The constantly increasing number of cars and, consequently, growing air pollution are a major problem for the environment. The task of protecting the air is not made any easier by the widespread modern trend of urban concreteosis. After all, it is the trees that capture pollution from the atmosphere.

Numbered in billions

There are approximately 6.5 billion trees in Poland and over 3 trillion in the world. They enrich and beautify the landscape, but unfortunately, they are disappearing more and more from urban spaces. A 60-year-old pine tree produces enough oxygen to support the life of three people, while a hectare of forest produces up to 700 kg of oxygen in 24 hours. Trees also release phytoncides, which fight micro-organisms circulating in the air.

It is commonly believed that coniferous trees are better at purifying the air, but that is not always the case. Some deciduous species often have a higher capacity to capture particulate pollution. This is because, in their case, the surface area that captures contaminated particles is greater than the surface area of thin needles.

The results of research by scientists from the University of Gothenburg shed new light on this issue. The research indicates that the most effective air filter may be a combination of trees because not every tree species is equally good at handling the same type of pollution.

The researchers analyzed leaves and needles

The university team, led by Prof. Håkan Pleijel, analyzed the leaves and needles of 11 species of trees growing in the same area of the botanical garden in Gothenburg. Deciduous forests included aspens, beeches, cherry trees, rowans, oaks, birches, and walnut trees, whereas coniferous trees included larches, spruces, fir trees, and black pines.

As a large portion of air pollution in cities comes from exhaust gases released by cars, the researchers focused on polycyclic aromatic hydrocarbons (PAHs). They considered 32 PAHs, some of which were bound with particles, while others were in gaseous form. It was found that PAHs were best absorbed by the needles of fir trees and black pines. Unlike deciduous trees, coniferous trees keep their needles throughout the year, which is why they can capture polycyclic aromatic hydrocarbons for the entire 12 months. The best species for capturing solid particles was the larch.

Pollution and photosynthesis capacity

The researchers observed that pollution most likely does not affect the capacity of photosynthesis, but they do not rule out the possibility that pollution levels in larger cities may affect this process. Another issue is the falling of leaves or needles saturated with PAHs. Contaminated matter can also accumulate in the soil until the decomposition of the deciduous-coniferous mixture. Small leaves from lindens, birches, or fruit trees take approximately 12 months to decompose, while large leaves from chestnut trees, beech trees, or oaks, with a high content of tannins, take twice as long.

The team also suggests that simply planting many trees along the streets is not always the best solution. If the street is too narrow, low-hanging trees may restrict air flow, keeping the pollution cloud close to the road. To avoid this, it may be more efficient in such surroundings to use lower trees, similar to hedges.

Miłosz Magrzyk