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.