# Estimation of Gross Primary Productivity (GPP)
## 2 Materials and Methods
### Measurement of CO\({}_{2}\) Fluxes and Estimation of GPP  
The study site was equipped with an EC system to monitor continuously the CO\({}_{2}\) net flux between the surface and atmosphere, also called Net Ecosystem Exchange (NEE). The system was composed of a Gill-HS50 sonic anemometer (Gill Instruments Ltd., Lymington, UK) for measuring 3D wind speed components and sonic temperature and of an LI-7200 enclosed-path infrared gas analyzer (LI-COR Inc., Lincoln, NB, USA) for measuring CO\({}_{2}\) and H\({}_{2}\)O concentrations. Both instruments collected measurements at a frequency of 10 Hz. CO\({}_{2}\) and H\({}_{2}\)O concentrations and fluxes of latent and sensible heat were estimated every 30 min using a standard scheme for processing the raw EC data. Briefly, flux data collected during conditions of well-developed turbulence and a good level of stationarity with sustained mechanical turbulence were used for the analysis according to CARBOEUROPE methodology [(77)]. The location of the tower was on the north-east edge of the plantation (Figure 1). To maximize the representativeness of flux measurements collected at the EC tower, data with wind direction between 50\({}^{\circ}\) and 250\({}^{\circ}\) were retained for the analysis [(78)] (Figure 1, wind rose). A more detailed description of the experimental setup, the calculations applied to the raw EC data and the criteria used to filter the entire data set have been previously reported by [(11; 19)]. Complete details of the EC methodology, processing schemes and corrections are also available [(77; 79)].  
Figure 1: Normalized Difference Vegetation Index (NDVI) Sentinel-2 image of the short-rotation coppice (SRC) plantation in Lochristi acquired on 7 July 2016, at 10 m resolution. The red polygon is the area of the plantation. The same area was extracted for calculating VIs. The white dot indicates the location of the flux tower. The black pixels represent the outbuildings. On the left, the wind rose for 2016 representing the wind direction distribution for wind speed classes.  
Other meteorological variables were continuously measured and averaged every 30 min at the site: global solar radiation (Rg) was measured using a CG3 pyranometer (Kipp and Zonen B.V., Delft, The Netherlands), air temperature and relative humidity were measured using an HMP155A thermohygrometer (Vaisala Oyj, Vantaa, Finland) and soil water content at a depth of 20 cm was measured using CS650 time-domain reflectometric probes (Campbell Scientific Inc., Logan, UT, USA).  
GPP was estimated from the measured NEE by partitioning it into its two main components: GPP and ecosystem respiration \((R_{cco})\), following the well-established relationship \(NEE=R_{cco}-GPP\), where Reco is modeled from NEE night-time data as a function of measured air temperature [80, 81]. The full time-series of the three variables have been obtained by filling unavoidable gaps in the measured NEE time-series, using the methodology for gap-filling and partitioning described by [PERSON] et al. (2005). Numerical results were obtained using the REddyProc online tool provided by the Max Planck Institute ([[https://www.bgc-jena.mpg.de/bgi/index.php/Services/REddyProcWeb](https://www.bgc-jena.mpg.de/bgi/index.php/Services/REddyProcWeb)]([https://www.bgc-jena.mpg.de/bgi/index.php/Services/REddyProcWeb](https://www.bgc-jena.mpg.de/bgi/index.php/Services/REddyProcWeb))). We averaged half-hourly GPP values between 10:00 and 14:00 solar local time to allow for direct comparison with the Sentinel-2 satellite data acquisition. These values are indicated hereafter as GPP, while daily sums of GPP are indicated as daily GPP.