TENAFLY, N.J. — A chain saw's drone drowned out the trill of migrating warblers in the Greenbrook Sanctuary woods as arborists conducted major surgery to save an extremely aged patient — a 163-year-old American elm tree.

Dutch elm disease has wiped out some of the few remaining stands of native elm trees in the region.

Greenbrook Sanctuary recently lost four of its 12 remaining elms. That spurred Nancy Slowik, the sanctuary's director and naturalist, to lobby the organization's board to start an aggressive campaign to save the elms that are left.

"They're a piece of ecological diversity that we don't want to lose," Slowik said.

So on a recent Tuesday, Jose Jimenez stood in the extended bucket of his green and white SavATree truck, about 80 feet up, cutting off the dead, diseased branches.

The pruning will be followed in about a month by a treatment called macroinjection. Arborists drill small holes into the tree's flares — or raised, exposed roots — and inject a chemical solution. It kills the disease in a sick tree and helps healthy trees fight off future infection.

"It's almost like a vaccine for trees," said Brice B. Dorwart, the SavATree arborist overseeing the Greenbrook project. The procedure normally costs about $600 to $700 per tree, but Wyckoff-based SavATree is doing the job for free.

American elms were for generations the shade tree of choice in the United States. The tree grows tall, and its branches fan out to create a vase-shaped profile perfect to plant along urban and suburban streets for a welcome canopy that doesn't interfere with utility lines.

Dutch elm disease is caused by a fungus that travels from tree to tree on the bodies of bark beetles. It can also spread by the root system to adjacent trees.

It is called "Dutch" elm disease because scientists in the Netherlands first identified it in the early part of the 20th century, but the fungus actually originated in Asia. It came to the United States in 1930 on a diseased shipment of wood. It quickly spread, killing millions of American elms.

Early signs of the disease are subtle and occur when small patches of leaves wilt or turn yellow, a process called flagging, Dorwart said. But it can often occur first in the upper crown of the tree, and not be visible from the ground.

"If people have an old elm on their property that is important to them, it's always best to get an arborist to look it over and see if it's at risk," Dorwart said.

When Jimenez came down from the old elm in Greenbrook, he handed Slowik a dead branch, about four feet long. It had two large, perfectly round holes in it.

"Oh yes, a red-bellied woodpecker lived up there," Slowik said.