Soil compaction due to mechanized forest harvesting: quantification of ecosystem effects and exploration of recovery potential

During the last decades, manual felling and logging of forest trees by animals or small tractors evolved towards mechanized harvesting, using heavy tractors or specialized forestry machines with increasing masses. This development may cause soil degradation in forest ecosystems as the resulting soil compaction modifies soil characteristics that are important for the sustained provision of ecosystem services. Consequently, soil conditions may become unfavourable to soil fauna, herb and tree layer and in the long-term it may lead to a loss of biodiversity, soil fertility and stand productivity. The impacts of site and stand characteristics, machine weight and traffic intensity on the compaction degree were examined in a first field trial, performed in eight Flemish forest stands. If based on bulk density and penetration resistance, we generally found low compaction degrees, even on the vulnerable soil textures. Effects could be explained by high soil water contents in the clayey soils (leading to clear rut formation) and a high precompression stress (as indicated by the high initial bulk densities) on the sandy and loamy to silt loamy soils. Results showed that soil water content and initial compaction status (as an indicator for the precompression stress) should always be taken in consideration when evaluating the influence of texture on the compaction degree. Higher machine masses and traffic intensities increased the compaction degrees. Although compaction degrees remained low, increased carbon dioxide concentrations within tracks on the sandy soils showed that soil aeration was severely affected by machine traffic. These results indicated that quantification of the soil impact based on bulk density and penetration resistance may lead to an underestimation and should take more sensitive soil variables such as soil carbon dioxide concentration into account. The vulnerability to compaction is often regarded as negligible on sandy forest soils and was therefore examined in detail in a second field trial. Significant increases of bulk density and penetration resistance and a positive (logarithmic) relationship with traffic intensity were found. The application of a brash mat reduced the compaction degree. The results of both field trials were combined with international study results to draw general conclusions on the impact of mechanized harvesting. A meta-analysis was performed to examine the effects of soil texture, machine weight and traffic intensity. It showed clear compaction degrees for both clayey as sandy textures and confirmed the significant impact of the initial compaction status and machine mass. A lot of interesting studies could not be implemented due to lack of important information. Recommendations for future research were therefore formulated. Abiotic changes may yield biotic effects and these were examined in a second meta-analysis, focussing on survival, height and diameter growth of seedlings of mainly light tolerant tree species. The influence of soil compaction on seedling growth and survival was predominantly insignificant, due to strong variation in the datasets. However, they indicated a different response in accordance to soil texture, with negligible to slightly positive impacts on sandy to loamy soils and more negative impacts on silty to clayey soils. Again a lot of the performed studies lacked important information. As soil compaction induces biotic effects, fast recovery is desired. Compaction status of nine forest stands on three soil textures where the last harvesting activity took place seven to nine years ago was determined by measuring penetration resistance along transects. In all forest stands, traces of former machine traffic were found in the shape of locally increased or overall high penetration resistance. This means that complete recovery of compacted forest soils was certainly not achieved within seven to nine years after the last machine impact. As this is a common period between two harvesting activities, effects will accumulate and expand at subsequent harvests in case machine traffic is not restricted to permanent skid trails. A fourth field trial examined whether stimulating biological activity by means of a manipulation of litter quality, soil acidity and earthworm populations could accelerate recovery. Liming and the application of calcium-rich litter positively influenced the numbers of inoculated anecic earthworms that were retraced, with a positive feedback on soil acidity and litter decomposition. Within the short study period, small reductions of the compaction degree due to anecic worms could only be shown on the non-trafficked soil beside the wheel tracks. Unfavourable soil acidity and nutrient status probably hampered ecological restoration. We hypothesize that ecological restoration of compacted soils is possible though time-consuming, stipulating that soil conditions are favourable, particularly to anecic earthworms. An increase of their survival rate and activity is best achieved through an admixture containing species with high quality litter, which induce lower soil acidity and a better nutrient status. In this thesis we gained insight into i) the abiotic and biotic effects of soil compaction as influenced by stand, site and harvesting characteristics, ii) the compaction status of Flemish forest soils, and iii) the potential of ecological restoration options for compacted forest soils. Results showed that the risk for soil compaction should be taken into account for all texture classes when planning and preparing harvesting activities. We recommend performing harvesting activities on sandy soils at intermediate soil water contents, while on medium- to fine-textured soils very dry conditions are optimal for limitation of the soil impact. The machines used should always be tuned to the intensity and the demands of the harvesting activity and the field circumstances. We emphasize to concentrate the traffic on designated skid trails. In this way only a restricted portion of the area is damaged, enabling the soil between trails to recover from the compacted status applied during previous harvesting activities. A brash mat may be very effective to further reduce the degree of soil compaction on these trails. Admixtures with tree species that provide good quality litter, perhaps combined with liming may imply stimulation of biological activity and in the long-term a decrease of the compaction degree