Has water use efficiency improved in fortes with rising carbon dioxide concentrations?

Published: Tuesday July 23 2013, 15:00:50

Carbon dioxide (CO2) is a vital element for all photosynthetic plants as it is the building block for their growth. Plants take up CO2 via opening in their leaves but at the same time water is lost to the atmosphere (transpiration). Water is another vital element for plants and they want to restrict this loss. So plants need to achieve a balance of taking in enough CO2 for growth while at the same time limiting the loss of water vapour. The more efficient the uptake of CO2 the better this is for photosynthesis. One argument that has been put forward is that under higher concentrations of CO2 plants may be more efficient in their water use since less is likely to be lost when CO2 is taken in.

A resent paper in Nature investigates whether water use efficiency has improved in forests, by conducting a cross-data analysis. The authors started with the premise that increasing CO2 levels should lead to increases in photosynthesis and water use efficiency. This process is often referred to as ‘the CO2 fertilization effect’. There is a large interest in understanding whether (or how) this process occurs as it has implications on carbon uptake and storage by terrestrial plants. Previously there have been experiments to reconstruct higher CO2 exposures to plants and computing models have also been used to explore whether plants are more efficient as CO2 concentrations rise. However the evidence from these has often been inconclusive or provides an inconsistent image.

The work described here uses direct and continuous long term data of CO2 measurements and water vapour fluxes. Study sites were in seven locations in the Midwestern and north-eastern USA; that are not subject to strong management practices. Analysis of this region has shown that water use efficiency did increase at each site over the measurement period. The authors also tested for other factors that might influence this observed increase in water use efficiency, these include: nitrogen deposition, changes in leaf area, height of the canopy, climate change etc. these competing strands were examined and found that over the observation period the CO2 was the only driver that was changing consistently and strongly enough to explain the changes in water use efficiency.

The observed water use efficiency described in this paper is important for improving our understanding of ecosystem functioning as well as feedbacks to the climate system. For example on timber yields where other studies have indicated yields could increase with rising CO2 concentrations. Additionally more efficient use of water means could influence the water cycle by increasing freshwater runoff on land surfaces. Further work is necessary to better understand the interactions with the climate and water systems, but this work represents a positive step in the discussion.

Additional information:

The paper, “ Increase in forest water-use efficiency as atmospheric carbon dioxide concentrations rise” by Trevor F. Keenan, David Y. Hollinger, Gil Bohrer, Danilo Dragoni, J. William Munger, Hans Peter Schmid & Andrew D. Richardson1is published in Nature (2013), doi: 10.1038/nature12291