Canadian Biomass Magazine

Features Harvesting Sustainability
Biomass Sustainability: Cleaner is better?


August 7, 2009
By Evelyne Thiffault

Topics

Over the past year, I have been touring Canada to talk about the consequences on the ecosystem of removing too much biomass, presentations mostly given to industry and biomass promoters. Afterwards, I was often pressed about the consequences 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 perceptions?

Over the past year, I have been touring Canada to talk about the consequences on the ecosystem of removing too much biomass, presentations mostly given to industry and biomass promoters. Afterwards, I was often pressed about the consequences 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 perceptions?

Perception 1:
Harvest residues left on site will decompose and emit CO2; we might as well harvest them and emit carbon producing energy.

This is partly correct. In forest ecosystems under temperate and warm climates, the carbon in harvest residues is gradually degraded by soil organisms and respired as CO2; 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 decomposition of harvest residues and respiration of CO2 likely occur too, but at a rate so slow 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 different from the burning of biomass: over a long period of time vs. now.

Perception 2:
Harvest residues produce methane, a greenhouse gas even more damaging than CO2. This is likely not true. Methane is indeed a very potent GHG, with a global warming potential higher than CO2. However, methane is produced from organic material under anaerobic conditions (i.e., without oxygen), which do not occur in most harvest slash conditions. If there are tightly packed piles of rapidly decomposing residues, or buried piles, this might generate anaerobic conditions for a small period of the year, but this phenomenon should be pretty rare and of little consequence. On the other hand, methane is produced in considerable quantities from the decaying 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. Eutrophication is an increase in nutrients, mostly nitrogen (such as nitrates) and phosphorus compounds, which overloads the nutrient processing capacity 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 water. 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 nutrients already in the ecosystem. Natural disturbances 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 residues 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 biomass 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 lower risks of fires that threaten human safety or wood supplies for industries (wildfire by itself being part of the natural cycle of many ecosystems). 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. Harvesting of biomass in the form of unmerchantable 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 forest ecosystems need us to grow and thrive, because, well, most often they don’t.

Want to know the best science-based reason 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 contributes thoughts on biomass harvesting sustainability to Canadian Biomass on behalf of the Canadian Research Group on Ecosystem Sustainability.


Print this page

Related



Leave a Reply

Your email address will not be published. Required fields are marked *

*