  Multiple mechanisms have been proposed to explain how greenery enhances their
mental wellbeing. Mediation studies, however, focus on a limited number of
mechanisms and rely on remotely sensed greenery measures, which do not
accurately capture how neighborhood greenery is perceived on the ground. To
examine: 1) how streetscape and remote sensing-based greenery affect people's
mental wellbeing in Guangzhou, China; 2) whether and, if so, to what extent the
associations are mediated by physical activity, stress, air quality and noise,
and social cohesion; and 3) whether differences in the mediation across the
streetscape greenery and NDVI exposure metrics occurred. Mental wellbeing was
quantified by the WHO-5 wellbeing index. Greenery measures were extracted at
the neighborhood level: 1) streetscape greenery from street view data via a
convolutional neural network, and 2) the NDVI remote sensing images. Single and
multiple mediation analyses with multilevel regressions were conducted.
Streetscape and NDVI greenery were weakly and positively, but not
significantly, correlated. Our regression results revealed that streetscape
greenery and NDVI were, individually and jointly, positively associated with
mental wellbeing. Significant partial mediators for the streetscape greenery
were physical activity, stress, air quality and noise, and social cohesion;
together, they explained 62% of the association. For NDVI, only physical
activity and social cohesion were significant partial mediators, accounting for
22% of the association. Mental health and wellbeing and both streetscape and
satellite-derived greenery seem to be both directly correlated and indirectly
mediated. Our findings signify that both greenery measures capture different
aspects of natural environments and may contribute to people's wellbeing by
means of different mechanisms.
