Comments submitted to the Nova Scotia Renewable Energy Stakeholder Consultation Process in response to the Interim Report To Stakeholders (December 15th 2009)

David Patriquin
Professor of Biology, Dalhousie University (Retired)

Dec 15, 2009.

From the Report to Stakeholders (Dec. 15, 2009):

There is an emerging consensus between the various parties (DNR, forest managers, academics and industry) that there is sufficient forest biomass to support up to 150 MW of electricity generation. But more discussion regarding forestry management standards and the assurance of ecological integrity of Nova Scotia's forests is required.

I suggest that in addition, we need to give careful consideration to the implications of various regimes for net carbon sequestration. Because of the history of forestry in Nova Scotia, our forests are relatively young, averaging perhaps 40 years¹. Left undisturbed after clearcutting, forests in northeastern North America continue to accumulate carbon and sequester atmospheric carbon dioxide for well over 100 years². If we clearcut our forests for biomass, whether by whole-tree or stem-only harvest, the implications for net carbon sequestration must be taken into account - at least if we want to reduce CO2 emissions, as well as substitute for petroleum based energy generation.

Whether or not harvesting biomass for energy is carbon neutral depends very much on site and process specific factors.³ I suspect that detailed carbon accounting would show that clearcutting forest for biomass would be far from carbon neutral for the typical Nova Scotian scenario; it would in fact reduce net carbon sequestration substantially. The contention that biomass is carbon neutral is based on the assumption that the carbon dioxide released when biomass is burned (or respired) is taken up stoichiometrically when the biomass crop re-grows. For biomass crops such as switchgrass or sugarcane, the CO2 released on burning can be recaptured within one growing season; if it is grown on degraded land with fertilization, there can even be net carbon sequestration. Harvesting standing forests for biomass is, however, a quite different matter. If we clearcut a 40 year old forest now for biomass energy, all of the harvested biomass carbon is going into the atmosphere now; then it will take a full 40 years to take up an amount of carbon dioxide equivalent to that released, assuming that the forest recovers to its previous state. So, in the short term, e.g., over the ensuing decade at least, burning of the forest biomass will result in net carbon emissions.

Further, in order to realize carbon neutrality over 40 years, we have add to the carbon that needs to be recaptured: (i) losses of soil carbon associated with clearcutting (ii) carbon costs of harvesting and processing the biomass, (iii) the additional carbon that would have been taken up had the forest not been cut. To the extent that these amounts (including the initial biomass carbon) are not recaptured, there will be net emissions of carbon dioxide to the atmosphere.⁴

It is for this reason that biomass energy schemes of this sort are generally considered to cause net emissions of CO2 unless there is (i) significant carbon capture and storage associated with the combustion of the biomass and/or (ii) conversion of a significant portion of the harvested biomass into a slowly degrading form (carbonization, biochar) and/or (iii) biomass production sites are fertilized to substantially increase productivity over background levels.⁵

In the short term (2015), forest biomass projects in N.S. are not likely to involve any of these three conditions. Further, in considering forest biomass as a substitute for fossil fuels, the lower efficiency of biomass compared to fossil fuels in generating electricity must be taken account. Given the potential of N.S. forests to sequester carbon if they are NOT harvested, a full carbon accounting would likely indicate that we could reduce carbon emissions much more by substantially reducing clearcutting in Nova Scotia than we could by substituting clearcut forest biomass for fossil fuels in power generation.

At the very least, we need to do this sort of carbon accounting before embarking on an ambitious forest biomass cutting to meet 2015 substitution goals.

Recent studies/modeling by forest and ecology scientists in relation to using forest biomass in the well-studied Massachusetts forests provide an example. Thompson et al.⁶ modeled the implications of potential future demand for biomass electricity of around 165 MW, which would require up to 2 million Mg of woody biomass annually from Massachusetts forests:

Changes in species composition were small, but present, under the biomass energy scenarios, with white pine and red oak increasing relative to the baseline scenario, and black birch, beech, and hemlock decreasing. Living aboveground biomass increased by 2.0%, from 225 to 229 Mg/ha under the baseline scenario, while decreasing to 207 Mg/ha (-7.9%) and 201 Mg/ha (-10.7%) in the two biomass scenarios. The difference in standing biomass translates to a net carbon sequestration of 1.9Tg over 50 years under current trends, compared to a 7.3 and 9.9Tg of net emissions in the biomass energy scenarios. In spite of this, the amount of biomass feedstock harvested in the biomass future scenarios was only enough to generate 90 and 100 MW of power, well short of potential future demand. These results indicate that demand for biomass energy is likely to greatly increase the importance of harvesting as a disturbance on the forest landscape. Furthermore, pursuing a renewable energy policy that relies heavily on biomass power is likely to come at the cost of a diminished forest carbon sink

The Massachusetts forests occur on better soils than in Nova Scotia and have been subject to much less clearcutting. We could expect equivalent or larger effects on carbon sequestration in N.S. forests.

Sustainability

As emphasized by Goldmsith in 1980¹, an important factor to be considered in relation to repeated clearcutting, whatever the use, is loss of nutrient capital in the harvested forest biomass and through enhanced erosion and leaching: at some time in the future, that will result in lower forest productivity and reduced uptake of carbon dioxide. Susceptibility of N.S. forests to this type of degradation is especially high in SW Nova Scotia because of acid rain and the poor buffering capacity of soils on granitic bedrock.⁷ Loss of calcium is a key concern for both forests⁸ and the downstream riparian and aquatic systems.⁹ Other regions of N.S. might be able to withstand repeated clearcuts for a longer period, but that is only a matter of degree and, as illustrated by the Massachusetts study, clearcutting for biomass even on better sites is likely to increase carbon emissions, not reduce them.

Conclusion

There is a role for forest biomass in energy production in N.S., e.g., using processing wastes, selective cutting for firewood, growing fast growing trees coupled with use of biosolids as fertilizers, especially when combined with energy efficient conversion technologies and/or carbon capture/biochar production. However, clearcutting forests for biomass energy is a questionable strategy. Indeed, given the generally degraded state of Nova Scotia's forests and evidence that our forests go on accumulating carbon for well over 100 years after a clearcut, a case could likely be made for gaining carbon credits by substantially reducing the current annual cut in Nova Scotia, even with ongoing use of fossil fuel to generate energy (where we might otherwise substitute clearcut forest biomass).

References