Scoping study lays groundwork to show trees benefit our health
by Missouri LSEN Staff
I think I shall never see
A poem lovely as a tree.
So begins the most notable verse penned by American journalist and poet Joyce Kilmer. As with many of Kilmer’s works, “Trees” celebrates the beauty of the natural world. But is there something more than intangible, anecdotal, eye-of-the-beholder beauty in trees and green space? Perhaps something with quantifiable, measurable, and positive health effects?
A systematic scoping review, “Urban Trees and Human Health,” published in the June 2020 International Journal of Environmental Research and Public Health tends to make some definitive associations between trees and health. The authors reviewed and sorted 201 studies into a three-part framework to develop categories and subcategories:
-Reducing Harm, representing 41 percent of the studies, includes topics such as air pollution, ultraviolet radiation, heat exposure, and pollen.
-Restoring Capacities, at 31 percent, includes attention restoration, mental health, stress reduction, and clinical outcomes.
-Building Capacities, at 28 percent, includes topics such as birth outcomes, active living, and weight status.
A guide for policymakers.
The study posits that the urban forest is a green infrastructure system with environmental, economic, social, and health services, and functions in cities, yet no review has specifically examined how urban trees affect human health. The “Urban Trees and Human Health” review is a comprehensive study on existing literature that can help inform and guide future research, policy, and public health interventions based on green space and the natural environment. Specifically, urban planners may want to consider strategically promoting trees as a social determinant of public health to achieve, among other aims:
- The ability of trees to reduce greenhouse gases through carbon storage.
- Decrease stormwater runoff through interception and absorption of rainwater.
- Mitigate the urban heat island effect.
It’s also important to note that the scoping study was a global review of literature and 39 percent of the studies included were conducted in North America. The authors also discuss the generalizability of the conclusions as being most fit for temperate climates.
Variables among all literature and research to be cautious about in concluding include study size, age, gender, personality types, baseline health conditions, and even size, number, and species of trees. The report also lists and outlines the limitations of the research and ongoing research needs.
Breaking it down.
Taking a closer, abbreviated look at the scoping study’s three categories can provide insight into policy actions, additional study and research, and ongoing conversation about the benefits of green space — particularly trees.
Reducing Harm.
- Air pollutants and respiratory conditions.
- Positive outcomes included lower prevalence of lung cancer associated with green space covered with trees in residential areas; lower prevalence of asthma among children associated with greater street tree density; and reduced asthma hospitalization associated with tree density.
Ultraviolet radiation.
- UVR exposure is a major risk factor for most skin cancers. “In general, the studies found that trees can reduce exposure to UVR,” though studies also noted that visible shade is not the best indicator of UVR protection, as numerous factors determine the degree of protection provided by shade.
Crime.
- Studies focused on this social, rather than environmental, source of harm in the relationship between trees and crime. Some mixed findings, but generally findings indicate that trees may reduce the incidence of various types of crime. Influencing factors also include the size, location, and health status of trees.
Restoring Capacities
Cognition and attention restoration.
- A study centered on a Chicago public housing complex determined: “Adult residents having nearby trees and lawns versus paved outdoor surfaces showed better attentional functioning and life management effectiveness — and less household aggression and violence.”
- Though not all findings were consistent, physical activity — whether indoors or in a “forest bathing” setting — generates better short-term memory, reduced fatigue, and reduced tension, anger, and depression.
Mental health, anxiety, and mood.
- Forest settings produced lower measures of anxiety, depression, anger, confusion, and fatigue.
- Near-infrared spectroscopy (NIRS) of adults revealed a more relaxed brain state.
Not surprisingly, concerning workplaces, self-reported job satisfaction and stress were improved with window views of forests. Similarly, however, indoor handicraft interventions in a therapy setting showed positive effects in non-natural environments that promote creative engagement.
Clinical Outcomes
- There were many positive results in studies that looked at physiological conditions with exposure to urban trees. Among the results: Significantly lower blood glucose, HbA1c, and blood pressure were noted in a longitudinal study of non-insulin-dependent diabetic patients who participated in forest walks on nine occasions over six years.
- Another study observed that patients with views of a tree through their window (versus view of a brick wall) had significantly shorter recovery times following gallbladder surgery. In general, exposure to trees among clinical populations produced mainly positive responses.
Building Capacities
Birth outcomes
- Lower odds of preterm births in areas with more street trees. Other studies were not as conclusive. The relationship between trees and neonatal health warrants further exploration.
Immune system.
- “Spending time in forest settings, even short visits, may promote healthier human immune systems, though underlying pathways are not completely understood.”
Active living.
- The presence of urban trees tends to result in increased physical activity, particularly among seniors.
Cardiovascular function.
- The majority of published articles provide evidence for the positive influences of urban trees on cardiovascular health.
Overall, the study found that exposure to trees is associated with multiple health benefits. Again, there are additional factors and even competing interests — primarily, funding for urban treescape planning and parks — as well as challenges to ensure equitable distribution of trees to provide residents and neighborhoods with access to the health benefits of trees.
Missing link: Climate change.
With all the discussion and review of the literature to quantify the health benefits of trees, it’s a bit surprising that climate change was not included as a topic. After all, the research presents a dual picture of fast-growing trees as a result of warmer temperatures, as well as tree degradation as a result of climate change. (See “Climate change accelerates growth of urban trees in metropolises worldwide.”)
Trees in urban settings may experience accelerated growth due to higher levels of CO2 in the atmosphere and warmer temperatures (due to urban heat island impacts). The trees can also be impacted by climate change due to increased insect/pest pressure. On the flip side, urban trees filter air pollutants and cool the area directly surrounding the trees, so they may be a helpful tool in mitigating the impacts of climate change in urban areas.
The City of Columbia, Mo., adopted a comprehensive Climate Action and Adaptation Plan (CAAP) in June 2019. (Download the plan here.) While the primary focus of the CAAP is the reduction of greenhouse gas emissions, the plan includes some treatment of climate change versus trees conundrum.
In summary, “Warmer temperatures and increasing drought will likely stress trees and shrubs in Columbia, making it more difficult for new plantings to survive and increasing the risk of disease, pests, and invasive species infestations. The range of species will likely shift, altering the composition of trees and shrubs suitable for Columbia’s climate — for better or worse.”
