Technology and systems for stump harvesting with low ground disturbance

Tree stumps could make a significant contribution to the transition from a fossil- to a bio-based economy, but current stump harvesting operations have adverse ecological effects. The ground disturbance caused by the up-rooting leads to increased carbon emissions from the soil and increases risks of leaching of heavy metals and nutrients, while removal of stump wood increases nutrient removal and reduces amounts of dead wood in the forest. However, the ground disturbance could be reduced by introducing new techniques. The overall objective of the studies this thesis is based upon was to investigate possible future systems for stump harvesting capable of reducing ground disturbance, and estimate their economic sustainability. Studies were based on experimental field studies and simulations. The ground disturbance depends on the type of harvesting head, as harvesting the whole stump creates more disturbance than harvesting the central part of the stump; the ground disturbance is also larger on peat soil than on mineral soil, but does not depend on time since clear cutting; and the root breakage diameter is surprisingly small (5-30 mm) after whole stump harvests and is not affected by the time since clear-cutting. Twisting stumps loose requires large torques and cannot be considered a viable way of extraction. The ground disturbance and cost to industry was estimated for four systems: a conventional whole stump harvesting system (WSH), a stump centre harvesting (SCH) system and two possible future systems for integrated harvest and forwarding of stem and stump centres with separation of the stump centres at either the landing (IHL) or industrial sites (IHI). The IHI and SCH systems are estimated to be up to 100 % and 60 % more costly, respectively, than WSH. However, costs of IHL were estimated to be similar with WSH for large trees. WSH (up-rooting) caused five times more ground disturbance per hectare compared to the other systems. In conclusion, the conventional up-rooting system was estimated to be best from an economic perspective, but caused more ground disturbance then harvesting of stump centres only. If ground disturbance restrictions are introduced, and new technologies are developed accordingly, costs of utilizing stump wood will be higher than at present. In development of integrated stump centre harvesting systems, as described here, it is crucial to design techniques and methods that minimize possible risks of damaging the stem wood in order to secure timber quality. Finally, regardless of future regulations, whole stump up-rooting technologies will still be warranted for treating stands infected by root rot.