Sustainable Biomass Cleaner is better? Do forests need to be cleaned of biomass? The answer is not as simple as many believe. By Evelyne Thiffault much biomass, presentations mostly given to industry and biomass promoters. After- wards, I was often pressed about the con- sequences of NOT removing biomass from the forests. “Forests need to be cleaned of biomass; we are helping the ecosystem by removing it.” But how much scientific support is there for such common percep- tions? O perception 1: Harvest residues left on site will decompose and emit CO2 ; we might as well harvest ver the past year, I have been touring Canada to talk about the consequences on the ecosystem of removing too ent from the burning of biomass: over a long period of time vs. now. perception 2: Harvest residues produce methane, a green- house gas even more damaging than CO2 . This is likely not true. Methane is indeed a very potent GHG, with a global warming po- tential higher than CO2 . However, methane is “ There is no need to argue that forest ecosystems need us to grow and thrive, because most often they do just fine on their own.” them and emit carbon producing energy. This is partly correct. In forest ecosys- tems under temperate and warm climates, the carbon in harvest residues is gradually degraded by soil organisms and respired as CO2 produced from organic material under anaero- bic conditions (i.e., without oxygen), which do not occur in most harvest slash conditions. If there are tightly packed piles of rapidly de- composing residues, or buried piles, this might generate anaerobic conditions for a small pe- riod of the year, but this phenomenon should be pretty rare and of little consequence. On the other hand, methane is produced in considerable quan- tities from the decay- ing organic waste of waste landfills. perception 3: Decomposition of harvest residues leads to the production of nitrates that leach through soils and to lakes, causing eutrophication. Again, this is likely not true. Eutrophica- ; after some years very little carbon is thus incorporated into the soil, as most is lost to the atmosphere. Under wetter and cooler conditions such as those found in boreal coniferous ecosystems decomposi- tion of harvest residues and respiration of CO2 it is difficult to detect it, and more carbon is likely integrated into the soil (although this remains a challenging question among scientists). Yet even if the fate of carbon in residues may ultimately be its emission to the atmosphere, the timing is quite differ- 16 CanadianBIOMASS likely occur too, but at a rate so slow tion is an increase in nutrients, mostly nitrogen (such as nitrates) and phosphorus compounds, which overloads the nutrient processing ca- pacity of the ecosystem, and happens usually when nutrients are introduced from outside of the ecosystem. This causes excessive plant growth and decay, depleting oxygen in the wa- ter. For example, increased inputs of nutrients from the Mississippi and Atchafalaya rivers, which drain the U.S. fertilizer-rich agricultural heartland, cause ‘dead zones’ in the Gulf of Mexico, large areas of water in which oxygen levels are too low for most fish to survive. However, harvest residues contain nutri- ents already in the ecosystem. Natural distur- bances also transfer large amounts of nutrients to the ground. This flush of nutrients in soils following forest disturbance (both natural and man-made) causes a rise in soil nutrient availability, but not eutrophication. Studies have shown that the presence of harvest resi- dues on some forest sites does increase nitrate leaching compared with whole-tree harvested sites. However, absolute increases in leaching are generally small and disappear after three to five years. So, do forests need to be cleaned of bio- mass for their own sake? Natural disturbance such as forest fires, windthrows, insect attacks and self thinning produce great loads of slash, usually much more than harvesting, and have been doing so for millennia. Forests don’t need to be cleaned other than may be necessary to achieve some human-desired condition. For example, we may need to remove slash to low- er risks of fires that threaten human safety or wood supplies for industries (wildfire by itself being part of the natural cycle of many ecosys- tems). Slash removal may help to meet other silvicultural goals, such as clearing the way for the establishment of natural regeneration or to facilitate site preparation and plantation. Har- vesting of biomass in the form of unmerchant- able trees can also help enhance the quality of the stand. Removing biomass as part of a forest management strategy to meet human needs is legitimate. There is no need to argue that for- est ecosystems need us to grow and thrive, because, well, most often they don’t. Want to know the best science-based rea- son to remove biomass? It’s a renewable energy source to substitute for planet-damaging fossil fuels and coal (but you already knew that). • Dr. Evelyne Thiffault of Natural Resources Canada contrib- utes thoughts on biomass harvesting sustainability to Canadian Biomass on behalf of the Canadian Research Group on Ecosystem Sustainability. JUNE 2009
Canadian Biomass June 2009: Page 16
