MONTPELIER, Vt. — University of Vermont researchers have gathered scientific evidence that for the first time establishes the link between acid rain and upper elevation forest decline in the northern Appalachian Mountain chain.

The research found that acid rain — essentially rainwater and cloud water dirtied by polluted skies — causes subtle changes in red spruce seedlings, weakening them and making them susceptible to other diseases or damage.

Several researchers said the trees are injured in much the same way people are weakened by AIDS: The acid rain itself doesn’t kill the plants but it exposes them to conditions that might.

Scientists have been documenting a decline in northern forests, especially in the mountains of northern New York and New England, for decades. Theories about why that has happened have abounded.

But until University of Vermont and Tennessee Valley Authority researchers set up a sophisticated experiment on a mountaintop in Virginia, there was nothing to substantiate their suspicions, according to a summary of their findings in the scientific publication “Canadian Journal of Forest Research.”

“Everything before that was circumstantial and very controversial,” said Donald DeHayes, a Vermont professor who led the study. “Although people had a gut feeling that acid rain was responsible for forest decline that had occurred, it was very difficult to prove it.”

What researchers led by DeHayes and TVA scientist Frank Thornton found was that red spruce seedlings exposed to natural rainwater had considerably less tolerance to cold than protected seedlings. That, in turn, made the seedlings more susceptible to cold-weather damage.

The freezing injuries and subsequent widespread red spruce deaths at elevations above 2,500 feet in the Adirondack, Green and White mountain ranges have been well documented over 40 years.

“We have proven there has been recent mortality approaching 50 percent on certain mountaintops,” said Christopher Eagar, a scientist with the U.S. Forest Service. Finally, there is an explanation.

“The study that we did really made the connection that acid rain reduces the ability of trees to withstand cold temperatures,” DeHayes said. The resulting damage slowly has been killing off the trees, once the most abundant species atop the mountains.

Scientists at the U.S. Environmental Protection Agency and the U.S. Forest Service, which helped fund the Whitetop work, said the conclusions were valid and valuable.

“The information that was generated during this program, I think, has a lot of scientific credence behind it,” said Ralph Baumgardner, a project manager with the EPA.

Eagar said he presented the findings to 16 scientists contributing to a book about acid rain that he is editing. “They all agreed with our basic finding that acidic cloud water is a factor in the decline of red spruce,” he said.

The important difference between the Vermont-TVA study and previous research is that the new work was conducted in the real world.

“Most of the other ones have done controlled or lab studies,” Thornton said. “This was actually at the site. That’s what makes it significant.”

The TVA scientists had set up chambers atop the 5,573-foot Whitetop Mountain for other research. They were equipped with sensors that opened and closed depending on the presence of rain, fog or mist, all collectively referred to as “cloud water” containing acid rain.

That permitted DeHayes to set up three different plots of red spruce: those exposed to natural cloud water and unfiltered air; those subjected to unfiltered air and no natural cloud water; and those growing in the presence of some filtered air and no natural cloud water.

Pollutants in the atmosphere elevated acidity in the rainwater to a pH average of 3.4. Pure distilled water has a pH level of 7.0 and “clean rain” is usually about 5.5, DeHayes said.

The seedlings were monitored for two growing seasons.

Samples were shipped to laboratories at the University of Vermont on six occasions for testing to determine at what temperature freeze damage first occurred. Those exposed to acid rain and unfiltered air consistently showed damage at temperatures 2 degrees to 5 degrees Celsius warmer than the other plants.

Eagar said the research explains only declines at high elevations and not other environmental degradation ascribed to acid rain. And the importance of the findings “depends on your point of view,” he said.

“If you’re a recreationist that likes high elevation spruce and fir forests, it could be a concern to you,” he said. “As far as timber production out of New England, it’s not a consequence.” Market-grade timber is harvested at lower elevations where clouds, which are more heavily laden with acid than rainwater is, are less likely to settle.