The root of the problem
Getting to grips with the prospect of stump harvesting
Forest owners and managers may be forgiven for becoming increasingly confused over the long heralded benefits of stump harvesting. When a tree is felled a large proportion of biomass remains in the ground. Over a large clearfell area this translates into a huge wood resource offering much needed extra income and profit after decades of investment and inputs.

There is clearly caution and constraint over large-scale stump removal in clearfell areas due to concern for the local environment, through impact on soil structure and fertility and associated ‘run off’ or leaching into water courses. These considerations are described and discussed in a new Forestry Commission publication* which sets out the criteria used to assess clearfell sites for local environmental risk before embarking on large-scale harvesting of tree stumps. Stump harvesting and climate warming.

On the ‘wider world scene’ stump harvesting appears to be in favour with foresters encouraged to earn extra cash and do their bit for climate stability by relieving pressure on the world’s dwindling fossil fuel reserves. Huge volumes of tree biomass otherwise left in the
ground could be burnt in power stations, leaving more carbon in coal, crude oil and natural gas deposits, locked up safely underground. Extra income helps to finance restocking, machinery manufacturers and forest contractors thrive and everyone is happy within a seemingly sustainable situation. But not so fast because the ‘bigger picture’ watchers are starting to claim stump harvesting may not be so environmentally friendly after all.

Derek Nelson, senior industry advisor with the Forestry Commission, has said stump harvesting is undertaken principally to supply the growing woodfuel market and, if sustainably practised, has the potential to contribute to efforts in combating climate change by reducing net carbon emissions. He added a caveat saying, “We need to ensure that the carbon which is lost from the soil because of stump harvesting does not exceed the carbon emissions saved using the resulting woodfuel to replace fossil fuels.” (Forestry Journal May 2009).

At the Forestry Commission’s climate change conference in November 2007 one speaker apparently generated palpable palpitations in some delegates after questioning the contribution of stump harvesting to reduced carbon emissions and climate change, according
to Paul Ramsay who reported the event in Forestry Journal in July 2008. Professor Paul Jarvis of the School of GeoSciences at Edinburgh University said harvesting stumps and tree waste was no more productive than mining coal in tackling climate change. He released other climate warming gases into the equation and said these, in addition to much maligned carbon dioxide (CO2), must be thrown into the mix to get a complete and true picture of how growing and harvesting trees affects the environment.

New ‘bogeyman’ in climate change Though only the third ‘greenhouse gas’ in atmospheric concentration terms (behind CO2 and methane – CH4), nitrous oxide (dinitrogen oxide – N2O) commonly known as the dentist’s anaesthetic and ‘laughing gas’, was in Professor Jarvis’s firing line. And no one is apparently laughing because, although normal atmospheric concentrations are tiny, nitrous oxide is claimed to be 310 times more powerful as a ‘greenhouse’ gas than CO2. Atmospheric concentrations have risen 16% since 1750. Climate warming watchers clearly want the teeth of nitrous oxide pulled, even if the extraction causes economic pain to financially strapped foresters.

Concern arises because nitrous oxide is released during combustion at power stations, though at much lower emission levels than in agricultural soils with nitrogen fertiliser the most significant source. Professor Jarvis went on to question current fertiliser usage in forestry by saying, “We can grow the trees without the use of 300kg per hectare of nitrogen-fertilisers.” Oneoff climate change credit asked for and awarded at the power station for providing biomass from clearfell debris would be promptly negated by fertiliser application to subsequent replanting. Notice how the jargon has changed from ‘carbon credit’ to ‘climate change credit’ to bring other ‘nasty’ gases like nitrous oxide on board.

You can hear mass grinding of teeth in UK forestry, having sunk investment and inputs into sites for 40 years or more, only to find it is forced to harvest trees in the teeth of the worst recession for almost 100 years, and being made to leave a sizable proportion of tree biomass
and cash profit in the ground to rot away. And all this after foresters have been ‘fired up’ about the opportunities offered by woodfuel.

Gradual release inappropriate If stumps are left in situ, carbon, nitrogen and other nutrients and minerals are released gradually as biomass decays, eliminating the need for fertiliser to replant. That presumably is the theory, but in practice fertiliser is rarely an ‘all or nothing’ option and trees, whether conifers in the west of Scotland or rubber trees in West Africa, require different combinations and amounts of nutrients according to their stage of growth and development. Early rapid growth, in which nitrogen fertiliser plays an important part for fast seedling tree establishment, is essential for survival through adverse conditions, and to withstand weed competition and insect pest attack. For instance, early sustained growth of young conifers is key to surviving feeding by large pine weevil (Hylobius abietis), the principal restocking pest of UK forestry (Forestry Journal, June 2009).

Fast growth rate with increased bark thickness in young Sitka spruce and Scots and Corsican pine treated to high light and nitrogen regimes reduced deep bark feeding. Resin duct size (resin flow rate), which affords some protection from weevil feeding and damage, was positively related to plant growth and increased bark thickness in particular.†

Resin flow protects stems of young conifers from weevil feeding, not by affecting the total amount of bark eaten, but by limiting the depth of feeding, thus protecting inner phloem and cambium. Shallow feeding may increase the chances of effective wound repair. Widely reported effects of plant size, growing conditions and transplantation on susceptibility to attack by H. abietis may be due to underlying variation in resin duct size or flow rate, said the authors. The effect on weevils of superficial stem feeding is to increase the time taken by insects for reproductive maturation by reducing consumption of the nitrogen-rich inner bark.

Nitrogen fertiliser plays an important role in damage alleviation from other pests at much later stages in tree growth and development. Scots pine trees defoliated by pine sawfly (Diprion pini) in Finland were given various nutrient combinations in the growing season following defoliation to determine the effect on tree recovery. Nitrogen fertilisation enhanced radial growth even in the severely defoliated trees and increased the number of vertical resin ducts. Nitrogen fertiliser is clearly an important component of commercial forestry with clear implications for tree establishment, growth rate and alleviation of insect pest damage.

Attempts to dispense with nitrogen fertiliser application could have serious implications for stand density, tree size, vigour and health and consequences on the capacity to lock up atmospheric carbon. A wider joined-up picture Greenhouse gases and global warming are important but somewhat of a ‘red herring’ in a wider ‘joinedup’ picture. Until now UK forest industries have focused on woodfuel burnt industrially or domestically to generate energy.

Biofuel on the other hand is liquid chemical such as bioethanol and biodiesel produced largely (until now) from agricultural crops and for good economic and practical reasons. Bioethanol is the product of glucose sugar fermentation. Sucrose (cane sugar) is a disaccharide sugar with glucose and fructose units and starch is a storage polysaccharide composed of glucose units and the main constituent of cereal grains like wheat and maize. Both sugar cane and cereal grains are easy and cost effective to ferment into bioethanol.

Biodiesel manufacturers have gone for similar easy routes by using oilrich food crops like oilseed rape and oil palm to extract vegetable oil for processing into biodiesel and use in the transport sector. Wood as a feedstock for bioethanol fermentation is a ‘different kettle of fish’, being composed of structural polysaccharides like cellulose, hemicullulose and lignin, more complex and correspondingly more difficult to ferment into commercially sustainable quantities of bioethanol.

But the tide changed during the world food crisis of 2008 when eyes refocused on ‘second generation’ biofuels, manufactured from nonfood crop feedstocks with wood at the forefront pending appropriate fermentation technology. 2008 saw world food commodities including wheat, maize and rice shoot up in price. The price of most foods, including bread, rose rapidly
in UK supermarkets but the effect on developing world countries was unsustainable, causing widespread acute food shortages and food riots in regions as far afield as West Africa, the Middle East and South East Asia.

Many factors were involved, including large yield reductions in traditionally big grain producer/
exporter countries like Australia and Canada, and higher demand for food in increasingly rich Asian countries including India and China. But North America and Europe were stuffing large chunks of the world’s cereal grain and oilseed supplies into fermentation vats and extractors
to make bioethanol and biodiesel and this received most blame.

Bioethanol from wood
The world took note with the EU scaling back ambitions on biofuel use in its transport sector and switching attention to ‘second generation’ biofuel manufactured from anything except food and livestock feed crops or plants displacing such crops from the land. Wood now features high on the world’s agenda as a feedstock for bioethanol production, so why are UK foresters being advised to leave valuable biomass in the ground with the technology for efficient fermentation of wood into bioethanol just around the corner?

Harvested stump material is mostly secondary xylem tissue with plant cell walls containing a range of structural polysaccharides including cellulose, hemicelluloses and lignin. The cellulose microfibrils in the secondary cell wall are embedded in lignin and function like steel rods embedded in concrete, but with less rigidity. The target is cellulosic ethanol using hydrolytic enzymes to break down the cellulose into smaller units and finally glucose sugar before it can be fermented, but the lignin presents a formidable barrier because it prevents the enzymes from getting at the celluloses. Therefore, heat and/or chemical pretreatment processes are
being developed and used to break down cell walls into intermediates and remove lignin to expose the cellulose to the cellulase enzymes.

To produce cellulosic ethanol from wood the harvested lignocellulosic biomass is compacted (fresh or dry) and transported to a cellulosic ethanol refinery where it is stored ready for conversion. The biomass is then pretreated with extreme heat and/or chemicals to break it down into intermediates and remove the lignin. This is followed by detoxification, neutralisation and separation into its liquid and solid components. The latter are then hydrolysed using enzymes produced in microbial bioreactors from bacteria or fungi. Finally, sugars are separated and fermented to produce ethanol. Cell-wall polysaccharides can be converted into fermentable sugars through enzymatic hydrolysis using enzymes such as cellulases and hemicellulases.

Wider world benefits from stump harvesting
The bottom line is – do we want foresters to restock? The answer is clearly yes, and they must be allowed every financial incentive to do so. Stump harvesting does not require government grants and hand-outs, but are foresters looking after themselves by securing all
harvestable biomass and reaping rewards in return? If evidence of the need for incentives to boost restocking is required then you only need to look at figures relating to Wales (Forestry Journal, June 2009). Over the period 1990/91 to 2007/8 there were reductions of 34% and 25% in conifer area and total woodland area, respectively, calculated as per cent of area felled.

The carbon equation, global warming and commodity supply scenario is like a political opinion
poll. There are so many variables and parameters that experts (and politicians) can ‘fiddle’ the figures by extraction and compartmentalisation, so ordinary mortals only see what they are shown. Justification is that the ‘big’ picture is just too difficult to comprehend, but foresters are not fools. If you want to take away part of their livelihood here in the UK, due to global perspectives, then give them a complete and joined-up global picture and let them make up their own minds.

Politicians are urging us to think globally. This shows stump harvesting in the UK can do its bit to relieve biofuel pressure on world food and feed supplies and help ensure people in Africa and elsewhere don’t have to riot because they can’t access basic staples like wheat, maize and rice.

Stump harvesting might also help the orangutan in Asia. The last main refuge of man’s close cousin is the island of Borneo occupied mainly by the Indonesian territory of Kalimantan. The biggest single threat to the island’s forest and the orangutan’s home is Indonesia’s headlong rush into biofuel production using oil palm. The Indonesian government recently considered clearing mountainous forest from an area as large as the Netherlands, and containing the sources for a majority of the island’s main rivers, just to plant oil palm which is a lowland species from West Africa totally unsuited to high elevation planting and cultivation.
Terry Mabbett

* Nisbet T, Nicoll B and Perks M (2009).
Stump harvesting: Interim guidance on site selection and good practice. www.forestresearch.
gov.uk/stumpharvesting

† Wainhouse, D, Staley, J, Johnston, J and Boswell, R (2005). The effect of environmentally
induced changes in the bark of young conifers on feeding behaviour and reproductive development of adult Hylobius abietis (Coleoptera – Curcilionidae). Bulletin of Entomological Research, 95:151-159.

Clearfell areas offer foresters potentially lucrative supplements to their original timber harvest. Felling leaves a considerable amount of tree biomass at and below soil level.