Deep-rooted plants have much greater impact 

on climate than experts thought

By Robert Sanders, Media Relations

BERKELEY - 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.

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