Sarah Charlop-Powers was comparing New York City’s forests to its subways.

The city has more than 840 miles of tracks for one. It has 10,542 acres of the other, about half as much as the Congaree Swamp in South Carolina, a small national park with an old-growth forest and, according to the website I Love National Parks, “more bugs than you can imagine.”

No doubt there are many bugs in New York’s forests, even cockroaches that have yet to find their way to somebody’s basement or bathtub. But Ms. Charlop-Powers, the executive director of a Manhattan-based nonprofit group called the Natural Areas Conservancy, is not focused on them. She sees the trees — and the forest. Most of the city’s forest is deep in parks, and on the worry spectrum, she is “concerned” about them.

“The situation is not dissimilar to the subways, in that we’re at a crucial moment,” she said.

Fortunately, urban forests appear to be at the point the subways reached decades ago, before transit policymakers decided that maintenance could be deferred.

But there is another concern for urban forests: climate change.

Ms. Charlop-Powers wants to prepare the city’s forests for a changing climate. If nothing else, forests can slow rising temperatures by reducing what foresters call urban “heat-island effects.” Trees lower the nearby temperature by up to nine degrees. There is also the worry that forests could be overrun by invasive species that can change soil conditions — another contributing factor on the climate change checklist.