Why might urban tree canopy reduce dementia risk? A causal mediation analysis of 109,688 adults with 11 years of hospital and mortality records.

Urban tree canopy is associated with lower dementia risk, but no mediation analysis has been attempted to reveal potential mechanisms. We examined 3,639 dementia diagnoses in 109,688 participants of the Sax Institute's 45 and Up Study. Adjusted models indicated ≥20% tree canopy lowered the odds of developing dementia by 14% over 11 years (Odds Ratio = 0.86, 95%CI = 0.79-0.93). Association between tree canopy and dementia was partially mediated by physical activity (4.5%) and absences of psychological distress (5.7%), social support (2.9%), sleep duration (2.3%) and diabetes (1.8%). Social loneliness and absence of heart disease or hypertension did not mediate the tree canopy-dementia association.