New studies show large forest areas alone may not be able to counter the effects of increased carbon dioxide in the atmosphere.
IT IS WIDELY believed that higher
carbon dioxide levels in the atmosphere will stimulate plant growth
and thus help crops and forests fix
more carbon - a positive feedback
effort that may partially help control
the global warming problem. But two
recent studies indicate'forests may
not be as efficient in filtering out carbon dioxide as many people
think. Researchers at
the University of Basel
in Switzerland have
found that elevated carbon dioxide levels in
the atmosphere only
lead to a greater "carbon turnover" and not
to greater "carbon
sequestering" by terrestrial ecosystems like
forests (Science, Vol
257, No 5077).
Christian Kbrner
and John.Arnone III
constructed four model
humid tropical ecosystems in polyethylenecovered houses and exposed two to
higher levels of carbon dioxide and
two to ambient conditions. Both
treatments resulted in vigorous plant
growth, but exposure to elevated carbon dioxide levels did not increase
plant growth any more than exposure
to ambient levels.
The absorption of carbon dioxide
by the canopy increased in the elevated carbon dioxide experiments,
but the scientists found that this was
offset by the subsequent increase in
carbon dioxide lost frorn'the soil. The
elevated carbon dioxide did not lead
to increased plant growth and carbon
sequestering from the atmosphere as
compared to ecosystems exposed to
ambient carbn dioxide levels, but led
only to more rapid carbon turnover.
The scientists warn that carbon
dioxide fertilisation could promote
osses of soil carbon and the release
'of soil nutrients over a period of
time. The deleterious levels of starch
produced in leaves at the top of
canopies under elevated carbon dioxide levels may also cause changes in
the structure of plant communities.
Another recent analysis done by
Titus D Bekkering of the Netherlands
shows national forestry action plans
introduced in some countries will
fail to counter the effect of greenhouse gases if deforestation is
not stopped first (Ambio, Vol 21,
No 6). Only a few of the forestry
action plans of 13 countries,
including Argentina,
Bolivia, Tanzania and
Indonesia, implemented
under the Tropical
Forestry Action Plan
(TFAP) have formulated
strategies to halt deforestation.
The study found that
even if reforestation
efforts in 11 of the 13
countries are stepped
up, the total area reforested -annually would be
much smaller than what
is needed to have a global impact on carbon
dioxide levels in the
atmosphere. New plantings would fix about
12.4 million tonnes of
carbon annually, compared to more
than 282 million tonnes of carbon
released every year by the 11 countries.
Bekkering, currently advisor to a
social forestry project in northern
Pakistan, doubts that the huge costs
involved are justifiable simply for the
sake of fixing carbon. He says,
"Forestry alone will not solve the
greenhouse effect."
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