## 2 Materials and Methods  
### Sites Description, Instrument Set-Up, and Data Sources  
The sites used in this study are Mediterranean tree-grass ecosystems, composed predominantly of an herbaceous layer and low-density evergreen broadleaf oak trees (Quercus ilex; \(\sim\)20 tree ha\({}^{-1}\); Figure 1). Three sites are located approximately 500 m apart from each other in Majadas de Tietar, Caceres, Spain (39\({}^{\circ}\)56\({}^{\prime}\)24.68\({}^{\prime}\)N, 5\({}^{\circ}\)46\({}^{\prime}\)28.70\({}^{\prime\prime}\)W), while one site is located in La Albuera, Spain (38\({}^{\circ}\)42\({}^{\prime}\)6.48\({}^{\prime\prime}\)N, 6\({}^{\circ}\)47\({}^{\prime\prime}\)9.24\({}^{\prime\prime}\)W). The experimental sites in Majadas de Tietar belong to a large-scale manipulation experiment, where the three areas of approximately 20 ha were manipulated with addition of nitrogen (FLUXNET ID ES-LM1), nitrogen and phosphorous (FLUXNET ID ES-LM2), and the last was kept as control (FLUXNET ID ES-LMa). The experimental site in the La Albuera (FLUXNET ID ES-Abr) is a natural ecosystem with no manipulation. In this study, we did not focus on the fertilization, but only on the evaluation of the effectiveness of different vegetation indexes to represent the ecosystem functions. The Majadas de Tietar and La Albuera are characterized by a long-term annual mean air temperature of 16.7 \(\pm\) 0.2 \({}^{\circ}\)C and 18.3 \(\pm\) 1.5 \({}^{\circ}\)C, respectively; while mean annual rainfall is ca. 650 mm and 400 mm, respectively. The rain falls typically from November to May with a very dry summer [49].  
In each site, an EC system was installed at 15 m of height to measure the carbon, water and energy fluxes (Section 2.2 for more details). The fluxes data are available from March 2014 in ES-LM1, ES-LM2, and ES-LMa; and from October 2015 for ES-Abr. Two broadband Decagon SRS (spectral reflectance sensor) sensors with a FOV of 36 degrees were installed on a rotating arm in each tower area. Downwelling irradiance and upwelling radiance at 650 nm (red spectral band) and 810 nm (near-infrared spectral band) were measured every 5 min for tree and grasses from 30 October 2015.  
A NIR-enabled digital camera (Stardot NetCam 5 MP), was mounted at the top of the EC tower (facing north) at each site. Images were collected every 30 min (from 10:00 to 14:30 UTC) as JPEG format. The camera settings were defined according to the \"PhenoCam\" protocol ([[https://phenocam.sr.unh.edu/webcam/tools/](https://phenocam.sr.unh.edu/webcam/tools/)]([https://phenocam.sr.unh.edu/webcam/tools/](https://phenocam.sr.unh.edu/webcam/tools/))). Sequential red, blue, green (RGB) and RGB + NIR images were collected by the Stardot camera according to [PERSON] et al., ([PERSON] et al., 2014). FOVs of cameras in ES-LM1 and ES-LM2 were stable during the study period (from 1 August 2014 to 31 July 2017), whereas the FOV of ES-LMa was not constant and the camera experienced a white balance problem until 3 December 2015. At the ES-Abr site, images were available from 1 January 2016. Hence, RGB and RGB+NIR images were available for the analysis from 1 August 2014 to 31 July 2017 for ES-LM1 and ES-LM2; from 3 December 2015 to 31 July 2017 for the ES-LMa site; and from 1 January 2016 to 31 July 2017 for the ES-Abr site. This guarantees a total of 9 site-years for the following analysis.