3.5 | Comparison of GPP estimates
Our first objective was to compare GPPiso/SF to GPPPRELES for 2012 and 2013. To simplify the figure, we chose to represent only the assumption that gm/\(g_{C\hat{\alpha}}\) = 2.67 (Figure 5), which allows gm to vary during the season. The seasonal GPP patterns were similar between PRELES and the sap flux/isotopic method (Figure 5). Recall that GPPPRELES included understorey vegetation. Correlation coefficients among methods and plots were all high, with minimum r = 0.91 (Figure S4). However, the fit was nonlinear; in 2012 and 2013 GPPiso/SF approached an asymptote at high levels of GPPPRELES (Figure S4). The highest GPPPRELES values did not match with the highest GPPiso/SF values; the peak of GPPiso/SFoccurred earlier in the season than those of GPPPRELES. Interestingly, confidence intervals for GPPiso/SF and GPPPRELES overlapped most of the time, even during the fall, when the offset was bigger than the rest of the year. However, the VPD filters removed many values during the fall, which allowed us to draw a confidence interval only during small periods at that time. As previously mentioned, the GPP values were gapfilled to draw a complete seasonal pattern, at least during the thermal growing season. The resulting annual sums were higher for GPPiso/SF than for PRELES on the control plot, but not on the fertilised plot (Figure 6).