PS 48-239 - Where is carbon stored in the landscape of a midwestern city?

Abstract: Cities consist of a patchwork of land covers that exist on a spectrum of natural to highly modified. Given this high level of heterogeneity, the amount and pattern of carbon (C) stocks in urban landscapes may exhibit fine-scale variability. However, carbon density and the relative contribution to total carbon storage of different land-cover categories are not well known for cities, and few studies simultaneously consider terrestrial, aquatic and wetland components. The goal of this study was to quantify carbon storage in different components of an urban ecosystem, Madison, WI, to determine if land cover types contribute to carbon storage proportionately to their area. Madison encompasses 26,000 ha and while dominated by developed areas (70% of total area), agriculture, wetlands and forest make up 17%, 7% and 6% of the land area, respectively. We synthesized carbon per unit area values from local measurements, publicly available datasets, and published literature to estimate carbon densities within soils, sediments, and vegetation for the developed, forested, herbaceous, cultivated, and wetland cover classes categorized by the National Land Cover Database (NLCD). For total soil carbon storage, these densities were scaled up using the total cover of each NLCD class type. For aboveground biomass, aerial imagery was used to determine the vegetative composition of developed land cover classes so that carbon densities could be scaled to the major classes within city limits. Initial results show an average of 0.2 kg-C/m² of aboveground vegetative biomass for medium-intensity developed areas consisting of turfgrass, residential trees, and impervious surface. Aboveground live C in developed areas is thus low compared to literature-derived ranges of 0.5 – 1.3 kg-C/m² for herbaceous wetlands, and 3.8 – 13.0 kg-C/m² for forests. For soil carbon, emergent wetlands had a particularly high density of 14 kg-C/m² compared to roughly 6.0 kg-C/m² for low-intensity developed areas or urban forests. Using estimates of soil carbon for all land cover types, the percentage of total carbon stored in developed, agricultural, wetland and forest soils are 61%, 20%, 14%, and 6% respectively. The high proportion of soil carbon storage in wetlands, as well as the high density of aboveground vegetative carbon in forests, demonstrate that less common cover types can play an outsized role in urban carbon storage. This variation highlights the importance of a whole ecosystem approach to quantifying urban carbon stocks