Recently, we wrote about the importance of setting a tree canopy cover goal for San Francisco, a city that should be a green leader. We’d like to see such a goal incorporated into the Urban Forest Master Plan, which unfortunately watered down its goals from the first public draft. San Francisco has an tree canopy cover percentage of only 13.7% – as against an ideal of 25%. (We’re writing to the Planning Commission at firstname.lastname@example.org – and if you would like to add your voice, please do the same. Tell them the Urban Forest Master Plan needs a canopy cover goal!)
Urban trees are a public asset. They benefit us all in many ways: Green infrastructure; fighting climate change; improved public health; reducing crime; improving economic values. Recently, we found an excellent note from Alliance for Community Trees (ACTrees) that compiled all these benefits – and documented the scientific data. We’re summarizing and including the PDF with permission.
You can read it here as a PDF: benefits_of_trees – Actrees There’s 19 pages of information in bullet points.
TWENTY REASONS TREES BENEFIT US
This is based on the note from ACTrees, using excerpts and summaries to bring out key points.
- GREEN INFRASTRUCTURE BENEFITS
- Economic benefits: The 3.8 billion trees in the US have a structural asset value of around $2.4 trillion.
- Reducing storm water runoff and maintenance costs: Urban forest can reduce storm water runoff by 2-7%, and a mature tree can store 50-100 gallons of water during storms. Portland is planting 4,000 trees to implement a gray-green storm water management solution – and saving $64 million.
- Improving air quality: Trees clean the air by absorbing carbon dioxide, sulphur dioxide, nitrous oxides, and trapping particulates.
- Improving water and soil quality: Trees divert captured rainwater into the soil, where micro-organisms filter out impurities. Trees can also help remediate contaminated soil, absorbing many contaminants.
- PUBLIC HEALTH BENEFITS
- Improving attention: Kids with Attention Deficit Disorder function better in green settings – as do college students in dorms with a green outlook.
- Decreasing asthma and obesity: Columbia University researchers found that asthma rates fell by 25% for every extra 340 trees per square kilometer [247 acres]. Kids in greener neighborhoods have a lower Body Mass Index.
- Improving physical and mental health: Visual exposure to settings with trees helps recovery from stress within 5 minutes. And in one study, workers without nature views from their desks claimed 23% more sick days.
- Reducing hospital days: Patients in post-op recovery had shorter hospital stays and needed less pain medicine if they had green views, compared with rooms facing a brick wall.
- Protection from Ultra-violet rays: A person takes 20 minutes to burn in full sun, but 50 minutes in part shade, and 100 minutes in full shade.
- Noise reduction: Trees absorb noise. A belt of trees 98 feet wide and 49 feet tall can reduce highway sound by 6-10 decibels.
- ROAD AND TRAFFIC BENEFITS
- Traffic calming and accident reduction: Trees improve driving safety. One study found a 46% decrease in crash rates after landscape improvements were installed. Drivers reduce speeds by an average of 3 miles per hour in the presence of trees. Trees can also reduce road rage by reducing stress.
- Reducing road maintenance costs: Trees prolong pavement life. Shaded roads can save up to 60% of paving costs.
- BUSINESS BENEFITS
- Business districts – Increased sales, desirability, and rents: Shoppers prefer districts with high-quality trees, and spend more time there. They are willing to pay 7-10% higher prices. Commercial offices with trees have a 7% higher rent.
- Jobs: In 2002, distributing, planting, and maintaining trees added about 2 million jobs. [Now – it can only be higher.]
- PROPERTY VALUE BENEFITS
- Increasing property values: Studies have found up to 37% increase in residential values.
- CLIMATE CHANGE AND CARBON BENEFITS
- Storing carbon and reduction of carbon emissions: Urban trees in the US store 700 million tons of carbon, and sequester 22.8 million tons of carbon per year. Urban trees sequester more carbon than wild forests because they grow faster. In California, if 50 million trees were planted, they would sequester 4.5 million tons of CO2 annually, and could reduce air-conditioning energy use equivalent to 1.4 million ton of CO2 in addition. That would be like retrofitting every household with energy-efficient devices.
- Carbon mitigation programs: In Los Angeles, the ‘Million Trees LA” campaign plans to plant one million trees, aiming to reduce carbon equivalent to taking 7,000 cars off the street each year.
- Reducing the heat island effect: Trees reduce the heat island effect. Shaded surfaces may be 20-45 degrees F cooler than unshaded ones. Trees cool city heat islands by10-20 degrees, reducing ozone levels and helping cities meet air quality standards.
- ENERGY USE BENEFITS
- Trees reduce energy consumption: Trees can reduce both cooling and heating costs by providing shade and acting as windbreaks. A 25-foot tree can reduce annual heating and cooling costs of a typical residence by 8-12%.
- COMMUNITY BENEFITS
- Less violence and crime: Public housing with nearby trees and nature reported 25% fewer acts of violence. Apartment buildings with high levels of greenery had 52% fewer crimes than those without any trees.
- Improves community: In buildings with trees, people report significantly better relations with their neighbors. People report a stronger feeling of unity and cohesion with their neighbors.
- Wildlife and biodiversity: Urban forests help create and enhance animal and bird habitat.
HOW MUCH TREE CANOPY COVER DO WE NEED?
How much tree cover a city needs depends on local climate. Eastern cities ideally need 40% and western cities need 25% canopy cover.
[San Francisco has 13.7%, a city estimate updated from USDA’s 2007 estimate of 11.9% using a different methodology. We will summarize the excellent USDA report on San Francisco’s Urban trees another time, but you can read the whole report here: SF Urban Forest fs fed US]