MONTPELIER, Vt. – Red spruce trees around New England suffered severe damage last winter, University of Vermont researchers have found.

Scientists have not determined the cause of the damage but say a number of factors, including acid rain, may have weakened the trees.

In a widespread study at 28 sites in Vermont, New Hampshire, Massachusetts and New York, more than half of the trees lost foliage and some lost buds.

“In 2003, an average of 46 percent of current-year spruce foliage was killed at our study sites, and injury was about 65 percent for the largest forest trees,” said UVM researcher Brynne Lazarus. “What was worse was that many buds were also killed.”

The research was published in the September issue of the Canadian Journal of Forest Research. Red spruce are most common in higher elevations and there also are stands at lower elevations in the northeast corner of Vermont.

The damage, marked by red needles and sparse branches, was the most widespread since 1989, the most recent year when such damage was measured, scientists said.

The injuries were worse in trees at higher elevations and on western slopes of mountains, said Paul Schaberg, a research plant physiologist with the U.S. Forest Service.

“It’s the same kind of pattern that we see for acid rain in the area,” said Schaberg.

In the 1970s and early ’80s, some of the region’s mountaintops were scarred by dying trees from acid rain. That’s rainwater polluted by sulfur emissions from Midwest power plants and other sources.

Since that time, the red spruce has looked healthier, indicating to scientists that the most vulnerable trees had died off and were replaced with stronger trees, Schaberg said.

That theory made last year’s damage puzzling.

UVM researchers first noticed the damage while skiing and hiking at higher elevations.

The trees were ablaze in a brownish red, Schaberg said.

The color emerges after the foliage freezes, the cells die, and the chlorophyll goes away.

“These trees are very, very injured,” Schaberg said. “The buds on these trees were damaged. Spruce trees keep many years of needles on it. Losing one year is bad.”

The loss of foliage reduces the amount of food taken in and stored, he said.

“It just kind of wears the tree out, it’s a downward decline. You start having a tree without much foliage on it,” he said.

A lack of nutrients in the soil may have contributed to the damage.

Acid deposits in soil from acid rain tend to deplete calcium in soil, Lazarus said.

Scientists visited Hubbard Brook in the White Mountains in New Hampshire, an experimental site where calcium has been added back into a mountain watershed.

“The spruce are much less injured,” said Lazarus.

Lazarus, who conducted the research as part of her master’s thesis, plans to continue to analyze the trees.

“We think that some will probably recover and some will die. Some have already died,” she said.