Figure 1 Schematics of a radiation index (RI) based on the radiation balance and its relationship with gross primary productivity (GPP). Rn is the net radiation which indicates the net energy balance of an ecosystem; DSR, DLR, USR and ULR are the downwelling short- and long-wave radiations and upwelling short- and long-wave radiations, respectively; and I is the effective radiation which reflects the net heat transmissions within a system through long-wave radiations.