We have long associated that more plants equal cooler and cleaner local climate.  

But what if more plants make things worse for air quality? Have we been blindsided by plants’ biological reaction relatively unknown to us? 

And ultimately, a question asks: should we not plant more trees in the future then? 

Tom Sharkey, a professor at Michigan State University, has taken the question a step further. “Should we cut down all the oak trees?” 

This question came up after Sharkey conducted research with his team. Their work has been published in the journal Proceedings of the National Academy of Sciences. 

While Sharkey doesn’t actually suggest cutting down oak trees, the question is an honest one. To map out possible scenarios and outcomes, one would ask the same question, too. 

Trees and their compound 

According to the team, some plants like poplars and oaks will emit a compound more. This happens when the planet (or local environment) warms up, and that compound makes air quality poorer. 

Additionally, the compound also contributes to harmful particulate matter and low-atmosphere ozone. 

The compound is called isoprene. Now, what’s weird and interesting is that it actually improves the quality of clean air. And at the same time, it also makes plants more resistant to stressors including insects and high temperatures. 

 

 

So, what gives? Does it help with the air quality or not? 

That’s what drives the researches to find out about this contrasting capability. 

Sharkey said, “Do we want plants to make more isoprene so they’re more resilient, or do we want them making less so it’s not making air pollution worse? What’s the right balance? 

“Those are really the fundamental questions driving this work. The more we understand, the more effectively we can answer them.” 

Killing more insects 

If trees get problematic for sure quality when it’s hot, then it’s bad news for insects, too. 

That’s according to Professor Mark Elgar at the University of Melbourne. Elgar is also the co-author of a new study published in Nature Communications. 

The study examines the impact of air pollution on insect population.  

“While we know that particulate matter exposure can affect the health of organisms, including insects, our research shows that it also reduces insects’ crucial ability to detect odours for finding food and mates,” Professor Elgar said. 

It goes without saying that such effects could result in the decline of populations. The research suggests that this rush may also happen to areas affected by bushfires. It can even occur in habitats far from the pollution source. 

 

 

How exactly does air pollution affect insects? 

When we talk about smoke, we sort of intuit how such type of pollution leaves a negative impact. But what about the “invisible” type of air pollution?  

Well, insect antennae have receptors that detect odour molecules that come from a lot of things. For example, a food source, a potential mate, or a good place to lay eggs. 

If an insect’s antennae are covered in particulate matter, there will be a physical barrier. This prevents contact between the smell receptors and air-borne odour molecules. 

Professor Elgar said, “When their antennae become clogged with pollution particles, insects struggle to smell food, a mate, or a place to lay their eggs, and it follows that their populations will decline.” 

The co-author added that around 40% of our planet’s landmass is exposed to particle air pollution. That’s above the World Health Organisation’s recommended annual average. 

“Surprisingly, this includes many remote and comparatively pristine habitats and areas of ecological significance,” Professor Elgar said. 

Now that we know how some plants emit isoprene when it’s hot, maybe that can also contribute to putting insect population at risk. 

Isoprene 

In the 1970s, when Sharkey was a doctoral student at MSU, he began his study on isoprene and how plants produce it. 

It may sound shocking, but Isoprene from plants is the second-highest emitted hydrocarbon on Earth. Its volume is only behind methane emissions from human activity.  

However, despite its importance and nature, most people (me included) have never heard of it.  

Well, to be fair, it did some sort of recognition in the 1980s, when then-president Ronald Reagan stated that trees were emitting more air pollution than cars.  

Environmentalists might think that Reagan’s assertion is a stretch, but the researchers said that his statement wasn’t entirely wrong. 

 

 

Isoprene interacts with nitrogen oxide compounds. This interaction is prevalent with compounds found in air pollution produced by coal-fired power plants. It also happens in pollution cause by internal combustion engines in vehicles. 

These reactions create ozone, aerosols and other byproducts that are unhealthy for both humans and plants. 

Sharkey explained, “There’s this interesting phenomenon where you have air moving across a city landscape, picking up nitrogen oxides, then moving over a forest to give you this toxic brew. The air quality downwind of a city is often worse than the air quality in the city itself.” 

So now, the research team is trying to understand the plants. They want to know how plants utilize biomolecular processes to make isoprene. 

To be specific, they want to understand how the environment and climate change affect such processes. 

Carrying the legacy if isoprene research 

Before Sharkey’s recent research, the team had understood that certain plants produce isoprene as they carry out photosynthesis.  

Moreover, they knew how the environmental changes that we have now were having competing effects on isoprene production. 

Increasing carbon dioxide in the atmosphere drives the isoprene production rate down. But in contrast, increasing temperatures accelerate the rate.   

Therefore, in the research, the team raised a question about the chances of either one of these effects winning out. 

Lead author Abira Sahu said, “We were looking for a regulation point in the isoprene’s biosynthesis pathway under high carbon dioxide. Scientists have been trying to find this for a long time. And finally, we have the answer.” 

 

 

More heat = more isoprene 

According to Sharkey, the paper had identified the specific reaction slowed by carbon dioxide, CO2. The temperature effect proves to be stronger than the CO2 effect. 

“By the time you’re at 95 degrees Fahrenheit—35 degrees Celsius—there’s basically no CO2 suppression. Isoprene is pouring out like crazy,” Sharkey said. 

The team’s experiment used poplar plants. There, they found that when a leaf experienced warming of 10 C, its isoprene emission increased more than tenfold. So, to refer back to Reagan’s statement, plants do emit a lot of isoprene—when it’s hot. 

This discovery could help future researchers anticipate how much isoprene plants will emit. So they can be better prepared for the impacts of such emission. 

At the same time, the team hours that this discovery can help people make better choices in today’s time. 

Do we cut down trees or what? 

The research suggests that we could plant fewer oak trees or other isoprene-emitting plants in the future. That way we can help limit isoprene production. 

But what about now? What about the oak trees and the plants we have now? Well, Sharkey suggested an idea that doesn’t involve cutting down trees. 

“My suggestion is that we should do a better job controlling nitrogen oxide pollution,” Sharkey said. 

 

Sources:

https://phys.org/news/2023-10-worsen-air-pollution-planet.html  

https://www.sciencedaily.com/releases/2023/07/230712124747.htm