Trees Become More Efficient at Saving Water as CO2 Levels Increase, Scientists Find

Most of us know the service trees, and other plants, do for almost all living things on the planet by converting carbon dioxide into oxygen. Now, scientists have discovered that when carbon levels have been made artificially higher, trees adjust and ramp up their absorption while increasing their rate of photosynthesis.

The findings of the new study, published by researchers at the UK’s University of Birmingham, show that, not only do trees increase their rate of absorption and photosynthesis, they also become more efficient at conserving water by adjusting the rate of the opening and closing of their stomata — cell structures in the epidermis of their leaves, stems and needles.

“The process of photosynthesis is driven by enzymatic reactions using the enzyme RuBisCO. Increasing the concentration of CO₂ in the air surrounding the plant means that there is more substrate available for the enzyme RuBisCO. More substrate means more enzyme activity and in this case, a higher processing/fixing of carbon,” said leader of the study Dr. Anna Gardner of the Birmingham Institute of Forest Research (BIFoR) in an email to EcoWatch.

The researchers found that the responses were similar for all types of trees they studied.

The study, “Optimal stomatal theory predicts CO₂ responses of stomatal conductance in both gymnosperm and angiosperm trees,” was published in the journal New Phytologist.

The results offer new insights for scientists regarding how forests will react to increased carbon dioxide in the atmosphere at the levels likely to be standard in 2050, a press release from the University of Birmingham said.

Carbon dioxide is the main source of food for plants, so it makes sense that they would want to take in as much as possible. To absorb it, their stomata open. As they do so, water leaves the plant through a process called transpiration. While the plant wants to absorb as much carbon as it can, it also wants to lose as little water as possible. Guard cells regulate the stomatal opening in a delicate balance between carbon absorption and water loss.

“If the stomata are open, they allow CO₂ to enter and if they are closed, CO₂ does not enter. [T]he plant does work to convert and fix the CO₂ into carbon that is then used around the plant,” Gardner told EcoWatch. “Importantly, there is a compromise that occurs when the leaf has its stomata open. That is, opening stomata for CO₂ to enter also means water escapes out into the environment (i.e. transpiration). With an increase in CO₂, there is more CO₂ entering the leaf and being fixed as carbon for each unit of water lost. This is defined as an increase in water-use efficiency.”

Gardner said the research team used an infrared gas analyzer to measure the rate of carbon dioxide absorption and transpiration.

“The instrument measures the uptake of CO₂ and release of water (H₂O) by the leaf using differential infrared absorption spectroscopy. Based on the difference of input and output of two infrared spectra, the system can calculate CO₂ absorption and transpiration,” Gardner told EcoWatch.

The study was conducted at 16 different sites around the world over a span of 20 years and assessed data at the leaf scale, whole tree scale and even forest scale.

Considering all the data, the scientists found that, when carbon dioxide levels were doubled — the same carbon dioxide increase over the pre-industrial average that is predicted by 2050 — the water use efficiency of trees’ leaves went up by 85 percent.

The “g1 number” — or how much water it would cost for the carbon increase for each type of tree — was calculated using the data. The researchers found that the g1 number remained the same when the carbon dioxide levels were increased.

“The g1 number expresses the water cost of carbon gain for any particular species,” Gardner told EcoWatch. “The g1 number is a value that acts as a useful tool to describe tree-leaf responses which we use in climate and vegetation models when predicting or forecasting vegetation responses to environmental change. We have found that under higher CO₂, the trees are getting more carbon gain for each ‘buck’ of water spent.”

The interrelation between trees, carbon storage, water and carbon cost is important for the conservation of resources in years to come.

“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,” Gardner said in the press release.

One way we can help support our environment is to keep in mind that we are all part of interdependent ecosystems and work to strengthen their diversity.

“A key way that we can better support trees and, more generally, ecosystems is by increasing diversity in these ecosystems. Increasing diversity increases the resilience of these ecosystems, important when times are changing fast. For trees and forests, diversity means many things: diverse (i.e. unrelated) individuals of different ages; tree species and diverse communities of other organisms living with the forest,” professor Rob MacKenzie, founding director of BIFoR, told EcoWatch.