Community Trees and Human Health: What is the Evidence?

The environmental services and benefits of urban trees are diverse and well recognized. For example, trees can reduce greenhouse gases through carbon storage, decrease stormwater runoff through interception and absorption of rainwater, and reduce the urban heat island effect. Each of these services has public health implications.

However, what is the direct relationship between urban trees and human health? Knowledge about this connection is still developing, and research has expanded rapidly over the past two decades.

Our international team of authors published a scoping review of 201 studies in June 2020. A scoping review is a science exploration that displays the depth and breadth of a topic, to help guide future research and promote best practices.

The systematic search for articles used keywords representing human health, environmental health, and urban forestry, and were entered in search systems that serve environmental, natural resources, and medical audiences. After collecting and screening several thousand articles, 201 studies were sorted into a three-part conceptual framework.

• Reducing Harm, representing 41% of studies, addressed the broader environmental conditions that can cause illness or reduce wellness.
• Restoring Capacities, at 31%, included studies about how nature experiences can be used to address disease or if one is unwell.
• Building Capacities, at 28%, are the studies that demonstrate how nature experiences support and encourage health and wellness.

Figure 1 shows the 3 part conceptual framework, and the number of studies that we found for each of the specific health outcomes associated with experiences of trees and forests. Note that many studies found positive outcomes. Others showed mixed results as perhaps not all measures in a study detected change. Some studies showed negative response to trees, such as those testing tree pollens and allergic response. Interestingly, tree pollens were not as severe allergens when studies compared other common respiratory irritants, such as dust mites and molds. Also, the medical databases turned up more of the pollen articles and fewer of the other benefits articles, suggesting that the medical community may not be fully aware of the potential of community trees and health benefits.

Figure 1: Scoping review of research on urban trees and human health: summary classification of effects and outcomes (credit: Sharon Lam)

What are the policy implications of our findings? Many possibilities emerged:

• Trees and the urban forest are essential for health-supportive environments in communities.
• Health and environmental professionals (e.g., urban foresters, town and city planners, and urban designers) should collaborate to promote urban trees as a social determinant of health.
• Evidence supports the need for equitable distribution of trees across towns and cities so that all residents can experience health benefits.
• Tree planning and management in communities is a long-term, even multi-decade, investment for better public health.
• The costs of individual and community healthcare are substantial; investing in urban trees can pay human well-being and economic dividends.