Not All Tree Planting Programs Are Great for the Environment

Some tree species can lead to more ozone production than others, especially in urban areas.

A crew plants trees above Houston's Buffalo
Bayou river in April 2014 (Pat Sullivan/AP Photo) 

Countless cities have launched ambitious tree planting projects in the name of improving the environment—to suck up carbon dioxide and storm water, to provide cool shade on sunny days. But many trees also produce compounds that contribute ozone to the local atmosphere. That’s why communities considering planting large numbers of new trees in the next decade should pay special attention to exactly what they’re growing.

Scientists have long known that trees emit volatile organic compounds (VOCs). In the presence of sunlight, VOCs combine with NOₓ (mono-nitrogen oxides commonly produced by vehicles and power plants) to form ozone. The diagram below illustrates this process.

VOCs combine with NOₓ in the presence of sunlight to produce ozone, some of which does diffuse back into the tree’s leaves. (Diagram courtesy of Galina Churkina/Institute for Advanced Sustainability Studies

More recently, researchers have been trying to zero-in on how exactly this ozone production process works in urban areas, as well as which tree species emit more VOCs than others. Turns out, not all trees are equal. The unit for measuring VOC emissions from trees is micrograms per gram of leaf mass per hour. This table lists average VOC emission rates for several popular urban trees under standard conditions of temperature and light.

Tree species Avg. VOC emissions rate (μg *g-1*hr-1)
Black gum 77
Poplar 70
Cedar 11
Birch 5
Standard conditions is defined as 30 degrees Celsius, with photosynthetically active solar radiation of 1000 mol*m2*sec-1.  (Data courtesy of Galina Churkina/Institute for Advanced Sustainability Studies)

Black gum trees emit over 15 times more VOCs than birch trees. Since the reaction needs sunlight to occur, a concentration of high-emitting tree species planted in extremely sunny cities would be the worst combination. According to Galina Churkina, a senior fellow at the Institute for Advanced Sustainability Studies in Potsdam, Germany, the ozone production process will also intensify with climate change, since the warmer it is, the more VOCs the trees will emit. 

According to Churkina, there have been plenty of simulations of ozone levels for different cities, but the interplay between VOCs and ozone levels have not been as rigorously investigated. That's why her current work involves studying the detailed limits of this reaction in urban areas—for example, how the reaction rate changes with rising levels of atmospheric carbon dioxide, or whether regions with already high ozone concentrations will see a feedback loop in which high ozone levels stimulate greater VOC emissions. 

This summer, Churkina’s team is collaborating with the city of Berlin to simulate VOC emissions from trees. Since Berlin keeps a rather detailed inventory of vegetation, they can investigate which conditions lead to certain ozone concentrations. One specific condition she hopes to understand better is how VOC emissions and ozone production change during heatwaves, when there’s a high temperature for a short span of time.  

So what should cities do in the meantime? According to Churkina, removing already-planted trees is not a wise strategy, but “cities have to be careful about what they plant, especially in big quantities.” She says Berlin had recently considered starting a poplar plantation in the city. The project did not end up moving forward, which from a VOC emissions perspective, is good news. Otherwise, she says, they could become ozone hotspots. 

Since VOCs need NOₓ to form ozone, cities should probably avoid planting high-emitting trees along streets with heavy traffic. And of course, any measures to reduce NOₓ emissions would go a long way. 

(h/t Scientific American)

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