A Meta-Analysis to Evaluate the Reliability of Depth-to-Water Maps in Predicting Areas Particularly Sensitive to Machinery-Induced Soil Disturbance

Abstract

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.