4.4 | Difference between fertilisation treatments
We found a slightly higher GPP in the fertilised plot than in the reference plots with the sap flux/isotopic method. Indeed, WUEi in the F plot was higher than in the R plot, although \(g_{C\hat{\alpha}}\) was not different. This means that photosynthetic rates were higher on the F plot, as demonstrated in previous studies in coniferous boreal forest: photosynthetic activity, which is the product of gS for CO2 and the [CO2] gradient between the atmosphere and the sub stomatal chamber (Ca-Ci) increases only if the CO2 gradient increases for a given gS value (Duursma & Marshall, 2006; Marshall & Linder, 2013).
The difference between the F and the R plots was only significant at the daily time scale, perhaps because of the large number of repeated measurements (Figure 7). However, this sap flux/isotopic result, corrected for autocorrelation, was validated with the daily PRELES estimates (Figure 7). However, it should be recognized that these daily estimates are not independent and may exaggerate our ability to detect a difference. In contrast, the annual sums did not detect a difference (Figure 6), perhaps because we were able to compare only two years, limiting the power of ANOVA. Thus, our annual sums did not find a significantly higher GPP in the F plot compared to the R plot, agreeing with previous studies focused on photosynthetic activity at shoot (Tarvainen et al., 2016) and stand scale (Lim et al., 2015). The daily estimates did not agree. Based on these mixed results, we suggest that GPP under the F treatment might be slightly higher, but that a replicated study would be necessary to settle this question.
However, the magnitude of the GPP increase differed between PRELES and sap flux/isotopic methods. The 8% increase in GPPiso/SFdue to fertilisation was nearest to Lim et al (2015), who inferred a 3% difference in GPP between the same F plot and the R plot based on biometric measurements. In contrast, the GPPPRELES value in the F plot was 16% higher than in the R plot, almost twice the increase estimated from GPPiso/SF and five times higher than in Lim et al. (2015).