People have a hard time breathing when ozone levels are high.
So do plants.
"It is very similar to people with asthma," said David Karnosky, a forestry professor at Michigan Tech University in charge of a long-running experiment near Rhinelander in northern Wisconsin where some trees get extra doses of ozone and others do not.
"This is the canary in the coal mine," Karnosky warned, explaining that the levels of ozone the trees in the experiment are experiencing are comparable to the levels that could exist in northern Wisconsin by 2050.
If that happens, the consequences are potentially serious for both plants and people.
Asthmatic plants grow more slowly, which is bad news for crop production, and affected trees and plants also consume less carbon dioxide, which leaves more of it in the air where it traps heat on the planet and may harm people if breathed in high concentrations.
The pollutant ground-level ozone, commonly known as smog, is a product of nitrogen oxide and volatile organic compounds produced in high temperatures from vehicle and industrial emissions -- and overall temperatures have been warming.
Human beings -- especially children, the elderly and those with breathing problems -- are warned to avoid outdoor activity on high ozone days, because the pollutant can restrict airways, cause coughing and sneezing, increase risk of asthma attacks, and worsen diseases.
But plants can't just pick up their roots and go inside. So they close the little mouths -- called stomata -- on their leaves that take in carbon dioxide. Though carbon dioxide in high amounts is bad for people, it is vital for plants, which use it with light and water to produce food and oxygen. If they don't get enough carbon dioxide, they don't grow as much as they should.
"The ones that close up are less affected by the ozone because they take in less, but they also take in less carbon dioxide, so it reduces photosynthesis," said Thomas Sharkey, a former University of Wisconsin-Madison botany professor who is now a professor at Michigan State University and has done research at the Rhinelander site.
But the ozone that does get in through the stomata has the same effect that it would have on a human being running on a high ozone day, said Massachusetts Institute of Technology researcher John Reilly, who has studied ozone's effects on crops as well as trees. "That damages human lungs, and in the same sense, plants are damaged."
Almost 5,000 trees planted 10 years ago at the 40-acre research site on U.S. Forest Service land near Rhinelander have been tested via a complex system of pipes and valves that apply carbon dioxide to three "rings" of trees, ozone to three other rings and a combination of the two gases to another three rings. A control ring group just receives the local air.
Trees that got those extra shots of ozone have only three-fourths as much growth and yield as those that did not receive the pollutant, Karnosky said.
"It knocks your eyes right out," Sharkey added.
At the beginning of the experiment years ago, existing trees were removed and new aspen, birch and maple trees planted, so growth could be watched from the beginning.
"No one had ever done a forest face experiment from ground zero," Karnosky explained. "We have looked at the whole life history of these stands."
The researchers found that ozone at fairly low levels actually occurring in large parts of the United States was already detrimental to the growth of the trees and their ability to withstand insect and disease attack.
Trees that received extra carbon dioxide had high productivity, but when ozone was added as well, the beneficial aspects of carbon dioxide were canceled, Karnosky said.
Researchers also have found that elevated carbon dioxide and ozone and their combination influences the incidence and severity of insect and disease problems. "In 2100, pests and diseases may become very important interacting stressors for aspen stands," one study by Karnosky and others found.
Aspen-dominated forests currently occupy nearly 2.8 million acres in the state, and various studies say they presently absorb enough carbon dioxide to counter much of the state's carbon dioxide emissions from commercial and industrial sectors.
But a January study funded by the state of Wisconsin's Focus on Energy program estimates that a potential 30 percent increase of ozone in coming years could force the expenditure of more than $50 million per year (in 2008 dollars) to control excess carbon dioxide because trees wouldn't be able to process as much of it.
"We as a society would clearly be remiss if we ignore the importance of tropospheric ozone as an agent of global change," the Focus on Energy report concluded.
But it's not just aspen stands that would suffer if ozone levels continue to rise, researchers say. Perhaps more significantly, crops would be dramatically affected by ozone in the future, according to recent research by MIT's Reilly.
The generally accepted standard is that 40 parts per billion of ozone is the threshold where crop and plant damage starts, and Reilly said in an interview that according to his computer model, places where ozone was over that level for some part of the day would increase by five or six times from now to the year 2100.
That would mean a 43 percent reduction in plant yields in the United States and a more than 60 percent drop in China by 2100.
"Crops are more susceptible to ozone than forests and natural grasslands because crops tend to be near some of the urban areas that tend to have higher levels of ozone," said Reilly, who earned B.S. degrees in economics and political science at UW-Madison.
But learning how to control that problem is difficult because the pollution isn't all local.
"There is transboundary pollution. Just as Wisconsin can't control pollution from Illinois, the U.S. can't control from China or India," Reilly said. "We used to think it could be controlled locally and regionally, and now it is turning into a hemispheric problem."