Trees are one of the planet’s most powerful carbon sinks. But if global temperatures continue to rise, some trees could actually contribute to poor air quality.

Such is the conclusion of a recent study out of Michigan State University in the United States, published in the journal “Proceedings of the National Academy of Science”. This is because certain types of trees emit isoprene, the second-most common hydrocarbon released into the atmosphere after methane.

Isoprene is naturally present in plants to help them resist insect pests and high temperatures. But when produced in overly large quantities, it can come into contact with nitrogen oxides from pollution generated by car engines and coal-fired power plants, thereby worsening air pollution.

“These reactions create ozone, aerosols, and other byproducts that are unhealthy for both humans and plants,” the authors said.

The researchers studied the reactivity of isoprene to CO2 as a function of light and temperature. Their goal was to gain a better understanding of the biomolecular processes used by plants to produce isoprene, and in particular to assess how these processes are affected by climate change.

The scientists based their work on the observation that increasing atmospheric CO2 decreases the rate of isoprene production, while rising temperatures accelerate it.

Using poplar seedlings, the team observed a 42% reduction in isoprene emission when the leaves were exposed to a high concentration of CO2. But when the temperature reached 35°C, there was virtually no further suppression of CO2.

“Isoprene is pouring out like crazy,” co-author Tom Sharkey wrote, adding: “With that, we can say the temperature effect trumps the CO2 effect.”

The researchers also found that when a leaf experienced warming of 10°C, its isoprene emission increased more than tenfold. They further pointed out that plants’ increase in isoprene emission can be observed both in greenhouses and in the wild.

The authors hope this new data will help scientists better anticipate the quantity of natural hydrocarbons emitted by plants in the future, for example by planting fewer oaks or poplars, and to avoid felling trees that already emit large amounts of isoprene.