Sustainable Lessons from Sweden Biomass harvesting guidelines must take into account both science and practicality in the field. By Evelyne Thiffault recently came across an interesting report produced in a country that is further down the road relative to Canada in the develop- ment of its bioenergy-from-biomass indus- try and its regulation. In a fine example of adaptive forest management, Sweden is at the stage where it reassesses its harvest- ing guidelines using ecological knowledge gathered over the years. The report, En- vironmental Effects of Logging Residue Recovery and Ash Recycling in Sweden, was published in Swedish in 2006 and is available from the Swedish Energy Agency. I am told that an English version will be available in the coming months. Let’s have a look at some issues in this report that I find of particular relevance. The Swedish National Board of For- I estry advised in 2002 in its Recommenda- tions for the Extraction of Forest Fuel and Compensation Fertilizing that when stems and tops of trees are harvested, the needles should be left on-site and evenly spread about. This could be done by letting tops dry in small piles on the site before re- moving them. This was meant to limit the amount of nutrients exported from the site and mitigate potential impacts of biomass harvesting on soil fertility. The recommendation appears to be sensible, as needles contain high con- centrations of nutrients compared with woody parts of trees and are of negligible interest for bioenergy because of their low calorific value. However, in practice, a large proportion of needles is actually ex- tracted during biomass harvesting, despite efforts to leave them on-site. For example, the efficacy of drying is highly weather de- pendent, and pine needles remain firmly attached to branches even after a whole n my last column, I discussed various guidelines adopted by Canadian prov- inces for forest biomass harvesting. I summer of drying. Also, the development of bundling machines for branches and tops, which allows for bundled biomass and roundwood transport using the same vehicles, will likely reduce the logistic possibility of letting residues dry on-site before picking them up. Thus, although the recommendation to leave most needles on-site is a reason- able one from a soil fertility perspective, its practicality is questionable. Another recom- mendation in the 2002 guidelines is that compensatory fertilization, in the form of ash recy- cling, should be car- ried out when it is not possible to leave needles on-site, and on peatlands and highly acidified areas. Ash recycling has caught the eye of many biomass developers in Canada because it is seen as a convenient way to offset nutri- ent extraction and soil fertility loss while providing a waste disposal opportunity. However, field trials in Sweden show that ash recycling may not be the panacea that we wish it to be. Whereas ash recycling on biomass-harvested sites was shown to stimulate tree growth in the southern, more temperate part of Sweden, growth was actually decreased by ash application in the northern part of the country. These effects should not be interpreted northern stands somehow reacts negative- ly to ash addition by reducing the amount of nitrogen available to trees, whereas the contrary may be true in temperate stands. One of the key messages here is that as far as tree growth is concerned, there is no obvious need to provide ash to counteract effects of biomass harvesting. However, Biomass “Field trials in Sweden show that ash recycling may not be the panacea that we wish it to be.” there are other reasons for spreading ash on biomass-harvested sites. For instance, it could counteract acidification of lakes in areas affected by acid rain. In other words, maintaining tree productivity is not a strong argument for ash recycling after biomass harvesting, whereas mitigat- ing soil and water acidification in already acidified areas is a more relevant goal. Another key message that I would like as direct reactions of trees to the nutrients in ash (mostly calcium, magnesium, and potassium). Rather, they probably reflect indirect effects of ash on the availability of nitrogen, the most likely driver of tree growth. The thick layer of undecomposed organic material on the forest floor in to add is that biomass is much more than just nutrients: It is organic matter with physical and chemical properties, nutri- ents embedded in organic structures, and it is also the living ground for a myriad of organisms. On sensitive sites, biomass may play an overwhelming role in the maintenance of tree growth and forest eco- system functioning that ash simply cannot replace. So, what’s in there to learn for Canada? Continued on page 31 canadianBIOMASS 9
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