Bob Polomski, Ph.D. | Associate Extension Specialist–Horticulture/Arboriculture |Dept. of Plant & Environmental Sciences | Clemson University
As Secretary of the Urban Tree Growth & Longevity Working Group, which is affiliated with the International Society of Arboriculture, I had the opportunity to interview Michelle Kondo, Ph.D., lead author of a groundbreaking study that investigated the existence of a connection between tree canopy or greenspace and premature human deaths: Health impact assessment of Philadelphia’s 2025 tree canopy cover goals. Lancet Planet Health, 2020 Apr;4(4):e149-e157.
Q:Most of us are aware of the ecosystem services provided by the urban forest, but I feel that few are aware of the benefits of the urban tree canopy on people. Would you discuss the scientific evidence regarding the importance of urban vegetation, notably trees, on our health and wellbeing?
Kondo:There is so much new and rigorous evidence of the benefits of urban greenspace in general on human health. There is some evidence that suggests that it is trees in particular, as opposed to other forms of urban vegetation, that affect human health and well-being. A study of the association between self-reported health and near-residence trees, grass, and total vegetation found significantly higher reporting of very good health for participants with high tree cover, and no association with grass cover.Another recent study found that tree cover, and not grass, lowered risk of dementia. With widespread death of ash trees due to emerald ash borer in the mid-western US, USDA Forest Service researcher Geoff Donovan found reduced mortality due to lower respiratory illness and cardiovascular disease.
Trees can be more effective at reducing land and air temperatures than grass or other vegetative cover. Trees also filter pollutants from the air and can reduce noise pollution. Trees (and tree planting and stewardship activities) bring people together, reduce social isolation, which can have immense benefit for human health.
Efforts to increase urban tree canopy cover, or the proportion of land covered by tree crowns when viewed from above, are a predominant form of urban greening policy and therefore I am hopeful that more greenspace-health research will make use of new datasets that indicate tree canopy specifically.
Q:In your groundbreaking study, you investigated if there was any connection between tree canopy or greenspace and premature human deaths. What prompted you to conduct this study and why did you choose the city of Philadelphia?
Kondo: First, I am a scientist of the USDA Forest Service [Northern Research Station, Philadelphia], in which scientists typically focus on forested ecosystems. But my station in Philadelphia focuses on trees in urban areas and the role they play in the lives and livelihoods of urban residents.
This study relied on strong partnerships with city agencies and local nonprofits. My Forest Service colleagues and I have spent many years developing partnerships within the city and metropolitan area to answer questions about why and how trees and green spaces are important to urban residents.
Philadelphia also had available data resources to conduct this health impact assessment. It is one of few cities that has a high-resolution tree canopy cover dataset developed for it, based on LiDAR-derived land cover assessments. This data is very resource intensive to produce.
Philadelphia has one of the most extensive parks system of any large city in the US. But it is also an older city, and so many of the residential neighborhoods are very dense, with attached rowhouses lacking yard space and little tree canopy. Furthermore, there are equity concerns here as there are in many other similar post-industrial cities. In Philadelphia, socioeconomic status (SES) and tree canopy are closely tied; median household income strongly coincides with overall greenness levels in neighborhoods, and lower SES neighborhoods tend to have less trees or other vegetation than higher SES areas.
Q: In your study you found significant differences in tree canopy cover between low and high income neighborhoods. Was socioeconomic status a factor that affected annual death rates?
Kondo:It could be that socioeconomic status affects mortality rates, but in this case the main reason we saw more benefit to low-income neighborhoods was because low-income neighborhoods have fewer trees than high-income neighborhoods in Philadelphia. So bringing low-income neighborhoods to the 30% tree canopy cover goal would benefit them most of all.
Q:What are the costs of increasing tree canopy cover compared to the savings in human lives and healthcare costs?
Kondo:In the paper, we write:
“Although we can predict economic values associated with prevented loss of life… we were not able to compare the costs and benefits of increasing tree canopy directly. Predicted values do not correspond to direct costs or cost savings assumed by any one entity, such as the city. Moreover, it is very difficult to estimate cost for tree canopy goals because of many uncertainties, eg, related to unknown losses to development or natural causes. Health benefits can also come from greening interventions other than tree planting (eg, landscaping or gardens). Few, if any, experimental studies have compared the effects of tree planting versus other forms of greening or public health interventions on morbidity or mortality.”
In other words, while we can estimate economic gains from improvements in health, it is nearly impossible to estimate costs of increasing tree canopy cover. There are too many unknowns and potential set-backs.
Q:As we discovered during the pandemic, a wide gap exists in public health and health inequalities relative to socioeconomic status. Based on your research, can increased tree canopy cover bridge this gap?
Kondo: This is an important question that is not easily answered. But I can say that some of my own research provides some evidence that yes – increasing/improving neighborhood greenspace can help to reduce health inequalities. My colleagues and I have run a randomized-controlled trial of a vacant lot cleaning-and-greening intervention (managed by the Pennsylvania Horticultural Society under their LandCare program) that is improving many disadvantaged neighborhoods of Philadelphia with maintained grass and trees. We found significant declines in crime in general and gun violence around these improved lots, as well as significant declines in feelings of depression and worthlessness among near
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