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

incidence of Lyme disease. Instead, there is unexpectedly a weakly positive relationship, offer- ing new evidence for the debate on the dilution hypothesis [46,54,55]. Given these analyses, enhanced immune functioning seems the most likely candidate as the pathway through which biodiversity might affect cardiovascular disease. As already stated, car- diovascular diseases encompassing diseases of the heart and stroke are closely tied to immune function [56–58]. The nature of our data is better suited to address the effects of acute exposure to tree diversity, as we do not control for movement of individuals within our dataset across time. Although this is a common issue [see 17,47], it limits the weight of our discussion on the long-run effects of biodiversity exposure on chronic cases of cardiovascular disease. Simulated benefits from tree planting in the city of Los Angeles Estimated environmental benefits from planting a street tree in Los Angeles range from $38- 56/year in the city of Los Angeles [59,60]. These and other benefit estimates focus on ecosys- tem services provided by urban forests, such as air and water purification, stormwater manage- ment, carbon sequestration, habitat provision, soil erosion protection, and the reduction of the urban heat-island effect [61–68]. Benefits from planting trees tend not to account for urban forest diversity, though there is a recognition that biodiversity may increase ecosystem resil- ience from environmental stressors [69,70]. To get a sense of the relative importance of biodiversity benefits in urban tree-planting ini-

tiatives, we simulated five planting scenarios for the Mayor of Los Angeles’ commitment to plant ninety thousand trees between 2020–2021 [71]. Our analysis is necessarily speculative, with uncertainty on several margins, the two most important of which are that our study only documents associations and it is unclear whether these relationships are causal and there is a risk that our data on forest characteristics is not representative. We present our simulation with the goal of encouraging a discussion about the relative value of increasing biodiversity in tree planting in preventing cardiovascular disease using different realistic planting scenarios, but want to be clear about its limitations. In this spirit, for each scenario, we calculated the change in the heart disease and mortality rate and implied dollar values of these benefits. The five scenarios we considered fell into two types, planting a single genus or planting multiple genera. For planting a single genus, we com- pared planting the most commonly encountered genus in our data for the city of Los Angeles (“Top”) with planting the fifth most commonly encountered genus (“Fifth”). For planting mul- tiple genera, we considered splitting new plantings evenly across the top two, five, and ten gen- era. Next, we scaled the ninety thousand trees down by 3.6% to be consistent with our sample. The result was 3,278 trees total to be planted in our simulation. To be consistent with the city’s emphasis on planting in areas with less urban forest, we targeted zip codes below the median,

in terms of the number of trees observed in our data. The result was a total of 61 targeted zip codes (this includes censored zip codes not included in the analysis). We split our 3278 trees evenly among the 61 targeted zip codes and rounded to 50 trees per zip code. For each scenario, we calculated the Shannon index, shown in Fig 4. We used the change in the Shannon index and the estimated coefficient from the linear model (from Table 2) to cal- culate the implied change in the heart disease and stroke mortality rate and used the value of a statistical life of $5.6 million to get implied benefits [72]. Note that this is a simplification that likely overstates benefits. It would be more appropriate to use quality-adjusted life years. How- ever, the dose-response function for exposure to biodiversity and prevention or delay of car- diovascular disease is unknown. Keeping these important caveats and others in mind, results from our simulation for the five scenarios are reported in Table 4. We see that planting a single genus, whether the top or PLOS ONE Biodiversity and cardiovascular disease PLOS ONE | https://doi.org/10.1371/journal.pone.0254973 November 3, 2021 14 / 23