Scientists have long worried that air pollution could amplify climate change by hurting the ability of forests to grow and absorb atmospheric carbon dioxide (CO2). Source: Science Mag
But a new study questions that conventional wisdom when it comes to one key pollutant, low-level ozone.
Past studies have tended to assume all trees react equally to ozone, but the new research suggests prolonged ozone exposure creates winners and losers in forests, tilting the balance toward smog-hardy species without hurting the forest’s CO2-trapping abilities.
The study is “a very important step” toward a more sophisticated understanding of how forests respond to environmental stress, says David Schimel, an ecoclimatologist at the Jet Propulsion Laboratory in Pasadena, California, who was not involved in the work.
“It’s a very clear example of where the identity of the trees matters, not just how many there are,” he said.
Ground-level ozone has long been a target of pollution regulators, because it can damage human health. But many lab studies have shown that the pollutant also hurts trees by damaging cells or reducing their ability to take in CO2 to make food.
A growing body of research shows that not all tree species are affected the same way, however. Some species, including oak trees, emit a compound called isoprene that is known to at least partially help them resist ozone’s effects (though, paradoxically, isoprene may itself get converted to ozone in the air).
Non-isoprene-producing species, such as pines, are more likely to be ozone-vulnerable.
To understand whole forests however, researchers need to study more than just single trees. And understanding how whole forests respond to ozone is tough to study in the real world, in part because trees tend to live for decades.
That means a pollutant’s impact can take a long time to propagate across an ecosystem.
Past modeling work has largely assumed, for simplicity’s sake, that all trees in a forest react the same way to ozone. In general, that research has shown that forest “productivity” – the rate at which trees take in CO2 and water and then turn them into organic matter – drops under prolonged ozone exposure.
That assumption ignores the complex ways in which ecosystems function according to ecologist Manuel Lerdau of the University of Virginia in Charlottesville.
To grapple with that complexity, Mr Lerdau and his colleagues, Bin Wang, Herman Shugart and Jacquelyn Shuman decided to adapt an existing computer model that simulates forest species dynamics, but had lacked the ability to simulate ozone’s impacts.
They hoped to upgrade the model so that it could give researchers a sense of ozone’s effects, revealing how a forest’s species composition, tree mass, and isoprene emissions would change over time.
Getting the model to play nice with ozone however, wasn’t easy. Ozone’s cellular effects happen within minutes for instance, but the effects on whole forests can take decades or centuries to play out.
“Creating that coherency was one of the real challenges that we had to meet and overcome,” Mr Lerdau says.
As their guinea pig, the researchers chose to focus on typical forests of the southeastern United States. First, they collected a number of existing studies that show how key species in these forests interact and respond to ozone’s effects.
Then, the researchers divided the trees into three categories, depending on how vulnerable the studies suggested they were to ozone. That simplified the model, without going so far as to assume all trees respond the same way.
Finally, the researchers told their model to grow a typical southeastern US forest under two different ozone treatments, one without any ozone from human activities, and another with ozone at roughly 80 parts per billion, a level just above current US ozone standards for human health.
To account for how long it takes for forests to grow, researchers let the model simulate 500 years of growth.
After half a millenium, the mass of all tree life in the two simulated forests leveled out at roughly the same amount, the researchers found.
Somewhat paradoxically, the forest with higher ozone levels briefly spiked to a higher level than the nonpolluted forest.
The ozone-polluted forest experienced greater changes in species composition, however, showing a shift toward ozone-hardy species.
Because the forest as a whole managed to maintain the same total mass under higher ozone levels, its ability to suck up CO2 didn’t suffer, the findings suggest.
That could mean ozone’s climate impacts might not be as high as previously thought. The findings do suggest, however, that less-diverse communities, such as croplands, could be far more vulnerable to the pollutant than diverse forests are.
One take-home message, Mr Lerdau says, is that “if we want to make policy to preserve ecosystems, we need to be focusing on those ecosystems that are going to be sensitive, which are the low-diversity ecosystems.”
“This is an intriguing result, underscoring the need for such tree community–level analyses,” Stephen Sitch said, he is a climate researcher at the UK’s University of Exeter, who wasn’t involved in the work. But Sitch says more studies, using other models and examining other ecosystems and locations, are needed to thoroughly understand the environmental and climate implications.
Mr Schimel agrees with Mr Sitch, but also suggests that the amount of data on how specific tree species respond to ozone, though growing, are still somewhat patchy. As a result, the findings are useful “not so much in terms of taking immediate action,” Mr Schimel said, but in terms of helping guide “what people should be looking for.”