Forest trees find a new watery 'sweet spot' when carbon dioxide levels are high, shows study

Trees living in conditions where carbon dioxide (CO₂) levels have been artificially elevated are likely to become more efficient in conserving water.

According to new findings, published by University of Birmingham researchers, the trees under increased CO₂ treatment were able to increase their water use efficiency by increasing their carbon uptake while, simultaneously, conserving water by adjusting the opening and closing of pores on the leaves, called stomata.

In a study published in New Phytologist, the researchers found that, contrary to previous assumptions, these responses were similar across all the tree types studied.

The results offer fresh approaches to modeling and predicting plant behavior under increased CO₂ conditions. It offers a new piece of the puzzle for plant scientists working to build a more complete picture of how our forests will respond to atmospheric conditions expected to be the norm by 2050.

The trade off—important to plants across the globe—is between carbon gain and water loss. This compromise exists as a result of the plant's structure: stomata open and close to allow plant to take in CO₂ for growth, but as the stomata are open, water is able to leave the plant via transpiration. That means the plant must compromise between absorbing the maximum amount of CO₂, while also minimizing water loss.

The researchers analyzed data on long-term elevated CO₂ experiments in trees over the past 20 years. The analysis spanned 16 different sites worldwide and included data from experiments looking at whole trees to smaller branch and leaf data collection.

Taking all the data into account, the researchers found the efficiency of water use in the trees' leaves increased by 85% for a doubling of CO₂, which is the CO₂ increase expected by mid-century compared to the pre-industrial average.

The team then used this data to calculate the 'g1 number', which expresses the water cost of carbon gain for each tree type. They found the g1 number did not change under elevated CO₂, making it a very 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 behavior of stomata is one of these," explained Dr. Anna Gardner, of the Birmingham Institute of Forest Research, who led the study.

"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."

She added, "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."