Election Section

While we’re waiting for the first delicate ash-tree leaves to unfurl, and their later-rising neighbors to follow their example, I’m going to talk about why trees in a city are a good thing, and why we should think long and hard before cutting down a healthy one.  

Trees are efficient providers of habitat. A city with nothing but humans, our pets, rats, mice and cockroaches (OK, maybe Argentine ants) is not a very pleasant place. That’s aside from the thoroughness of destruction it represents. We’ve made the discovery that we can stack more of ourselves into small spaces if we build up: high-rises, skyscrapers. The same works for trees. On a footprint of a mere few square feet, it can stack yards and yards of habitat and food for birds, butterflies, squirrels and such pleasant company. Planting them in cities, we mitigate some of the insults we offer the land with paving, buildings—even exotic plantings that are useless to local animals.  

That vertical advantage works for us, too, in quantifiable ways. Along with their shade, city trees cool their surroundings by transpiration, as they exhale enough water vapor to drop the temperature around them by a few degrees, and lower it more by spending energy in the process of vaporizing the water in their leaves. Cop a feel of a healthy tree’s leaves, and you’ll notice how cool they are. Urban surfaces of stone and steel and concrete heat the air enough to affect weather in some places; we need that bit of cooling.  

Trees reduce energy needs, if we plant them right. Deciduous trees shade and cool what’s just north or east of them in summer and let sunlight through to warm the place in winter. A staggered row of evergreens on the north side of a building buffers the wind and reduces heating needs in winter. That means PG&E burns less fuel and spares the air—and there are other measurable ways trees improve air quality. 

Some are quite local. Trees in parking lots not only keep cars cool and reduce the need for A/C; they slow the rate at which asphalt paving volatilizes its petroleum compounds. Like all plants, trees produce oxygen and consume the greenhouse gas carbon dioxide; furthermore, they sequester the carbon part of it, by using it to build themselves, and, being long-lived, they sequester it for a long time.  

They keep more noxious gases and particulates out of human lungs, too. A preliminary study of Chicago air in 1991 estimated that the trees that shade 11 percent of the city filtered out 210 tons of ozone, 98 tons of nitrogen dioxide, 93 tons of sulfur dioxide and 17 tons of carbon monoxide. The study also showed trees removing 234 tons of particles less than 10 microns in size. These, especially the ones smaller than 2.5 microns, are strongly associated with lung diseases from asthma to lung cancer, with heart disease—even with strokes.  

Dust like this has been a feature of cities ever since we invented them. They are kicked up from the soil by foot, draft animal, and wheeled traffic; rubber tires and internal combustion engines are late additions to the grind. Industries have strewn the air with particles since the first days of blacksmithing; smokestacks are more a difference in degree than kind.  

Trees take up this dust both by absorbing it through their stomata—the respiratory pores in their leaves—and just by trapping it on leaf surfaces, especially if they’re fuzzy. Surface particles might blow away again, or be washed off harmlessly in rain, or just stick and be incorporated into leaf litter: mulch, compost—useful soil nutrition instead of dangerous pollution. In cities, disposable, compostable lungs become a good idea.  

When we cut a tree down, we immediately destroy all its efficient habitat, and even replacing it with a younger tree doesn’t mitigate that until years later, when the replacement gets as big as its predecessor. Meanwhile, a few generations of displaced small wildlife dies out. We’ve also eliminated an oxygenating air filter, and reversed the process of carbon sequestration as the dead wood decays. Trees are mortal, and sometimes we must cut dangerously ill individuals. But whenever we cut a city tree, we’re eliminating one of our best citizens.