Photo courtesy of Ponsse
Photo courtesy of Ponsse

Transporting the forest residues located in slash piles out of the woods requires vehicles and equipment to roll and vibrate over bare ground.  This activity can cause soil compaction and disturbance, which can lead to adverse soil erosion and limit plant growth.

Numerous studies and harvesting policies have been initiated to lessen the impact of soil compaction and are based on the harvesting methods and equipment used at the time. As new harvesting methods and equipment are developed, a reassessment of their impact on the soil is warranted. New equipment is being developed for ground-based harvesting on steep slopes that offer an alternative to expensive cable harvest systems.

An article recently published in Forest Science, and partially funded by NARA, evaluates how forest-thinning operations on steep slopes affects the soil. For their study, authors Rene Zamora-Cristales, Paul Adams and John Sessions, evaluated thinning operations on two sites with steep slopes (greater than 50%) and similar soil types. Both thinning-operations used the same harvester equipment that cut and delimbed trees in a single downhill pass; however, one operation employed a skyline cable system to take out the harvested material and the other used a harvester-forwarder. Both the harvester and the forwarder were fitted with band tracks to provide improved traction. The soils on both sites were similar. The trails created represented 10% with the harvester-forwarded and 15% with the harvester-cable. The soil moisture levels were at ~30%.

To read “Ground-Based Thinning on Steep Slopes in Western Oregon: Soil Exposure and Strength Effects” view here.

Study results

Once thinning operations were complete, soil exposure and strength were estimated on the trails created by the thinning and harvesting equipment. For soil exposure, the harvester-forwarder operation resulted in over two times the soil exposure than the the harvester-cable operation with 7% of the sample points recording soil exposure verses 3%.  For soil strength, a single harvester pass caused soil strength to increase by 19-34% in the upper depth classes (25-200 mm). A second pass using the forwarded increased the soil strength to 33-40% greater than undisturbed soil. Soil strength can affect forest productivity. Higher soil strength can limit tree and vegetative root penetration. These measurements were biased and recorded the  soil strength of soil under the track ruts where the maximum compaction occurred. Total trail soil strength did not exceed 15%. It is interesting to note that approximately 30% of the trails created were covered with slash. Comparing the soil strength on trails with and without slash cover showed a 14% decrease in soil strength for slash-covered trails. This information suggests that the slash cover helped buffer the effects of compaction by dispersing the vehicle weight.

Forest residual harvest implications

Evaluating the soil conditions after timber harvest on step slopes provides a baseline of data that can be used to evaluate the affects of transporting forest residuals from a harvesting operation on steep slopes. In addition, this study indicates that residual slash cover can affect the severity of soil compaction and may influence how slash piles are managed and transported relative to the sequence of harvesting operations. The authors acknowledge that further site evaluations could also provide a better understanding to how the changes in soil strength can affect water retention and erosion on steep slopes.