Soil and vegetation carbon in urban ecosystems: The importance of urban definition and scale

There is conflicting evidence about the importance of soils and vegetation in urban carbon metabolism that is caused, in part, by inconsistent definitions of 'urban' land use. In Massachusetts, the US census estimates that 36% of the state is 'urban', yet remote sensing observations reveal that 50% of this urban area is forest or forested wetlands. While both of these estimates can be correct, the importance of soils and vegetation on the carbon metabolism of urban areas is clearly dependent on whether municipal, physical, or social definitions of urban are applied. We quantified urban ecosystem contributions to terrestrial C pools in the Boston Metropolitan Statistical Area (MSA) using several alternative urban definitions. Aboveground biomass (DBH ≥ 5 cm) for the MSA was 7.2 ± 0.4 kg C/m2, reflecting a high proportion of forest cover. Vegetation C was highest in forest (11.6 ± 0.5 kg C/m2) followed by residential (4.6 ± 0.5 kg C/m2) and then other developed (2.0 ± 0.4 kg C/m2) land uses. Soil C (0 to 10 cm) followed the same pattern of decreasing C concentration from forest, to residential, to other developed land uses (4.1 ± 0.1, 4.0 ± 0.2, and 3.3 ± 0.2 kg C/m2, respectively). Soil N concentrations were higher in urban areas than non-urban areas of the same land use type, except for residential areas, which had similarly high soil N concentrations. Urban soil (1 m depth) and vegetation C stocks spanned a wide range, from 14.4 to 54.5 Tg C and from 4.2 to 27.3 Tg C, respectively, depending on the urban definition that was used. Conclusions about the importance of soils and vegetation in urban ecosystems are very sensitive to the definition of urban used by the investigators. Urban areas are rapidly expanding in their extent; a systematic understanding of how our development patterns influence urban carbon metabolism, including vegetation and soils, is necessary to inform future development choices.