Urbanization, the carbon cycle, and ecosystems: an exploration of coupled dynamics and feedbacks

Abstract

Urban areas are responsible for the majority of global anthropogenic CO2 emissions. Urbanization has altered the structure and function of terrestrial ecosystems and is increasing rapidly, further modifying global carbon cycling. The three research papers in this dissertation explore the role of urban vegetation in the carbon cycle using a combination of atmospheric observation, field measurements, remote sensing, and modeling. First, I characterized the spatiotemporal patterns of observed atmospheric CO2 mixing ratios and compared these data to estimated CO2 fluxes at three sites across Boston's urban-to-rural gradient. Total fossil fuel emissions estimates ranged from 1.5 to 37.3 Mg C ha-1 yr-1 between rural Harvard Forest and urban Boston. Despite large differences in emissions, atmospheric CO2 concentrations only differed by approximately 5%. The growing season length in Boston was approximately 31 days longer than in Harvard Forest, enhancing the period for biological carbon uptake. In Boston, gross primary production was 3.8 Mg C ha-1 yr-1, which was ~75% lower than gross primary production at Harvard Forest and ~10% of total anthropogenic carbon fluxes in Boston. Second, I assessed how forest-to-urban land cover change affected both aboveground biomass and productivity across eastern Massachusetts. I found that urban land covers contained less than half the biomass of adjacent forests, but the mean basal area increment of existing trees nearly doubled with development over time from 17.1 ± 3.0 to 35.8 ± 4.7 cm2 yr-1. Scaling this increase in growth suggests an aboveground biomass growth rate of 1.8 ± 0.4 Mg C ha-1 yr-1, a rate similar to that found in Harvard Forest, despite having only ~1/3 the standing aboveground biomass. Last, I assessed how above- and belowground ecosystem characteristics changed as a function of time since development and development intensity. I found that soil C and aboveground biomass showed significant differences with time since development. My data suggests that soil C, N, and bulk density are dependent on land use history, with previously agricultural sites consistently showing higher rates of soil N and C accumulation than previously forested and grassland sites. Taken as a whole, this dissertation highlights the potential consequences of altered ecological and environmental conditions on tree growth, the legacy effects of land use history, climate, and land management practices on below ground soil C and N, and the importance of vegetation in the C cycle in urban areas.