Lunch Seminar "Environmental effects of stump harvesting – what have we learnt since 2011?"

Abstract

Stumps harvested on clear-cuts can be used as a biofuel in power plants and replace burning of oil, coal or natural gas. Because 80% of stumps and coarse roots will decompose and be emitted as CO₂ within 40 years, burning of stumps will theoretically reduce the CO₂ emissions to the atmosphere by 75% (80% due to stump/root burning – 5% energy input in fossil fuel for stump handling) of a given amount of energy. However, stump removal also means soil disturbance, and mixing of soil is known to increase CO₂ release from the soil organic matter. The magnitude and duration of this increase is now intensively studied in 0-5, 5-10 and 30-40 year-old field experiments. The loss of soil organic C through increased CO₂ release is counteracted by better tree regeneration and growth. Stump removal increases N availability, which explains better tree growth in combination with less competition from ground vegetation, but this also seems to favour nitrate leaching to the water courses. Clear-cuts have higher ground water table than intact forests. In combination with higher N availability, this favours formation of nitrous oxide, which is a potent green-house gas. Also methane emission is favoured by the clear-cut situation but not necessarily by stump harvesting. The same appears to formation and transport of methyl mercury to surrounding streams. Stumps are important habitats for lichens (stump surface), fungi (wood and bark), beetles (wood and bark) and a range of other organisms. Removal of stumps will reduce the abundances of these organisms, but also intensive stump harvesting will not be a threat to other species than those almost entirely dependent on stumps. Some beetle and fungal species belong to this category. Also intensive stump harvesting means that 30-40% of the pine and spruce stumps and 100% of the broadleaf stumps remains after stump harvesting. Still there is a need to know if there is a threshold/minimum value of stump density when stump-dependent species have difficulties in keeping viable populations