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CAMBRIDGE, Mass.--An MIT scientist and colleagues have found the first direct evidence that metal particles carried in the wind from industrial sources are involved in forest decline in the mountains of New York, Vermont, and New Hampshire.
Heavy metals like copper, cadmium and lead have been implicated before in the death and damage of trees, but until now there has not been a direct link. Past studies have only shown that patterns of where metal pollutants are deposited correspond to patterns of damaged trees.
In the new work, reported in the May 2 Nature, the researchers measured the concentration of a compound produced by all plants, from redwoods to yeast, that is an indicator of metal stress. The concentrations of this compound, known as phytochelatin, matched observations of tree health: higher amounts were found in sicker stands.
The researchers are James E. Gawel, a doctoral student in the Department of Civil and Environmental Engineering and lead author of the paper; Beth A. Ahner and Francois M. M. Morel of Princeton, and Andrew J. Friedland of Dartmouth.
"Phytochelatins are produced by the plants themselves in response to metals that actually get into the cell," Mr. Gawel said. The compounds bind to the metals to keep them from poisoning the plant. As a result, they serve as "physiological markers" for exposure to heavy metals. The higher the phytochelatin concentration, the more metal stress the plant is facing.
Key to the research was a sensitive test developed by Dr. Ahner when she was a doctoral student at MIT in Professor Morel's lab (both are now at Princeton). The test measures phytochelatins at very low levels. Working with Dr. Friedland at a Dartmouth research facility at Whiteface Mountain in New York, Mr. Gawel used the test to measure phytochelatin concentrations in the needles of red spruce and balsam fir. He found that phytochelatin levels in red spruce were consistently higher than in balsam fir. "That was very interesting to us because spruce are in decline and fir are not," Mr. Gawel said.
The scientists also found a correlation between phytochelatin concentration in red spruce and altitude. "The phytochelatin concentration increased very strongly with elevation," Mr. Gawel said. The number of dead and damaged trees also increases with elevation.
When the scientists extended the study to red spruce on nine other mountains in New York, Vermont, and New Hampshire, they got similar results. In addition, higher phytochelatin concentrations and more tree decline were found in the Adirondack mountains of New York and the Green Mountains of Vermont than in the White Mountains of New Hampshire.
"The New York and Vermont mountains receive more pollution from the Midwest," explained Mr. Gawel. By the time the winds reach the White Mountains, a large percent of the pollutants have already been deposited on the other mountains.
Mr. Gawel is currently studying how the metals get into the trees (whether through roots or leaves), and what their full impact is. "We haven't yet been able to show to what degree heavy metals are only part of the problem or are an essential causative factor in general," Mr. Gawel explained. Acid rain and ozone are also thought to be important factors in forest decline.
"We expect that the metals by themselves aren't killing the trees," Mr. Gawel continued. "Rather, they're predisposing trees to damage from other sources, such as harsh winters." The production of phytochelatins themselves, for example, is known to deplete stocks of another important compound. This depletion "results in an increased sensitivity to oxidative damage" from other pollutants, the authors write. "In the mountain environment, anything that puts a tree under stress will impact its ability to survive," Mr. Gawel concluded.
In continuing work, Mr. Gawel is growing red spruce seedlings in the lab to study the effects of known concentrations of one heavy metal, cadmium, on tree health. Laurel A. Schaider, a junior in civil and environmental engineering, is working with him through the Undergraduate Research Opportunities Program.
The work is supported by the National Institute of Environmental Health Sciences, the NSF, the EPA, the Andrew W. Mellon Foundation, and the US Forest Service Northern Stations Global Change Research Program.