Years before creating his award-winning designs for urban plazas and parks, legendary landscape architect Lawrence Halprin studied trees. Street trees, to be precise. In a meticulously detailed article for an architectural magazine, he sketched the patterns, colors, and shape of their canopies, leaves, and seeds, noting nuisances or special features in loving detail. In a way, I find it fitting. Trees are an important component to the construction of outdoor urban space, a living framework to the city itself.

The urban tree can be an irreverent challenge to city order: obscuring our beloved buildings, taking root where they are unwanted, and up-heaving our delicate sidewalks with their persistent growth. Else, they can be embraced by the hardscaping, celebrated as landmarks.

Their form itself contains an architectural quality, the branches supremely evolved supports for a delicate canopy of leaves, the city’s ceiling.

A tree can be a memory of a past pattern or time: in Cape Town, South Africa, an effort to eradicate a mixed-race neighborhood during apartheid was survived by a hardy date palm, surviving to the present day amid acres of grassy wasteland. In Georgetown, the rhythm of trees seen below documents the former route of a street long cut short.

The tree provides shape and structure to the street; an unpleasant street and a treeless street are nearly synonymous, a point not lost on a Greensboro, North Carolina urbanist who finds trees the major difference between a desolate street an an inviting one. Here, the lived experience of an urbane street is revealed to contain a highly regular rhythm of trees in plan view.

The tree takes on additional burdens in the suburban context, defining the very boundary of the street. Is this a street lined with homes, or lush foliage?

Trees have been recognized as indispensable to livable cities: they are natural air conditioners and purifiers, and perhaps necessary to help combat global warming. The organization Casey Trees has gone so far as to assign dollar values to the trees in Washington, D.C.: according to their formula, a Willow Oak outside my front door is worth $12,226.

Many cities celebrate their trees with surveys and press releases announcing their calculated value. Their benefits come with one caveat – at least one allergist fears the over planting of male trees may boost pollen levels in urban areas.

The subtle impact of trees on cities can extend far beyond aesthetics and air. One researcher found that people are willing to spend more in business districts with trees than without them. Another found that, among 28 architecturally identical high-rise apartments, residents of buildings with trees had better relations with neighbors and experienced less violence.

As central as trees are to a city’s life, they have a role to play in death. Ailanthus altissima, made famous at Betty Smith’s tree growing in Brooklyn, can grow in highly polluted soils with no human care. In Detroit, German artist Ingo Vetter found the trees (known locally as “ghetto palms”) helped him understand the decaying cityscape: “… from their height, you could guess about the time these places were left abandoned.”

One imagines when our cities shrink, in forested areas it will be our garden and street trees who will repopulate the place, obscuring the remnants of our industry with their quiet canopy.