Take a look at this shot from Google Maps and see if you notice anything:
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If you haven't found anything, feel free to turn on the labels to show road names. Find Beals Street. (Yup, where JFK was born.) Now turn off the labels. Zoom out a bit if you want. And note that Beals Street has more treecover than any other street in the area. (Once you zoom out, you can also follow the Brighton-Brookline border by where the treecover changes south of Commonwealth Avenue, too.)
Beals Street is lined by American Sycamore trees. This isn't obvious from above, but as you walk down the street the mottled, almost camouflage bark is striking as some of the trees are several feet in diameter. Their canopy spreads over the street to the extent that sunlight rarely breaks through to the road. Most other streets in the area have less continuous tree cover, and certainly none appear as straight lines of green from an aerial view.
The question is, how long will this last? Beals Street has a monoculture tree of the same age. Even if the trees are not felled by wind all at the same time, they will likely succumb within a few years of each other (there are already a few notable gaps along the street). If there is a blight or other disease, they could die even more quickly. At that time, Beals Street will be opened to sunlight like never before. And the houses, especially those on the northwest side of the street, will no longer be shaded through most of the day. And there seem to be no efforts to plant new trees in the sycamores' stead.
Street trees have a long history, but the American Elm is probably the most telling example of what happens when a tree quickly dies off. The imported dutch elm disease began killing off elms in the 1930s in Ohio; by 1970 most of the tree's range was infected. Streets which had previously been lined with the cathedral-like elms had nothing but stumps. Many such streets have still not recovered their tree cover—Kansas City had planted mostly elms and many streets became devoid of trees in ten years.
Some elms survive along street. Luzerne Street in Johnstown, Pennsylvania has been painstakingly preserved for years as infected trees are razed and replanted. (Yes, they have a Facebook page.) Winnipeg, where the disease only spread in the late 1970s, has successfully kept the effects of Dutch elm disease manageable in the last three decades. But it doesn't come cheap: Winnipeg has significant legislation and capital costs to keep the disease at bay.
Still, the results are remarkable: on Google Maps, streets appear as strips of green between houses. Going forward, we should savor such resources, but when new street trees need to be planted, age diversity and biodiversity should be goals.