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CO₂ in an Urban-Rural Gradient

by Diana Hsueh

Spring 2009

Last year, for the first time in history, 50% of the world’s population lived in urban centers. By 2050, this is expected to rise to 70%. As urbanization increases globally, it is important to investigate how our natural ecosystems respond to this change. Urban pollutants and acidic soils may hinder plant growth, while higher night-time temperatures, nutrient deposition, and carbon dioxide (CO₂) concentrations may stimulate plant growth.

To study these effects, Consortium scientists established an urban-to-rural transect of oak seedlings (Central Park, Lamont-Doherty Earth Observatory (LDEO), Black Rock Forest (BRF), and Catskills) in 2004 [Ed. Note: See “Field Season Yields Plant Growth Data,” Winter 2007]. Oak seedlings in the city grew larger than those in rural sites; however, the cause is still unclear. We know CO₂ concentrations play a crucial role in plant growth, but the distribution of CO₂ concentrations at small temporal and spatial scales is still poorly understood.

For that reason, I examined both historical CO₂ concentrations from the past 100-150 years using tree cores from the four sites and current CO₂ concentrations to decipher diurnal and seasonal patterns. Historical CO₂ concentrations can be determined by measuring the radiocarbon (¹⁴C) content in tree rings. Current CO₂ concentrations are measured at a network of atmospheric monitoring stations, the  Lamont Atmospheric Carbon Observation Project. Each monitoring station also measures weather indices such as rain, air temperature, solar radiation, relative humidity, wind direction, and wind speed so researchers can study CO₂ trends in relationship with variations in weather patterns. Current weather conditions and the locations of the six project sites can be found at www.ldeo.columbia.edu/outr/LACOP/. Up-to-date CO₂ measurements from an instrument installed in the Forest in 2008 will be accessible on this website soon.

The tree core data show that historic CO₂ levels in Central Park, LDEO, BRF and the Catskills were roughly 15, 7, 4 and 2 ppm above ambient “clean” levels, respectively. I am still analyzing the current data; however, the overall trend is the same, with the city exposed to higher levels of CO₂.  Nevertheless, the CO₂ concentrations in the city are not as high as one might expect in such a large urban area, partly because some of city’s pollution is flushed out by cleaner air blown in by northwest winds.

By examining what CO₂ concentrations past flora were exposed to, current CO₂ levels, and weather data, we can better understand the basic physics, biology (notably plant physiology), and chemistry of the environment, and thus have a better understanding of how urbanization affects air quality and ecosystems.

Diana Hsueh is an M.A. Conservation Biology student in the Ecology, Evolution and Environmental Biology Department at Columbia Universit