Biomass Sustainability: Lessons from Sweden

In my last column, I discussed various guidelines adopted by Canadian provinces for forest biomass harvesting.  I recently came across an interesting report produced in a country that is further down the road relative to Canada in the development of its bioenergy-from-biomass industry and its regulation.  In a fine example of adaptive forest management, Sweden is at the stage where it reassesses its harvesting guidelines using ecological knowledge gathered over the years.  The report, Environmental 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 Forestry advised in 2002 in its Recommendations 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 removing 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 concentrations 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 extracted during biomass harvesting, despite efforts to leave them on-site.  For example, the efficacy of drying is highly weather dependent, and pine needles remain firmly attached to branches even after a whole 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 reasonable one from a soil fertility perspective, its practicality is questionable. 

Another recommendation in the 2002 guidelines is that compensatory fertilization, in the form of ash recycling, should be carried 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 nutrient 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 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 northern stands somehow reacts negatively 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, 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 mitigating soil and water acidification in already acidified areas is a more relevant goal. 

Another key message that I would like to add is that biomass is much more than just nutrients: It is organic matter with physical and chemical properties, nutrients 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 ecosystem functioning that ash simply cannot replace.
 
So, what’s in there to learn for Canada?  Well, there is no mitigation measure (drying of residues, ash recycling) that can be used easily as a blanket solution to offset all impacts of biomass harvesting on every type of site.  Also, guidelines aimed at conserving a certain amount of residues on-site, e.g., one in five tree tops left on fertile sites, and one in two tops left on less fertile sites, are probably the most practical to apply in the field.  This is actually acknowledged in Sweden’s guidelines.  Such guidelines are also likely the most relevant from an ecological point of view because they exclude any assumptions of how forest residues help in the functioning of the forest ecosystem.  Is it their nitrogen content?  Calcium?  Organic matter?  Bug-and-fungus-friendly dwellings?  No one knows exactly.  But to take a leaf from my book as well as that of Sweden, it surely is better to be on the safe side. •

Dr. Evelyne Thiffault is a research scientist in forest biomass at Natural Resources Canada and provides Canadian Biomass with her thoughts on sustainable biomass harvesting.