Measurements were taken from 253 flux towers around the world, including this one high above the tropical forest in Ghana. (Science/AAAS)

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 of geography and earth sciences at McMaster University in Hamilton, Ont.

He was one of 25 co-authors of the paper, based on a study led 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 at 253 measurement platforms in different environments around the world between 1998 and 2006. Many are placed high above forest canopies.

They included a network of platforms in Canada, known as the Canadian Carbon Program, where measurements were taken 10 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. He was in charge of the measurement stations at Turkey Point, located at a 70-year-old pine forest near 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 of carbon dioxide while 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 respiration together control a large part of the carbon exchanged between the land and air. That may partly offset some of the carbon released through the burning of fossil fuels, estimated to be around seven 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 per square metre.

Rainfall important

The combined data from around the world showed that availability of water, including rainfall, plays a large role in the amount of photosynthesis that plants undergo.

It also showed 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 the measurement stations was also used in a second paper, also published Monday in Science Xpress, about respiration.

Past research has provided conflicting information about how much respiration is affected by temperature in different ecosystems 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.