Same for MODIS data. There are hundreds of MODIS products. Which one exactly are you using and where can I find it? It would be nice to show a local map showing the nature of the data you are using here. Is it smart to use a single pixel? This kind of data is inherently noisy. Could an average of a larger area around your flux tower yield better results? Explain what indices you use and how they work in principle. Why can they serve as proxy for GPP?