"Greening" and "browning" of the Earth's land surface: is there evidence from FLUXNET observations?

There is compelling evidence that many parts of the northern middle and high latitudes have been exhibiting "greening" trends since the early 1980s. Most of these studies are based on the satellite-derived normalized difference vegetation index (NDVI). Some studies have also shown "browning" trends for some areas of the world. The trends in ecosystem productivity inferred from the NDVI record have not been well evaluated or confirmed mainly because of the lack of long-term observations of ecosystem productivity. The new FLUXNET database released in December 2015 consists of long-term observations of ecosystem-atmosphere CO2 exchange from a large number of eddy covariance sites across the globe and provides a timely dataset for directly examining the long-term trends of ecosystem productivity. We use the new FLUXNET database to examine whether there is any evidence from the eddy covariance data. We analyze the gross primary productivity (GPP) and net ecosystem exchange (NEE) data for all the sites with >= 7 years of data. Although many sites exhibit upward or downward trends in GPP (and/or NEE), fewer sites have significant increasing or decreasing trends. We also analyze the underlying mechanisms of the observed trends in NDVI and flux tower GPP. The trends and climatic correlates of flux tower GPP are compared against those of the NDVI record. We also analyze the trends of the NDVI for the pixels in which these sites are located. Inconsistency for greening or browning trends between the NDVI and GPP data is observed for many sites/pixels. The translation of fluxes from the tower footprint to the global scale through upscaling can minimize the scale mismatch between the tower footprint and the NDVI grid cells and better evaluate the "greening" and "browning" trends inferred from the NDVI record. The new FLUXNET database in its full capacity ( 400 sites) will also better help evaluate these trends.