Ecosystem Fluxes at the Wind River Canopy Crane AmeriFlux Site
Washington, USA
Overview
The 85 m tall Wind River AmeriFlux canopy crane is located in a 500-hectare old-growth, conifer forest in the T.T. Munger Research Natural Area (latitude N 45° 49' 13.76'' and longitude W 121° 57' 06.88'', elevation 371 m above sea level), a protected section of the Gifford Pinchot National Forest that has been unmanaged for centuries since originating from a natural fire disturbance. Despite the surrounding complex terrain of the western Cascade Mountains, the forest is located in a relatively flat valley (slope is 3.5%) and maximum micrometeorological fetch reaches 1-2 km in the westerly direction over homogeneous forest type. The two dominant tree species are Douglas-fir and western hemlock. Secondary tree species include western red-cedar, Pacific silver fir, western white pine, noble fir, and grand fir. Trees at the site are up to 500 years old and reach maximum heights of 60 meters. Canopy leaf area index (LAI) has been estimated between 8.2 - 9.2 m2 m-2.
We have been measuring carbon dioxide, water vapor and energy fluxes above the Wind River old-growth forest since July 1998. Emphasis has been placed on identifying the environmental drivers of interannual net ecosystem production (NEP) variability, including looking at water use efficiency, light use efficiency, vapor pressure deficit, air temperature, soil temperature, soil moisture content, and fraction of intercepted photosynthetic active radiation (PAR) within the canopy.
Essentially, NEP is the difference between ecosystem photosynthesis (GPP) and ecosystem respiration (Reco) and that net transport of CO2 between the forest canopy and the atmosphere is what we directly measure with eddy covariance instrumentation. Positive annual NEP estimates show that the forest ecosystem was a net carbon sink for atmospheric carbon dioxide while a negative annual NEP indicates that the ecosystem released more carbon dioxide to the atmosphere than it up-took during photosynthesis.
Instrumentation and Data Processing
The eddy-covariance (EC) system consists of a sonic anemometer (Solent HS, Gill Instruments, Lymington, England, UK) and a closed-path infrared gas analyzer (IRGA) (LI-6262, LiCor, Lincoln, Nebraska, USA, replaced with the LI-7000 in 2006), which measure the wind velocity vectors, sonic temperature, and concentrations (mixing ratios) of H2O and CO2 at 10 Hz. The IRGA and sonic anemometer are mounted on a boom at a height of 67 m on the crane tower so that the anemometer faces west, the predominant wind direction. The four IRGA channels are run analogue through the sonic anemometer and all raw data are stored in binary format using WinFlux on a desktop computer in a building at the base of the crane tower. Line-power is available at the crane.
Carbon dioxide (?mol CO2 m-2 s-1) and water vapor (mmol H2O m-2 s-1) fluxes are computed using a 30-minute averaging time on the 10 Hz turbulence data. Half-hourly CO2 fluxes are further screened for outliers, gap-filled (using a running-mean approach, Reichstein et al. 2005) for missing values, and ustar (u*)-corrected at night using a u*-critical value of 0.3 m s-1 to replace carbon dioxide fluxes measured under low, nighttime turbulence conditions.
30-minute fluxes and turbulence statistics from the Wind River AmeriFlux tower are available at http://cdiac.ornl.gov/ftp/ameriflux/data/Level1/us-sites/preliminary-data/
If you would like gap-filled and higher QC-ed data from Wind River, please e-mail me at swharton@ucdavis.edu for those datasets. I also have gross primary production (GPP) and respiration half-hourly fluxes available, starting in January 1999.
Highlighted Results
Since July1998, the old-growth forest ecosystem has on average been a weak, net sink of carbon (9-year NEP average = + 65 g C m-2 year-1). But we have also observed a high degree of interannual variability and the forest ecosystem has switched between carbon sink and carbon source strength during our measurement period. Highest net carbon uptake occurred in 1999 (NEP = + 207 g C m-2 year-1) during a cool and wet La Niña phase. Highest net carbon release occurred in 2003 (NEP = - 100 g C m-2 year-1) during a warm El Niño phase event.
The mature forest is not at steady-state for carbon exchange and instead acts as either an annual source of carbon (NEE = +) to the atmosphere or a sink (NEE = -) in response to climatic drivers.
Respiration flux anomalies appear to be driving the interannual variability in net ecosystem productivity at our site. Higher annual net ecosystem production (NEP) is associated with cooler, wetter years while increased GPP fluxes are associated with warm air temperature anomalies. It is not an increase in GPP which is causing increased carbon uptake during the cooler years. Instead, higher annual NEP results from decreased respiration fluxes during years of cooler than normal temperatures.