A Shield of Leaves

On 911 Greenwich Street in Lower Manhattan, a water-droplet-shaped tree rooted in a puny patch of soil stretched its branches sunward, painting the adjacent New York City street in dappled shade. Among a handful of other trees also boxed in by sidewalks and streets, this individual Callery pear tree absorbed carbon dioxide produced by the zipping cars, the ambling people, the humming buildings. It shaded and cooled the fast-movers and weary businesspeople. Its limbs offered refuge to critters that skitter and fly.

But life in the city is hard for a tree. The dirt is too compressed to share water and nutrients. Foundations, pipes, and concrete stifle root systems. Invasive pests and toxic chemicals threaten harm to the entire system. Trees didn’t evolve to survive this jungle of glass and steel. Yet, this Callery pear tree pressed on. Then, on September 11, 2001—rooted in Ground Zero—cement, glass, office supplies, and the ash of human catastrophe smothered out the tree’s life-giving sun.

Now, 20 years later, another human-led tragedy is encroaching on cities and their trees across the world. The climate is warming fast—and cities are warming faster. They are becoming “urban heat islands.” Unlike the surrounding areas with cooling vegetation and bodies of water, cities suffer the sun’s power more than outlying areas. Synthetic materials absorb the sunshine, reaching temperatures upward of 20 to 45 degrees Fahrenheit hotter than shaded areas, and then the city releases its collection of the sun’s heat until the surrounding air rises by two, five—as much as seven degrees Fahrenheit.

In the concrete jungle, there are not enough trees to cool the environment and sequester the growing mass of carbon dioxide from the atmosphere.

Urban trees can be our allies against the largest human-caused threat we face. And as global temperatures continue to rise, the right mix of trees can help bring the cool back to cities for future generations. As we rush to plant more, we must confront an innate challenge: Humans built cities for humans—not for trees. Cities are hard enough for trees to survive in, and climate change makes their struggle even harder. If we want to reap the full benefits and services of our tree companions, we need to take greater consideration of trees’ needs against the changing environments, while calling in a diversity of species for help.

In a world of climate-weird, the rules of what grows well in different cities and regions are changing. We can no longer differentiate “native” or “non-native,” experts are cautioning. The Monterey white oak from Mexico is getting along well in a hotter Austin, Texas. The Chinese Akasha orchid and golden bell trees are surviving the heat while withstanding the frigid touches of the polar vortex in the U.S. South. The historically Southeastern tulip popular is finding a milder Minnesota quite tolerable.

And no one tree species will be a savior for urbanites, despite cities tending to play favorites with cherries, maples, or oaks. Trying to replicate the natural world within the developed has never made any sense. In a manmade ecosystem like a city, there is the question of what team of trees can survive and thrive within the changing limits of the local concrete—and how we can improve conditions for our leafy companions even by a little.

Horticulturists chant “right tree, right place,” hoping city planners, developers, and homeowners heed the call. Check out the “likes to soak in one rain after another” pin oak, they say to cities becoming more prone to floods. Consider the “barely thirsty in a drought” hedge maple for a drier West. Learn more about the “not fazed by much” hackberry. When in doubt, turn to the “I can take the heat and then some” trident maple.

We need our trees to grow big and old to do their best work—shielding kids, grandkids, and future generations beyond. We have the tools and the willpower to lighten the weight of the city so the right trees can tighten their hold on the earth against the stronger storms, the flashier floods, and the increasingly violent winds. We have the insight and wisdom to diversify our canopies to withstand the problems we know are coming—and even those issues we don’t understand yet—so most of our trees inhaling carbon dioxide can remain, even if some fall. We have an obligation to loosen and nurture the compacted soil with every new planting so trees can reach deeper and stretch higher into a durable shield we can stand beneath together, stronger.

Humans are resilient. Lucky for us, so too are many species of trees—with a bit of our help. September 11, 2001, should have been the end for that Callery pear tree, as it had been the end of nearly three thousand human lives that day. Yet in the weeks of rescue and repair thereafter, strong, tired human fingers tugged aside the rubble and found its stripped woody remains. Parks and Recreation employees removed the tree and tended to it for the next nine years until it was ready to return to its post—to fight for life in the wake of one human-caused disaster just as another is making a slow-motion landing. A “survivor tree,” they call it. I like to think all trees can be survivor trees with our help—and in their survival, they can help our species survive this changing world, too.