We are reprinting, with permission, an article on the website of the San Francisco Forest Alliance about the many ecological functions performed by our urban forests and the plans of the Natural Areas Program to destroy over 18,500 mature, healthy trees. Please visit the website of the Forest Alliance to read about their efforts to save the urban forest in San Francisco from needless destruction.
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Urban trees are hugely important, not just for their beauty, but for environmental reasons. The [Natural Areas Program’s] NAP’s SNRAMP plans to cut down 18,500 trees (and a whole lot more under 15 feet in height, plus whatever is lost to wind-throw when the wind-break of the other trees is gone).
What are these 18,500++ trees doing for us? Here are nine ways in which urban forests help us.
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The Nature Conservancy's Carbon equivalence graphic Storing Carbon. Trees store carbon and keep it out of the atmosphere. The bigger the tree, the more it stores. CLICK HERE for a 3-minute video from the Nature Conservancy about the carbon stored by a red oak tree that’s 18 inches in diameter. Eucalyptus may store even more, because it grows taller than a red oak and is more dense (Eucalyptus, around 50 lbs/cu ft; red oak, about 41).
- Providing Oxygen. Trees produce more oxygen than they use. When they’re felled, they decay and use oxygen instead of making it.
- Trapping and removing air pollution. Tree leaves capture air pollution, and help clean our air. The trapped pollution stays on the leaves or falls to the ground – where we don’t have to breathe it.
- A windbreak. In its pre-European state, San Francisco was a place of windblown sand that got into everything from railway tracks to people’s lungs. With a city and a major park atop the former dunes, we don’t have to worry about sand so much, but the wind still sweeps across our city. The eucalyptus forests and other trees act as a windbreak, and improve the micro-climates not only of the forest, but of surrounding areas.
- Buffering noise. Trees absorb sound, in much the way that fabrics and soft materials do. Once they’re felled, everything becomes noisier. Thinning a forest lets in the sounds of the city and its traffic. When Laguna Honda Hospital felled some 200 trees in conjunction with its new building, neighbors in Forest Knolls and Midtown Terrace noticed increased noise.
- Slowing runoff. When it rains, the roots of the trees, and the duff made by their shed leaves and the understory beneath them, soaks it up like a sponge. Then it slowly lets it out again, allowing plants and vegetation to use it over time, replenishing ground water, and fighting erosion. (If you want to see the difference – drive by Christopher, below Mount Sutro, during heavy rain – and then drive up Twin Peaks Boulevard. The latter’s like a river when it’s pouring.) [See “Rainfall Interception” data from USDA]
Preventing erosion. Many of these trees grow on very steep slopes, and below them are our neighborhoods. Their roots function now like a geo-textile, holding the slopes in place – particularly in forest areas, where the roots are intermeshed and intergrafted. On Twin Peaks, where the vegetation is thinner, landslips occur every season of heavy rain. In Forest Knolls, clearing of slopes below the houses has resulted in landslides requiring months of tarping to stabilize them. This is a particularly insidious problem; it may take 6-8 years for the root system to die and decay, and by then the homeowner may not even know or recall that trees once held the slope together.
Provide habitat. Trees provide cover, places to perch and hide, and food by way of nectar and leaves and the insects attracted to the trees. Eucalyptus, in particular, flowers in winter providing nectar for bees, butterflies, and birds – and attracting birds that prey on these insects. It’s a nesting site for owls and hawks and feral bees, and a hunting ground for birds small and large. Our city would have far fewer birds, animals, and bees without these trees.
- Boost property values. People like trees. Homes near forested areas are valued by owners and potential buyers. Realtors often mention these settings in their listings. Some studies show mature trees nearby can add up to 30% to property values.
Conservation Sense and Nonsense 
Here is an interesting study of an urban forest. Trees do 8 times better in the New York City Urban Heat Island.
They claim that heat is the main factor helping trees grow based in part on laboratory experiments. They mention that nitrogen from pollution may have been a factor, but they did not mention the elevated levels of CO2 also found in cities. However, if one reads narrative descriptions of flora during the Medieval Warm Period it is clear that plants grew where they don’t grow today, so heat is probably the main factor.
Do Urban ‘Heat Islands’ Hint at Trees of the Future? http://www.earth.columbia.edu/articles/view/2967