An effort to selectively remove branches from nearly a thousand trees in the Back Bay Fens is underway and will continue into March.

The tree pruning project by the Emerald Necklace Conservancy, a nonprofit stewardship organization for the 1,100 acres of the park system, will altogether cover more than 900 trees.

The work in part addresses tree health, public safety and park aesthetics, according to Jack Schleifer, field operations manager for the conservancy.

“We’re right in the heart of the pruning right now, making noise but making sure that it’s safe and that we’re taking care of all the trees,” Schleifer said.

The organization inspects and prunes trees in the several parks of the Emerald Necklace on a seven-year cycle. With this year’s project in the Back Bay Fens, more than 2,400 trees were assessed for size, health condition and species among other qualities. Out of that figure, 930 trees were deemed to need selective branch removal, according to Schleifer.

“Definitely a lot of trees, and some of them are quite large and quite noticeable, so they require pretty big equipment and work,” he said, Some trees have big, dead branches that pose safety hazards if they fall. Some of the branches grow too low and block views or make roads and light fixtures unsafe. And some trees are too dense. Thinning their branches increases their vitality and helps to keep them growing, Schleifer said.

The Back Bay Fens remains open throughout the project, which is expected to continue into mid-March. Visitors may see small worksites as the arborists move throughout the park each day, according to Schleifer.

He said winter is the best time for pruning because the trees are dormant, so the branch trimming creates less strain and stress, making it easier for the trees to heal and resist disease and infection. The park, quieter now with fewer people, is also currently frozen, which minimizes disturbances to the ground.

With hundreds of trees to prune and each tree’s care costing hundreds of dollars, the contracts for the project are in the six figures, according to Schleifer.

“It’s expensive work,” he said. “It’s a big priority.”

The pruning project is supported in particular by Liberty Mutual Insurance and the Boston Planning Development Agency.

The import of the effort to remove certain branches from certain trees in the Emerald Necklace extends beyond the visible, physical park aesthetic and safety benefits. There’s a public health impact as well.

Trees filter pollutants out of the air, sequester carbon, manage stormwater and reduce urban heat, according to Schleifer.

The goal of pruning the trees then is partly “to invest in these ecosystem services,” he said.

“We want the trees to continue providing these benefits to the community for as long as they possibly can,” Schleifer said. “We know that their benefits increase drastically as the trees get bigger and older, so we’re really here trying to do whatever we need for our old trees so that they can keep growing and keep cooling our city and keep making Boston a healthier, more beautiful place.”