Trees respond to rising CO2 levels by conserving water

A new study led by the University of Birmingham has found that trees living in conditions where carbon dioxide has been artificially elevated are likely to become more efficient in conserving water. The experts discovered that when trees are exposed to elevated levels of CO2, they increase their water use efficiency through higher carbon uptake. At the same time, the trees conserve water by adjusting the opening and closing of pores on their leaves (called “stomata”). 

The findings will contribute to modeling and predicting plant behavior under the increased CO2 conditions that are expected to be the norm by mid-century.

Plants worldwide are characterized by a trade-off between carbon gain and water loss, as a result of their structure. While stomata open and close to allow plants to absorb CO2 for their growth, as the stomata open, water is leaving the plant through a process called “transpiration.” This means that plants must compromise between absorbing as much CO2 as possible while minimizing water loss.

The scientists analyzed data on long-term elevated CO2 experiments in trees over the past two decades, spanning 16 sites worldwide. The investigation revealed that the efficiency of water use in the trees’ leaves increased by 85 percent for a doubling of CO2 (the expected rise by 2050). Then, they used this data to compute the so-called “g1 number,” which expresses the water cost of carbon gain for each tree type, and discovered that this number did not in fact change under elevated CO2 conditions.

“The models we use to predict the responses of trees to future atmospheric CO2 levels still contain lots of uncertainties – and the behavior of stomata is one of these,” explained study lead author Anna Gardner, an expert in Tropical Tree Ecophysiology at Birmingham.

“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 behavior 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 behavior of these resources in the future,” she concluded.

The study is published in the journal New Phytologist.

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By Andrei Ionescu, Earth.com Staff Writer

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