results (see Supporting Information). The results are not
sensitive to most parameters, and the general trends of the
impacts of biomass harvest on carbon stocks and their
contribution to overall emissions were not found to be
impacted by uncertainty in the parameters. The pellet
pathway results were found to be most sensitive to assump-
tions related to the quantity of biomass used for drying during
pelletization (15% of input biomass in base case) (see
Supporting Information Figure S-3). Reducing the consump-
tion of biomass during the drying stage increases pellet output
and fossil fuel displacement per unit of input biomass. Co-
location of pelletization facilities with processes generating
waste heat could reduce the drying energy requirement. If
no input biomass is required for drying, there are larger
emissions reductions associated with pellet use and the time
before reaching break even with the fossil energy system is
reduced from 16 to 11 years (residues) and from 38 to 29
years (standing trees). When forest carbon is excluded from
the analysis, biomass utilization for drying energy has a
minimal impact on LC emissions (6).
Study Implications. The simplified assumption of im-
mediate biomass carbon neutrality has been commonly
employed in bioenergy studies, owing in part to emissions
from the energy and forest sectors being reported separately
in national inventories (17). This study, however, shows that
increasing biomass removals from the forest significantly
reduces carbon stocks and delays and lessens the GHG
mitigation potential of the bioenergy pathways studied.
Ignoring the complex relationship between forest carbon
stocks and biomass harvest by employing the carbon
neutrality assumption overstates the GHG mitigation per-
formance of forest bioenergy and fails to report delays in
achieving overall emissions reductions.
Combining LCI analysis and forest carbon modeling as
an analytical approach provides a more accurate represen-
tation of the role of forest bioenergy in GHG mitigation. When
forest carbon dynamics are included in the case study, the
use of forest-based bioenergy increases overall emissions
for many years and, in the worst-performing scenario
(standing tree harvest for ethanol production), does not yield
any net climate mitigation benefit over the 100 year period.
Carbon implications of bioenergy production are not limited
to forests, and these results should not be taken to suggest
that agricultural biomass is inherently preferable. Land use
impacts associated with agriculture-sourced bioenergy can
greatly increase LC emissions (7). Nonbioenergy systems can
also impact carbon stocks (e.g., overburden removal in coal
mining). While the contribution to total emissions may not
be significant in all situations, a comprehensive evaluation
of any fossil or renewable system should consider impacts
of life cycle activities on terrestrial carbon stocks.
Do our results support continued reliance on fossil fuels
for electricity generation and transportation? Fossil fuel use
transfers carbon from the Earth’s crust to the atmosphere;
moving beyond reliance on these energy sources is imperative
to address climate change and nonrenewable resource
concerns. Bioenergy offers advantages over other renewable
options that are limited by supply intermittency and/or high
cost. However, effective deployment of bioenergy requires
the thoughtful selection of appropriate pathways to achieve
overall emissions reductions. Harvesting standing trees for
structural wood products has been reported to reduce overall
emissions: storing carbon in wood products and displacing
GHG-intensive materials (steel, concrete) exceeds associated
forest carbon impacts (14). In comparison, using standing
trees for bioenergy immediately transfers carbon to the
atmosphere and provides a relatively smaller GHG benefit
from displacing coal or gasoline, increasing overall emissions
for several decades. Identifying biomass supply scenarios
that minimize forest carbon loss will improve the emission
mitigation performance of forest bioenergy. Residues em-
ployed for bioenergy reduce emissions from coal after a much
smaller delay than standing trees, while other forest biomass
sources (e.g., processing residuals) could offer near-term
emission reductions if used to replace GHG-intensive fossil
fuels. Industrial ecology approaches (e.g., utilizing end-of-
life wood products as a biomass source; integrating bioenergy
production with other wood products to utilize waste heat
for processing) could reduce forest carbon implications of
bioenergyproduction andare deservingoffurther consideration.
Utilizing bioenergy to displace the most GHG-intensive
fossil fuels minimizes initial emissions increases and reduces
the time required before net GHG benefits are achieved.
Ethanol production for gasoline displacement, under the
modeled conditions, is not an effective use of forest biomass
for GHG reductions. Displacing coal in electricity generation,
in comparison, is superior in reducing emissions. However,
this does not indicate that electricity applications are always
preferable. The mitigation performance of biomass-derived
electricity depends on the displaced generation source.
Further, these results represent the expected near-term state
of energy system technologies and do not consider changes
in either the reference or the bioenergy pathways over the
time frame studied. Performance improvements are inevi-
table with technological maturation and commercialization.
Technological developments regarding thermal electricity
generation (e.g., efficiency improvements; viable carbon
capture and storage) would be applicable to both biomass
and coal, while improvements in pellet production would
not greatly influence total emissions. Emissions from pro-
ducing ethanol, regarding both the ethanol production
process and the appropriate reference pathway in the future
given the limited petroleum supply and associated price
volatility, is uncertain and in the future could prove a more
effective means of emissions reductions than reported here.
Ethanol can also play an important role in addressing
economic and energy security concerns related to petroleum
dependency.
Although the method demonstrated in this research is
generalizable, site-specific characteristics of forests prevent
the generalization of specific results from this study. Numer-
ous factors would influence forest carbon dynamics and must
be considered in specific analyses. Intensifying silvicultural
practices (e.g., planting instead of natural regeneration,
utilization of fast-growing species) could shorten, but not
eliminate, the period of net emission increase found in our
results. In some jurisdictions, residues are burned during
site preparation for forest regrowth. Using such residues for
bioenergy would not significantly impact forest carbon stocks.
While GHG mitigation is an important consideration of
forest resource utilization, numerous other factors must be
considered in the decision-making process. In particular,
declines in Ontario’s forest sector have negatively impacted
communities that would welcome the investment and
employment opportunities associated with bioenergy. Other
environmental factors and technical constraints must be
considered before implementing bioenergy production.
The potential of forest-based bioenergy to reduce emis-
sions from fossil fuels must be balanced with forest carbon
impacts of biomass procurement. This perspective is of
particular importance as policies related to climate change
mitigation, deployment of renewable energy, and the forest
bioeconomy are developed and implemented. Considering
bioenergy in isolation of its impact on forest carbon could
inadvertently encourage the transfer of emissions from the
energy sector to the forest sector rather than achieve real
reductions. Accounting methods must be designed to
measure the complete impact of mitigation options on the
atmosphere. By considering the broader impacts of bioenergy
production on the forest, particularly forest carbon pools,
794
9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 45, NO. 2, 2011