Evaluating distributional equity of tree canopy cover in Sacramento, CA

Background/Question/Methods

Tree canopy has been associated with a myriad of ecosystem services including temperature mitigation, improved water quality, reduced air pollution and carbon sequestration.  As a result, tree canopy is generally perceived as an environmental amenity or good.  In response, many urban areas have initiated tree planting programs as a way to mitigate the impact human-dominated systems have on the environment, specifically global climate change.  Sacramento California, known as the “city of trees”, is part of an ambitious effort to double the region’s tree canopy cover in the next 40 years by planting an additional 5 million trees.  With the current focus on planting trees, it’s important to examine the distributional justice associated with this environmental amenity.  Environmental justice theory suggests that low income communities of color will live in areas with less tree canopy.  We examined this assumption for the city of Sacramento, CA.  Tree canopy, as quantified by the National Land Cover Dataset, was examined in relation to social variables derived from the US Census at two spatial scales: the Census Block Group and the Census Tract. 

Results/Conclusions

Consistent with environmental justice theory, the percent of the population that is white was positively correlated with tree canopy cover at both the Census Block Group and Census Tract levels (r = .409, p <.001, r = .520, p <.001) while the percent of the population that is black was negatively correlated with tree canopy cover (r = -.311, p <.001, r = -.489, p <.001).  At the Census Block Group scale, these patterns persist when controlling for income; however, the patterns are non-significant when controlling for indicators of educational attainment.  At the coarser scale of the Census Tract, associations between canopy cover and race are significant even when controlling for income and education.  The results of this study demonstrate that relationships between race and environmental amenities can change depending on the scale of analysis.  This finding could have important implications for environmental justice studies, which often analyze data at the Census Tract scale.  In addition, identifying changing associations between race and tree canopy cover at different scales can help us understand the dynamics of socio-ecological systems and may be used to identify the scales at which social processes that drive the distribution of tree canopy are operating.