RESEARCH LOCATIONS:
Imnavait Creek, Alaska 2003: Measurements were made on 8 flux plots in different vegetation types along the topographic sequence of the west facing slope of Imnavait Creek catchment. A light response curve was measured on each plot several times during early July and several times during late July/early August.

Imnavait Creek, Alaska 2004: Measurements were made on 15 plots along the topographic sequence of the west facing slope of Imnavait Creek catchment, in the same area as 2003. A light response curve was measured on each plot once in late June/early July and once in August.

Toolik Lake, Alaska 2004. Measurements were made on a total of 20 flux plots in treatment (N +P) and control blocks of the LTER fertilization experiment at Toolik Lake, including the moist acidic tussock, non-acidic tussock, non acidic non-tussock, inlet wet sedge and heath sites. Light curves were measured on each plot once in late June/early July and once in August.

Abisko, Sweden 2004. Measurements were made on 11 plots at the "Stepps" site, each measured once in July and once in August. Measurements were made on 13 plots at Paddus and 12 plots at Latnjajaure, once in July and August.

Abisko, Sweden 2005. 15 plots were located at the "Stepps" site, measurements were made on 3 occasions through June/ early July and once in mid August. During the 3rd phase of measurements in June we conducted a diurnal series on five of the plots, measuring a light curve once every 4 hrs for 28 hrs.

Svalbard 2005. Measurements were made on 41 plots at various sites in Adventdalen, one light curve was measured on each plot during July.

Zackenberg 2006. Data were collected on 35 1 x 1m plots and 25 0.3 x 0.3m plots across a range of vegetation types. 13 of the 0.3 x 0.3m plots were located within 1m x 1m plots (identified by plot name with suffix "b1, b2 or b3"), the remainder were independent of the 1 x 1m plots (B14-25). A light curve was measured on each plot once, all measurements were made from 8 July through 1 Aug 2006.

Toolik 2009: Data were collected on nine 1m x 1m flux plots, three in each vegetation type. Vegetation types included moist acidic tundra (MAT), dry heath (HTH) and wet sedge (WSG). Each plot was measured at approximately 2 weeks intervals from mid-June to mid-August 2009.

Anaktuvuk River Burn 2009: Data was collected on five 0.7m x 0.7m flux plots along a burn severity gradient in mid-June 2009.

Barrow 2009: Data was collected on thirteen 0.7m 0.7m flux plots along a moisture gradient during late-July to early-August 2009.

EXPERIMENTAL DESIGN AND METHODS:
We measured net CO2 flux using a Licor 6400 photosynthesis system (Li-Cor Inc., Lincoln, Nebraska, USA) connected to a plexiglass chamber. For most sites, the chamber measured 1m x 1m x 0.25m, but a 0.3 x 0.3 x 0.15m was used in 2006 and a 0.7 x 0.7 x 0.25m was used in 2009 due to limited accessibility at these sites. In Alaska in late season 2004 and at Zackenberg from 25th July 2006 onwards, we used a LiCor 6200 photosynthesis system. Flux data from the LiCor 6200 were corrected for the effects of water vapor build-up according to Hooper et al 2002.

At each plot we took measurements to create a light response curve: 2-3 measurements were made at ambient light conditions, followed by 2 flux measurements at each of 3 successive shading levels, with finally 3 measurements under complete darkness. We shaded the chamber by layering 3 fine mesh net cloths and used a tarpaulin to block all light from the chamber. Flux measurements under complete darkness represent ecosystem respiration. At each light level a flux measurement lasted 45 - 60 secs in total, CO2 concentrations in the chamber being recorded by the Licor every 2-3 secs. Incident radiation, average temperature and NDVI were also measured on each plot for each light curve.

A saturating (Michaelis-Menton type) function is fitted to the measured data. Best fit parameters describing each light curve are calculated by minimizing the sum of squared errors between measured and modeled NEP values.

In 2003, all flux data from a plot on one day are grouped into one light curve.

We measured NDVI on each flux plot using either a portable 2 channel field sensor (Skye Instruments) or Unispec spectral analyzer (PP Systems) or both:

Toolik 2003: Skye sensor
Toolik 2004: Unispec and Skye sensor
Abisko 2004: Unispec and Skye sensor (Unispec only at Stepps site phase 2)
Abisko and Svalbard: Unispec and Skye sensor (Unispec only during June at Abisko)
Zackenberg: Unispec
Toolik 2009: Unispec
Barrow 2009: Unispec

The unispec spectral analyzer measures reflected light intensity in 256 portions of the visible spectrum from ~300nm to ~1100nm. A foreoptic cable transmits light reflected from the target to the instrument, a measurement scan lasts for ~10ms. 9 scans were measured in a regular grid for each of the flux plots. The end of the fiber optic was kept approximately 1m vertically above the ground surface resulting in a view of the vegetation of approx. 20cm diameter. For the 0.3m x 0.3m plots, 3 scans were taken from directly above the plot with a field of view of approximately 30cm diameter. The average of these three scans was then taken. Incident radiation was measured using a reference standard in order that reflectance from the vegetation could be calculated as a percentage of incoming solar radiation. The program Multispec5.1.5.exe was used to compile Unispec reflectance spectra from the raw target spectra.

The skye NDVI sensor measures total radiation reflected from the vegetation (without measuring incident radiation) in the wavebands 570nm-680nm and 725nm - 1100nm. NDVI is then calculated as below.

Calculations:
The light response curve is modeled as a rectangular hyperbola:

NEP = Re - [(Pmax*PAR)/(K + PAR)]

where Re = ecosystem respiration [μmol C m-2 ground s-1]
Pmax = theoretical light saturated photosynthesis [μmol C m-2 ground s-1]
K = half saturation constant [μmol PAR m-2 ground s-1]

The initial slope of the light response curve (light use efficiency or E0) and the light compensation point (LCP) are calculated from these parameters as:

E0 = (Pmax/K)

LCP = (RE*K)/(Pmax-RE)

where Re = ecosystem respiration [μmol C m-2 ground s-1]
Pmax = theoretical light saturated photosynthesis [μmol C m-2 ground s-1]
K = half saturation constant [μmol PAR m-2 ground s-1]
E0 = ecosystem light use efficiency [μmol C μmol-1 PAR]
LCP = ecosystem light compensation point [μmol PAR m-2 ground s-1]

NDVI = (RII-RI)/ (RI + RII)

where RI = average reflectance from 570nm to 680nm
RII = average reflectance from 725nm to 1000nm

COMMENTS: all times are given as local times. Sorting variables and curve ID numbers match the flux data from which the curve is calculated, in the ITEX CO2 flux data file 2003-2009gsflux.dat

FOR MORE INFORMATION CONTACT: Gus Shaver, The Ecosystems Center, Woods Hole, MA, 02543, USA

REFERENCE CITATIONS:
Hooper D.U., Cardon Z. G., Chapin III F. S. Durant, M. (2002) Corrected calculations for soil and ecosystem measurements of CO2 flux using the LI-COR 6200 portable photosynthesis system. Oecologia 132:1–11

Street, L.E., Shaver, G.R., Williams, M., van Wijk, M.T. (2007) What is the relationship
between changes in canopy leaf area and changes in photosynthetic CO2 flux in arctic ecosystems? Journal of Ecology, 95, 139-150.

Williams, M., Street, L.E., van Wijk, M.T., and Shaver, G.R. (2006) Identifying Differences in Carbon Exchange among Arctic Ecosystem Types. Ecosystems, 9, 288-304.

FORMAT OF DATA FILE: ASCII

See Methods