Incorporating vegetation to build a comprehensive urban carbon budget

Abstract:

Efforts to accurately monitor, report, and verify anthropogenic CO₂ emissions using atmospheric measurements require reliable partitioning of anthropogenic and biogenic sources/sinks. Anthropogenic emissions peak during the daytime, coincident with biogenic drawdown of CO₂. In contrast, net biogenic respiration emissions peak at night when anthropogenic emissions are lower. This temporal aliasing of fluxes requires careful accounting of both biogenic and anthropogenic fluxes for accurate source attribution. Relative to non-urban ecosystems, vegetation in urban areas is subject to longer growing seasons, reduced competition, greater nitrogen availability, and unique management practices. Ecosystem models and vegetation maps often omit urban areas despite potentially extensive vegetation coverage. For example, in Massachusetts (MA), models assume no biological flux over as much as 40% of land area.

To improve our understanding of the role of vegetation in the urban carbon cycle, we combine a new, comprehensive inventory of anthropogenic emissions, the Vegetation Photosynthesis and Respiration Model modified to incorporate altered urban growing conditions such as impervious surface area (ISA) and urban heat island, and an improved map of urban vegetation at 30m resolution. All fluxes are estimated at an hourly time scale for a 1km² resolution grid extending across MA. We combined Landsat-derived estimates of canopy coverage and EVI of vegetation with ground surveys of tree growth and mortality to improve estimates of aboveground biomass and biogenic flux rates in urban ecosystems.

Modeled biogenic fluxes vary significantly between sites along a gradient of urban density from downtown Boston to the heavily vegetated Harvard Forest in western MA. Our vegetation map demonstrates that vegetation in urban ecosystems may contain up to ~300 Tg of C. Net biogenic C fluxes in urban areas are small relative to anthropogenic emissions but exhibit substantial temporal aliasing. While urban vegetation may have limited potential to directly offset anthropogenic emissions, ignoring biogenic fluxes will adversely impact the accuracy of our efforts to monitor, report, and verify anthropogenic C emissions.