Page 142 - Materials Chemistry, Second Edition

Climate change responses: carbon offsets, biofuels and the life cycle assessment contribution

subsequent replanting, there is an abrupt adjustment to the balance, and forests become net 129
emitters of atmospheric carbon for up to 20 or even 40 years (Marland and Schlamadinger
1997; Reijnders and Huijbregts 2003). Trees only become carbon ‘sinks’ when they are well-
established. There are several sources of carbon emissions after harvesting. The roots and the
trunk parts that remain after cutting can range from 15% to 40% of the total tree biomass,
while tops, branches and bark residues typically account for another 15% to 20% of the biomass
(Fraanje and Lafleur 1994; Reijnders and Huijbregts 2003). Therefore, 30% to 60% of total
biomass can remain in the forest, where it degrades to form carbon-containing gases, the exact
amount depending on harvesting practices such as the removal of branches. In addition,
carbon contained in the soil is released due to soil disturbance associated with harvesting (Rei-
jnders and Huijbregts 2003).
Forests do, of course, absorb atmospheric carbon and store it in solid wood, vegetation,
litter and soil (Oneil et al. 2007). Carbon accumulation is typically linear in time during the
initial phases of growth until half the maximum carbon sequestration has occurred, at which
time accumulation slows and progresses asymptotically (without ever being completed)
(Marland and Schlamadinger 1997). Different species of trees, as well as localised soil and
climate conditions, also affect growth rates. Therefore, the age of a forest, its tree species and
localised conditions will all have impacts on total carbon sequestration rates.
Land use prior to and following a forestry plantation can be expected to have a major role
in net sequestration, as different land uses result in different soil carbon levels. If the previous
land use is long-term forestry, the carbon level in the soil will be similar after afforestation.
However, if the previous land use is agriculture, forestry will typically be expected to lead to a
net increase in soil carbon (Reijnders and Huijbregts 2003). Similarly, where post-harvest land
use introduces a change to agricultural land, the amount of carbon sequestered in the soil is
likely to fall due to soil disturbances and adjustment to a new lower carbon soil regime (Rei-
jnders and Huijbregts 2003). Moreover, as already mentioned, levels of other more potent
greenhouse gases such as nitrous oxide can be important and should be included in calcula-
tions. Moisture content can locally determine nitrous oxide emissions, which can vary between
fields as well as between regions (e.g. Beer et al. 2005; see also Chapter 9). This further compli-
cates the problem of identifying the actual ‘savings’, and accounting systems allow for aggre-
gate calculations of these based on standard conditions.
As indicated above, the fate of the harvested biomass also needs to be taken into account.
Durable forestry products such as building materials store embodied carbon over their func-
tional lifetime. At the end of their life, however, some of the embodied carbon is released,
although rates of release depend on how products are disposed. Burning and aerobic compost-
ing will release carbon rapidly, while burial in landfill sequesters more carbon for longer and
leads to conversion of the remaining biomass to carbon dioxide and methane. Shorter term
products such as paper will typically be burned (emitting carbon dioxide) or buried in landfill
where they will decompose more quickly and more thoroughly than durable forestry products
in landfill – emitting carbon dioxide and methane. Depending on the landfill technology,
more than half the methane will be captured for power generation or flared to reduce safety
risk and greenhouse potential.
Clearly, time – including forest life span and age – also affects total carbon sequestration
potential. However, this needs qualification as, again, there are complicating variables. Older
forests in Australia generally present increased risk of wildfire from a combination of natural
mortality and expected increases in temperature due to global climate change (Gedalof et al.
2005; Oneil et al. 2007). Climate change can also be expected to increase the risk of insect and
disease outbreaks in forests, increasing mortality and hence the risk of wildfire (Oneil et al.
2007). Wildfires release stored carbon, turning forests from carbon sinks into carbon sources
(Westerling et al. 2006; Oneil et al. 2007).

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