3.4 | Intrinsic water use efficiency (WUEi)
WUEi is a key variable in the GPPiso/SFestimation procedure (Figure 4).For all three gmassumptions, WUEi showed a significant seasonal pattern (‘date’ effect, p-value < 0.0001, df = 1, F = 29), decreasing sharply as the thermal growing season began and increasing as it ended (Figure 4). It also decreased gradually over the summer. In 2012, the mean values on the fertilised plot were 6% higher for gm∞, 7% higher for gm/\(g_{C\hat{\alpha}}\) = 2.67, and 9% higher for, gm = 0.31 mol CO2 m-2s-1 respectively. In 2013, the relative increase in WUEi on the F plot was similar: 6%, 7% and 8% respectively. For both years, there was a significant ‘plot’ effect (p-value < 0.0001, df = 1,) and a significant effect of the gm assumptions (p-value < 0.0001, df = 2) (Figure 4).