Interaction of Climate Change and Acid Deposition - Climate Change Impacts on Freshwater Ecosystems - page 172

temperature and S deposition to project future soil and water acidification. He
predicted that future climate change would mean increased DOC and also
increased sea-salt deposition, but that these would have counteracting effects
on acidification of soil and water. Increase in soil solution DOC will result in
transport of base cations from the soil, thus leading to greater soil acidification
but less water acidification.
Several other modelling studies have focussed on the role of DOC. Aherne
et al . (2008a) used MAGIC coupled with INCA-C (Integrated Catchments
model for Carbon), a model that simulates DOC concentrations in runoff from
catchments (Futter et al . 2007) to project future recovery from acidification of
a small tributary stream to Plastic Lake, Ontario. Downscaled climate scenarios
derived from GCMs were used. At this site drought-induced pulses of sulphate
play the dominant role (Fig. 7.12). As INCA-C does not provide information
on soil solution DOC, the MAGIC simulations were run without any future
change in soil DOC. The INCA-C model predicted increases in stream water
DOC as a result of warming but does not thus fully account for the full effect
of increased temperature on DOC and surface water acidity. As Futter et al .
(2008) point out, a full understanding of surface water DOC dynamics requires
incorporation of DOC processes into catchment-scale process-orientated
models, such as MAGIC.
Aherne et al . (2008b) and Posch et al . (2008) expanded this type of scenario
study to cover an entire lake population in Finland. They used the MAGIC
model framework and extensive soil, surface water and deposition datasets for
163 Finnish forested catchments to evaluate the water chemistry response to
several scenarios for acid deposition, climate change and forest harvesting.
Simulations suggested that only the maximum (technically) feasible levels of
reduction in emissions would result in significant recovery of soils and surface
waters and would return water quality close to pre-acidification values in the
studied catchments. The direct influence of climate change (temperature and
runoff) had very little impact on model simulations for the sites, based on current
process descriptions. However, two exploratory simple empirical DOC models
indicated that changes in S deposition or temperature could have a confounding
influence on the recovery of surface waters and that the corresponding increases
in DOC concentrations may offset the recovery in pH due to reductions in S and
N deposition.
The use of forest biomass for energy production has become an important
mitigation strategy to reduce greenhouse gas emissions. To meet this new demand,
future harvesting is expected to shift from stem-only to whole-tree harvesting.
This increased use of forest harvest residues for biofuel production (whole tree
harvest (WTH) scenario) was predicted to have a significant negative influence
on the base cation budgets causing re-acidification of the study catchments
(Fig. 7.15) (Aherne et al . 2008b). Sustainable forestry management policies must
consider the combined impact of air pollution and harvesting practices. Clearly,
there is a need for further emission reductions to mitigate the negative impacts of
WTH, if such a policy is implemented. Additionally, increased fertilizer use, such
as wood ash applications, may also be required to maintain soil nutrient status
and lake water quality in these forested ecosystems.