These spaces are filled with trees, and that’s what makes them great — lines of American elms or London planes, groves of mixed hardwoods, lofty mature campus canopies, or many-storied successional forest species competing for space and light. I would not want to live in the city without trees.

Tree-lined streets and shady parks or wilds make our cities livable. Shade cools the city’s surface by as much as 20 degrees, canopies and roots intercept rainfall and help harvest its benefits, healthy tissues provide for uptake of pollutants, leaves release oxygen to the air we breathe, and all this wood keeps massive amounts of carbon from escaping into the atmosphere and turning up the heat. And the part I like most: Trees define urban space like nothing else in our design arsenal.

Imagine Boston without trees. It’s easy; just check out East Boston, where canopy coverage amounts to about 6 percent (compared to US Forest Service recommendations of 35 percent). For contrast, think of Roslindale and West Roxbury, where larger home lots and mature shade trees bring the coverage up as high as 49 percent.

A 2014 study by researchers at Boston University and Hofstra reported that Boston’s overall canopy coverage measures at 25.5 percent, with an error rate of ±1.5 percent. That is significantly down from the previous mark of 29 percent coverage reported in 2006. The decline could in part result from recent improvements in the way data are analyzed, but an international downward trend is confirmed by other studies.

Yale Forestry Researcher Thomas Crowther estimates, for instance, that Canada possesses around 9,000 trees per inhabitant; the US, with its broad open plains and sprawling cities, around 700 per person. In the 2006 Boston study, the num- bers were thus: around 1.2 million trees in total, or two trees for every citizen. Not enough!

In 2006, then-mayor Thomas Menino responded with a proposal to plant 100,000 new trees in the city, an increase that would get Boston closer to that 35 percent goal. The Boston Globe reported in 2013 that the city was far behind. The great recession had taken its toll on canopy expansion because the city’s proposed increases rely substantially on private development, public park expansion, campus redevelopment projects, and homeowners. Now, with Boston’s boom in construction, it’s time to rally.

Here is what we need to do: Be vigilant in protecting the trees we have, and be opportunistic about where we can plant. Translate these goals into enforceable rigor in project approvals. Exceed Boston’s Complete Streets guidance, which outlines provisions for planting in all but the narrowest streets. Plant trees on every project, and give them good life support below grade. Think of the living biology in the soil and root world as the reciprocal of the tree we see above grade — you can’t have one without the other. Let’s aim for higher than 35 percent coverage. That’s the radically transformative project that will make the physical structure of Boston robust, resilient, and spatially beautiful.