Units for initial slope of CO2 response curve for C4 plants

[mpaiao]

I am on the process of removing the hard-coded jmax25top and kp25top definitions and turning both into parameters. In doing so, I noticed that the default kp25top is defined as kp25top = 18000 * vcmax25top. It is very possible that I am missing something, but I wonder if this factor 18000 is 6 orders of magnitude too high...

The original CO₂ limitation for C4 grasses was first described in Collatz (1992), who defined k_p = 0.7 mol m⁻² s⁻¹.

Later, in IBIS, Foley et al. (1996) converted k_p to be a function of V_max (k_p = k₀ × V_Cmax), by using Collatz (1992) value of V_Cmax (39 µmol m⁻² s⁻¹). In IBIS, V_Cmax is defined in (mol m⁻² s⁻¹), so that k₀ = 17949 mol mol⁻¹ ≈ 18000 mol mol⁻¹.

However, in FATES, it seems V_Cmax is not internally converted to mol m⁻² s⁻¹. Also, the units for CO₂-limited net photosynthesis (A_C) are µmol m⁻² s⁻¹ and units for CO₂ molar mixing ratio (C_i) are µmol mol⁻¹. If these are all correct, k_p should have units of mol m⁻² s⁻¹, because A_C = k_p × C_i. This means that k₀ should be defined as k₀ = 0.18 mol µmol⁻¹.

As I said, I may be missing something or made a mistake in the unit checks, but I thought I should point this out because it seems GPP for C4 grasses is too high and this could be one of the reasons.

[mpaiao]

My bad, units for C_i seem to be in mol mol⁻¹, sorry for the confusion.