Plants suck 123 billion tonnes of CO2 a year

Trees, shrubs and grasses around the world take in 123 billion tonnes of carbon dioxide a year through photosynthesis, an international research team has calculated.

The results published online in Science Xpress on Monday mark the first time researchers have based such a calculation on such a large number of actual measurements instead of mainly computer modelling, said Altaf Arain, an associate professor ofgeography and earth sciencesat McMaster University in Hamilton, Ont.

He was one of 25 co-authors of the paper, based ona studyled by Christian Beer at the Max Planck Institute for Biogeochemistry in Jena, Germany.

The researchers, based in 10 countries, found tropical forests account for the largest portion of worldwide photosynthesis 34 per cent. Savannahs account for 26 per cent, even though they cover twice as much land on Earth as tropical forests do.

As the basis for their calculations, researchers measured changes in carbon dioxide and water levels as well as weather conditions at253 measurementplatformsin different environments around the world between 1998 and 2006.Many are placed high above forest canopies.

Theyincluded a network of platforms in Canada, known as the Canadian Carbon Program,where measurements were taken10 to 20 times per second, 24 hours a day, all year.

"This is a huge amount of data," said Arain, director of the McMaster Centre for Climate Change. Hewas in charge of the measurement stations at Turkey Point,located at a70-year-old pine forestnear Lake Erie, and also took part in the data analysis and write-up.

During photosynthesis, plants take in carbon dioxide,storing it as energy in the form of sugars.However, they also release large amounts ofcarbon dioxidewhile consuming the sugars as energy for growth and sustaining themselves or when they die and decompose a process known as respiration.

Arain said plant photosynthesis and respirationtogether control a large part of thecarbonexchanged between the land andair. That may partly offset some of the carbon releasedthrough the burning of fossil fuels, estimated to be aroundseven billion tonnes a year. However, plants ultimately give off nearly as much carbon dioxide as they consume.

At Turkey Point, for example, Arain said,photosynthesis took in about 1,400 grams of carbon dioxide per square metre of surface area, while respiration was measured at around 1,250 grams of carbon dioxide per square metre, resulting in a net carbon dioxide intake of just 200 to 300 grams persquare metre.

Rainfall important

The combineddata from around the worldshowed that availability of water, including rainfall, plays a large role in the amount of photosynthesis that plants undergo.

It alsoshowed about 40 per cent of plant ecosystems and 50 per cent of croplands are sensitive to the amount of rainfall, which is expected to change in different areas as a result of climate change. In Canada, the southern boreal and temperate forest and prairie ecosystems are particularly vulnerable, Arain said.

He added that results are relatively close to those predicted by computer models, helping validate them.

Data collected at themeasurement stationswas also used in a second paper,alsopublished Mondayin Science Xpress, about respiration.

Past research has provided conflicting information about how muchrespirationis affectedby temperature in differentecosystems of the world.

However, research led by Miguel Mahecha at the Max Planck Institute for Biogeochemistry found that based on the data from around the world, respiration increases 1.4 times for every 10-degree increase in temperature across various ecosystems.

"Respiration response to the temperature was the same in various regions across the world, including tropics and the temperate forests," Arain said.