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Spruce stumps in Sweden, photo by Ylva Melin
In the very short term, the use of forest fuels such as tree stumps and roots larger emits larger emissions of carbon dioxide to the atmosphere than when the same amount of energy produced by burning coal. In the long term, however, carbon emissions on stumps and roots are reduced  instead of using coal, according to a thesis by researcher Ylva Melin from the Swedish Agricultural University.

At a logging about 20 percent of the trees biomass are left on the clear-cut in the form of stumps and roots. Using some of this renewable fuel would make Sweden more energy self-sufficient, but it would also affect the prospects of achieving the goal of reducing greenhouse gas emissions. In the long run it would be good for the climate to replace fossil carbon with stumps but for a the fist ten years there is no positive contribution.

One way to curb global warming is to replace fossil fuels with biofuels, another way is to bind the carbon in biomass such as tree stumps and roots. Ylva Melin has in his doctoral work compared different scenarios that use different units of the Swedish stump  production” to replace coal in power plants. For each scenario, she calculated how much carbon dioxide is emitted to the atmosphere and how much coal is left in stumps and roots, from year to year. Central to the work is how quickly the stumps decay, and how carbon attrition is postponed when the stumps are left in place or to be fired immediately.

If one were to pay half of the production at the Ringhals nuclear power plant in 2013 (14 TWh) of harvested stumps, it would take about nine years before this scenario became more climate-friendly than burning coal, according to Ylva Melin’s calculations. During an initial period gives stubble burning thus more carbon emissions than the use of fossil fuel. The cause is fossil fuels higher energy value – they give more energy per amount of carbon dioxide released.

In the longer term (> 10 years), however,  carbon emissions are reduced by burning  stumps and roots instead of coal. The reason for this is the different fuels tend to break down when left undisturbed in nature. When you burn coal instead of stumps and roots  emissions from both coal combustion and decay to accumulate in the atmosphere. The stump harvesting scenario of 14 TWh would in the long run reduce emissions of fossil carbonby 5 million tonnes a year, equivalent to 8.6 percent of Sweden’s current annual greenhouse gas emissions.

Phil. mag. Ylva Melin, Department of Forest Resource Management, defended her doctoral thesis Impacts of stumps and roots on carbon storage and bioenergy use in a climate change context Oct. 17 at SLU in Umeå. The opponent was Professor Annette Cowie, School of Environmental and Rural Science, University of New England, Australia.