by Tami Biddle, Clean Air Board member
The trees around us perform many functions; some of these are well-known and appreciated, others less so. Trees add beauty to our environment. Tree-lined streets, because they are a delight to the eye, add value to neighborhoods. But those trees are doing more than creating pleasurable scenes for residents. They are helping to clean the air, they are reducing temperatures created by paved surfaces, and they are assisting in the reduction of human stress.
Cleaning the Air
In an exchange of gases, trees take in carbon dioxide, convert it to food, and release oxygen into the air. The exchange occurs through pores on the surface of leaves, or stomata. But stomata can also take in gaseous pollutants from the air, including ozone, sulfur dioxide, nitrogen dioxide, and carbon monoxide. Leaves can trap particulates as well. Spines or hairs on a leaf’s surface can trap pollutants. Some leaves have waxy surfaces that work the same way. Naturally, larger groupings of trees can have the most significant effect on the environment. Pollution bearing winds passing through a tree canopy lose speed and some of their pollution load. But smaller collections of trees, including woodland glens, roadside parks—even trees along highways—can aid in improving local air quality.
Cooling the Air
Trees can also aid the environment by cooling air. California Forest Service researchers found that trees in parking lots could lower air temperature by up to three degrees. Those few degrees can reduce temperatures on the surface of cars, and inside the cabins of cars. Most importantly, gas tank temperatures can be lowered by up to seven degrees. Fewer hydrocarbon emissions result from gas that evaporates out of tanks and hoses with reduced temperatures. Paved spaces without trees create high temperatures, increasing pollutant emissions and, therefore, the formation of smog and ozone.
For trees to have a net positive effect on energy usage in buildings, proper placement is key. Well-placed trees can lower temperatures in buildings by shading them. But poorly placed trees can increase energy needs by shading in the winter or blocking summer breezes. With proper placement, trees can create significant energy savings. Homeowners and building owners see a reduced energy bill, and the entire local community benefits from lowered energy demand and usage. When energy demand decreases, pollutant emissions from the power plants supplying energy also decreases, and this helps improve air quality. It is also important to select long-lived trees so that minimal energy expenditure is required for planting and removal. And low maintenance trees are essential in urban environments so that the energy cost of tending them with motorized vehicles or tools (watering, trimming, etc) does not outweigh their positive environmental effects.
Reducing Stress and Violence
Research from a number of different universities has made a strong case for the stress-reducing effect of trees. A research team led by Dr Bin Jiang at the University of Illinois subjected 158 volunteers to mildly stressful scenarios. Afterwards the volunteers used a VR headset to view one of a selection of six-minute 360-degree videos featuring urban areas with variable amounts of visible tree canopy coverage. The participants’ stress levels were measured by physiological tests and by questionnaires; the results revealed a positive, linear association between the density of trees and recovery from stress. The study is one of many demonstrating positive psychological benefits from natural landscapes. Researchers at Glasgow University found that being around trees and grass lowered brain stress levels in humans. Similarly, a study done in Edinburgh, Scotland used lightweight, portable electroencephalograms to measure the brain patterns of volunteers while they were outside in different environments. In tree-barren urban settings, participants were more aroused and frustrated than they were in parklike settings, where their brain activity patterns became calmer and more meditative.
Studies in three American cities—Portland, Baltimore, and Philadelphia – indicated that well-placed trees can help people feel less stressed, safer, and more committed to the care of their local environments. In Philadelphia, a 2011 study found a substantial reduction in crime, including a 7 percent to 8 percent decrease in gun assaults across most of the city—as the result of a program to clean up vacant lots and plant trees on them. The study in Baltimore provided the strongest connection yet between trees and crime. Comparing neighborhoods that were otherwise similar in density and income level, it revealed that the ones with more trees had significantly lower crime rates. While the researchers did not claim causation, they did claim a strong association that was not explained by other socioeconomic and housing factors. Across the entire area of study, neighborhoods with 10 percent more tree canopy were shown to have nearly 12 percent less crime than comparable neighborhoods.
In the past, trees and vegetation have sometimes been implicated in crime because it was thought they provided hiding spaces for criminals. This prompted some urban planners to eschew them. The result, though, was barren urban space devoid of the calming effect of trees and plants — and devoid of the lower temperatures and community-bonding opportunities provided by summertime shade. Tree-lined neighborhoods reduce stress and encourage local pride. The shade they offer entices people outside, and being outside encourages people to build social relationships that, of themselves, can aid in crime reduction.
Boult, Adam, “Being Around Trees Makes You Less Stressed – Study,” The Daily Telegraph (London), 6 May 2016
Conniff, Richard. “Trees Shed Bad Rap as Accessories to Crime” Environment, Yale School of Forestry and Environmental Studies. 2012. http://environment.yale.edu/envy/stories/trees-shed-bad-wrap-as-accessories-to-crime
Downing, Adam, “Air: What’s a Tree Got to Do With It?” 2 September 2011, at http://www.ecology.com/2011/09/02/air-and-trees/
Forrest, Sharita, “Watching 3-D Videos of Trees Helps People Recover from Stress, Researchers Say” 21 Oct 2014, at https://news.illinois.edu/blog/view/6367/204489
How Trees Help Clean the Air, Forest Service, US Department of Agriculture Bulletin No. 412 (December 1977)
Nowak, David J., The Effects of Urban Trees on Air Quality, USDA Forest Service, Syracuse, NY (2002) http://www.nrs.fs.fed.us/units/urban/local-resources/downloads/Tree_Air_Qual.pdf
Reynolds, Gretchen, “Easing Brain Fatigue with a Walk in the Park,” The New York Times, 27 March 2013