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

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

ACOUSTIC METHODS FOR MAPPING AND CHARACTERIZING SUBMERGED AQUATIC VEGETATION USING A MULTIBEAM ECHOSOUNDER

Submerged aquatic vegetation (SAV) is an important component of many temperate global coastal ecosystems. SAV monitoring programs using optical remote sensing are limited by water clarity and attenuation with depth. Here underwater acoustics is used to analyze the water volume above the bottom to detect, map and characterize SAV. In particular, this dissertation developed and applied new methods for analyzing the full time series of acoustic intensity data (e.g., water column data) collected by a multibeam echosounder. This dissertation is composed of three separate but related studies. In the first study, novel methods for detecting and measuring the canopy height of eelgrass beds are developed and used to map eelgrass in a range of different environments throughout the Great Bay Estuary, New Hampshire, and Cape Cod Bay, Massachusetts. The results of this study validated these methods by showing agreement between boundaries of eelgrass beds in acoustic and aerial datasets more in shallow water than at the deeper edges, where the acoustics were able to detect eelgrass more easily and at lower densities. In the second study, the methods developed for measuring canopy height in the first study are used to delineate between kelp-dominated and non-kelp-dominated habitat at several shallow rocky subtidal sites on the Maine and New Hampshire coast. The kelp detection abilities of these methods are first tested and confirmed at a pilot site with detailed diver quadrat macroalgae data, and then these methods are used to successfully extrapolate kelp- and non-kelp-dominated percent coverages derived from video photomosaic data. The third study examines the variability of the acoustic signature and acoustically-derived canopy height under different tidal currents. Submerged aquatic canopies are known to bend to accommodate the drag they generate in response to hydrodynamic forcing, and, in turn, the canopy height measured by acoustics will not be a perfect representation of canopy height as defined by common seagrass monitoring protocols, which is usually measured as the length of the blade of seagrass. Additionally, the bending of the canopy affects how the blades of seagrass are distributed within the footprint of the sonar, changing the acoustic signature of the seagrass canopy. For this study, a multibeam echosounder, a current profiler and an HD video camera were deployed on a stationary frame in a single eelgrass bed over 2 tidal cycles. Acoustic canopy heights varied by as much as 30 cm over the experiment, and although acoustic canopy height was correlated to current magnitude, the relationship did not follow the predictive flexible vegetation reconfiguration model of Luhar and Nepf (2011). Results indicate that there are significant differences in the shape of the return from a deflected (i.e., bent-over) canopy and an upright canopy, and that these differences in shape have implications for the accuracy of bottom detection using the maximum amplitude of a beam time series. These three studies clearly show the potential for using multibeam water column backscatter data for mapping coastal submerged aquatic vegetation while also testing the natural variability in acoustic canopy height measurements in the field