The study examined a class of chemicals that are emitted as unburned fuel from automobile tailpipes and as vapors from industrial chemicals, but also which come naturally from tree leaves. These chemicals, known collectively as VOCs, react with other pollutants to form ozone, a bluish, irritating and pungent gas that is a major form of smog in the lower atmosphere.

While clean-air laws have reduced the level of man-made VOCs (volatile organic compounds), the tree-produced varieties have increased dramatically in some parts of the country, the study found.

The increase stems from intensified tree farming and other land use changes that have altered the mix of trees in the landscape, said Drew Purves, the lead author of the study that included scientists from four universities.

"There are seemingly natural but ultimately anthropogenic (human-caused) processes in the landscape that have had larger effects on VOC emissions than the deliberate legislated decreases," said Purves.

Although scientists knew that trees contribute substantial amounts of VOCs to the atmosphere, the rate of increase in recent decades was previously unrecognized.

"If we don't understand what's going on with biogenic (plant-produced) VOCs, we are not going to be able to weigh different air-quality strategies properly," said Purves. "It's a big enough part of the puzzle that it really needs to go in there with the rest."

The study may help explain why ozone levels have not improved in some parts of the country as much as was anticipated with the enactment of clean-air laws, Purves said. Environmental technologies such as catalytic converters and hoses that collect fumes at gas pumps have substantially reduced human-produced VOCs.

However, in some parts of the country -- particularly the area extending from Alabama up through the Tennessee Valley and Virginia -- these improvements may have been outweighed by increased VOC emissions from forests, mainly because of tree growth in abandoned farmland and increases in plantation forestry.

Purves emphasized that cutting the human-caused sources may have been worthwhile even if ozone levels did not decrease. "Even keeping the air quality the same might have been an achievement because if we hadn't done anything it might have worsened," said Purves.

The study did not measure actual ozone levels. Instead it focused on VOCs, a crucial part of the chemical reaction that produces ozone. The other critical ingredient is a class of gasses known as NOx (various combinations of nitrogen and oxygen), which are almost entirely man-made. The ozone-producing reaction happens most readily in hot weather, which is also when trees produce the most VOCs.

Further studies at Princeton and the federal Geophysical Fluid Dynamics Lab at Princeton are using sophisticated computer models to estimate the changes in ozone caused by the changes in tree-produced VOCs. Purves noted that interactions between VOCs, NOx and ozone are complex -- some may actually lower pollution -- so it would be premature to base environmental policy on studies of VOCs alone.

Purves, a postdoctoral fellow, wrote the article in collaboration with Stephen Pacala, professor of ecology and evolutionary biology at Princeton, as well as John Casperson of the University of Toronto, Paul Moorcroft of Harvard University and George Hurtt of the University of New Hampshire. The article is scheduled to be published later this fall in the journal Global Change Biology.

The scientists conducted the study by analyzing data collected by the U.S. Forest Service, which measured and cataloged 2.7 million trees on 250,000 plots of land across the country.

They calculated the VOC emissions for each tree and each plot and used their findings to map VOC levels nationally. The scientists compared survey data taken in the 1980s with those taken in the 1990s to determine how levels were changing over time.

They found that areas where farmland has been abandoned during the last century have early generations of trees that produce higher levels of VOCs than older growth forests.

In the South, pine plantations used for their fast-growing supplies of timber have proven to be havens for sweetgum trees, which are major producers of VOCs. Indeed, virtually every tree that grows fast -- a desirable quality for forestry production -- is a heavy emitter of VOCs.

"It's just one of those biological correlations," said Purves. "What you want is a fast-growing tree that doesn't produce a lot of VOCs, but that doesn't seem to exist."

The findings also could raise questions about potential strategies for developing "green" fuels. One idea for cutting greenhouse gas emissions is to create "biofuels" from renewable tree plantations; however, these plantations may lead to increased ozone levels, the authors note.

The findings lend support to proposals among environmentalists to shift attention to regulating NOx, the other main ingredient for ozone. They also underscore the pervasive effect of human activities on the environment, because something as seemingly benign as planting forests can have a substantial impact.

"You can't identify any of these processes as 'natural,'" said Purves. "The idea of natural versus human-caused is disappearing. Ten years from now, woodlands may have more of one species than another, and you could call that natural change. But here in New Jersey, the mix of trees is affected by the population of deer, and they are entirely under our control. In other parts of the country, it is the fire-suppression policies."

Noting President Ronald Reagan's notorious 1980 reference to trees causing pollution (Reagan said: "Approximately 80 percent of our air pollution stems from hydrocarbons released by vegetation."), the authors conclude: "The results reported here call for a wider recognition that an understanding of recent, current and anticipated changes in biogenic VOC emissions is necessary to guide future air-quality policy decisions; they do not provide any evidence that responsibility for air pollution can or should be shifted from humans to trees."

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