In the leafy Connecticut Avenue corridor of Northwest Washington, big old street trees are saying a slow farewell to a growing season that began too wet, turned too dry and ended too cold.

Some 60-foot oaks attest to years — even decades — of knowing such stressful conditions in tight root boxes, and they should be admired for their survival skills alone.

I say a slow farewell because this year many trees seems to be reluctant to let go of their leaves. Brown, shriveled leaves will often cling to oaks and beeches, but I’m talking about trees where the entire mantel of foliage remains firmly attached as we approach mid-December, albeit in fall coloration.

Sonja Dümpelmann, a landscape historian at the University of Pennsylvania, was recently in her native Germany and noticed the same phenomenon while she was riding the train to Weimar. When she joined me recently to look at street trees in Cleveland Park, we found the leaves still clinging — in the case of a sweetgum, they were just changing from green to yellow. What gives, we asked? Neither of us knew, but we live in strange times when shifting natural cycles seem bound up with unpredictable patterns of weather and climate.

One thing seems certain: The need for street trees is only going to get greater. They shade, they cool, they grab carbon from the atmosphere. Oh, and they can be beautiful if spared the butchery of life with the aerial power line. A 2015 study confirmed what we might have already known — people who live in the shelter of mature trees are healthier than folks in neighborhoods that don’t have them.

Even before the age of climate change, the value of the urban forest was understood, first in an aesthetic sense and later, with industrialization and urbanization, systematically and scientifically.

Dümpelmann tracks the history of the modern street tree in her book “Seeing Trees,” a deep, scholarly dive into urban society’s need for — and relationship with — trees that sought to return the natural world to the concrete jungle. Her focus is on two great sylvan cities — New York and Berlin — but she gives a nod too to Washington, the first city in the United States to implement a street tree planting program, she writes. By 1912, the capital had almost 280 miles of double plantings of street trees — approximately 100,000 trees and, crucially, a program of annual replacement.

Around the same time, civic-minded tree lovers in New York were pressing the city to create an urban forestry agency to plant and care for street trees, Dümpelmann writes, especially in poor and tenement neighborhoods in need of better sanitation. The effort was led by a physician, Stephen Smith, and other figures, the sculptor Augustus Saint-Gaudens and financier J. Pierpont Morgan among them. Smith had written an influential paper equating the lack of trees and summer heat with childhood mortality.

Today, the citizens and civic leaders of both Washington and New York embrace the urban forest with enthusiasm and resources. Public, private and nonprofit partnerships have led to a resurgence of tree planting. In New York, the city launched a program in 2007 to plant 1 million trees, a task that was finished in 2015. Trees were not always hugged like this. Older folks will remember the decline of American cities in the postwar decades, and in Washington, the city’s tree canopy withered as people abandoned the urban core for the suburbs. “Disinvestment” in communities, Dümpelmann says, “automatically meant disinvestment in the urban forest.”

Now, ironically, the sprucing up of the streetscape often precedes the gentrification of low- and moderate-income neighborhoods. “These are the big questions we need now to resolve,” she told me. “How do we see to it that everybody has access to nature and is not displaced when trees are planted?”

Some of the most poignant images from the book are of tree-planting efforts in Berlin and Kiel in the years after World War II, when both cities were still in ruins. One photo is of schoolboys planting saplings in Kiel in 1948 against the backdrop of a bombed-out building. The children and the young trees speak to a future being built out of a horrific immediate past.

“That’s where you can become very emotional about the trees,” Dümpelmann says. “For me, if you see trees suffering you automatically think about human beings suffering also.”

Other pictures show the re-planting of Berlin’s famous boulevard, Unter den Linden, against the backdrop of mountains of rubble. The avenue by then was in Soviet control. Many of the postwar trees declined before reunification and have since been re-planted.

During the Cold War, the communist authorities created a model, tree-lined boulevard in East Berlin named Stalinallee, which became part of a greening competition between the city’s two camps.

The East’s tree-planting frenzy of the early 1950s didn’t necessarily play out decades later in the form of a mature urban forest, though. Many died or declined as a result of excessive road salting and the use of gas lamps, which had the effect of poisoning plants.

Street trees can be alarmingly short-lived, so the mature oaks on Connecticut Avenue deserve to be cherished. But it is uplifting, too, to see a number of young trees in Cleveland Park getting established — beyond the sapling stage and on their way to healthy adolescence.

Dümpelmann seemed pleased by the abundance of street trees and their species diversity. “This isn’t scientifically proven,” she says. “But I think if people care about trees, they also tend to care about people.”

Tip of the Week

If you have spare refrigerator space, consider chilling tulip bulbs to force into early bloom in late winter. Triumph tulips are best for forcing. Set them in containers with potting soil, water and store in the fridge, not the freezer. After 12 weeks, bring the pot into a cool room to initiate growth.

— Adrian Higgins

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