Sustainable What ShouldWe Sustain? It’s not easy to define ecologically sustainable levels of forest biomass harvesting. By Evelyne Thiffault F or the past several years, I’ve been work- ing on defining sustainable levels of for- est biomass harvesting that can be used as guidelines for decision making. As a forest soil scientist in a team of mainly soil research- ers, I tend to look at sustainability in terms of soil characteristics such as carbon and nutrient content, acidity, and organic matter quality. In my world, we tend to think that if the soils are kept happy, then the whole ecosystem will be happy and function contentedly. However, if you dig a little deeper (pun in- tended!) and try to define a ‘happy soil,’ you find that soil scientists have various schools of thought and definitions of sustainability. For example, much Canadian research since the late 1960s regards sustainability as the balance between nutrient inputs and outputs. Nutrient budgets indicate how theoretical net gains or losses of nutrients differ under various harvest- ing intensities. This kind of model is the basis of biomass harvesting guidelines recently is- sued by the government of New Brunswick. Nutrient budgets are also important in de- fining critical acidity loads; they can predict the cumulative effects of acidic atmospheric depo- sition and forest harvesting on soils by account- ing for acidity inputs and outputs. In Sweden, estimations of critical acidity loads are used to define a site’s suitability for biomass harvesting. In these calculations, the focus of sustainability is on the maintenance of proper soil acidity. Other soil researchers gauge sustainability by using direct measurements of various fac- tors in the soil over time after harvesting. They compare these with measures done before har- vesting or in sites harvested at different intensi- ties, e.g., whole-tree vs. stem-only harvesting. The comparisons are used to understand the effects of a particular type of harvesting on soil processes and to establish how sustainable the practice is. maNy possible methods— maNy diFFereNt results The difficulty is that the different methods sometimes disagree on how ‘happy’ a particular soil should be after harvesting. For example, according to nutrient budgets, whole-tree har- vesting in jack pine stands on coarse sandy soils is fine, whereas the same practice in balsam fir stands on medium-textured soils is not. How- ever, soil measurements done in field trials 20 years after whole-tree harvesting conclude the opposite. So, sustainability is a moving target, as soil ‘happiness’ can be defined in many ways and at various timescales, all of which are scien- tifically sensible and logical at a certain level. And this is just one example from a soil-ori- ented viewpoint. Last December, I was asked to give a talk on biomass harvesting issues related to soils (about which I knew a little) and biodi- versity (about which I knew nothing). I consult- ed some biodiversity researchers and was startled to learn that what I considered to be adequate amounts of harvest residues left on site to keep the soils ‘happy’ were not nearly enough to keep communities of flies that live on dead wood thriving and well. And I’ll bet that fungus and arthropod special- ists would also have something to say about sustainable levels of biomass harvesting. People may say, “Who cares about soils and flies? Keep the trees growing big and happy: that’s the indicator of sustainability.” But is stable wood volume production the definitive index we’re looking for? One might say “yes!” not only because of the need for a steady wood supply for industry, but also because of the crit- ical role forests play in the fight against climate change. The International Panel on Climate Change states, “Maintaining or increasing for- est carbon stocks, while producing an annual yield of timber, fibre, or energy from the forest, will generate the largest mitigation benefit [to reduce greenhouse gas emissions].” Shouldn’t carbon sequestration be the ultimate goal driv- ing forest management in an era in which ongo- ing changes push global and regional climates beyond the bounds experienced over the last centuries? sustaiNable livelihoods Climate change presents us with an unknow- able and uncertain future in which trying to sustain levels of wood production might not be uniformly desirable or feasible—think of the pine beetle outbreaks in BC. As con- cluded at the International Conference on Climate Change and Adaptation last August in Sweden, it may be time to move away from sustainability based on maintaining past patterns of forest conditions and forest use to- wards a goal of sustainable livelihoods. This “ If you dig a little deeper, and try to define a ‘happy soil’, you find that scientists have various definitions of sustainability.” means that producing forest biomass to meet today’s need for bioenergy could be a transition phase from a fossil-fuel-based society towards a society based on yet-unknown green technolo- gies in which forest bioenergy might not be as important as other forest values such as soils, fungi, and flies! This requires local solutions that not only optimize and combine a multi- tude of goals and time frames, e.g., sequester carbon, maintain soil quality, preserve biodi- versity (justifying fundamental research in all of these sectors!), but that also sustain present and future communities and the people living in them.• Dr. Evelyne Thiffault of Natural Resources Canada con- tributes thoughts on biomass harvesting sustainability to Canadian Biomass on behalf of the Canadian Research Group on Ecosystem Sustainability. CanadianBIOMASS 21 Bioenergy
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