Planting Healthy Air got a lot of press when it was released at the American Public Health Association annual meeting in Denver that ran from late October to early November. The report dives into the research evidence base for how trees positively affect air pollution and urban temperatures and directly ties those benefits to public health.
Planting Healthy Air was written by The Nature Conservancy for C40 Cities Climate Leadership Group, which represents a network of 785 cities with 650 million residents (about 25% of the world population).
Particulate matter, a.k.a. the worst part of air pollution (especially fine particulate matter) kills about 3.2 million people a year worldwide. Urban heat waves are estimated to kill an additional 12,000 people a year and make cities hard to live in for millions more. However, the right, well-placed urban trees help to improve air quality and reduce urban temperatures.
The authors of Planting Healthy Air provide an evidence base for the effects of trees on air quality and temperature in cities and extrapolate how and where trees should be planted for the greatest benefit. They even include short case studies for specific cities like Denver, Beijing and Atlanta, among others. Their conclusion, in short: Planting more trees where there are more people provides the highest benefit.TrAir
The specifics are more complicated as the results of studies conducted worldwide provide a spectrum of data. As always, in looking at how plants behave and impact the world, the answers are “it depends.”
In general, trees with greater leaf surface area and rougher surface area are better at dry particulate matter deposition for reducing air pollution. Evergreens are generally good choices for this purpose, especially if air pollution doesn’t dissipate seasonally and continues to be a wintertime issue. As could be expected, however, when regarding the cost of trees in isolation only for particulate matter removal, trees aren’t always the least-expensive option. Co-benefits back to the community like stormwater mitigation, aesthetics, higher property values, etc. make the proposition pencil better. There was no parsing of how tree structure, pruning or maintenance would impact net benefits, although when trees are planted with the primary purpose of improving air quality, the authors noted they shouldn’t be planted in such a way as to block airflow.
Research into the temperature-reduction effects of urban trees shows that most benefits are felt within about 300 ft. of the tree. Trees cool based primarily on providing shade and transpiration. Some studies have demonstrated that a 1.5F rise in extreme temperatures results in approximately a 3% to 5.5% increase in all-cause mortality and a 1.1% to 26% increase in cardiovascular mortality. The August 2003 European heat wave was well documented in Paris. In those neighborhoods where temperatures were the warmest, there were more deaths. The authors write that “each increase of 1C (1.5F) raised the odds of death during this particular heat wave by 21%.” As high temperature extremes continue, the World Health Organization (WHO) estimated that deaths from high temperatures could reach 100,000 worldwide by 2030 and 250,000 by 2050. Trees effectively reduce nearby air temperatures, as do cool and living green roofs, and cool pavements.
The report focusses exclusively on street trees and on particulate matter and temperature reduction. While the authors state that the benefits of urban trees go far beyond these two specifics, they don’t delve into the other co-benefits of urban forests.
Planting Healthy Air provides a great evidence-based case that can be leveraged by urban foresters, urban planners, landscape architects, garden designers and others that seek to provide health and environmental benefits in cities through vegetated systems. The information is meant to engage the public health community as well, creating more stakeholders and allies. The professional document supports the arguments, while allowing plenty of room for urban foresters and local authorities to provide the specifics of design, tree selection, planting and maintenance. There are a couple of visuals that quickly convey how trees do the work of impacting particulate matter and temperature (Figures 7 and 10), that practitioners will find beneficial in stating tree benefits in an easy-to-understand way.
“Street trees can be part of a cost-effective portfolio of interventions aimed at controlling particulate matter pollution and mitigating high temperatures in cities,” the authors write, adding that “trees cannot and should not replace other strategies to make air healthier,” but instead be used “in conjunction with these other strategies …”
The take away: Plant more trees in areas where they make a difference .
A selection of some of the publicity that the release of “Planting Healthy Air” generated: Urban Trees Can Save Tens of Thousands of Lives Globally by Reducing Air Pollution and Temperature–Nature Conservancy Study Projects Impact of Tree Planting in 245 Cities on the Nature Conservancy website; The Big Green Payoff from Bigger Urban Forests-Trees clean and cool the air, but just how much depends on where you are, a new report finds by Laura Bliss for The Atlantic’s CityLab; How planting trees in cities can save thousands of lives by Chelsea Harvey in The Washington Post; and Why Public Health Researchers Are Looking to Urban Trees by Katharine Gammon on Smithsonian.com.
November 15, 2016 NewTerrain.