4.3 | Mesophyll conductance influenced
GPPiso/SF estimates
The calculation of WUEi would not have been valid if
gm had been considered infinite (Seibt et al., 2008;
Wingate et al., 2007). Yet gm is still frequently
ignored by some global photosynthesis models and ecophysiologists (Hu et
al., 2010; Rogers et al., 2017; Zhao et al., 2005), or is embedded
within a constant empirical adjustment (Cernusak et al., 2013) most
likely due to the challenges in its measurements (Flexas et al., 2008;
Pons et al., 2009). Likewise, the global modelling community has been
reluctant to account for it because of the lack of consensus about how
to measure or model it (Rogers, Medlyn, & Dukes 2014).
We compared three different ways of accounting for gm.
Simplest would be to assume a constant mean value (Keenan, Sabate, &
Gracia, 2010). For example, we estimated GPP with a constant
gm = 0.31 mol CO2 m-2s-1 measured at the site (Stangl et al., 2019). The
GPPiso/SF from the assumptions of gm =
0.31 mol CO2 m-2 s-1was not different from the GPPiso/SF from the
gm∞ assumption. Perhaps this is because the constant
gm value was estimated during sunny days in the
summertime and therefore represents the maximal gm,
under optimal conditions.
We therefore based our comparison with PRELES on a constant ratio:
gm/\(g_{C\hat{\alpha}}\) = 2.67. The ratio has the
advantage of allowing gm to vary seasonally. Variation
responds to environmental factors (Bickford, Hanson, & McDowell, 2010;
Cano, López, & Warren, 2014; Han et al., 2016; Xiong, Douthe, &
Flexas, 2018); both diurnal (Bickford et al., 2010; Peguero-Pina et al.,
2017; Stangl et al., 2019) and seasonal (Montpied, Granier, & Dreyer,
2009) variations have been reported. The use of a constant
gm/\(g_{C\hat{\alpha}}\) ratio was certainly artificial
(Xiong et al., 2018), but it is a relatively common assumption (Klein et
al., 2016; Maseyk, Hemming, Anger, Leavitt, & Yakir, 2011). We suspect
that the higher discrepancies between the GPPiso/SF and
GPPPRELES in the fall and to a lesser extent in the
spring occurred because the constant ratio did not adequately account
for seasonal dynamics in gm. The need to refine our
description of gm is confirmed by the uncertainty
analysis (Table S1 and Figure S3) The Sobol indices, which describe
sources of uncertainty, showed that almost 75% of the
GPPiso/SF uncertainty came from the
gm/\(g_{C\hat{\alpha}}\) estimate.