Figure 4.

Simulations of leaf stomatal conductance, g_w, in response to photosynthetically active radiation (A), ambient CO₂ concentration at the leaf surface, c_s (B), vapor pressure deficit at the leaf surface at 101 kPa atmospheric pressure, D_s (C), and leaf temperature (D). Shown are results for the BB model (Eq. 1; Ball et al., 1987), with g₀ = 0.01 mol m⁻² s⁻¹ and g₁ = 9; the MED model (Eq. 3; Medlyn et al., 2011), with g₀ = 0 mol m⁻² s⁻¹ and g_1M = 4.45 (kPa)^0.5; and the WUE model (Bonan et al., 2014), with λ = 1.33 mmol water µmol⁻¹ CO₂. All simulations used the Farquhar et al. (1980b) photosynthesis model as implemented in CLM4.5 (Bonan et al., 2011) with parameter values for the broadleaf deciduous tree plant functional type. The ratio of potential electron transport rate to maximum RuBP carboxylation rate, J_max/V_cmax, is 1.67 (at 25°C). Standard conditions for the simulations were as follows: c_a = 380 µmol mol⁻¹, photosynthetically active radiation = 2,000 µmol m⁻² s⁻¹ (or 0–2,000 µmol m⁻² s⁻¹ for g_w light responses), relative humidity = 0.8, air temperature = 25°C, and leaf temperature = 25°C. Environmental factors were varied individually. For the temperature simulation, vapor pressure was adjusted so that relative humidity remained constant (80%) or vapor pressure deficit remained constant (0.6 kPa) depending on the model. For these simulations, boundary layer conductance was 2 mol m⁻² s⁻¹, so D_s is comparable to leaf-to-air vapor pressure difference.