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Deep-Rooted Trees Contribute To The Earth's Climate Much
More Than Scientists ThoughtMain Category: Biology
/ Biochemistry News
Article Date: 13 Jan 2006 - 8:00am
(PDT)
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Trees, particularly those with
deep roots, contribute to the Earth's climate much more than
scientists thought, according to a new study by biologists and
climatologists from the University of California, Berkeley.
While scientists studying global climate change
recognize the importance of vegetation in removing carbon
dioxide from the atmosphere and in local cooling through
transpiration, they have assumed a simple model of plants
sucking water out of the soil and spewing water vapor into the
atmosphere.
The new study in the Amazonian forest
shows that trees use water in a much more complex way: The tap
roots transfer rainwater from the surface to reservoirs deep
underground and redistribute water upwards after the rains to
keep the top layers moist, thereby accentuating both carbon
uptake and localized atmospheric cooling during dry periods.
The researchers estimate this effect increases
photosynthesis and the evaporation of water from plants,
called transpiration, by 40 percent in the dry season, when
photosynthesis otherwise would be limited.
"This
shifting of water by roots has a physiological effect on the
plants, letting them pull more carbon dioxide from the
atmosphere as they conduct more photosynthesis," said
co-author Todd Dawson, professor of integrative biology at UC
Berkeley. "Because this has not been considered until now,
people have likely underestimated the amount of carbon taken
up by the Amazon and underestimated the impact of Amazonian
deforestation on climate."
As the largest forested
area on the planet, the Amazon plays a major role in removing
carbon dioxide from the atmosphere and thus impacts the
climate globally, according to lead author Jung-Eun Lee, a
former UC Berkeley graduate student and now a post-doctoral
fellow here.
Dawson, Lee and their colleagues,
including Inez Fung of UC Berkeley, reported their findings
last month in the Dec. 6 issue of the Proceedings of the
National Academy of Sciences. Fung is director of the Berkeley
Atmospheric Sciences Center, co-director of the new Berkeley
Institute of the Environment, and professor of earth and
planetary science and of environmental science, policy and
management.
The researchers incorporated these new
details into the most widely accepted model of global climate,
and found that it accounts for a previously observed but
unexplained dip in Amazonian temperature during the dry
season.
"Evapotranspiration stays higher than
previously expected during the prolonged dry season because of
this private reserve of water banked during the wet season by
the tap roots," said Dawson. "Just as perspiration cools us
off, increased transpiration by trees in June and July
explains the drop in temperature in the Amazon."
This
effect changes the way the atmosphere heats and cools, and
will change the way rain is distributed, he noted. Depending
on the extent to which trees elsewhere in the world,
especially in Africa and other tropical and extratropical
areas, redistribute water in the soil, the impact on global
climate could be significant.
"The impact on
transpiration is greatest in the Amazon and Congo forests, but
our model also shows an impact in the United States and other
places that have dry and wet periods," Lee said.
Trees
have long been known to lift water from the soil to great
heights using a principle called hydraulic lift, with energy
supplied by evaporation of water from leaf openings called
stomata. Twenty years ago, however, some small plants were
found to do more than lift water from the soil to the leaves -
they also lifted deep water with their tap root and deposited
it in shallow soil for use at a later time, and reversed the
process during the rainy season to push water into storage
deep underground. Dawson discovered in 1990 that trees do
this, too, and to date, so-called hydraulic redistribution has
been found in some 60 separate deeply rooted plant species.
Earlier this year, Dawson's colleague and former UC
Berkeley doctoral student Rafael Oliveira of the Laboratório
de Ecologia Isotópica at the University of Sao Paulo, Brazil,
discovered that Amazonian trees also use hydraulic
redistribution to maintain the moisture around their shallow
roots during the long dry season. During the wet season, these
plants can store as much as 10 percent of the annual
precipitation as deep as 13 meters (43 feet) underground, to
be tapped during the dry months.
"These trees are
using their root system to redistribute water into different
soil compartments," Dawson said. "This allows the trees and
the forest to sustain water use throughout the dry season."
The process is a passive one, he noted, driven by
chemical potential gradients, with tree roots acting like
pipes to allow water to shift around much faster than it could
otherwise percolate through the soil. In many plants that
exhibit hydraulic redistribution, the tap roots are like the
part of an iceberg below water. In some cases these roots can
reach down more than 100 times the height of the plant above
ground. Such deep roots make sense if their purpose is to
redistribute water during the dry season for use by the
plant's shallow roots, though Dawson suspects that the real
reason for keeping the surface soil moist is to make it easier
for the plant to take in nutrients.
"Hydraulic
redistribution is definitely related to water, but it can't
really be discussed outside the context of plant nutrition,"
he said.
Dawson, Lee and Fung set out to incorporate
hydraulic distribution in the National Center for Atmospheric
Research Community Atmospheric Model Version 2 (NCAR's CAM2
model), one of the most respected models.
"Global
climate models don't do a very good job of capturing plant
effects on how climate might behave," Lee said.
Lee
accounted both for daily and seasonal dryness in the Amazon,
and showed that the two together have a large impact on the
climate over the region. The increased moisture in the soil
created by hydraulic redistribution during the dry season
allows the plant to carry on photosynthesis at a higher rate,
leading to greater carbon uptake. This also leads to greater
evaporation from the leaves of water, which takes heat with
it. Thus, the summer dry-season temperatures are cooler than
would be expected.
"When Jung-Eun incorporated this
into the global climate model, we were better able to explain
our observations and may be able to even predict future
climate behavior," Dawson said.
Because these plants
store water in the rainy season for use in the dry season,
decreased precipitation during the wet season, as occurred in
recent El Nino years, would be expected to lead to decreased
photosynthesis during the following dry season, according to
the researchers.
"There's this skin on the Earth -
plants - that has an effect on a global scale, pulling carbon
dioxide out of the atmosphere and letting water go, in a
dynamic way that has climatic implications," Dawson said.
Dawson and Fung plan to continue their collaboration
to improve the way that plants are represented in global
climate models.
The work was supported by the National
Science Foundation and the National Aeronautics and Space
Administration. Field research in Brazil was supported by the
Seca Floresta-FLONA-Tapajos program.
Robert
Sanders
rsanders@berkeley.edu
University of California -
Berkeley
http://www.berkeley.edu/
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