Regional eddy covariance measurements of CO2 exchange over an anthropogenically-altered landscape from a tall tower near Denver, Colorado

Background/Question/Methods

Global population growth is expected to accelerate throughout the coming century, leading to an increase in disturbances to natural landscapes through urbanization. These disturbances may have a variety of effects on ecological process, including on the balance of land-atmosphere CO₂ exchange in a region. To examine the regional impacts of urbanization on the Net Ecosystem Exchange of CO₂ (NEE), NEE was examined at the Boulder Atmospheric Observatory (BAO), a 300-m tall tower in Erie, Colorado. NEE was estimated as the sum of the eddy covariance CO₂ flux (F_c) and the storage flux (F_s) from August 19, 2010 to August 19, 2011. The turbulent flux footprint of NEE, as determined with an analytical diffusion model, extended to 15 km for 70% of the upwind source area and was comprised of a heterogeneous landscape. The flux footprint consisted of 27% vegetative cover, 33% bare fields, 32% residential areas, 6% roads, and 2% water, determined using Landsat 5TM imagery. In addition, an annual timeseries of the Normalized Difference Vegetation Index (NDVI) was constructed from MODIS imagery, and near-surface remote sensing techniques were employed to track seasonal changes in vegetation. Both NDVI and near-surface estimates produced an annual cycle of vegetation greenness. 

Results/Conclusions

Biological influences (i.e., soil temperature and moisture, incident solar radiation, and seasonal greenness estimates) and anthropogenic influences (i.e., land cover class) were examined for their effects on NEE. Biological variables were shown to have relatively little effect on NEE in comparison to anthropogenic factors. Land cover class, however, had a significant overall effect on NEE, although the impact of individual land classes on NEE could not be determined due to the well-mixed landscape and to poor spatial resolution in footprint modeling. In addition, the system was a strong net annual source of CO₂ to the atmosphere, implying a dominant influence of anthropogenic activity over biological CO₂ uptake, despite only 32% of the turbulent flux footprint consisting of residential areas. The results align with estimates of NEE from other suburban studies, suggesting that tall tower eddy covariance methodology may be an effective method for deriving regional estimates of CO₂ exchange over complex landscapes. These results indicate that the impact of urbanization on CO₂ exchange may be observed well outside of city centers and that tall tower eddy covariance measurements may be a powerful new important tool for examining CO₂ exchange at a regional scale, even in areas with mixed land cover.