Relationship between the RI and GPP at monthly and daily scales
The monthly GPP showed positive relationships with the RI in all the ecosystems. The correlations were significant (p < 0.05) in most of the sites except for the tropical rainforest (MY_PSO), where the correlation was weak (R2 = 0.15, p = 0.07) (Figure 4). Besides the tropical rainforest site, the RI explained 73–88% of the GPP variance at the monthly scale, with the greatest explanatory degree presented in the subtropical evergreen forest.
The daily variations of the GPP and RI at the selected sites also showed significant correlations (p < 0.05), except in the tropical rainforest (MY_PSO) (Figure 5). Besides the tropical rainforest, the explanatory degrees of RI were between 40% and 71%, which were lower compared with those at the monthly scale.