Ecosystem fluxes Net ecosystem exchange (NEE) and ecosystem
respiration (ER) were measured on the field while gross ecosystem
production (GEP) is calculated as GEP = NEE - ER. Carbon fluxes are
measured with a closed loop chamber system connected to an infrared gaz
analyser (INGRA; Li-840A, Li-cor). The plexiglas chamber (25x35x40 cm)
is connected to the INGRA with plastic tubes (inlet and outlet, 3m long
and 4mm diameter each). A filter prevents water droplets and small
particules from entering the INGRA and an air pump ensures a constant
flow of 1L/mn. The chamber is equiped with a fan to mix the air, a
thermocouple (Pt1000, Delta-T) to measure air temperature and a PAR
sensor. In 2020 the air temperature was measured with an iButton.
Airtightness is ensured by laying a heavy chain on the “skirt” of the
chamber (a piece of tarp taped to the lower part of the chamber). NEE is
measured with a transparent chamber. ER is measured with a dark chamber,
in our case the transparent chamber was covered with a thick dark tarp.
The CO2 concentration was measured every seconds and logged
in a datalogger (Squirrel 2010). The chamber was put on the plot during
three (two in 2020) minutes for each measurements and then aired during
one minute. Since the logger was logging continuesly, the start and end
time of each measurement was noted (it is necessary to precisely
synchronise the logger clock with the watch used on the field and to
regularly check it). The function match.flux() is matching the time in
the logger file with the plot being measured at that time (using the
time recorded on the field). It attributes the concentration of
CO2 measured every seconds to the correct measurement. A
linear regression is fitted to every measurements and the slope is used
to calculated the flux.
Where:
- flux: the flux of CO2 at the surface of the plot (mmol/m2/h)
- slope: slope of linear regression fitting the CO2 concentration versus time (ppm-1)
- P: pressure, assumed 1 atm
- V: volume of the chamber and tubing (L)
- R: gas constant (0.082057 L*atm*K-1*mol-1)
- T: chamber air temperature (K)
- A: area of chamber frame base (m2)