DURHAM, N.H. (AP) -- A University of New Hampshire forest program has noticed that whitepine trees in northern New England seem to be losing more needles lately.

The Forest Watch program says the trees maintained vigorous growth during the late 1990s as the Clean Air Act took effect and ozone levels fell. Ozone is an oxidant that accelerates aging in foliage.

But data shows that since 2010, the trees have not done a good job of retaining thoseneedles.

"Something very serious is stressing the trees," said Forest Watch founding director Barrett Rock of UNH's Institute for the Study of Earth, Oceans and Space. "Not since the early to mid-1990s, when ozone levels were extremely high, have we seen these kinds of measurements of stress in the pines."

One possible cause is air pollution from wildfires. Rock said he believes that a powerful oxidant in wildfire smoke from Canada, in combination with unusually high temperatures, might have contributed to damaged sugar maple trees in the region.

"The event might also have stressed the pines, and other pollutants from a growing number of wildfires might be causing further stress," Rock said.

Another theory is that unusually wet weather in 2009 caused an explosion of fungi that are clearly now feasting on the pine needles. They appear as orange-looking "blisters" on theneedles.

"Such fungi normally only attack needles that have been weakened by some other factor, and the fungi usually only damage a small percentage of the needles, not the large percentages we're seeing," Rock said.

Forest Watch takes K-12 students and teachers out of their classrooms to study air pollution and forest health. Since 1991, more than 350 schools across New England have helped researchers at UNH gather samples and measurements of white pine needles to monitor the impacts of the ozone levels.