What might we learn from urban trees? Five years ago I started an unusual experiment to explore this question. I wandered haphazardly through the streets of the Upper West Side in Manhattan until I found a street tree that was, on the surface at least, thoroughly ordinary and unexceptional. I then started my watch at the tree, listening to its stories, watching how the tree was woven into the life of the city. I continued these observations in all seasons and in all weather over several years, whenever I visited New York.

From my watch I learned how deeply human lives are linked to the lives of trees in the city. These connections exist at many levels: Cellular, physiological, psychological, and economic. The tree even mediated the social dynamics of people on the street. And what was true for one ordinary tree was then magnified thousands of times for the urban “forest”, making trees in cities vital parts of human well-being.

The tree that I watched in New York grows from a small rectangular opening in an otherwise unbroken expanse of sidewalk and roadway concrete. Stray pieces of rubbish litter the meagre soil around its trunk. The tree is a Callery pear, an import from China, and its branches carry glossy green leaves to the third floor of the apartment building next door. A street vendor and news stand sit under the arches of its limbs. Most people walk past without giving the tree a glance.

The tree seemingly stands as a separate individual. But this is an illusion. The tree’s life and the life of people in its neighbourhood are inseparably connected. Every breath of air that we inhale has been partly cleansed by trees; their leaves absorb and detoxify air and the complex architecture of twigs and bark also serves as a soot-catcher, filtering the air. Our lung cells are therefore directly connected to the lives of trees. I rest my hand on the bark and see how my palm has been stained by the tree-captured pollution. The dark smear on my skin is a reminder of the benefits my lungs derive from the tree’s presence.

When the rains come, trees act as sponges, drawing water through their roots and holding it on and in their leaves. As I stand next to the tree in a storm, I see the water trickling down its bark to the soil, water that would otherwise join the sluicing flood on the pavement. The surge of floodwater that comes to cities in the aftermath of rainstorms is thus reduced through the actions of trees. People, rivers, and wetlands downstream are therefore linked to city trees.

The tree that I studied also changed how people interact on the street. On a sidewalk without trees, the only possible movement for people on the busy thoroughfare is to keep moving. Like a raging river, the pedestrian street flow carries all along in its power. But a street tree creates a pool, a space where the flow is broken. People can step into this quieter zone to talk, to rest. City trees diversify the possibilities for human movement and interaction on the street, for the benefit of all.

Our senses also benefit. Street trees change the smell and taste of the air, a sylvan balm for our noses and tongues. Leaves reflect sound, creating a small auditorium under the branches, a friendlier acoustic than the open street. Light finds its many leafy moods among the branches, refreshing our harried eyes with a memory of the forest.

In summer, I placed a thermometer under the tree’s shade, then moved the thermometer a few metres away to the unshaded ground. The tree cooled its surroundings by 10°Celsius. No wonder people love to congregate there in the afternoon and the street vendor is the envy of his unshaded colleagues. If one tree can create such a microclimate, imagine the benefit over the whole city. In New York City, street trees save at least $10 million a year in air-conditioning costs. Not only do trees save money, they also reduce the demand for electricity, thus reducing pollution from power stations.

Trees in cities also reduce stress and open our imaginations. Urban life is often psychologically taxing, increasing anxiety and other forms of distress. The sight, sound, smell, and feel of trees ease our minds and emotions, drawing us into community and interconnection with others. Urban trees also draw our imagination into places beyond the street, serving as portals into a green world not centred exclusively on humans.

Life is about relationships. No species can live alone. No individual can survive without interconnection to others. The pear tree in New York taught me this, that even though we may seem to be separate, we’re in fact deeply and inseparably intertwined.

David George Haskell, professor of biology at The University of the South, Tennessee, US, is the author of The Songs of Trees and The Forest Unseen

Related Topics
comment COMMENT NOW