Street Tree Diversity and Urban Heat

Higher diversity within a city's street trees may offer greater cooling benefits than less diverse urban forests. California’s urban forests are among the most diverse in the world and offer an opportunity to test the relationship between diversity and cooling at a large scale. For 136 urban ZIP codes, we connect the most comprehensive data to date on California’s urban forests to both local station and satellite weather data for the period 2010-2018. We test whether biodiversity, measured by the Shannon-Wiener index and the new Top Diversity 50 index, is correlated with extreme heat in summer. After controlling for local averages in weather and tree canopy cover, we find that urban forest biodiversity is associated with lower maximum and higher minimum temperatures for June to September. Our specification makes it unlikely that reverse causality drives our result. Instead, we suggest that greater tree species diversity may boost daytime cooling through several pathways, including mutualism and greater aboveground biomass, a mechanical relationship where greater biodiversity implies a greater likelihood of having species with excellent shade, and cooling benefits from structural diversity in urban settings.