Ozone pollution could exacerbate global warming, drought impact on forest ecosystems

Researchers try to pinpoint impacts of ozone on forest ecosystems.

Large-scale study suggests ozone-stressed trees use more water

By Summit Voice

FRISCO — The impacts of ozone of trees and other plants are well documented, but new large-scale studies suggest the heat-trapping gas may intensify the effects of warmer temperatures and reduce streamflow from forests to rivers, streams, and other water bodies.

Such effects could potentially reduce water supplies available to support forest ecosystems and people in the southeastern United States, according to a new study by the U.S. Forest Service and Oak Ridge National Laboratory.

Using large-scale models, the researchers found that increased concentrations of ozone in coming decades will aggravate drought and change stream flows, especially in dry seasons.

“From previous studies, we know a lot about ozone’s influences on crops and leaves of young trees. However, no studies have investigated the impacts of ozone on water flow in large forested watersheds,” said Ge Sun, research hydrologist with the Forest Service Eastern Forest Environmental Threat Assessment Center. “Our studies show that ozone has a possible connection in the reduction of streamflow in late summer when flow is generally lowest, particularly in areas with high ozone levels such as the Appalachian Mountains in the Southeast.”

Researchers developed models based on 18 to 26 years of data and observed streamflow in response to climate and atmospheric chemistry during the growing season across six southeastern forested watersheds ranging in size from 38 acres to more than 3,700 square miles.

When they crunched all the numbers together, they found that ozone concentrations can affect stream flows by anywhere from 7 percent to as much as 23 percent in areas with high levels of ozone exposure.

The findings from this study along with a wide range of previous field studies challenge assumptions derived from small controlled studies that ozone exposure reduces water loss from trees and forests. The present study of mature forests under moderate ozone exposure shows however those ecosystems may react in a different way than can be predicted by short-range, intensive studies.

“We’re predicting that forests under high ozone conditions will use more water instead of less, as was previously assumed,” said Samuel “Sandy” McLaughlin, scientist emeritus from the ORNL Environmental Sciences Division. “The concern is that ozone-induced increases in plant water loss could aggravate drought impacts on forests, and reduce the water available for people and stream life dependent on water flow during the dry seasons.”

Forest Service and ORNL scientists also collaborated with researchers from the University of Virginia, Lamont Doherty Observatory, and the University of Gothenburg, Sweden.

The research was published in the November issue of the journal Global Change Biology.

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