Tree Coverage

Tree canopy and house size

I will take a controversial position: trees are good. Given the amount of needless tree destruction we see, I sometimes feel in a minority. The City recently took the elm in front of our house, diminishing our shade. It was nearing end of life, and I assume diseased or at risk of disease. They have not planted any replacement yet.

Of course, many people like trees. That is why most of us live under a canopy of trees, even in relatively urban areas. We constrain our houses to be two or three stories to fit under trees, so that we may get shade, reducing summer cooling costs and to a lesser extent winter heating costs. Commensally, we benefit many of the trees we keep because we send trees the extra water runoff that does not get absorbed by our houses’ impervious roofs, and our houses radiate excess heat, keeping the trees warmer.

The City of Minneapolis has a tree canopy of 31.5% (11,569 acres), according to a recent University study. Coincidentally, that is the city’s goal. That goal is too small.

The study says trees could cover an additional 37.5% of possible UTC (urban tree canopy, not university transportation center), including grass and impervious surfaces. Minneapolis has about 200,000 boulevard trees on over 1000 miles of street, leaving aside trees on private property. I believe we should as a society raise our target, maximize tree cover, so much so that during the summer we can hardly see the city from an airplane for the coverage.

There are other benefits to trees. Each tree we plant absorbs one ton of carbon in its lifetime. A carbon calculator tells me I need to plant 17.7 trees to offset one year of driving a gasoline-powered fuel efficient car (1000 miles per month). Fortunately I walk, because that seems like a heckuva a lot of trees to offset driving, certainly more than I could do on my own property (assuming they refer to full size trees). PBS tells me “Densely vegetated areas of a city can create cool islands within an urban center. Plus, shady sidewalks encourage people to walk rather than drive.” Those all seem good things.

Montgomery County, Maryland gives trees to qualifying CBD property owners. There the market failure seems to be the more general problem of private provision of a public good, namely sidewalks. People have little incentive to upkeep beyond the minimum required by law. What if property owners were required to build roads and the public took care of sidewalks?

Chicago pays people to grow trees through its Sustainable Backyards program. Is it really necessary to pay people for trees in their own yard? The main benefits to planting trees should be captured by property owners directly through lowered energy (heating/cooling costs). While I will admit there are some externalities (see Carbon paragraph above), they are small relative to the purported personal benefits of tree planting. Not every good idea should be a government program.

Trees have their risks. In Florida, palm trees can act like missiles during hurricanes. But they also have benefits, such as slowing erosion.

Improve your street, plant a tree.