| 05 April 2010
Energy conservation is perhaps the most valuable of the ecosystem services that trees provide. Why? Because it has trickle-down effects that amplify other benefits, like air quality improvement and carbon sequestration. But we’ll get to those later this week. For now, let’s talk about how trees help save energy.
Trees help reduce energy used for cooling in three main ways:
- Shading reduces the amount of heat that reaches buildings.
- Shading of heat-absorbing surfaces like parking lots, roads, roofs, and sidewalks helps reduce the “urban heat island” effect. These manmade surfaces can increase city temperatures as much as 8-10° above the surrounding natural areas.
- As trees take in and give off water (evapotranspiration), they use up solar energy that would otherwise heat the air. Over large areas of trees, this effect can be substantial.
Trees also help reduce energy used for heating:
- Reducing windspeed reduces the movement of colder outside air into buildings.
What kinds of impacts can we expect in San Francisco?
In all honesty, the energy conservation benefits of trees in San Francisco will likely be lower than just about anywhere else! Why? Mainly because of our very mild climate. Few San Francisco buildings even have air conditioners, so there aren’t so many opportunities to reduce air conditioning use, and it’s rarely cold enough to require heat.
A 2003 study [PDF] on the street trees of San Francisco estimated that the average tree here reduces energy consumption by only about $1. Compare that with the results of a similar study in Berkeley [PDF], where warmer temperatures and bigger trees yieled energy benefits that were 15 times higher.
Still, there are a lot of ways we can plant trees with energy conservation in mind. In a future post, we’ll talk about how to get the most out of your trees with special emphasis on the unusual circumstances of San Francisco. And remember, one of the coolest things about the Urban Forest Map is that it will help estimate how much energy the trees of San Francisco are conserving. Once you've added a tree (with its trunk diameter and species), we'll calculate the energy benefits and add them to the city total.
For more information on how trees help conserve energy and how and what to plant to maximize benefits, visit the Center for Urban Forest Research’s website and check out these two research summaries [PDFs]:
