June
13, 2007
HUMAN ACTIVITIES
INCREASING CARBON SEQUESTRATION IN FORESTS
Human-caused
nitrogen deposition
has been indirectly “fertilizing” forests,
increasing their growth and
sequestering major amounts of carbon, a new study in the journal Nature
suggests.
The
findings create a more complex
view of the carbon cycle in forests, where it was already known that
logging or
other stand-replacement events – whether natural or not
– create periods of
5-20 years when there is a net release of carbon dioxide from forests
to the
atmosphere, instead of sequestration as they do later on.
The
end result is a highly
variable forest carbon cycle that appears to be heavily influenced by
the
footprint of humans, one way or another. It’s a complicated
process with
powerful driving forces that were poorly understood, said scientists
from 10
institutions in the U.S., Canada and Europe.
Until
this report, researchers had
never quantified the effect of continuous low levels of nitrogen
deposition –
about 5-10 percent of the amount used by a farmer each year - to spur
net
carbon uptake by forests and actually offset a significant amount of
greenhouse
gases into the atmosphere.
This
broad study analyzed the
carbon balance across a network of forest sites that represent nitrogen
deposition in most of Western Europe and the continental United States. Until now, it has been difficult to
separate the effects
of nitrogen deposition on forests from the many other variables that
affect
their carbon release or sequestration – things like forest
age, logging,
wildfires, disease or insect epidemics, or other causes. This study
attempted
to do that, and found that the net carbon sequestration by temperate
and boreal
forests was overwhelmingly determined by nitrogen inputs.
“What
is surprising is that the
net sequestration is quite large for a relatively low level of nitrogen
addition,” said Beverly Law, a professor of forest science at
Oregon State University, co-author of the study and director
of the AmeriFlux
monitoring network in North and South America.
“Through
our forests,
fertilization by nitrogen deposition is to some degree offsetting our
carbon
dioxide emissions – at least right now,” she said.
It
was first recognized in the
1980s that human activities, by releasing unprecedented amounts of
active
nitrogen into the atmosphere, were not just altering the global
nitrogen cycle
but also causing the eutrophication of large parts of the biosphere,
the
researchers said in their report. Nitrogen – produced by
automobile engines,
factories, and intensive agriculture – is often a key,
limiting nutrient in
forests and other ecosystems.
Early
forest growth puts a severe
nitrogen stress on the ecosystem initially, and then the forest
continues to
grow and remove carbon from the atmosphere for the rest of the
management or
life cycle, accumulating wood at a high rate on the small additional
nitrogen
inputs.
This
growth and sequestration is
achieved without applications of fertilizer that would likely result in
nitrous
oxide emissions, another greenhouse gas, that would offset the benefits
to the
atmosphere of carbon removal.
However,
it’s known that large
additions of nitrogen to ecosystems can also be damaging above a
certain
threshold, researchers say, and it’s unclear how long this
process will
continue.
“The
results demonstrate that
mankind is ultimately controlling the carbon balance of temperate and
boreal
forests, either directly through forest management or indirectly
through
nitrogen deposition,” the study authors said.
Ultimately,
mature forests, at
least in northern latitudes, absorb and sequester substantial amounts
of carbon
from the atmosphere. Forest protection and management options
have been viewed as one
mechanism to absorb carbon dioxide from the atmosphere and reduce
concerns
about the greenhouse effect and global warming.
##
Contact:
Beverly Law
bev.law@oregonstate.edu
541-737-6111
Oregon State University
This text
derived from:
http://oregonstate.edu/dept/ncs/newsarch/2007/Jun07/carbonsink.html
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