The twisted effect of trees in the city

Good idea to plant trees in town to fight against pollution. Except that the effect was not proven. It has just been studied in depth: as a result, trees only very slightly reduce the quantity of fine particles. Worse, they can locally increase pollution.

Trees in the city don’t only have positive effects. This is what explains to us today Mathilde Fontez, editor-in-chief of the scientific magazine Epsiloon. Planting trees in the city seems to be a good solution to combat heat and pollution.

franceinfo: Can trees in cities also have negative effects?

Mathilde Fontez: Yes, this is the astonishing result of a study, carried out by a Norwegian team from the Nature Research Institute, led by ecologist Zander Venter. A large-scale experimental study, based on analyzes of more than 2,600 air quality measuring stations, in Europe and the United States, between 2010 and 2019.

The researchers coupled these data with satellite measurements and aerial measurements of the vegetation cover of cities. They were able to monitor the evolution of the main air pollutants: nitrogen dioxide, ozone, fine particles. And Ddistinguish the different causes of decline. And they show that trees, in fact, have only a very small effect on the reduction in pollution, almost zero: only 0.8% of the reductions in pollutants on average over 10 years are due to trees.

Is there a reduction in pollution?

Yes, pollutants have indeed decreased, but this is the result of policies to reduce emissions, rather than trees. For comparison, the weather, for example, has much more of an impact than urban vegetation on pollution – humidity in the air plays a role 10 times more.

Ecologists have even found that trees can increase pollution. Not globally, but on a small scale, on the scale of a street, they increase pollutants by 20 to 96%. It’s logical when you think about it: they limit air flows, they trap particles at ground level which, without them, would be dispersed in the atmosphere…

Many policies are based on revegetation to fight against pollution?

Yes, urban forests seemed like a good idea, a simple, natural idea to fight pollution, but no. So you don’t have to throw the tree away. It remains very effective in reducing heat, absorbing water from heavy rain. Not to mention its effects on our daily well-being.

But you shouldn’t rely on them to filter the air. And we undoubtedly need to be more strategic in the greening of cities: plants lower than trees would be more effective, according to Zander Venter and his team. The ideal would be dense, continuous hedges, in the middle of the streets, and not on the edges – not sure that that fits with the architecture of our cities.

By paying attention to pollens and volatile compounds emitted by plants. The study showed that in Los Angeles, these volatile compounds contribute a quarter of secondary particles on hot days.