Biomass Sustainability: What Should We Sustain?

For the past several years, I’ve been working on defining sustainable levels of forest biomass harvesting that can be used as guidelines for decision making. As a forest soil scientist in a team of mainly soil researchers, 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 intended!) 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 harvesting intensities. This kind of model is the basis of biomass harvesting guidelines recently issued by the government of New Brunswick.

Nutrient budgets are also important in defining critical acidity loads; they can predict the cumulative effects of acidic atmospheric deposition and forest harvesting on soils by accounting 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 factors in the soil over time after harvesting. They compare these with measures done before harvesting or in sites harvested at different intensities, 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 harvesting in jack pine stands on coarse sandy soils is fine, whereas the same practice in balsam fir stands on medium-textured soils is not. However, 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 scientifically sensible and logical at a certain level.

And this is just one example from a soil-oriented viewpoint. Last December, I was asked to give a talk on biomass harvesting issues related to soils (about which I knew a little) and biodiversity (about which I knew nothing). I consulted 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 specialists 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 critical role forests play in the fight against climate change.  The International Panel on Climate Change states, “Maintaining or increasing forest 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 driving forest management in an era in which ongoing changes push global and regional climates beyond the bounds experienced over the last centuries? 

Sustainable livelihoods
Climate change presents us with an unknowable 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 concluded 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 towards a goal of sustainable livelihoods. This 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 technologies 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 multitude of goals and time frames, e.g., sequester carbon, maintain soil quality, preserve biodiversity (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 contributes thoughts on biomass harvesting sustainability to Canadian Biomass on behalf of the Canadian Research Group on Ecosystem Sustainability.