Residential tree canopy and allostatic load in US adults: a population-based cross-sectional study.

Background: Increasingly, studies emphasize modifiable elements of the neighborhood that could be intervention targets to reduce stress and lower allostatic load, and ultimately chronic disease. One such feature is residential greenery. Few studies have evaluated potential differences in the effect of greenery on allostatic load across sub-populations. Methods: The objective was to examine the cross-sectional relationship between residential tree canopy and allostatic load among US adults (n = 40,307, female = 52%, mean age = 46.8, non-Hispanic Black = 11.7%, non-Hispanic White = 67.3%, Hispanic = 13.8%, other race = 7.2%) in the National Health and Nutrition Examination Survey (NHANES), to determine whether relationships differ by race and ethnicity, or socioeconomic status (SES), and to examine potential mediation by physical activity. Findings: Over 80% of adults lived in neighborhoods that did not meet the recommended 30% tree cover. We found that greater tree canopy was associated with lower allostatic load. Physical activity mediated this relationship, such that higher tree canopy was associated with higher physical activity and higher physical activity was associated with lower allostatic load. Separate stratified analyses revealed significant negative associations between percent trees and allostatic load for non-Hispanic White and Hispanic subpopulations and higher SES individuals. Interpretation: In demographic groups where the association between higher tree canopy and lower allostatic load was detected, there may be more opportunities for greening urban design to improve health. There may also be other unmeasured individual or contextual stressors in neighborhoods occupied by non-Hispanic Black or low SES individuals, where the association was not detected. These national findings do not indicate that low SES groups benefit more from residential greenery than higher SES groups. Funding: No funding.