## 3 Data Collections
### Flux Tower Observations  
Flux towers use micrometeorological measurements and the Eddy Covariance (EC) method (first published by [57]) to directly observe air motion and gas concentrations at two or more heights, and to estimate the exchanges of gas, energy, and momentum between the terrestrial biosphere and the overlying atmosphere on an ecological scale from the covariance between fluctuations in those variables [58; 59; 60]. GPP (expressed in units of \(\mathrm{g_{C}\,m^{-2}\,d^{-1}}\)) is derived from an analysis of those data. Figure 2 shows the relation between the GPP and precipitations, in this case, estimated as part of the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA)-Interim reanalysis project. It is important to note, from the outset, that unmanaged, protected areas in a National Park are not monitored or used like an agricultural field: there is no planting or harvesting date, and nobody observes the emergence of leaves or flowers. This would be difficult in any case, given the diversity of species involved. Hence, the characterization of the phenology of the canopy must be derived from remote sensing measurements and access to local observations from an EC flux tower provides a unique opportunity to establish possible relations between remote and in situ measurements, even though these address quite different biogeophysical variables. The Skukuza EC flux tower is located at 25\({}^{\circ}\)1\({}^{\prime}\)10.54\({}^{\prime\prime}\)S, 31\({}^{\circ}\)29\({}^{\prime}\)48.6\({}^{\prime\prime}\)E within the Kruger National Park, near the Skukuza camp, at an altitude of 365 m above sea level. It has been in continuous operation since 2000, and stands on the ecotone between broad-leafed _Combretum_ savanna on sandy soil and fine-leafed _Acacia_ savanna on clayey soil [61]. Woody vegetation height ranges between eight and ten m, and the flux sensors on the tower are 17 m above ground, implying a sensor footprint of about 500 m [62]. The Skukuza flux tower currently makes use of the LiCor 7500 open-path infrared gas analyzer to measure carbon dioxide (CO\({}_{2}\), \(\mu\)mol mol\({}^{-1}\)) and water vapor (H\({}_{2}\)O, mmol mol\({}^{-1}\)) concentrations, as well as ultrasonic anemometers (Gill Wind Master Pro and Campbell Scientific CSAT3), to measure horizontal and vertical fluctuations in wind speed (m s\({}^{-1}\)) and air temperature (K). NASA erected this flux tower as part of the Southern African Regional Science Initiative (SAFARI-2000), an international field campaign investigating emissions, transport, transformation and deposition of trace gases and aerosols over southern Africa between 1999 and 2001 [61; 63; 64; 65]. The tower was subsequently donated to the Council for Scientific and Industrial Research (CSIR). The Skukuza EC flux tower measurements confirm the close inter-connection between the water and carbon fluxes: ecosystem respiration is highest at the onset of the wet season, continuously decreasing during the transition towards the dry season [6]. Variations in precipitation between years, which influence variations in the amount of green leaf area, seasonal patterns, and soil water availability are the main drivers of Net Ecosystem Exchange [62]. This approach has been used to estimate GPP [66], as well as to partition the energy fluxes, at the surface [67]. For the purpose of this study, the Skukuza flux tower data set processed by the FLUXNET organization (Site Id: ZA-Kru) was downloaded from the FLUXNET2015 data archive (Available from the portal [[https://doi.org/10.18140/FLX/1440188](https://doi.org/10.18140/FLX/1440188)]([https://doi.org/10.18140/FLX/1440188](https://doi.org/10.18140/FLX/1440188)), last accessed on 14 October 2020.). This organization coordinates regional and national networks to harmonize the compilation, archival and distribution of data to the scientific community.