Stomatal conductance models

The stomatal tool fits stomatal conductance to water vapour, gs (mol m⁻² s⁻¹), as measured by the LI-600 or LI-6800 (gsw). If the file has a gsw_corrected column from the LI-600 correction, PhytKit uses it instead of gsw.

Buckley–Turnbull–Adams (2012)

--model bta (default). A compact form of the Buckley–Mott–Farquhar hydromechanical model, driven by light and the vapour pressure deficit:

\[ g_s = \frac{E_m\,(Q + i_0)}{k + b\,Q + (Q + i_0)\,D_s} \]

• Q: photosynthetic photon flux density.
• Ds: leaf-to-air vapour pressure deficit as a mole fraction, in mmol mol⁻¹.

If your file has VPD in kPa, PhytKit converts it with \(D_s = 1000\,\mathrm{VPD}/P_{atm}\). It uses a Patm column if present, and 101.325 kPa otherwise. The parameters Em, i0, k and b are empirical.

Medlyn et al. (2011)

--model medlyn. The optimal stomatal model: the conductance that maximizes carbon gain per unit water lost.

\[ g_s = g_{s0} + 1.6\left(1 + \frac{g_1}{\sqrt{D}}\right)\frac{A}{C_s} \]

• A: net assimilation.
• Cs: CO₂ at the leaf surface.
• D: vapour pressure deficit, in kPa.

The 1.6 converts a CO₂ conductance to water vapour. D is floored at 1e-5 kPa, because the model diverges as D approaches zero.

The LI-600 doesn't measure CO₂, so a file without a CO₂ column uses Cs = 420 µmol mol⁻¹. That's the same ambient CO₂ the A estimate assumes, so you can fit Medlyn to the output of pk li600 estimate, which writes A_solved.

Units

Conductance above 10 is taken to be in mmol m⁻² s⁻¹ and divided by 1000, because leaf conductance in mol m⁻² s⁻¹ rarely exceeds 2. PhytKit reports every conversion it makes in warnings.

See Fit stomatal conductance.