Quantification of Urban Forest and Grassland Carbon Fluxes Using Field Measurements and a Satellite-Based Model in Washington DC/Baltimore Area

Cities are taking the lead on climate change mitigation with ambitious goals to reduce carbon dioxide (CO2) emissions. The implementation of effective mitigation policies will require accurate measurements to guide policy decisions and monitor their efficacy. Here, we present a comprehensive CO2 inventory of an urban temperate forest and unmanaged grassland using field observations. We estimate the annual storage of CO2 by vegetation and soils and place our biogenic flux estimates in the context of local fossil fuel (FF) emissions to determine when, where, and by how much biogenic fluxes alter net CO2 flux dynamics. We compare our hourly estimates of biogenic fluxes in the forest site to modeled estimates using a modified version of Urban-Vegetation Photosynthesis and Respiration Model (Urban-VPRM) in Washington DC/Baltimore area presenting the first urban evaluation of this model. We estimate that vegetation results in a net biogenic uptake of -2.62 +/- 1.9 Mg C ha(-1) yr(-1) in the forest site. FF emissions, however, drive patterns in the net flux resulting in the region being a net source of CO2 on daily and annual timescales. In the summer afternoons, however, the net flux is dominated by the uptake of CO2 by vegetation. The Urban-VPRM closely approximates hourly forest inventory based estimates of gross ecosystem exchange but overestimates ecosystem respiration in the dormant season by 40%. Our study highlights the importance of including seasonal dynamics in biogenic CO2 fluxes when planning and testing the efficacy of CO2 emission reduction polices and development of monitoring programs.