## 5 Results and Discussion
### Ecological Effectiveness
#### 5.2.3 Comparison Outcome  
These results call for various comments:  
* The GPP estimates derived from the flux tower measurements exhibit a very high day-to-day variability, as a result of fluctuations in local conditions, including turbulence. While progressively larger values are expected during the vegetation growth phase than during senescence, there is no expectation that successive values generate a smooth sequence.
* By contrast, vegetation growth and senescence occurs as a generally slow, smooth process, with more branches and leaves occurring during the rainy season, and the plants wilting progressively during the dry season. Hence, it makes sense to fit a smooth model through the FAPAR data, while this would not be appropriate for the GPP data, at least on a daily time scale.
* Inspecting again Figure 11, it is seen that FAPAR and GPP both increase quickly and together at the start of the rainy seasons, but that the decrease in FAPAR tends to lag behind the decline in GPP: plants continue to appear photosynthetically active from space longer than the in situ measurements of CO\({}_{2}\) indicate.  
\begin{table}
\begin{tabular}{l c c c c c c c} \hline \multirow{2}{*}{**SOS/EOS**} & **Vegetative** & **Integrated** & \multicolumn{4}{c}{**CFAPAR**} \\  & **Season** & **GPP** & **PDGF** & **PDHyTgF** & **PDLF** & **PDSF** & **Combined** \\ \hline
2001–09–10 & & & & & & & \\
2002–10-24 & 2 & 815.25 & 117.72 & 122.26 & 122.26 & 118.50 & 117.72 \\
2003–08–31 & 3 & 656.65 & 84.19 & 84.14 & 84.14 & 84.40 & 84.19 \\
2004–09–18 & 4 & 682.33 & 119.50 & 119.91 & 119.91 & 119.24 & 119.50 \\
2005–08–20 & 5 & 1043.88 & 105.95 & 111.70 & 111.70 & & 111.70 \\
2009–09–16 & 9 & 1522.03 & 130.12 & 130.87 & 130.87 & 130.44 & 130.12 \\
2010–08–27 & 10 & 1185.21 & 129.29 & 117.07 & 117.07 & & 129.29 \\
2011–09–06 & 11 & 1314.19 & 155.09 & 148.36 & 148.36 & & 155.09 \\
2012–09–01 & 12 & 1178.08 & 116.34 & 115.56 & 115.56 & & 116.34 \\ \hline \multicolumn{2}{c}{\(r\)} & \multicolumn{2}{c}{0.65} & 0.62 & 0.62 & 0.65 & 0.65 \\ \multicolumn{2}{c}{\(p\)-value} & \multicolumn{2}{c}{0.35} & 0.38 & 0.38 & 0.35 & 0.35 \\ \hline \end{tabular}
\end{table}
Table 3: Correlations coefficients (\(r\)) between the integrated GPP and Cumulated FAPAR (CFAPAR) obtained by integrating the PDGF, PDHyTgF, PDLF, PDSG, and combined models during the indicated vegetative seasons. Computations were skipped whenever fewer than half of the expected GPP observations were available during the season, or when the correlation between the daily GPP and model values dropped below 0.70 in Table 2 (see text for details).  
* GPP values measured at the Skukuza flux tower range from 0.0 to about 16 gC m\({}^{-2}\) d\({}^{-1}\), while FAPAR rarely drops below 0.1 or raises above 0.6 at this site: these variables experience quite different ranges of variability.
* As expected, the PDHyTgF (Hyperbolic Tangent) and the PDLF (Logistic) models generate essentially the same traces, since their parameters can be adjusted to generate the same values; however, they exhibit somewhat different levels of numerical efficiency, as will be seen shortly in Section 5.3.