[ktrizzo]

Adds the stomatal conductance model of Buckley et al. (2026), "A Simple Stomatal Model That Unifies the Metabolic and Hydraulic Control of Carbon and Water Flux", Global Change Biology 32: e70999, https://doi.org/10.1111/gcb.70999.

g_s = β s b (ψ_soil − ψ_c) / (1 + s b D_s / K_plant), with b = J/4 − R_d − A + b_o

Changes

stomatalconductance

• New Buckley2026coefficients {s, K_plant_at_25C, b_o, psi_c} with the same setters as the other models (all primitives, UUIDs, material label) and a species library (setBuckley2026CoefficientsFromLibrary, getBuckley2026CoefficientsFromLibrary): Almond, Elderberry, Cowpea, EasternRedbud, Grape, Olive, Pistachio.
• Uses the existing leaf-surface vapor balance (D_s), boundary-layer handling, dynamic mode, and optional outputs (model_parameters, vapor_pressure_deficit).
• K_plant is the whole-plant (soil-to-leaf) hydraulic conductance. It is read from primitive data plant_hydraulic_conductance when present; otherwise the coefficient is scaled to leaf temperature by the fluidity of water (Buckley et al. 2026, Section 2.2).
• Selecting the model sets global data buckley2026_stomatal_model_enabled = 1, which makes the photosynthesis and plant hydraulics plug-ins write the inputs below. Missing net_photosynthesis, electron_transport_rate, or dark_respiration is an error; a missing soil_water_potential falls back to −0.1 MPa with a warning.

photosynthesis

• New optional outputs electron_transport_rate (J) and dark_respiration (R_d) for the Farquhar model, also written automatically when the global data above is set.
• J and R_d are computed by a new helper, evaluateElectronTransportAndRespiration(), now also used by evaluateCi_Farquhar() (results unchanged).

planthydraulics

• New outputPlantConductancePrimitiveData(): writes plant_hydraulic_conductance (K_plant = root, stem, and leaf conductances in series, per unit leaf area) and soil_water_potential, also automatically when the global data above is set. soil_water_potential is written only for plants whose value was set with setSoilWaterPotentialOfPlant(), so a value set directly is not replaced.

Species library

Fitted to diurnal LI-600 porometer surveys (corrected per Rizzo & Bailey) with the first measured stem water potential of each day as ψ_soil, and ψ_c from the pressure-volume turgor loss point (fitted for eastern redbud). A, J, and R_d were computed with the Helios Farquhar library entry of each species, so each entry should be used with that entry. Fit statistics are in the documentation table.

Testing

• New self-tests in all three plug-ins; all existing tests pass (stomatalconductance 53, photosynthesis 65, planthydraulics 24 test cases).
• The steady state of photosynthesis and stomatal conductance run in alternation reproduces the coupled solution of the code used to fit the library to within 0.001% (g_s) and 0.000% (A).