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Trees Using Water More Efficiently Due To Increase in Atmospheric Carbon Dioxide

First Posted: Jul 11, 2013 05:04 AM EDT
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A paper published online in the journal Nature Wednesday, examines how a dramatic change in  atmospheric carbon dioxide levels makes trees thriftier with water.

It is a known fact that atmospheric carbon dioxide boosts the growth of plants.  To state that carbon dioxide is an important plant food is not a new argument. Many experts have expressed concerns over the fate of forests in the face of climatic changes taking place because such alterations directly and indirectly affect the growth and productivity of the forests.

The new research suggests that in a direct response to increasing atmospheric carbon dioxide levels , trees are using water efficiently in order to achieve the given amount of growth. 

Earlier studies have long predicted that plants would start using water more efficiently in response to the rise in carbon dioxide levels. 

The study was led by Trevor Keenan and Andrew Richardson of Harvard University.  Dave Hollinger, a plant physiologist, U.S. Forest Service's Northern Research Station is the co author of the study. The study was done in collaboration with scientists from the USDA Forest Service, Ohio State University, Indiana University and the Karlsruhe Institute of Technology in Germany.

The ratio of water loss to carbon gain is the key feature of ecosystem function that is fundamental to the global cycles of water, energy and carbon.

On analyzing the direct, long term measurements of whole ecosystem carbon and water exchange they noticed that over the past two decades there has been a rise in the plants using water efficiently, especially in the temperate and boreal forests of the Northern Hemisphere.

According to Hollinger, "Our analysis suggests that rising atmospheric carbon dioxide is having a direct and unexpectedly strong influence on ecosystem processes and biosphere-atmosphere interactions in temperate and boreal forests."

There are some suggestions for the functioning of the ecosystem as well as services and feedbacks to the climate system, based on how efficiently trees are using water. The implications include improved water availability, improved timber yields, and all this in return can balance the effects of future drought.

On the contrary, the drop in evapotranspiration that is a mix of evaporation and plant transpiration from land to atmosphere leading to greater water use efficiency could trigger increase in air temperatures, lower humidity and a decreased recycling of continental precipitation. This in return causes increased runoff of continental freshwater along with the drought in other regions of the world that depend on the water transpired in other regions.

To test this they worked in 21 different forests sites in the U.S. and measured the uptake of carbon and use of water over 20 years.  They worked on the technique called eddy covariance that depends on instruments mounted on the tall towers that extend over the forest canopy and permit the researchers to calculate the amount of carbon and water entering and exiting our ecosystem.

The towers placed in Harvard Forests are the best resource to study the forests' reaction to atmospheric carbon dioxide levels. Based on the data they calculated the total amount of carbon plants were locking up and the water they were losing, which was reducing drastically. This was not just limited to the research sites but also noticed in other regions as well by examining the long term data.

"We went through every possible hypothesis of what could be going on, and ultimately what we were left with is that the only phenomenon that could cause this type of shift in water-use efficiency is rising atmospheric carbon dioxide," Keenan said.

The results of experiments conducted in U.S. forests showed that the increase in efficiency was six times more when compared to the rise in the carbon dioxide levels.

The researchers state the need for further research prove these findings and a study that will work on data collected from various sites around the world.

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