Ecosystem services provided by urban forests, such as carbon sequestration and temperature regulation, are of growing importance in municipal-scale efforts to mitigate local greenhouse gas emissions and cope with the effects of climate change. However, urban tree growth rates and canopy spatial configuration can differ widely from better-studied rural forests, contributing uncertainty to our understanding of the services these forests provide and to the scope for protecting or enhancing services through targeted policies. To assess selected urban ecosystem functions connected with key services provision, this study estimated the scale and spatial patterning of current and future net C uptake, biomass, and canopy cover in a temperate-zone city.
We combined high resolution remotely sensed data on tree canopy cover and biomass in the city of Boston, Massachusetts, with models of tree growth rates based on local stem measurements in forest fragments and open-grown street trees to create a spatially explicit (30 m) estimate of annual net C uptake potential in Boston. This study then projected biomass, annual net C uptake, and canopy cover to the year 2040 under alternative policy scenarios to assess the services impact of planting and preserving urban trees.
Results/Conclusions
Tree canopy was predominately edge- or open-grown, with 85% of canopy area within 10 m of an edge. Median net C uptake was 0.5 (0–2.3) MgC ha-1 yr-1 and total uptake within the city limits was 9.8 (6.2–14.3) GgC yr-1, roughly 50% higher than estimates using models derived from rural forests. Open-grown trees in dense residential areas represented a large fraction of total tree biomass (43%), net C uptake (52%), and canopy cover (46%) for the city.
Policy reducing mortality rate in larger urban trees (>40 cm diameter at breast height) was more effective in expanding canopy cover (+20% (5–48%)) and C stocks (216 (178–310) GgC) by 2040 compared to the trajectory in canopy cover (-3% (-13–16%) and C stocks (161 (136–220) GgC) under present-day growth and mortality. Tree planting along road buffers offered the greatest potential for increasing annual net C uptake in non-forest trees (6.2 (4.6–10.2) GgC yr‑1) compared to the current trajectory (5.2 (3.8–8.8 GgC yr-1)). Our results suggest that ecosystem function in urban forests merits specific consideration to assess the services they provide. Municipal policy toward reducing mortality or raising tree numbers may meaningfully enhance ecosystem function and the generation of crucial services as cities confront the issue of climate change.