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

the fifth most popular, decreases evenness and exposure to biodiversity because the change in the Shannon Index is -0.39 or -0.30. The effect is similar for both scenarios, suggesting there are not large biodiversity benefits to uniform plantings of slightly less popular genus. In con- trast, if plantings are spread evenly across just two genera, this increases exposure to biodiver- sity and brings potential avoided mortality benefits of $89 per zip code for heart disease and $32 per zip code for stroke. Further increasing the diversity of plantings to ten trees each of five genera results in very large benefits of $1,183 per zip code for heart disease and $425 per zip code for stroke. Net benefits from tree planting in Los Angeles We add benefits from avoided mortality to benefits from eco-services from trees in Los Ange- les, with the low estimate at $38 per tree per year to the high estimate of $56 per tree per year in the city of Los Angeles [59,60]. We can compare total annual benefits to total annual costs to calculate net benefits. Annualized lifetime planting and maintenance costs were estimated at $52–79 per tree per year [73,74], assuming a discount rate of 7%, allowing for extra costs associated with concrete tree wells, poor soil, and supplemental watering, and using an esti- mated Los Angeles street tree lifetime of 30 years based on allometric equations for eight spe- cies in our dataset. We begin by assuming there are no additional costs from increasing the diversity of street tree plantings by up to ten genera, though this may be unlikely if local nurs-

eries fail to stock a sufficiently diverse set of seedlings. Given these assumptions, the total direct cost for planting fifty trees is $2,599–3,950 per zip code. When considering total benefits, from avoided mortality and ecosystem services, increasing diversity increases the return on a tree-planting investment. For the lower planting cost esti- mate, net benefits (benefits–cost) are positive when planting is evenly split across two or more genera. For the higher cost estimate, net benefits are positive when planting is evenly split across five or more genera. Note that these benefits do not include those associated with self- reported positive emotions and restoration in physical and mental health [75,76], encouraged physical activity [77,78], lower obesity reports and improved social cohesion [6], and reduced asthma in young children [79] with exposure to greenspace or benefits to other species, such as birds [80]. Cost-effectiveness of increasing biodiversity to combat cardiovascular disease Next, we consider the cost-effectiveness of this intervention within the context of preventing cardiovascular disease. Cost-effectiveness compares the incremental change in the cost of a policy, as compared to the standard approach, and the incremental change in benefits. Given large uncertainties, assessing the cost-effectiveness of a policy to prevent cardiovascular disease is difficult to assess in the best of circumstances [81]. In our context, we are working outside of an experimental framework, so our estimates should be interpreted with caution.

We begin by assuming a tree-planting initiative without a biodiversity mandate is the stan- dard approach. We simplify this to be planting the top genus, only. The incremental change in policy is increasing the biodiversity of plantings. The cost of increasing the diversity of plant- ings is unknown. A very rough estimate could be to include a 10–50% surcharge on the cost of a seedling to reflect administrative and transport costs to locate a vendor selling seedlings locally unavailable and transporting the seedlings to Los Angeles. This changes the planting cost range to $53–88 per tree or $2626–4380 per zip code, giving an incremental change of $27–417 per zip code. The gain would be the difference in benefits from planting only the top genus and the alternative scenario. This works out to $1,129–3,782 in total benefits from PLOS ONE Biodiversity and cardiovascular disease PLOS ONE | https://doi.org/10.1371/journal.pone.0254973 November 3, 2021 16 / 23