2.5 | gm assumptions
We used three different assumptions to obtain gm values:
- (i) constant gm/\(g_{C\hat{\alpha}}\) = 2.67. This
approach allowed the gm estimate to vary during the
growing season.
- (ii) constant gm. This value was determined during
several summer days, but was used throughout the year.
- (iii) infinite gm (gm∞), meaning that
in Eqn. 12, the \(\frac{g_{C\hat{\alpha}}}{r\times g_{m}}\) term
tends to 0.
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).