Street trees and mental health: developing systems thinking-informed hypotheses using causal loop diagraming

We considered the relationship between street trees and mental health with the aim of developing systems thinking-informed hypotheses to improve the implementation and evaluation of this popular nature-based solution (NBS). We integrated qualitative and quantitative evidence using causal loop diagraming (CLD), and then further analyzed and extended these diagrams with the aid of systems archetypes to identify key system structures. From these CLDs, we identified three systems thinking-informed hypotheses: 1) although there are many ways in which street trees may improve mental health, tree health is critical in realizing many of these benefits and minimizing dis-benefits; 2) communities which have benefited from street trees in the past are more likely to be able to advocate for additional trees, further entrenching historical inequities in street tree distribution; and 3) efforts to address these inequities through new tree planting initiatives may ultimately fail or even exacerbate existing challenges if they do not include sustained resources for tree maintenance, with direct and indirect impacts on inequities in mental health. Using a systems thinking lens was a useful way to deeply consider a purported but under-theorized co-benefit of a popular nature-based solution and identify policy-relevant hypotheses to guide future research.