Study shows why trees won't benefit much from extra CO2 in the air
The amount of carbon dioxide in the atmosphere is increasing steadily, having recently reached the highest concentration in human history. While that’s undoubtedly bad news for the planet, one argued silver lining is that plants are better off due to more of their food being in the air. But a new study has dashed those hopes, finding that the more extreme heat and drought brought on by climate change would cancel out most of the benefits for trees.
Carbon dioxide is the main nutrient for plants, who pluck it out of the atmosphere and, with the help of sunlight, convert it into chemical energy so they can grow. So it follows that more CO2 would be a good thing for our photosynthesizing friends, right?
To an extent that’s true, but things are more complex than that. More CO2 means a warmer and drier planet, and these factors would also have an effect on plants. And just how much of an effect was the focus of a new study, conducted by researchers at Karlsruhe Institute of Technology (KIT), Ludwig-Maximilians-Universität München, the University of Vienna, and Weizmann Institute of Science.
The researchers grew a series of Aleppo pines from seeds under two different concentrations of CO2 – 421 parts per million (ppm), which is a little higher than the current atmospheric level, and an elevated level of 867 ppm.
When the trees were 18 months old, the team began testing them. For the first month, they watered one group well, while leaving others without, to simulate drought conditions. Then, they planted both groups in chambers where they could control the temperature. Over 10 days, the heat was gradually increased from a pleasant 25 °C (77 °F) to a sweltering 40 °C (104 °F), while the scientists measured the trees’ responses.
The team found that higher CO2 levels did help the trees use their water more efficiently, and lose less of it, as the heat rose. That was largely thanks to root proteins becoming more stable, and the trees closing their stomata – the pores in leaves that allow for gas exchange.
But that’s where the benefits end, unfortunately. Closed stomata meant the stressed-out trees took up significantly less carbon from the air, and the heat had a negative effect on their metabolism as well.
“Overall, the impact of the increased CO2 concentration on stress reactions of the trees was rather moderate,” says Nadine Rühr, lead author of the study. “With increasing heat and drought, it decreased considerably. From this, we conclude that the increasing CO2 concentration of the atmosphere cannot compensate the stress of the trees resulting from extreme climate conditions.”
Of course, this is just one brief study on one species of tree, so there’s no assurance that the results apply to all other species. But it does feed into what we already know about tree biology, and contributes to the complex picture of how plants are responding to climate change, and what effects that will have.
Other recent studies have found that more CO2 is making trees grow faster and die younger, so they return their captured carbon to the atmosphere earlier than usual. And even when more are growing, the types of trees and the environments they’re growing in are also factors. For instance, another study found that “woody” plants are on the rise, covering more of the world’s savannas and tundras, but these changing biomes could have unexpected negative consequences.
The new study was published in the journal New Phytologist.