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ORNL researchers publish tree growth article in nature publication

OAK RIDGE, Tenn., May 23, 1995 — Two senior research scientists at the Department of Energy's (DOE) Oak Ridge National Laboratory (ORNL) have reported the first definitive link between ozone exposure and reduction in the rate of growth in mature forest trees.

In an article published in the March 16, 1995, issue of the publication Nature, Dr. Samuel McLaughlin and Dr. Darryl Downing report the results of a five-year study conducted on the growth of 34 mature loblolly pines on DOE's Oak Ridge Reservation. Variations in short-term growth rates were measured in relation to trees' exposure to varying levels of ozone under different environmental conditions, including temperature, rainfall and soil moisture.

The study, sponsored by the Southern Global Change Program (SGCP), a cooperative of the U.S. Department of Agriculture's Forest Service-Southern Research Station, used statistical techniques to isolate the ozone effect. Growth rates of trees were found to be reduced during multiday episodes when ozone exposure levels increased. When the ozone level decreased, growth increased. However, ozone interacted strongly with both low soil moisture and increasing air temperature to reduce growth.

McLaughlin and Downing measured the circumference of the trees' trunks twice a week during the five-year period. The trees were located in two different areas of the DOE's Oak Ridge Reservation. One site was near the Clinch River while the other was along Lou Cagle Road near White Wing Road. McLaughlin found some variation in the sensitivity of the approximately 60-year-old loblolly pines during the test period, but growth directly corresponded to the level of ozone in the atmosphere.

Findings of the study support earlier conclusions about growth losses in younger trees exposed to ozone under controlled experimental conditions. Those results are scheduled to be published this summer.

"This study shows we need to understand this subject better," McLaughlin said. "We found short-term growth responses that are clearly related to fluctuating ozone levels. These responses indicate that ozone is increasing the sensitivity of trees to drier soil conditions. How this response is produced and how it translates into longer-term growth trends is an area that requires further study."

SGCP commissioned the study to determine if atmospheric pollutants contributed to reported growth reductions of loblolly pines, a tree species predominant in Southern forest landscapes, that contributes billions of dollars to the region's economy annually.

McLaughlin said he hopes to conduct similar tests in the future with other tree species in an attempt to evaluate the relative sensitivity of pine and other tree species and to better understand the mechanisms responsible for ozone effects on longer term growth. Predictions of a warmer, drier climate with higher ozone levels for some areas of the United States during the next century indicate that this information will be increasingly important.

Susan Fox, SGCP manager, said results of the study will produce useful information to regulatory agencies, geneticists and forest managers.

"This just reinforces our belief in the value of cooperation among scientific organizations, whether from government agencies, universities or even private institutions," Fox said. "The pooling of science dollars and talent allows the flexibility of finding the quickest and most cost-effective ways of addressing critical research needs - a premise that is at the core of the global change program."

Peter J. Roussopoulos, Southern Research Station director, said results of the ORNL study provide a building block in an understanding of the interactions among forest ecosystems, air pollutants and climate change in the South.

ORNL, one of DOE's multiprogram national research and development facilities, is managed by Martin Marietta Energy Systems, a Lockheed Martin company, which also manages the Oak Ridge K-25 Site and the Oak Ridge Y-12 Plant. .