Trees efficient at conserving water when CO2 is high

The response was similar across 17 tree species studied

A new study by a team of researchers at the University of Birmingham have found that trees which are subjected to elevated levels of CO₂ were able to increase their water use efficiency by increasing their carbon uptake and simultaneously conserving water by adjusting the opening and closing of stomata.  Interestingly, they noticed that these responses were similar across 17 tree species that were studied.  These findings offer an insight into how forests could respond to atmospheric conditions that are expected to occur by the mid-century.

The compromise between carbon gain and water loss exists as a result of the structure of the plant. For example, as the stomata open and close, CO₂ is taken in and used for growth, but when the stomata are open, water is able to leave the plant via transpiration. The plant must therefore make a trade-off between absorbing the maximum amount of CO_2, while also minimising water loss.

The team analysed data on long-term elevated CO₂ experiments in trees over the last 20 years.  The analysis spanned 16 different sites worldwide and included data from experiments that studied the whole tree, to smaller branch and leaf data collection. 

The team found that the efficiency of water use in the trees’ leaves increased by 85% for a doubling of CO₂ which is equivalent to the level of CO₂ increase that is expected to occur by 2050 when compared with the pre-industrial average. The data was then used by the team to calculate the ‘g1 number’, which relates to the water cost of carbon gain for each tree type.  Under elevated CO₂ they found that the g1 number did not change.  This presented the team with a useful tool to describe tree-leaf responses in circumstances beyond those measured directly.

"The models we use to predict the responses of trees to future atmospheric CO₂ levels still contain lots of uncertainties and the behaviour of stomata is one of these," explained study co-author Dr Anna Gardner, from the Birmingham Institute of Forest Research. "In elevated CO₂ we might expect water consumption to reduce because stomata are taking in CO₂ at a higher concentration so do not have to be open as long. But actually, we found the increased CO₂ also caused an increase in photosynthesis and this behaviour was a stronger driver to increase water use efficiency. In effect, under higher CO₂, the trees are getting more carbon gain for each 'buck' of water spent."

"One reason trees are so important to ecosystems is because they store carbon, but water is also a valuable resource, so we need to find ways to accurately calculate the carbon cost of that water. All this data helps us build a more accurate picture of the likely behaviour of these resources in the future."

Looking for further information on this topic? Subscribers to the Forest Science database can access over 650 records by using the search string stomata AND "water use efficiency" AND "carbon dioxide" AND "plant water relations"

Journal Reference:

Anna Gardner, Mingkai Jiang, David S. Ellsworth, A. Robert MacKenzie, Jeremy Pritchard, Martin Karl‐Friedrich Bader, Craig V. M. Barton, Carl Bernacchi, Carlo Calfapietra, Kristine Y. Crous, Mirindi Eric Dusenge, Teresa E. Gimeno, Marianne Hall, Shubhangi Lamba, Sebastian Leuzinger, Johan Uddling, Jeffrey Warren, Göran Wallin, Belinda E. Medlyn. Optimal stomatal theory predicts CO 2 responses of stomatal conductance in both gymnosperm and angiosperm trees. New Phytologist, 2022; 237 (4): 1229 DOI: 10.1111/nph.18618