Arbor Day and the Importance of Chicago's Urban Forest


sketch from City of Chicago press photo

We’ve been talking a lot about trees lately – which is only appropriate as last friday was Arbor Day.  While protecting the rainforest and preserving our National Parks is vitally important to the health of our planet, we can’t forget the trees right around us.  Urban trees are important.  Dan Burden of Walkable and Livable Communities Institute estimates that a $250 to 600 dollar planting and initial maintenance cost can return $90,000 in various benefits  during the lifetime of a tree not including “aesthetic, social and natural.”  Check out his VERY informative 22 point benefit list for more info and/or read on for a number of interesting tree-related stats cribbed from his presentation.

Here in Chicago, the city celebrated Arbor day with a ceremonial tree planting – Mayor Emanuel hoisted a shovel to plant trees with school children from Jahn World Language Elementary  and announced that the city plans to plant 5400 new trees in 2014.   Chicago is home to 3.5 million public and private trees which cover 17 percent of our land area.  Here’s why that matters:

Slow Drivers and Make Streets Safer

Street trees increase safety BOTH by separating pedestrians and cars more effectively AND by creating a narrower visual field for drivers and reducing the speed cue effect generated when a suburban street looks like a highway.

This point is made well by Roman Mars in his 99% Invisible episode, “Built for Speed.”   Urban engineers in the mid-century era tried to make residential streets safer by widening the field of view … moving trees back from the street.  But they ended up having the opposite effect.  As he puts it, “the wider a road is or is perceived to be” the faster drivers’ speed increases.  The difference between a densely treed (and parked up) urban street and a wide-open-spaces suburban drive without trees or sidewalk makes a big difference in the way people drive on those streets.

Cut down our Need for Storm Water Infrastructure

Trees absorb the first 30% of most precipitation through their leaves and another nearly 30% at the root.  This absorption reduces run-off, compensates for the drainage problems of nearby hard surface and generally reduces the necessary drainage infrastructure (Burden).

Forest Products Laboratory studies of Trees and Storm Water Runoff estimate that city trees can reduce total annual runoff by 2-7 percent.  In residential developments they up that estimate to 65% of runoff diverted when trees are combined with other natural landscaping features (via ACTtrees, the Alliance for Community Trees).

FPL tree_stormwater diagram

Trees are “Cool”

Outdoor areas under tree shade are 5 to 15 degrees cooler (again Burden).  Nearby shade trees can reduce energy costs (Air Conditioning energy) for residential or commercial buildings by 15 to 50% according to So they make the area feel more pleasant and save money directly too.  That’s a win, win.

And they Clean the Air we Breathe 

Every school child learns that trees (and other plants) absorb CO2 and transform it into the O2 we like to breathe.  But how much good do they do in the face of the CO2 we humans generate all day long?  Lest we think that a tree lined boulevard is offsetting all the pollution produced by the cars driving along it … here’s an alarming pair of statistics.

A tree can absorb up to 48 pounds of carbon per year (credit here)

Burning one gallon of gasoline releases 20 pounds of CO2 (credit, Car Talk)

So that’s one tree we need for every two gallons of gas used per year.  We need a lot more trees (and less driving) if we’re hoping for a total carbon offset.  Suddenly all those seemingly serene trees seem like they might be working pretty hard to get their job done.

Having them near by DOES increase their benefits.  “Trees in street proximity absorb 9 times more pollutants than more distant trees” meaning that we are getting cleaner air by standing near trees and we’re also limiting the global spread of pollutants by having as many trees as possible near the emissions source (once again, Burden).

City Trees

The trees along this city street,

Save for the traffic and the trains,

Would make a sound as thin and sweet

As trees in country lanes.


And people standing in their shade

Out of a shower, undoubtedly

Would hear such music as is made

Upon a country tree.


Oh little leaves that are so dumb

Against the shrieking city air,

I watch you when the wind has come, –

I know what sound is there.


Edna St. Vincent Millay