Sap and the City: How to Help Urban Trees
In the wild, trees can live a long time. Many hardwoods can survive a century or more, and some conifers can live into their thousands. And yet, a study by Lara Roman of the University of Pennsylvania found that the average lifespan of a tree planted on the streets of Philadelphia was between 20 and 30 years. Cities are stressful to trees, and everything from road salt to foot traffic to car exhaust can prune the trees' lifespans. It doesn't help that most city arborists respond to complaints rather than giving trees check-ups that can catch problems before they turn fatal.
Yet trees are a huge boon to our city streets. They not only sequester CO2 by the ton, but also filter other forms of air pollution, such as particulates, nitrogen dioxide, and sulfur dioxide. The soil they sit in helps absorb runoff after rains, reducing water pollution and the load on our storm drains. They shade homes in summer and block wind in winter, reducing energy spent on heating and cooling. They cool streets with their shade, reducing the "urban heat island" and slowing the formation of ground-level ozone (an air pollutant). They make streets more pleasant to walk on, encouraging pedestrian traffic. Trees can even reduce crime! Not to mention trees just plain look nice, which increases property values.
For cities, green trees translate directly into green cash. The air cleaning alone is worth millions of dollars in a large city. And a bigger, older tree is worth more than a baby sapling: a tree 30 inches in diameter removes 70 times the amount of air pollution as a tree 3 inches in diameter. So if most urban trees are dying in their youth, we're missing out on the biggest benefits.
So what can we do? Instating better tree maintenance programs in cities can help, but there's a big gap in what we know about improving the health of urban trees and what we need to know. But some organizations are establishing citizen-science programs, using social media to promote knowledge of, appreciation for, and engagement with our hardworking city trees.
Friends of the Urban Forest, the Sacramento Tree Foundation, the California Center for Sustainable Energy, Friends of Grand Rapids Parks, and many other organizations have been using an interactive software called "Open Tree Map" to map out every tree in their region. Citizens can mark, identify, measure, and photograph trees in their neighborhoods, then upload them to the map.
Currently, tree maps are available for Grand Rapids, MI; San Francisco County, San Diego County, Yolo County, Yuba County, Placer County, Stutter County, and El Dorado County, CA; Burlington County, Camden County, Gloucester County, Mercer County, and Salem County, NJ; Delaware County, Bucks County, Chester County, Montgomery County, and Philadelphia County, PA; and all of Delaware. All in all, over 11 million Americans live in areas covered by tree maps. Who knows, you might be one of them!
With this knowledge, we can learn about tree health, age and species, and teach people how trees help fight pollution and save money. We can monitor how much CO2 the trees suck up. And we can look for neighborhoods in cities where trees live longer, and find out what these places are doing right.
--image by iStockphoto/yuriz
--Rachael Monosson is an editorial intern for Sierra and a recent graduate of Stanford University, where she studied Earth Systems. She lives in San Mateo.