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While Some Greenhouse Gases May Fertilize Plants, Benefit Is Offset by Increased Air Pollution
DURHAM, N.H. -- Professor Scott Ollinger of the Institute for the Study of Earth, Oceans, and Space at the University of New Hampshire and his colleagues have thrown a new wrench into the debate over whether forests can take in enough carbon dioxide to offset global warming.

He finds, while the greenhouse gas carbon dioxide seems to increase plant growth and the ability of forests to act as carbon sinks, the simultaneous impacts of land use and ozone pollution decreases plant growth, often resulting in no net gain of carbon uptake.

Ollinger asserts, "You can't look at the fertilization effects of carbon dioxide without realizing that we also have lots of pollutants in the air that have the opposite affect on growth. If you do this you are only scratching the surface of things that affect real ecosystems growing across real landscapes."

In typical studies of forest health, the impacts of carbon dioxide, nitrogen deposition, ozone and land use on forests are examined one variable at a time. This new study models the interactive impacts of all of these variables at once, as would happen in a natural environment.

"As we learn more about real ecosystems, we learn about all the factors which impact forest health," Ollinger says. "One of these factors is ozone pollution, which scientists find reduces tree growth. On the other hand, you have people saying that carbon dioxide increases growth. When we look at these factors individually, both of these camps are right, but because they rarely talk to each other it has taken a long time to put together a more complete picture."

Many predict that carbon sinks are increasing due to the fertilization effects of carbon dioxide and nitrogen deposition. However, Ollinger's research suggests this theory is incomplete. When the effects of ozone, or smog, and past land use are included, there has been almost no change in growth rates over the past 300 years. Since carbon is taken in only by growing plants, scientists can study the ability of forests to absorb carbon dioxide by looking at growth rates.

"The negative effect of ozone and historical land use across New England forests was predicted to be, on average, equal to the positive effect of carbon dioxide and nitrogen deposition," Ollinger explains. "This makes a lot of sense when you consider that ozone comes from nitrogen oxides (NOx) in the air. When nitrogen ends up in the soil, it often acts as a fertilizer rather than as a growth inhibitor."

NOx comes from the burning of fossil fuels, with automobile emissions as the leading source. Both carbon dioxide and NOx also are naturally occurring; however records from the past 300 years show nitrogen deposition in New England to be about 10 times higher and carbon dioxide to be 28 percent higher than they were historically.

"Some people may interpret these results to mean that if we continue to emit all those pollutants, it doesn't matter due to the equal but opposite effect on forest health," Ollinger notes. "But growth rates aren't the only factor we should be concerned with. Other factors -- including biodiversity and water resources -- make air pollution an increasingly important issue. If we were able to reduce the harmful effects of ozone pollution, we would realize the beneficial fertilization effect and be able to include forest sinks in national carbon accounting."

This research, which utilized Harvard Forest, Mass., and Hubbard Brook, N.H., as case studies, was presented at the annual American Geophysical Union meeting and has been accepted for publication in the Journal of Global Change Biology. Ollinger and co-authors, including UNH Professor John Aber, recently received a Department of Energy grant to expand upon this research

By Amy Seif
Communication and Information Coordinator
Institute for the Study of Earth, Oceans, and Space
603-862-5369