2.5 | gm assumptions
We used three different assumptions to obtain gm values:
The values for gm/\(g_{C\hat{\alpha}}\) = 2.67 and a constant gm = 0.31 mol CO2m-2 s-1 were calculated from discrimination against 13C measured at our site with a Picarro isotopic CO2 analyser (G2131-I, Picarro Inc., California, USA) and standard gas exchange according to Stangl et al. (2019).
Gross primary production (g C m-2 ground area d-1) was then calculated from Eqn. 10 and Eqn. 12:
\(\text{GPP}_{\text{iso}/\text{SF}}=\ \text{WUE}_{i}\times g_{\par \begin{matrix}C\hat{\alpha}\\ \\ \end{matrix}}\times\frac{M_{C}}{10^{6}}\) Eqn. 14
with MC the molar mass of C (12 g mol-1).