Just Street Trees? Street trees increase local biodiversity, are larger, and offer more unique species in higher income, denser neighborhoods

Background/Question/Methods

Increasing tree canopy cover is a primary goal in the pursuit of healthy cities. Canopy-based ecosystem services are related to the physical structure and species composition of trees, and planting initiatives often focus on small tracts of municipally-owned and available urban land, such as in parkways along streets. Street trees experience different selection and management than trees in private yards, yet both types of trees interact within a heterogeneous and inequitably distributed urban forest. Some neighborhoods have more canopy cover and consequently experience higher ecosystem services than others, and these disparities often follow racial and class-based patterns. Here, we sought to understand what street trees add to the urban forest in Baltimore, Maryland, USA. We evaluated data from the 2018 street tree census in conjunction with long-term tree composition data from 202 permanent plots. We compared size, density, and species composition of street trees to the adjacent non-street tree community. We further examined how street tree planting has augmented the urban forest across different socio-demographic neighborhoods and highlight how differential street tree contribution to increasing abundance, canopy cover, and biodiversity may further exacerbate environmental inequities.

Results/Conclusions

Street trees made up the entire urban tree canopy at nearly 25% of sites. Across the landscape, street trees were generally larger than non-street trees and species composition was almost completely distinct between street and non-street tree communities at the local scale. When plots had both types of trees, street trees always added at least one species, but typically added more (mean ± SD: 6.30 ± 4.77 species). There was very little overlap between species of street trees and non-street trees, with the exception of Acer rubrum. Street trees were much more abundant, larger, and contributed higher species richness in neighborhoods that have higher population density and higher median household income. These results suggest that municipally-managed trees may further exacerbate income disparities, with resulting consequences for inequitable ecosystem services provisioning. In all, street trees are an important and unique component of Baltimore’s urban forest and must be examined both independently as well as in context with other trees across the urban landscape. By interrogating how street trees add to the urban forest and how these patterns differ across a city, we can improve tree planting goals and outcomes, and ideally contribute to environmental justice work through street tree planting.