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4.5.2. CO2 Sequestration versus Fossil Fuel Substitution
Since Dyson (1977) and Dyson and Marland (1979), numerous analysts have studied
the potential for mitigating GHG levels by sequestering carbon in standing forest.
Forest land management can impact GHG levels in four ways: through carbon stored
in standing biomass and soils, carbon stored in durable biomass/ wood products
and landfills, fossil fuel left underground because biofuels are used instead,
and forest products and other bio-products displacing fossil fuel-intensive
materials. Matthews (1996) discusses the importance of capturing the full system
impacts.
Using biomass for energy and other products holds great long-term potential
for GHG mitigation but generally means less carbon is stored than would be under
a pure sequestration strategy. There is a need to consider the rate and character
of carbon flows and their short- and long-term benefits. Read (1996, 1997) suggests
that larger stocks of standing timber can increase short-term carbon sequestration
while building a "buffer stock" of wood fuel for future biofuel systems, when
biofuel market penetration is greater. Short-rotation biofuel production could
follow when biofuel-based infrastructure is in place and the initial rotation
is harvested.
Managing long-rotation forests for timber is also complementary with some biofuel
production. Suppression of fires, insects, and other disturbances may result
in large accumulations of fuel components in the forest. Wildfires that begin
in this litter can damage living trees (Fact Sheet 4.15),
so the long term well-being of the forest, as well as its carbon storage, is
enhanced by thinning and removal of potential fuel materials.
Modeling of forestland use for net CO2 mitigation shows that the merits of
different options depend on current use, potential productivity, how biofuel
is substituted for fossil fuels, and the time horizon. In terms of CO2 benefit
alone, low-productivity mature forests are best conserved as carbon stores;
low-productivity unforested land is best reforested and managed for carbon storage;
and more productive land is best forested and managed for biofuel with modern
conversion technologies, as well as for products that displace fossil fuel-intensive
materials. Longer time horizons tip the balance toward harvesting and replanting
(e.g., Hall et al., 1991; Marland and Marland, 1992; Schlamadinger and
Marland, 1996; Marland and Schlamadinger, 1997).
Strategies that rely only on sequestering carbon eventually find the reservoirs
filled: trees grown to maturity with increased risks of natural disturbances.
This prospect is distant, however, with large potential for sequestration before
saturation (see Fact Sheet 4.20). In addition, market
forces will eventually shift the pattern to biofuel production, with rising
costs as reservoir saturation is approached. Ultimately, net CO2 impacts require
consideration of carbon flows in the energy, forest product, and land-use sectors.
Optimal answers can be explored through dynamic modeling of these sectors jointly
(Read 1998, 1999; Fact Sheet 4.20).
Substitution of biomass products for energy-intensive materials is less researched,
but prospects for CO2 mitigation appear to be promising (Schopfhauser, 1998).
Wood used in place of aluminum, concrete, or steel saves fossil fuel used to
process these materials. Estimates of the gain must allow for production energy
requirements and service lifetimes of alternative products (Marland and Schlamadinger,
1997).
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