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

Abstract

Enhanced immune functioning in response to biodiversity may explain potential health benefits from exposure to green space. Using unique data on urban forest biodiversity at the zip code level for California measured from 2014 to 2019 we test whether greater diversity of street trees is associated with reduced death from cardiovascular disease. We find that urban forests with greater biodiversity measured via the Shannon Index at the genus level are associated with a lower mortality rate for heart disease and stroke. Our estimates imply that increasing the Shannon Index by one standard deviation (0.64) is associated with a decrease in the mortality rate of 21.4 per 100,000 individuals for heart disease or 13% and 7.7 per 100,000 individuals for stroke or 16%. Our estimates remain robust across several sensitivity checks. A policy simulation for tree planting in Los Angeles based on our estimates suggests that if these relationships were causal, investment in planting for a more biodiverse set of street trees would be a cost-effective way to reduce mortality related to cardiovascular disease in urban areas.

Fig 1. Variation in Urban tree diversity in California.

Notes: Data for 857 zip codes in Heart Disease sample California from consortium of private tree maintenance companies.

Fig 2. Histograms of Shannon index and trees per hundred thousand individuals.

Notes: Data for 857 zip codes in Heart Disease sample California from consortium of private tree maintenance companies.

Fig 3. Urban forest diversity and mortality.

Notes: Data for 857 California zip codes for Heart Disease and 241 zip codes for Stroke for 2010–2018 from California Department of Public Health. Shannon index calculated from data from consortium of private tree maintenance companies.

Fig 4. Change in Shannon index by number of genera targeted for simulated LA planting.

Figure Notes: Data for 61 zip codes that are at least partly within the boundaries of the city of Los Angeles, California, and are in the bottom 50^th percentile in terms of total trees. The simulation evenly splits a total of 50 newly planted trees per zip code under five scenarios, each corresponding to a box plot: A single genus (the most common in LA, “top” and the fifth most common), as well as evenly splitting plantings across two, five, and ten genera. For example, in the top five genera scenario, each genus would increase the total trees by 50/5 = 10 trees. The Shannon Diversity Index is recalculated for each scenario and boxplots show the distribution in the change in the Shannon across 61 zip codes.