Dense deciduous forests form the backdrop of childhood memories. West Virginia–my home–is covered in trees. I don’t think I was ever outdoors without a tree being within sight. And usually not one, but tens of thousands covering the hills with canopies thick and heavy with foliage in summer, then a cacophony of reds and orange in fall, and labyrinthine branches in wintertime. Trees seemed as abundant and eternal as the earth itself. And I of course paid the trees no mind. Like the air or water, trees were simply an unchanging, integral part of the world that needed no explanation.

I never spent any time in cities as a child. I remember visiting my grandparents on my father’s side who live just north of Seattle, on the coast. We would always fly into Seattle, but always immediately drive right out; and as a child I was usually in a long-travel stupor between the flight and the car ride to their home so never noticed much of Seattle. I also remember visiting Branson in the Ozarks when we spent time with my grandparents on my mother’s side, usually at Christmastime. But we were always in Branson for an amusement park, so trees weren’t on my mind. I remember also a trip to Washington D.C. in the eighth grade. The focus of the trip was history, not trees, so I can’t recall whether or not I saw any at all. But besides these few city visits, my childhood world was uniformly green with trees.

My first experience of urban life was in Northern India. I lived for 7 months or so in the city of Varanasi in Uttar Pradesh. During that time, I traveled to cities in Rajasthan–Jodhpur and Jaipur among them–as well as Lucknow in Uttar Pradesh and Kolkata in West Bengal. Each city was uniquely beautiful and interesting, like any city in the world. But that abundant and eternal feature of experience in West Virginia–the presence of trees–was radically different in these urban spaces. Urban trees in India were few and far between. Sometimes they lined a street or were clustered together in a park. Occasionally you saw a good outcropping of them on a nearby hill. But, for the most part, the world was made up of concrete, glass, and steel. Trees were sojourners, occasionally desirable for ornamental value but otherwise tolerated only if they didn’t get in the way.

I didn’t think much about trees while living in urban India. The myriad new people, foods, and experiences were far more interesting. And, frankly, besides generally appreciating their beauty, I never thought much about trees until this past year. My interest in trees–and specifically urban trees–began when I joined a research lab in my junior year at Princeton. The lab was then called the “Sustainable Urban Cities” lab (which was often abbreviated as “the SUS lab,” much to my delight), but is now called the “Urban Nexus Lab.” The research group is run by a remarkable woman named Anu Ramaswami who also directs the university’s “M.S. Chadra Center for Global India.” I had taken Professor Ramaswami’s class on “Sustainable Cities in the US and India” the year before. She had incredibly clear, nuanced, and expansive insight into complex urban systems and all their moving parts. I wanted to learn more from her, so I joined her lab.

The Urban Nexus Lab integrates research on what Professor Ramaswami calls “key provisioning systems” that exist in any given urban space and make it possible for people to live in those spaces. These provisioning systems are: “water, energy, shelter (buildings), mobility and/or connectivity, food, sanitation and waste management, and green and public spaces” (from Ramaswami, “Unpacking the Urban Infrastructure Nexus with Environment, Health, Livability, Well-Being, and Equity,” 2020). I joined her lab without any specific project in mind and offered myself as a set of hands to do whatever they needed. Incidentally, the lab was just launching a new research project on urban trees in India, which fits into the “green and public spaces” provisioning system. I joined the project at its beginning and from that point began to think a lot more about urban trees. I worked with the lab through the Spring (2021) and then full time through the summer. Now, over a year later, the India urban tree project has become my senior undergraduate research project. Needless to say, I’ve thought a lot about urban trees this year! And I’m convinced that they’re awesome. I hope you’ll think so too.

Urban trees around the world are kind of like lone cowboys who shoulder the burden of the whole world as they carve out a crude existence in a barren landscape. Alone and isolated from their families, these trees grow up fast and hard. They have more canopy space than they would have if their seed were dropped in a traditional forest, so they take full advantage and eat up as much sunlight as they can. This means that they don’t need to grow as tall to get their nourishment, so they tend to grow shorter and wider than traditional forest conspecifics (trees of the same species). Their roots, however, tend to be quickly circumscribed by hard concrete, pipes, and other impervious surfaces. In a traditional forest, the roots would link up to other trees through vast networks of fungi, forming a subterranean social network that passes information and goods from tree to tree (for a wonderful depiction of this forest dynamic, and for a good discussion on urban trees read The Hidden Life of Trees, by Peter Wohlleben). But these urban trees have to get by alone and with limited underground space.

My research has focused on quantifying one of the many benefits urban trees convey to city dwellers: carbon offset. Climate change is, of course, one of the biggest challenges facing the world today, and cities tend to be the worst perpetrators of greenhouse gas emissions. Trees are wonderfully designed carbon vacuums: they absorb the carbon dioxide that humans emit when fossil fuels are burned for energy consumption and transportation and convert this carbon, using energy in sunlight, into sugars that support the tree’s growth, maintenance, and reproduction. This is the very familiar process we call “photosynthesis.” Some trees use these photosynthesis-produced sugars to regularly generate very tasty fruits. Some trees divert more sugar into their vertical growth and become behemoths like the redwoods of California. But in all cases, the source of carbon that forms the chemical backbone for these sugars is the very atmospheric carbon dioxide that humans both produce in high quantities and are now very concerned with removing from the atmosphere. Very convenient!

When carbon dioxide is emitted within a city boundary (such as from a car’s combustion engine or from a local power plant’s energy generation process), that carbon dioxide molecule has a chance of being sucked up by local urban trees. Of course, the chances that the molecule is sucked up by an urban tree depends on the number of trees in the city. The more trees, the higher the chance. Think of the all the leaves on all the urban trees in a city together like a big fish net: the more leaves, the more little carbon fish you can catch every time you cast the net. The rate at which carbon dioxide is “caught” by urban trees (often called “sequestration rate”) can be approximated by the growth rate of those trees. If some carbon dioxide is taken up by a tree and turned into sugar, the majority of that sugar is used to generate more wood and grow the tree. And the wood stored by the plant over time is directly related the net amount of carbon removed from the atmosphere. The faster a tree gains weight, the more carbon dioxide it’s taking out of the atmosphere. So, at a city scale, the rate at which all the trees in the city are growing is a good approximation for the rate at which carbon dioxide is being removed from the atmosphere by those trees.

The carbon is stored long-term in the tree’s wood, so each tree can also be thought of as a carbon battery. The notion of “carbon battery” may feel foreign, but it’s actually the kind of battery we put in our cars and make electricity with. Most coal and oil is plant biomass (old tree bodies) condensed for a really long time at really high pressures. Dry wood is also the most common “biofuel” in the world for activities like cooking. To us humans, burning wood is a very natural and intuitive way to produce heat and light. When wood is cut and decays or burned, it releases all of its stored carbon back into the atmosphere in its oxidized carbon dioxide form. The lesson for cities: if trees are planted and kept, more carbon is both taken out of the atmosphere and stored. But if trees are cut down for development, all that carbon they’ve stored is emitted back to the atmosphere. So urban trees can actually be net emitters in a city if that city is undergoing a lot of construction projects or expansion at its edges.

You may think: well, one tree growing or being cut down doesn’t affect the situation very much, and you’d be right. On the city scale, one tree doesn’t make or break any carbon emissions portfolio. But on a city scale, and even though the trees may be few and far between, we’re talking about hundreds of thousands or millions of trees within the city boundary. The most recent study on urban forestry done by our lab showed that for some cities, the carbon sequestered by these trees offset up to 3% of that city’s carbon emissions, whereas for other cities the carbon released by trees cut down for development constituted over 20% of total emissions. Urban tree management makes a big difference.

So for the past year and a half I’ve studied urban tree carbon storage and sequestration. My thesis is in the final stages of writing and I hope to share it once I’m done. But for now, I just wanted to share one reason urban trees are so awesome: carbon!

The next time you’re out and about in your favorite urban space, keep an eye out for the local tree population. How are they doing? Do they look strong, healthy, and well-cared for, or neglected? Do you see many trees at all? What species are they? How big are they, on average? Would you like to see more trees around your city, or less? Trees are one of those things that we just take for granted, but human activity puts them, like everything in the world, at jeopardy. I believe that human activity can also bless trees and help them flourish, but only if we think and act carefully.

As a concluding thought, consider the following vision of the heavenly city in Revelation, ch. 22: “the river of the water of life, bright as crystal, flowing from the throne of God and of the Lamb through the middle of the street of the city; also, on either side of the river, the tree of life with its twelve kinds of fruit, yielding its fruit each month. The leaves of the tree were for the healing of the nations.” God includes trees in his perfect urban master-plan. And although this vision is of the heavenly Jerusalem, perhaps the leaves of urban trees today also provide real healing of the nations through the twin miracles of photosynthesis and atmospheric carbon sequestration.