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

The mechanisms driving the association between biodiverse nature-exposure and health are unclear. Candidates include psychological restoration via the biophilia hypothesis [8] and dilution of the risk of infectious disease transmitted through an arthropod vector [9]. Summa- rizing the literature, Kuo [10] proposes a central pathway by which exposure to biodiverse nature would promote human health: enhanced immune functioning. Environmental biodiver- sity may contribute to enhanced immune function by improving the resiliency and diversity of human commensal microbiota [11–13]. If true, exposure to biodiversity could mitigate the autoimmune and inflammatory errors that contribute to cardiovascular diseases [13,14]. Despite potential links, to our knowledge no studies have assessed whether exposure to more biodiverse urban forests is associated with cardiovascular benefits. Most work has focused on green spaces, including absence and accessibility, or the size and number of trees in the landscape; the role of biodiversity is untested [15,16]. Within this literature, several studies find that exposure to trees is associated with better cardiovascular health. Donovan et al. [17] use changes in tree exposure from tree loss due to the Emerald Ash Borer and county-level mortality data and find a robust relationship for lower respiratory disease mortality and weaker results for heart disease mortality. A study with a similar design but individual-level data, which mitigates the risk of ecological bias, also found that tree loss from Emerald Ash

Borer was associated with increased cardiovascular disease [18]. Using a cross-sectional design, several studies find that neighborhood greenness is associated with lower cardiovascular dis- ease [19–21] and overall non-accidental mortality [22]. In our study, we assess the relationship between urban forest biodiversity and cardiovascu- lar health, while controlling for total tree exposure, using California data from 2010–2018 at the zip code level. California’s urban forests are among the most diverse in the world. In con- cert with a diverse set of biomes and socioeconomic settings, California’s forests offer an unmatched setting for an initial assessment of the relationship between urban forest biodiver- sity and health outcomes. Our diversity measure, the Shannon Index, captures both the genus richness within a zip code and evenness of abundance across genus, e.g. the dominance of one or many genera. To calculate the Shannon Index, we use a unique dataset of individual street trees shared with us by a consortium of tree maintenance companies. Our results show a statistically significant and robust negative association between genus diversity and cardiovascular disease mortality prevalence (heart disease and cerebrovascular disease). A one standard deviation increase in our diversity measure, the Shannon Index at the genus level, 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%. We also investigate

the potential causal drivers behind this relationship and fail to find support for the restoration or dilution hypotheses, leaving improved immune function as the most likely pathway. Using our estimates, we simulate avoided mortality monetary benefits, net monetary bene- fits, and cost-effectiveness for a proposed tree planting quota for the Los Angeles, CA. With our simulation, we find that increasing the biodiversity of existing tree planting initiatives may yield a return of $2.70-$140.07 per dollar invested. We join several studies that find that exposure to trees is associated with better cardiovascu- lar health. Donovan et al. [17] use changes in tree exposure from tree loss due to the Emerald Ash Borer and county-level mortality data and find a robust relationship for lower respiratory disease mortality and weaker results for heart disease mortality. A study with a similar design but using individual-level data, which mitigates the risk of ecological bias, also found that tree loss from Emerald Ash Borer was associated with increased cardiovascular disease [18]. Using a cross-sectional design, several studies find that neighborhood greenness is associated with lower cardiovascular disease [19–21], and overall non-accidental mortality [22]. In our study, we assess the relationship between urban forest biodiversity and cardiovascular health, while PLOS ONE Biodiversity and cardiovascular disease PLOS ONE | https://doi.org/10.1371/journal.pone.0254973 November 3, 2021 2 / 23