The Commonwealth Avenue Mall is one of Boston’s signature sights, with its grand boulevard of majestic trees where residents and visitors can sit, stroll, or just enjoy some midday shade. 

It’s green spaces like this that contribute to Boston’s rich canopy of urban trees. But their real contribution goes beyond what we can see.   

"That vegetation is storing carbon," said Boston University scientist Lucy Hutyra. "If that vegetation was removed, that carbon would be in the atmosphere."

Hutyra is part of a team studying how cities like Boston can reduce carbon dioxide, a major greenhouse gas. She has a rooftop lab where she measures CO2 emissions from buildings and factories and the amount that plants take up. A robust urban forest is critical in controlling emissions, she said, since mature trees naturally absorb the gas, as well as reduce levels in the atmosphere. 

"The biggest contributions that the vegetation in the city has to our climate here is the cooling that the trees provide and the change in what the building energy demands are," she said. "And you put all of this together, and it does have a real climate impact."

As part of the city’s broader plan to reduce greenhouse gas emissions, former Mayor Tom Menino launched an ambitious effort in 2007 to plant 100,000 new trees by 2020. That would, according to Boston’s chief of environment, Brian Swett, increase canopy coverage from 29 percent to 35 percent. 

In the last six years, the city has planted 20,000 new trees in parks and along streets to provide shade and comfort. But that's just the first step; helping them thrive has proven more challenging, said city arborist Max Ford-Diamond, because of insufficient rainwater.

"I would say we have seen a lot more dead trees, trees with no leaves at all, than we've seen in previous years before," he said.

Meanwhile, Arnold Arboretum fellow Ailene Ettinger is studying the impact of climate change on trees there, in particular, the arrival of pests like the wooly adelgid that she hadn't seen in great abundance before. 

If average temperatures continue to rise, Ettinger said the city’s tree canopy may one day be completely different, and that's what she'll study at the Arboretum, where she can look at a bunch of different species growing in the same place.

Boston is expected to update its climate-change action plan in December, and city officials have said planting a wider variety of trees and maintaining those we still have would be a major part of it. 

Michael Dosman, curator of the Arnold Arboretum, talked about how climate change is affecting the trees under his care and what it means for everyone, on Greater Boston: