Annual APAR and GPP/NPP are correlated in a grid-by-grid manner, as shown in Fig. 7. A single line does not represent the relationship, although there is a positive correlation between absorbed PAR and productivity (r2 - 0.69 for GPP and 0.68 for NPP). This is simply because the LUE by Sim-CYCLE is not homogeneous over the terrestrial biosphere. Fig. 8 shows the seasonal change in the APAR-GPP relationship (similar for APAR-NPP, not shown); the correlation coefficient is highest in DJF (r2 - 0.79) and lowest in MAM (r2 - 0.38). The lowest correlation coefficient in MAM coincides with lower LUEs in the same period (see Fig. 6). Additionally, LUEs for GPP and NPP were correlated with the productivities (Fig. 9), showing
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Figure 7: Correlation between annual absorbed energy (APAR) and CO2 assimilation rate (GPP and NPP) for each of the 0.5° x 0.5°grids (n - 58 772).
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Figure 7: Correlation between annual absorbed energy (APAR) and CO2 assimilation rate (GPP and NPP) for each of the 0.5° x 0.5°grids (n - 58 772).
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Figure 8: Same as Fig. 7, but showing the seasonal change in the APAR-GPP relationship.
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Figure 8: Same as Fig. 7, but showing the seasonal change in the APAR-GPP relationship.
considerably higher correlation coefficients (r2 — 0.81 for GPP and 0.75 for NPP). Again, this relationship indicates that higher productivities are attributable to higher light-use efficiency, as well as higher incident PAR. Compared with a similar correlation analysis with 12 global models by Ruimy et al. (1999), Sim-CYCLE showed a moderate degree of correlation between APAR and NPP (r2 — 0.26-0.98 among the 12 models) and a high correlation between LUE and NPP (r2 — 0.06-0.71). The behavior of Sim-CYCLE is relatively close to that of process-based models such as the Frankfurt Biosphere Model, rather than that of remote-sensing-based models. Such a correlation analysis and model intercomparison are apparently effective in clarifying the characteristics of the models.
Figure 9: Correlation between LUE and productivity, for GPP (top) and NPP (bottom).
Figure 9: Correlation between LUE and productivity, for GPP (top) and NPP (bottom).
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