Three replicate blocks of multiple 5 meter by 20 meter plots were established. In 1996 an herbivory experiment was established on a previously undisturbed plot within each block. Within each block, one plot had a 5x10 m unfenced portion (CT), and a fenced portion (5x10 m). The fenced area was surrounded by a large-mesh fence to exclude caribou and a small-mesh fence to exclude both large and small herbivores. Within each treatment within a plot, we selected three subplots arbitrarily and measured each LAI (via NDVI) and carbon flux.

We collected reflectance data from each of our subplots using a field portable spectrometer (Unispec, PP Systems, Haverhill, MA) calibrated with a measurement on a 99% reflectance standard (Spectralon, Lab Sphere, North Sutton, NH) before each measurement. Data were taken at shoulder height (approximately 1.4 m above the ground) for a sample area of 0.18 m2 and five repeat measurements were taken and averaged for each subplot. We calculated NDVI from the red and near infra-red reflectance values as shown below: 

(NDVI=(R_800-R_660)/(R_800+R_660 ))

 Carbon flux measurements were taken from 10 am to 4 pm over 5 discontinuous days (weather permitting). We measured CO2 exchange using a Li- 6400XT (IRGA, Li-Cor, Lincoln, NE) infrared gas analyzer operating in closed mode and connected to a clear, polycarbonate, cylindrical chamber with a clear lid (height 31 cm, diameter 75.5cm). We placed the chamber over each subplot to measure changes to CO2 concentration, water vapor, photosynthetically active radiation (PAR), and air temperature over an interval of 40 seconds following the establishment of stable environmental conditions. To minimize air leakage between the chamber and the ground, we attached a thick plastic skirt tightly to the bottom of the chamber and weighed down by a heavy chain. Gas flux measurements for each subplot were made at 5 different light levels ranging from full sun to complete darkness. Light levels were changed by covering the chamber with shade cloths of different thicknesses (for intermediate light levels), a blackout cloth for dark measurements, and leaving the chamber exposed to full sun. We made a minimum of three measurements for each light level. Only measurements made under stable environmental variables for the duration of the measurement interval, particularly the light level, were used for further analysis. Measurements with obvious leaks (e.g. negative NEE during dark measurements) were discarded.

  We calculated Net Ecosystem Exchange (NEE) (μmol m-2 s-2) for each subplot using equation 3 (Shaver 2007, Street 2007)

(NEE=(ρ∗V∗dC/dt)/A)

Air density, ρ, is equal to P/(RT) where P is pressure, R is the universal gas constant and T is temperature in K. V is the volume of the flux chamber, A is the surface area the chamber covers and dC/dt is the change in CO2 concentration adjusted for water vapor. A negative NEE value indicates a carbon flux from the atmosphere to the environment.

Shaver GR, Street LE, Rastetter EB, Van Wijk MT, Williams M. Functional convergence in regulation of net CO2 flux in heterogeneous tundra landscapes in Alaska and Sweden. J Ecol. 2007;95(4):802–17.

Street LE, Shaver GR, Williams M, Van Wijk MT. What is the relationship between changes in canopy leaf area and changes in photosynthetic CO2 flux in arctic ecosystems? J Ecol. 2007;95(1):139–50.

Shaver GR, Rastetter EB, Salmon V, Street LE, van de Weg MJ, Rocha A, et al. Pan-Arctic modelling of net ecosystem exchange of CO2. Philos Trans R Soc B Biol Sci. 2013;368(1624):20120485–20120485.