4 Species to Avoid and 3 Species to Plant It’s well known that trees remove pollutants while purifying and humidifying the air. While this is true in many cases, it turns out that there's another side to the story. And, like so many stories, this one came from Hollywood.

Starting in the early 1940s a foul smelling, throat-burning, plant-killing fog would roll into Los Angeles without warning and for no apparent reason. Residents were understandably upset. It ruined the magnificent Los Angeles weather. It took many years to solve the puzzle of why this apparent mixture of smoke and fog, which became known as smog, formed in this area. And when the mechanism was finally revealed it involved… believe it or not… trees!

Trees produce isoprene which seems to protect the leaves against damage at high temperatures. As the air temperature goes up, trees produce more isoprene and much of it escapes through the stomata on leaves and wafts into the atmosphere. During beautiful hot, sunny southern California weather large amounts of isoprene were being injected into the air by trees. Isoprene is totally natural and there is no environmental threat when trees emit it in natural settings. However, when isoprene is chemically altered through exposure to solar radiation it can then react with nitrogen oxides in vehicle exhaust to create smog. And in the 1940s with cars becoming more common, the Los Angeles basin with its low winds had all the essential ingredients for smog formation.

In suburban and rural areas the amount of nitrogen oxide is typically very small and the potential for smog production is also very small. So the species of trees growing in these locations do not need to be low isoprene producers. But in high-urban areas with large volumes of vehicular traffic and reduced wind speeds it might be wise to avoid trees that produce a lot of isoprene.

So how can we know which trees to plant and which to avoid? You can refer to a long list of trees and their ‘ozone-formation potential’ (OFP) in Landscape Architecture Graphic Standards, in an article written by Terry Gillespie and me. But here’s a shorthand version of the list:

Four genera with high potential to produce smog: Carya, Populus, Salix, and Quercus.

Three genera with low potential to produce smog: Acer, Fraxinus, and Tilia.

While it would be incorrect to say that trees actually cause smog, it certainly is correct to say that some trees contribute to the level of ozone smog in urban areas. We hope that trees will live for many decades, and while we don’t know what the level of nitrogen oxides will be in the future, it’s probably wise to use low OFP trees in high-urban areas.

Hopper, Leonard J. 2006. Landscape Architecture Graphic Standards. John Wiley & Sons, New York.