Urban forest biodiversity and cardiovascular disease: Potential health benefits from California’s street trees

mortality from cardiovascular disease, where the lower number reflects the Top 2 scenario and the higher number the Top 10 scenario. In terms of cost-effectiveness, this is a return of $2.70- $140.07 for each dollar invested. The lower estimate is similar to results from a simulation that estimated medical cost savings in return for investing in parks and trails [82] and a compre- hensive workplace wellness program [83]. This simulation suggests that if our effect sizes are replicated within a causal framework, increasing the diversity of tree planting may be a cost- effective way to prevent cardiovascular disease. Conclusion Cardiovascular disease is the most common cause of mortality in the United States. Though management of blood pressure has improved over the last two decades, risk factors like obesity and diabetes remain challenging to address. Many population-based epidemiological studies have found an association between exposure to greenspace and lower cardiovascular risk. Numerous experimental studies of Shinrin-yoku and similar means of sustained forest expo- sure point to acute physiological changes after walking in a forest. Beyond the extent or quan- tity of general greenspace exposure, the quality of biodiversity may influence health outcomes via enhanced immune functioning and thus reduce mortality risks of diseases connected to proper immune functioning. Our paper examines whether greater biodiversity within urban forests in California is asso- ciated with lower cerebrovascular and cardiovascular mortality. Using mortality data from

2010–2018, we find that increasing the diversity of an urban forest by one standard deviation (0.64) is associated with a decrease in the mortality rate of 21.4 per hundred thousand for heart disease or 13% and 7.7 per hundred thousand for stroke or 16%. The relationship is linear, though having very diverse forests may offer additional protection. We explore possible mech- anisms and fail to find support that our effects are driven by psychological restoration or dilu- tion of disease risk. We conclude that enhanced immune functioning seems the most likely pathway. As stated earlier, the role of actual biodiversity within green spaces and urban forests on human health outcomes remain highly unexplored. Our paper is the first to assess how bio- diversity exposure is associated with cardiovascular health. Future research should use a causal framework to test the relationship between biodiversity and human health and potential mechanisms. The strong association between urban forest diversity and reduced cardiovascular disease mortality suggests that tree-planting may be a useful policy lever to address cardiovascular dis- ease. Benefits from planting street trees typically do not include avoided mortality from heart disease and stroke. A policy simulation of different planting scenarios for the city of Los Ange- les found that even small efforts to increase planting diversity, such as splitting new plantings evenly across more than two genera, may be a cost-effective policy tool to reduce the risk of

cerebrovascular and cardiovascular disease. Though our estimates include many uncertainties, we find that increasing the biodiversity of existing tree planting initiatives may beget a return of $2.70-$140.07 for each dollar invested. The lower estimate is similar in magnitude to the return on investment parks and trails and a comprehensive workplace wellness program. Future research should use more comprehensive data on urban forest biodiversity, such as the future TreeKeeper data from LA, and an experimental design that allows for causal infer- ence to directly test the relationship between biodiversity and prevention of cardiovascular dis- ease and mortality. Work is also needed to better understand the dose-response function for chronic exposure to biodiversity when living near greenspace. Moving forward in this way could enrich our conception of “green prescriptions,” improve policy targeting to communi- ties with inequitable access to biodiversity, and encourage cost-effective policy investments. PLOS ONE Biodiversity and cardiovascular disease PLOS ONE | https://doi.org/10.1371/journal.pone.0254973 November 3, 2021 17 / 23