NEWPORT, R.I. — In the Gilded Age, the rich built marble palaces here, surrounding them with exotic trees they acquired with the same ardor they brought to assembling their fabulous collections of art.

Their favorites were European beeches — green, copper and weeping beeches — trees they prized for their dramatic shapes and colors. Soon the streets of Newport’s mansion district were filled with the trees.

Today, many of them tower as high as 80 feet. “They are icons of Newport, the signature trees of the Gilded Age,” said John R. Tschirch, an architectural historian who directs conservation programs at the Preservation Society of Newport County, which owns many of the mansions.

But the trees are in trouble. Planted more or less all at once about 120 years ago, they are aging all at once now, a process hastened by insect and fungus infestations they can no longer fight off. Though the mansion district’s main street, Bellevue Avenue, looks almost as elegant as ever, here and there stands a skeleton tree, bereft of leaves, or a stump perhaps five feet across, all that remains of a vanished giant....