Carbon Sequestration Rates and the Energy Balance of Turf in the Denver Urban Ecosystem and in an Adjacent Native Grassland Under Contrasting Management Practices

Dean E. Anderson1, Katherine M. Powell1, Gergo Szanko1, Carol Mladinich1, Susan Curry1, Anne Griebel1*, and Thomas S. Thienelt1,2*

1. U.S. Geological Survey, Denver, CO, United States. 2. Martin-Luther-Universitaet Halle-Wittenberg, Germany

What are the differences in carbon flux?Denver Urban Turf

Native Tallgrass Prairie

CO2

560 g C/ha

Aug-Sept

Remote Sensing

Denver Urban

Leaf Area Index (LAI)

How is energy distributed at the surface?

The grasses found in urban turf are well watered and fertilized and usually act as a carbon sink. In addition, sensible heat advection from adjacent hotter surfaces, increasing evapotranspiration rates, cooling the surface temperatures in the turf area.

CO2

32 g C/ha

Aug-Sept

July

NDVI

Aug

Sept

Fluxes of CO2, water vapor, and sensible heat were measured by eddy covariance at both sites. Key instruments deployed at 2m above surface at the Ft Logan (FL) site were a CSAT3 sonic anemometer (Campbell Scientific), Li-7500 (LiCor Inc), Q*7 net radiometer (REBS), and in the soil- REBS heat flux plates (at 5cm), thermocouples (above the plates), and CS616 moisture probe (Campbell Scientific). Similar equipment was at the Rocky Flats (RF) site except that a Li-7200 was used. We used EdiRe to calculate fluxes from the 10Hz time series.

Site Histories

NDVI (normalized difference vegetation index) was derived from ARCHER hyperspectral imagery flown at low altitude (2000’ AGL) providing 15cm resolution.

NDVI = [NIR — VIS]/[NIR + VIS](where NIR=855nm & VIS=655nm)

Leaf area index was measured destructively from 320cm2 (at RF) or 45cm2 (at FL) area samples and analyzed with a Li-3100 leaf area meter.

The urban site is Fort Logan National Cemetery (87 ha), located near the southwestern Denver. The cemetery was established in 1887 although active lawn management for the area of our flux work began around 2005. Data for fertilization, irrigation, mowing, and seeding are documented and managed by the Veterans Administration.

The adjacent steppe site (a tallgrass prairie) is located at the Rocky Flats National Wildlife Refuge on the northwest side of the Denver metropolitan area (29km northwest of Fort Logan). Rocky Flats was formerly a nuclear weapons plant although the area within a 1-2 kilometers of the flux measurements was not disturbed and saw only light agricultural grazing use for the past 100 years.

Bluegrass and RyeBig Bluestem, Switchgrass, Indian Grass, Cheatgrass, Mullein

Preliminary ResultsCarbon fluxes, measured as NEE (net ecosystem exchange) over the turf and native grassland, were strongly influenced by water inputs in this semi-arid environment. For the urban lawn, irrigation which was begun mid-season led to a marked increase in leaf area (as seen from NDVI and LAI measurements), carbon uptake, and evapotranspiration (an order of magnitude greater). Since little precipitation fell over the area at this time, the NEE for native grassland was at a much lower rate and green LAI dropped as these grasses began senescence. Brief precipitation events led to some apparent recovery earlier in the season at the grassland but led to increased rates of respiration later in the season. Although these are preliminary data, varying irrigation rates of an urban lawn led to measurable shifts in carbon sequestration rates and magnified the contrast in sequestration rates between native and managed lands. The partition of energy budget terms also differed strongly, with much smaller sensible heat flux over urban lawns- implying a reduction in energy costs to cool urban residents. Acknowledgements: project supported by USGS Climate Effects Network funds. A. Griebel and T. Thienelt supported by the German Academic Exchange Service (DAAD)

Thanks to Mr. T. Jackson (Rocky Flats National Wildlife Refuge) and Mr. C. Hutchinson (Fort Logan National Cemetery) for facility acess

Native GrasslandDenver Urban

Precipitation

Precipitation

Irrigation