Stump Harvesting for Bioenergy – A Review of the Environmental Impacts

In the majority of modern forest management systems, mature trees are harvested above the root collar, leaving stumps in the ground. Due to widespread concern about climatic change caused by anthropogenic emissions of greenhouse gases, increasing prices of fossil fuels and a desire to improve energy security, there is a growing interest in the exploitation of alternative sources of energy, including forest biomass ( Bjorheden, 2006 ). Commercial operations are now effectively utilizing the entire above-ground tree biomass to produce both timber and fuel ( Hakkila, 1989 ). A more recent development is interest in the potential for utilizing stump biomass as an additional source of renewable fuel ( Hakkila and Aarniala, 2004 ). Historically, stumps were harvested both for fuel and for other uses including providing material for the horns of sledges, knee timber in ships and boats and ploughs ( Gayer, 1896 ). Stump wood was also used in Sweden for the production of tar between 1850 and 1950 ( Karlsson, 2007 ). Declining demand for woodfuel coupled with the industrial production of alternative materials such as steel led to reduced requirements for stump wood in central Europe towards the end of the nineteenth century. Driven by concerns about a looming fi bre crisis (associated with a shortfall forecast in small roundwood supplies) as well as the oil crisis (associated with increasing concerns about energy security) stump harvesting became the focus of a research programme in Scandinavia in the 1970s ( Hakkila, 2004 ; Bjorheden, 2006 ; Egnell et al. , 2007 ). The predicted fi bre crisis failed to materialize, while the cost of stump harvesting proved to be excessive ( Hakkila, 2004 ) and commercial stump harvesting for fi bre and fuel production was largely abandoned. Since then demand for other forest fuels has continued to grow, leading to a re-evaluation of the potential of stumps as an energy source. Karjalainen et al. (2004) estimated that across Europe, there is the potential to source up to 9 million m 3 year 1 of forest chips from stumps per annum, out of a total potential annual increment of 78 million m 3 year 1 . One hectare can produce upwards of an additional 100 m 3 of woodfuel, equivalent to 100 MWh ha 1 ( Flynn and Kumar, 2005 ), with some reports of 200 MWh ha 1 ( Hakkila and Aarniala, 2004 ) and 250 MWh ha 1 (Rolfsson, 2006 , cited in Eriksson and Gustavsson, 2008 : 897). The biomass available from the stump – root system offers the potential to gain an additional 20 per cent beyond that obtainable from the stem ( Richardson et al. , 2002 ). Stump Harvesting for Bioenergy – A Review of the Environmental Impacts