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

controlling for total tree exposure. We find that the tree exposure itself is less important than the biodiversity of neighborhood trees, though the two are correlated. As the first paper, to our knowledge, to assess the relationship between urban biodiversity and cardiovascular health, we highlight this potential pathway by which greenness may affect health and offer a way to mea- sure biodiversity, the Shannon Index. Conceptual framework Aerts, Honnay, and Nieuwenhuyse [16] propose three hypotheses as to how biodiversity may affect cardiovascular disease: enhanced immune functioning, psychological restoration due to biophilia, and dilution of disease risk for vertebrates. In this section, we consider each in turn and then summarize our hypotheses. Exposure to nature may affect health in many and varied ways, with enhanced immune functioning as a central pathway. Kuo [10] identifies 21 potential pathways and argues that among potential mechanisms, only enhanced immune functioning sufficiently (1) accounts for the empirically observed magnitude of nature’s impacts on health, (2) links to numerous specific observed health outcomes tied to nature, and (3) subsumes other identified pathways. Biodiversity may improve immune function through regulation of the composition of the human microbiome [16]. Immunological diseases such as asthma and diabetes, as well as ail- ments not traditionally considered immunological (obesity and depression) are gradually being found to be linked to the state of one’s microbiome [11,12,23–27]. Modification of the

human microbiome may prevent chronic disease through enhanced immune response [28]. For example, among children, Hanski and colleagues [29] found that exposure to biodiversity was associated with greater diversity of commensal microbiota. Consistent with the “biodiver- sity hypothesis,” exposure to nature has been shown to reduce chronic autoimmune and inflammatory disorders and disease, including diabetes, cardiovascular diseases, and depres- sion [29–31]. Experimental studies of exposure to nature offer indirect support of the link between biodi- versity and improved immune function. Li [32] measured changes in substances vital to immune functioning, notably cancer-fighting natural killer (NK) white blood cells, before, during, and after sustained exposure to a forested area and found substantial increases in these beneficial compounds that lasted well after exposure. Park et al. [33] found that a short walk through a forest lowers blood pressure. These findings are reinforced by numerous other experimental studies of forest-bathing or “Shinrin-yoku” [34–36]. A second notable pathway by which urban greenspace may affect human health is through “psychological restoration” via stress reduction or recovery of directed attention [37,38]. Bio- diversity has been shown to predict the restorative benefit of greenspace, suggesting diversity may play a role in the quality or extent of psychological restoration. Dubbed the “biophilia hypothesis,” in his 1984 book titled Biophilia, E.O. Wilson described restoration from biodi-

versity as “the innate tendency [in human beings] to focus on life and lifelike processes” [39]. Stemming from evolutionary history, humans may have innate preferences for the natural environment and exposure to biological diversity is restorative [40]. However, unlike the pathway of improved immune functioning, “psychological restora- tion” may also depend on an individual’s perception. The relation between biodiversity (actual and perceived) and indicators of psychological wellbeing is inconsistent [41], likely because biodiversity’s effect depends on an individual’s value judgments, as well as the time spent within the space, its location, and the activity performed within the space [42]. Health benefits from urban greenspace will depend on temporal exposure and spatial context. For example, for those spending most of the daylight hours away from their residential environment (i.e. PLOS ONE Biodiversity and cardiovascular disease PLOS ONE | https://doi.org/10.1371/journal.pone.0254973 November 3, 2021 3 / 23