We are all aware that trees help purify the air in cities, but which ones are the most effective for this purpose?
According to new research from Sweden, a blend of conifers and deciduous trees is your best bet.
The perfect mix of trees exists
In the study just published in the journal Ecological Indicators (I link it here), a team from the University of Gothenburg led by professor Hakan Pleijel analyzed the leaves of 11 types of trees that grew in the same area of the Gothenburg Botanical Garden.
The group of deciduous trees included aspen, cherry, rowan, beech, oak, walnut, and birch. That of conifers with persistent leaves consisted of firs, black pines and spruces. Then there were the larches, conifers but with deciduous leaves.
Since much of the deadly air pollution in urban areas comes from cars, scientists have focused on polycyclic aromatic hydrocarbons (PAH), generated almost entirely by urban traffic. The team considered 32 of them.
Of all the trees, black pines and spruces absorb gaseous PAHs best with their leaves. And by keeping their leaves year-round, they can trap PAHs in both summer and winter.
In absorbing the particle-bound PAHs, on the other hand, it was precisely the larch that proved to be the most suitable. Next, all the deciduous trees (also due to the fact that they have wider leaves and therefore greater "absorbing" power).
There is a "but" as big as a house, though. Actually two
While the pollutants don't appear to affect trees' ability to photosynthesise, scientists do have two major concerns.
The first: in heavily polluted cities this ability could be compromised. The second: trees that lose leaves and needles could absorb PAHs, but also pollute the soil by accumulating and decomposing as they fall. The third: the team simply planting trees along the roads may not always be the best solution. If a road is too narrow, for example, trees could hold pollutants in a hood, ultimately making matters worse. In those cases, I read in the study, hedges and lower plants are better.
Let's put it this way: this is an excellent study, but it takes time and above all strategic skills to understand, from time to time, how to distribute this "magical mix" of trees in the right way.
And if we really want to reduce pollution, I'm not offended.