All too often, when older homes are demolished to make room for larger new homes, the trees that shaded the old homes, sometimes for generations, are cut down.

That could have been the fate of a mighty oak on the corner of North Nottingham and 27th streets. More than 18 feet in circumference, the Willdenow’s oak (a natural hybrid between a black oak and southern red oak) escaped the widespread felling of Arlington’s trees during the Civil War for fuel and building materials and is listed among Arlington’s 100 designated “champion” trees. Thought to be the largest Willdenow’s oak in the state, with its acorns in the Smithsonian, “it’s truly irreplaceable and a living part of Arlington’s history,” said local plant ecologist Rod Simmons.

That pedigree, however, offered no protection against developers’ chain saws. To make room for a larger house, “normally it would have been taken down,” said DS Homes project manager Bill Nichols. DS Homes bought the property for redevelopment in 2014, after the longtime homeowner died.

But this story has a happy ending, thanks to the activism of local residents and the willingness of DS Homes to alter its original plans. Neighbor Vicki Arroyo, my wife, collected scores of signatures on a petition to save the tree.

“It was clear that Arlington residents are frustrated by the loss of mature trees to development,” she said. “People of all ages wanted to do what they could to save this special tree, but there was also a sense that ‘enough is enough.’ ”

When Paula Kelso, another neighbor, an editor at The Post and a volunteer for TreeStewards of Arlington and Alexandria, became aware of the threat, she swung into action. “I made a couple of urgent calls to the developer, knowing that the heavy machinery could come up at any minute,” she said. Kelso organized meetings between neighbors and DS Homes. As a result, “before we even started the house, we knew that there had been a petition and that the neighborhood wanted the tree saved,” Nichols said. “When we go into a neighborhood, we want to get along with everyone. So we decided that DS Homes was going to do what it could to save the tree.”

The designers worked around the huge oak, which is near the edge of a wide lot, siting the garage on the tree side of the property. Because there would be no basement under the garage, the tree’s roots would have the maximum room and minimum disturbance, leaving plenty of room for a large house.

Protecting the oak saved DS Homes thousands of dollars in tree removal costs, as well as the expense of planting new trees to meet Arlington County’s requirement that tree canopies cover 20 percent of a redeveloped property within 20 years of construction. “It was a whole lot better that we saved the tree,” said Nichols. “It is so appealing — and a heck of a conversation piece.”

In fact, the oak, and what it represented, was one of the key attractions for the buyers of the new house, Nicholas and Lisa Solinger. “We had lived in a similar older neighborhood in Minnesota, with a similarly grand tree in the front yard,” said Nicholas Solinger. He found an article about the neighborhood petition to save the tree, he said. “We were just tickled by the tree and the story itself of the neighborhood coming together.”

The Solingers hung a tire swing from one of the tree’s massive branches for their two boys to play on, and they and their friends are loving it.

The tree’s immense canopy offers shade, and the oak also intercepts tens of thousands of gallons of rainwater that otherwise would pour each year into storm sewers and streams — and perhaps into basements.

What’s frustrating to Arlington’s many tree lovers, though, is that such stories are unusual. “It’s rare to preserve of tree of this specialness,” said Arlington County’s acting urban forest manager Vincent Verweij. The key to preserving the Nottingham Street Willdenow’s oak? Neighborhood activism, Verweij said.

“For me and for this neighborhood, it has become a unifying emblem, and a reminder of this area’s past,” said Kelso. “I hope it will serve as encouragement to others who want to do the same thing in their neighborhoods.”

The writer is a member of Arlington County’s Urban Forestry Commission.