Nic Meyer from the Paul Scherrer Institute in Villigen, Switzerland, was inspired to think about how efficiently logs burn while running through his local forest. He noticed that the fraction of bark in a log varied, and wondered how much that affected the quality of the log and the emissions it produced while burning.
"In our previous investigations we had found that the emissions from burning bark had sent all of our test instruments into overdrive," Meyer told environmentalresearchweb.
Combustion of bark with wood tends to produce more particulate matter, black carbon and organics than combustion of clean wood alone.
Together with colleague Marco Mina, Meyer modelled and analysed beech and spruce plantations in Switzerland, to investigate how forestry management (thinning practices and timing of harvest, for example) affected the volume of fuel wood and its bark fraction. In the case of beech trees they found that altering management practices could reduce the bark fraction by up to 20%.
Choosing when to thin the trees turned out to be key. By delaying thinning until the branches reached a respectable diameter (20 to 40 cm), bark production was significantly reduced. Their findings are published in Environmental Research Letters.
Spruce bark percentages, by contrast, were barely affected by forestry management practices. "Spruce plantations, which tend to be composed of trees of the same age, grow evenly together, which means that the density of the stand produces natural self-pruning along the stems," explained Mina. As a result the branches rarely get a chance to grow large enough to minimize their bark ratio.
Meyer and Mina are also interested in seeing if other tree species, particularly those with high bark fractions, such as oak, larch and black locust, respond in a similar way to beech. If this is the case then optimizing management practices for these species would also contribute to reductions in atmospheric pollution.
"In Switzerland alone, the damage costs from direct emissions from wood combustion activities are estimated to be of the order of 100 million Euro per year," said Meyer. If rolled out across other regions with similar forests – much of Europe and North America – the benefits through reduced health impacts could be significant.
Reducing the fraction of bark around a log will also help it to burn more cleanly, leading to fewer black-carbon emissions, which is potentially good news for climate change too.
"Black carbon, a main constituent of wood combustion emission, is the highest contributor to anthropogenic climate change after carbon dioxide," said Meyer. "Any reduction in black-carbon emissions will be beneficial in stemming the impact of combustion emissions upon climate change, although the influence of organic coatings upon black carbon during its atmospheric lifetime is still not clear, and requires further research."
Rising energy prices mean that log-burning stoves are an increasingly popular domestic heating option. Optimizing log production will help to ensure that we don't revert to the pea-souper smogs of the 1950s. Many questions remain to be answered, but Meyer and Mina have demonstrated that controlling bark production in fuel wood is at least possible.
