Trees and forests are critical for sustainability, resilience, and well-being in cities, yet remain under-appreciated as formal green infrastructure elements. Managing and conserving urban forests to provide ecosystem services requires an improved understanding of the ecological functional of forests across urban contexts. For example, trees mitigate stormwater runoff in cities, but the amount of stormwater that trees can remove through hydrological functions in real urban settings is not well characterized. This limits the use of trees and forests as strategies to manage stormwater runoff. To address this gap, we introduce a novel research framework that uses ecohydrological approaches to assess the stormwater retention benefits of urban trees in different management settings. Using this framework, we have established monitoring sites in Montgomery County and Baltimore City, MD. Monitoring sites include patches of urban forests, a cluster of trees over mowed grass, and single trees over mowed grass and along a street. At these sites, we use sensors to measure tree transpiration, canopy interception, infiltration, and soil moisture dynamics. We compare these ecohydrologic variables for different species and management contexts using data collected in summer and fall 2018. We also explore how environmental drivers of transpiration vary across management contexts.
Results/Conclusions
We compare the stormwater retention performance of the same tree species across management settings. Urban forest patches have the capacity to infiltrate greater than half the precipitation events and volume that falls on them. Management context strongly drives stormwater ecosystem services provided by trees (transpiration rates) by influencing the physical environment and temperature, relative humidity, and VPD as drivers of tree physiology. We discuss the interaction of environmental drivers and management context in light of socio-ecological factors of the surrounding managed landscape that inform a typology of urban forests. These data ultimately help to inform guidelines for practitioners using urban forest patches to manage stormwater flows. Our results contribute to policy and practice by defining a nutrient reduction credit for urban tree canopies of different management settings in the Chesapeake Bay watershed.