The natural environment and social cohesion: Tree planting is associated with increased voter turnout in Portland, Oregon

Research suggests that social cohesion partially mediates the health benefits of being exposed to the natural environment. However, past studies have relied on self-reported measures of social cohesion that have well documented limitations. Therefore, we evaluate the impact of tree planting on social cohesion using voter turnout data as an objective metric of social cohesion. Our study area is the 141 Census tracts in Multnomah County, Oregon that fall within (>50% area) the City of Portland. We used biennial primary and general-election turnout data from 2002 to 2020, and tree-planting data from the Portland based non-profit Friends of Trees. We estimated mixed models of voter turnout including tract-level random effects and controlling for socioeconomic status. Voter turnout and tree planting may be codetermined, so lagged tree-planting variables were used in all regression models. We found that each tree planted 10-11 years before an election was associated with a 0.020% (95% CI: 0.013-0.027) increase in general election turnout and a 0.016% (95% CI: 0.0089-0.023) increase in primary turnout. As the length of the planting lag decreased, the association between tree planting and voter turnout, in both primary and general elections, declined in significance and magnitude. Our results add to the body of evidence suggesting that social cohesion partially mediates the health benefits of exposure to the natural environment.