Dr. Robert McDonald is lead scientist for the Global Cities program at The Nature Conservancy, holds a PhD in Ecology from Duke University and has published more than 50 peer-reviewed publications and a new book, Conservation for Cities.


Dr. Robert McDonald is lead scientist for the Global Cities program at The Nature Conservancy, holds a PhD in Ecology from Duke University and has published more than 50 peer-reviewed publications and a new book, Conservation for Cities.

Each year, millions die around the world because our cities are becoming unlivable — hotter, dirtier, more stressful. And addressing these challenges isn’t easy. Cities are complex ecosystems with countless interrelated challenges and few easy answers.

Yet something as simple as protecting and increasing the stock of urban trees can make a difference. The humble street tree is an ecological powerhouse. Study after study has shown multiple benefits to people and society.

Trees and other green spaces in cities can help manage runoff during rainstorms. They can help clean and cool the air, reducing harmful air pollutants and air temperatures on city streets — saving tens of thousands of lives each year. They lend beauty to our communities and significantly increase property values. And time spent in natural environments has demonstrated mental health benefits.

However, even as the evidence for the benefits of urban trees mounts, American cities are actually losing trees — even though they are one of the most cost-effective solutions to health issues available to city leaders. While urban trees alone can’t solve all the public health challenges that cities face, they’re an important piece of the solution that municipal leaders can implement today to improve the lives and health of their citizens.

The health benefits of urban trees
• In Louisville, KY, a research team planted three rows of mature serviceberries, pine, cypress and cedar trees in the front yard of St. Margaret Mary Elementary School. Air quality was monitored pre- and post-planting, and 60 students and 20 adults agreed to take part in the study. An initial analysis found that study participants had increased immune system functioning and lower inflammation levels.
• Another study in Los Angeles found that the more parks that were within 500 meters of a home, the lower children’s body mass index was at age 18. Additionally, multiple studies have found that spending time in nature decreases stress levels and improves mental focus.
• Trees also mitigate summer air temperatures. Thanks to the shade they provide and water they release into the atmosphere, trees reduce summer air temperatures by an average of 2-4° Fahrenheit, although under some circumstances the cooling effect can be even larger.
• A recent Nature Conservancy study, called “Planting Healthy Air,” found that investing $100 million per year in tree planting (and maintenance) globally could offer 68 million people measurable reductions in fine particulate matter pollution, a significant factor in cardiovascular and respiratory illness.

Despite these benefits, American cities are currently losing approximately four million trees each year, or 1.3 percent of the total urban tree stock.cover_trees4health_final

A cost-effective public health intervention
The Nature Conservancy collaborated with the Trust for Public Land and the Analysis Group to analyze what it would take to turn this trend around. Our report, titled “Funding Trees for Health,” quantifies the investment gap — how much more investment in trees we would need to maintain our current urban canopy and then significantly expand it to seize greater potential health benefits.

We estimate that an additional investment of around $8 per person annually would be enough to create a healthier, greener future in US cities. We emphasize that this is an average figure, and the situation will vary greatly in different cities. Nevertheless, it is enough to show that a green urban future is not an impossible dream. It is quite affordable, if policymakers and others decide to make the investment.

The last section of the report describes some specific solutions that can enable tree planting for public health:

• Establish code that sets minimum open space or maximum building lot coverage ratios for new development.
• Implement policies that incentivize private tree planting.
• Break down municipal government silos and facilitate various agencies working together to ensure effective and efficient policies.
• Link funding for trees and parks to achieving health goals and objectives.
• Invest the time and effort to educate the public about the tangible public health benefits and economic impact of trees.

We urge all cities to begin exploring ways to create links between the health sector and urban forestry agencies, both intellectual and financial. Working together, the health sector and the urban forestry sector can achieve a healthier, more verdant world. To learn more, download The Nature Conservancy’s in-depth analysis.