# Presentation: 034876

## Carbon Sequestration at the Niwot Ridge Ameriflux Site

**Carbon Sequestration at the Niwot Ridge Ameriflux Site**

- Russell Monson, Andrew Turnipseed, Jed Sparks, Travis Huxman

- Department of EPO Biology, University of Colorado, Boulder, Colorado

**Introduction**
- Three years of net ecosystem CO2 exchange (NEE) measurements are now available for the Niwot Ridge Ameriflux site in Colorado.  The site is a subalpine coniferous forest, with a lodgepole pine-dominated forest downslope from the primary flux tower and a subalpine fir-dominated forest upslope from the tower.  Current carbon sequestration studies at the site are focused on (1) the coupling of wintertime snow pack and soil respiratory fluxes, and (2) the cause of differences in ecosystem water-use efficiency between the pine-dominated forest and the fir-dominated forest.

**Upper panel.**  Cumulative NEE for the site varied from -80.5 to -57.6 g C m-2 over the three years, which included years with near-normal total precipitation (1998-1999) and lower-than-normal total precipitation (1999-2000). **Lower panel.**  The late-winter snowpack was relatively deep during 2000 compared to 1999 and 2001 -- note the concomitantly higher CO2 loss rates in the winter of 2000.  The summers of 2000 and 2001 were drier than the summer of 1999 -- note the concomitantly lower CO2 uptake rates in the summers of 2000 and 2001.

- The year 2000 had a higher snow pack during the winter, accompanied  by higher soil surface temperatures and higher respiration rates. Although we only have three years of data, the trend is suggestive of a strong coupling between winter hydrology and ecosystem carbon balance.

- This research was supported by the SouthCentral section of the National Institute for Global Environmental Change (NIGEC) and the Terrestrial Carbon Processes (TCP) program of the Biological and Environmental Research (BER) division of the U.S. Department of Energy (DOE).

- Approximately 60% of the carbon sequestered during the summer is lost through respiration the following winter.  More snow means warmer soil surface temperatures, higher soil respiration rates, and more carbon loss.

- To date, we cannot explain the higher water-use efficiency for the pine-dominated forest on the basis of needle characteristics. The explanation may reside in lower ecosystem respiration rates in the pine-dominated forest due to lower surface soil moisture contents. This hypothesis is being tested at the present time with chamber measurements and Keeling plots.

- Ecosystem water use efficiency (the ratio of CO2 flux to H2O flux) during periods when the wind direction was easterly (from the pine-dominated community) or westerly (from the fir-dominated community). Note the higher water use efficiencies for the pine-dominated community.

- Needle-level water-use efficiency did not differ among the three dominant tree species, as determined by carbon isotope ratio (upper table).  Ecosystem, nighttime respiration was significantly higher at higher temperatures during the summer in the fir-dominated forest, compared to the pine- dominated forest (lower figure).  The fir-dominated forest has higher needle nitrogen concentrations and slightly higher LAI (data not shown).