greater effect on the growth of broadleaf species and natural regeneration. This was, however, mostly true for belowground growth, that was typically more affected than aboveground growth. We discovered that there is a recovery in seedling biomass and lenght following harvesting, but this is a long process that requires about ten to twenty years to begin. We found that the number of machine passes and the machine mass did not directly correspond to a higher level of impact, indicating that the actual disturbance drivers are more complicated interactions that occur among the machine, the way in which forest operations are carried out, and the soil properties. It was finally demonstrated that soil compaction was the primary cause of disturbance to seedling growth, primarily affecting belowground biomass and lenght. In particular, we identified a minimum threshold of a 30% increase in soil bulk density to observe statistically significant negative effects on seedling belowground growth. Soil compaction proved to be the main factor that can jeopardize the development of forest regeneration in the skid trails. This suggests that the same best management practices that are used to reduce soil compaction caused by machinery should also be used to reduce the detrimental effects of ground-based forest operations on seedling development. As future research directions, long-term studies are recommended to assess the recovery process dynamics. Moreover, more research on broadleaf, natural regeneration, and Cut-to-Length machinery is strongly suggested.