LB 41-346 - Health of urban street trees

Cities across the world are taking the lead on climate change mitigation and adaptation with the creation of climate action plans. Many of these plans propose efforts to increase tree canopy cover as a nature-based solution to help cities become more climate change resilient. Urban trees provide a suite of ecosystem services to urban residents including helping to cool local temperatures and mitigate stormwater runoff. The ability of an urban tree to provide these ecosystem services in part depends on the health of the tree. Here we ask the question, what are the spatial trends in the health of urban street trees in Boston, MA? We define tree health as the growth, mortality, and recruitment (or re-planting) rates of street trees. We test this question using a city-wide street tree inventory that was conducted in Boston, MA in 2006 (n=38,765) and again in 2021 (n=42,091). The 2021 dataset was collected by a team of arborists working for The Davey Tree Company while the 2006 dataset was collected by a team of interns and volunteers organized by The Urban Ecology Institute. Before conducting our analysis, we compared each data point in the 2006 inventory to historical imagery in google street view to determine potential errors in 2006 dataset in tree identification or tree diameters and verify new plantings that occurred between 2006 and 2021. We compare our findings to a re-survey study of the 2006 tree inventory conducted in 2014 on a subset of trees (n=4,313) distributed across the city of Boston (Smith et al. 2019). We find on average lower growth rates (0.42 +/- 0.49 cm stem-1 year-1) of urban street trees compared to the Smith et al. 2019 study (0.78 +/- 0.02 cm stem-1 year-1). The high variability in tree growth rates puts into question what are the spatial drivers of urban tree health and the corresponding spatial heterogeneity in ecosystem services provided.