Ecological significance of the RI
Forests generally absorb more solar radiation than grasslands or shrub lands, resulting in a lower albedo at the surface. In particular, tropical rainforests, with high canopy wetness and darker leaves, have a canopy albedo as low as 0–5% (Yanagi & Costa, 2011). This explains the high ratio of Rn to DSR found in tropical rainforests compared with other ecosystems (Figure 3). At the same time, forests generally have high evapotranspiration rates and maintain a relatively cooler canopy surface temperature as compared with the ambient temperature (Helliker & Richter, 2008; Rotenberg & Yakir, 2010; Arora & Montenegro, 2011; Lee et al., 2011; Lawrence & Vandecar, 2014). Therefore, the amount of long-wave radiation transmitted from this cooler canopy surface (ULR) is low. On the other hand, due to the high air temperature, the DLR is high in tropical rainforests, resulting in an overall decrease in the effective radiation (i.e. ULR-DLR) (Figure 3). Hence, the large difference between Rn /DSR and I /DSR throughout the season, as shown in the tropical rainforest in Malaysia (MY_PSO; Figure 3), is likely a result of the combination of high photosynthesis and evapotranspiration. The opposite can be applied to ecosystems like grasslands, where low net radiation and high ULR are present, resulting in smaller RI values (Figure 3).
In this study, by analysing different radiation components measured at the surfaces of various ecosystems, we sought to understand the quantity of radiative energy retained in the ecosystem as a proxy to estimate GPP. Garbulsky et al. (2010) suggested that the global GPP is controlled by climatic constraints which depend on the specific ecosystem type. In this study, we demonstrated that the relationship between RI and GPP is independent of the ecosystem type, suggesting that the RI is a universal variable that depicts the productivity of an ecosystem through the overarching radiation properties without needlessly considering the complex biophysical processes in ecosystems with varying climate and vegetation.