Global CO2 concentration increase allows trees to become more efficient
Plants remove carbon dioxide from the air through photosynthesis, a process that goes along with the loss of water vapor from leaves. The ratio of water loss to carbon gain, or water-use efficiency, is a key characteristic of ecosystem function that is vital to the cycles of water, carbon and climate.
Professor Gil Bohrer was part of a team of scientists led by Andrew Richardson and Trevor Keenan from Harvard University that analyzed decade-long data of whole-ecosystem carbon and water exchange from many forest sites in the US and around the world. Their results, published in Nature, show a substantial increase in efficiency of water-use in forests of the Northern Hemisphere. They found that the observed increase is most consistent with a strong CO2 fertilization effect.
The team led by Professor Bohrer developed analysis software that determines the aerodynamic roughness of forests from long-term studies of wind in forest sites. This software was used to determine that there was no consistent change to forest structure that could explain the changes in water-use efficiency, while the effects of CO2 concentration increase were significant.
Large scale effects of CO2 fertilization were assumed to be a result of the increasing concentrations of CO2 in the air, but have never been confirmed in such an extensive way in natural ecosystems. CO2 and light are needed for the photosynthesis process that convert them into sugars and other carbohydrates that plants use and store. As the plants are "fertilized" by the increased availability of CO2, it is presumed that it will be easier to produce more sugars. This can either improve plant productivity, or allow plants to increase their efficiency and work less while maintaining constant productivity.
Imagine a car assembly factory that is working overtime to keep up with demand for the cars. If that assembly line would acquire a new robot that is much more efficient at assembling the cars it could use it to produce more cars. Another alternative, which the current study suggests that trees around the world seem to be adopting, is to use the more efficient manufacturing to control costs by reducing overtime while producing the same number of cars it did before the upgrade.
By analyzing decades of observations, the study shows that CO2 concentration increase leads to a partial closure of stomata – small pores on the leaf surface that regulate gas exchange – to maintain a near constant concentration of CO2 inside the leaf. This means that the amount of photosynthesis conducted stays near constant but takes place over a shorter time, while the overall amount of water lost in the process decreases leading to increase in water use efficiency.
The study is published on-line in Nature: Keenan TF, Hollinger DY, Bohrer G, Dragoni D, Munger JW, Schmid HP, Richardson AD. (2013) Forests are using water more efficiently as atmospheric CO2 concentrations rise.
The OSU-led study that presents the analysis method for forest roughness is published on line in Agriculture and Forest Meteorology: Maurer KD, Hardiman BS, Vogel CS, Bohrer G. (2013) Canopy-structure effects on surface roughness parameters: Observations in a Great Lakes mixed-deciduous forest. Agricultural & Forest Meteorology 177:24-34.
Recent New York Times regarding research can be found here http://www.nytimes.com/2013/07/11/science/earth/some-trees-use-less-water-amid-rising-carbon-dioxide-paper-says.html?_r=0
