Demystifying normalized difference vegetation index (NDVI) for greenness exposure assessments and policy interventions in urban greening

Abstract

Most nature and health research use the normalized difference vegetation index (NDVI) for measuring greenness exposure. However, little is known about what NDVI measures in terms of vegetation types (e.g., grass, canopy coverage) within certain analysis zones (e.g., 500m buffer). Additionally, more exploration is needed to understand how to interpret changes in NDVI (e.g., per 0.1 increments) for policy intervention in urban greening. This study aims to address such gaps in the literature. We, therefore, measured mean values of NDVI and other vegetation metrics for different buffer zones (100, 300, and 500 m) at sample locations within the Greater Manchester study area by applying focal statistics. For each scale, we fitted linear and nonlinear (i.e., GAM) regression models to explore (1) what vegetation types and amounts best predict the NDVI values in a multivariate model and (2) how changes in NDVI explain changes in different vegetation coverages in univariate models. We found that NDVI is most sensitive to tree canopy at 300 meters scale and that changes at different levels of vegetation values and types predict dissimilar changes in NDVI. The models with three vegetation types can explain approximately 80% variance in NDVI values (i.e., Pseudo R²) for buffer zones of 300 and 500 meters. We observed that forb and shrub density is most sensitive (12.21% change) to mid-range increments in mean NDVI (0.45 to 0.55) within 300 meters. These sensitivities usually follow nonlinear patterns for all the vegetation types in multivariate and univariate models. Our results indicate that NDVI values are more sensitive to certain types and amounts of vegetation within various buffer zones. Overall, interpreting changes in NDVI values for urban greening interventions would require careful evaluation of the relative changes in types and quantities of vegetation for different buffer zones.