2.9. Stomatal Resistance and Photosynthesis¶
2.9.1. Summary of CLM5.0 updates relative to the CLM4.5¶
We describe here the complete photosynthesis and stomatal conductance parameterizations that appear in CLM5.0. Corresponding information for CLM4.5 appeared in the CLM4.5 Technical Note (Oleson et al. 2013).
CLM5 includes the following new changes to photosynthesis and stomatal conductance:
Default stomatal conductance calculation uses the Medlyn conductance model
and at 25 ^{o}C: are now prognostic, and predicted via optimality by the LUNA model (Chapter 2.10)
Leaf N concentration and the fraction of leaf N in Rubisco used to calculate are determined by the LUNA model (Chapter 2.10)
Water stress is applied by the hydraulic conductance model (Chapter 2.11)
2.9.2. Introduction¶
Leaf stomatal resistance, which is needed for the water vapor flux (Chapter 2.5), is coupled to leaf photosynthesis similar to Collatz et al. (1991, 1992). These equations are solved separately for sunlit and shaded leaves using average absorbed photosynthetically active radiation for sunlit and shaded leaves [ , W m^{2} (section 2.4.1)] to give sunlit and shaded stomatal resistance ( , s m^{1}) and photosynthesis ( , µmol CO_{2} m^{2} s^{1}). Canopy photosynthesis is , where and are the sunlit and shaded leaf area indices (section 2.4.1). Canopy conductance is , where is the leaf boundary layer resistance (section 2.5.3).
2.9.3. Stomatal resistance¶
CLM5 calculates stomatal conductance using the Medlyn stomatal conductance model (Medlyn et al. 2011). Previous versions of CLM calculated leaf stomatal resistance is using the BallBerry conductance model as described by Collatz et al. (1991) and implemented in global climate models (Sellers et al. 1996). The Medlyn model calculates stomatal conductance (i.e., the inverse of resistance) based on net leaf photosynthesis, the vapor pressure deficit, and the CO_{2} concentration at the leaf surface. Leaf stomatal resistance is:
(2.9.1)¶
where is leaf stomatal resistance (s m^{2} mol^{1}), is the minimum stomatal conductance ( mol m ^{2} s^{1}), is leaf net photosynthesis (mol CO_{2} m^{2} s^{1}), is the CO_{2} partial pressure at the leaf surface (Pa), is the atmospheric pressure (Pa), and is the vapor pressure deficit at the leaf surface (kPa). is a plant functional type dependent parameter (Table 2.9.1) and are the same as those used in the CABLE model (de Kauwe et al. 2015).
The value for mol m ^{2} s^{1} for C_{3} and C_{4} plants. Photosynthesis is calculated for sunlit () and shaded () leaves to give and . Additionally, soil water influences stomatal resistance through plant hydraulic stress, detailed in the Plant Hydraulics chapter.
Resistance is converted from units of s m^{2} mol^{1} to s m^{1} as: 1 s m^{1} = mol^{1} m^{2} s, where is the universal gas constant (J K^{1} kmol^{1}) (Table 2.2.7) and is the atmospheric potential temperature (K).
PFT 
g_{1} 

NET Temperate 
2.35 
NET Boreal 
2.35 
NDT Boreal 
2.35 
BET Tropical 
4.12 
BET temperate 
4.12 
BDT tropical 
4.45 
BDT temperate 
4.45 
BDT boreal 
4.45 
BES temperate 
4.70 
BDS temperate 
4.70 
BDS boreal 
4.70 
C_{3} arctic grass 
2.22 
C_{3} grass 
5.25 
C_{4} grass 
1.62 
Temperate Corn 
1.79 
Spring Wheat 
5.79 
Temperate Soybean 
5.79 
Cotton 
5.79 
Rice 
5.79 
Sugarcane 
1.79 
Tropical Corn 
1.79 
Tropical Soybean 
5.79 
2.9.4. Photosynthesis¶
Photosynthesis in C_{3} plants is based on the model of Farquhar et al. (1980). Photosynthesis in C_{4} plants is based on the model of Collatz et al. (1992). Bonan et al. (2011) describe the implementation, modified here. In its simplest form, leaf net photosynthesis after accounting for respiration ( ) is
(2.9.2)¶
The RuBP carboxylase (Rubisco) limited rate of carboxylation ( mol CO_{2} m^{2} s^{1}) is
(2.9.3)¶
The maximum rate of carboxylation allowed by the capacity to regenerate RuBP (i.e., the lightlimited rate) ( mol CO_{2} m^{2} s^{1}) is
(2.9.4)¶
The productlimited rate of carboxylation for C_{3} plants and the PEP carboxylaselimited rate of carboxylation for C_{4} plants ( mol CO_{2} m^{2} s^{1}) is
(2.9.5)¶
In these equations, is the internal leaf CO_{2} partial pressure (Pa) and is the O_{2} partial pressure (Pa). and are the MichaelisMenten constants (Pa) for CO_{2} and O_{2}. (Pa) is the CO_{2} compensation point. is the maximum rate of carboxylation (µmol m^{2} s^{1}, Chapter 2.10) and is the electron transport rate (µmol m^{2} s^{1}). is the triose phosphate utilization rate (µmol m^{2} s^{1}), taken as so that for C_{3} plants (as in Collatz et al. 1992). For C_{4} plants, the lightlimited rate varies with in relation to the quantum efficiency ( mol CO_{2} mol^{1} photon). is the absorbed photosynthetically active radiation (W m^{2}) (section 2.4.1) , which is converted to photosynthetic photon flux assuming 4.6 mol photons per joule. is the initial slope of C_{4} CO_{2} response curve.
For C_{3} plants, the electron transport rate depends on the photosynthetically active radiation absorbed by the leaf. A common expression is the smaller of the two roots of the equation
(2.9.6)¶
where is the maximum potential rate of electron transport (mol m^{2} s^{1}, Chapter 2.10), is the light utilized in electron transport by photosystem II (µmol m^{2} s^{1}), and is a curvature parameter. For a given amount of photosynthetically active radiation absorbed by a leaf (, W m^{2}), converted to photosynthetic photon flux density with 4.6 mol J^{1}, the light utilized in electron transport is
(2.9.7)¶
where is the quantum yield of photosystem II, and the term 0.5 arises because one photon is absorbed by each of the two photosystems to move one electron. Parameter values are = 0.7 and = 0.85. In calculating (for both C_{3} and C_{4} plants), for sunlit leaves and for shaded leaves.
The model uses colimitation as described by Collatz et al. (1991, 1992). The actual gross photosynthesis rate, , is given by the smaller root of the equations
(2.9.8)¶
Values are and for C_{3} plants; and and for C_{4} plants. is the intermediate colimited photosynthesis. .
The parameters , , and depend on temperature. Values at 25 ^{o} C are , , and . , , , , and also vary with temperature.
at 25 ^{o}C: is calculated by the LUNA model (Chapter 2.10)
Parameter values at 25 ^{o}C are calculated from at 25 ^{o}C:, including: , and (C_{3}) and (C_{4}).
For C_{4} plants, .
However, when the biogeochemistry is active (the default mode), is calculated from leaf nitrogen as described in (Chapter 2.17)
The parameters , , , , and are scaled over the canopy for sunlit and shaded leaves (section 2.9.5). In C_{3} plants, these are adjusted for leaf temperature, (K), as:
(2.9.9)¶
(2.9.10)¶
and
(2.9.11)¶
Table 2.9.2 lists parameter values for and . is calculated separately for and to allow for temperature acclimation of photosynthesis (see equation (2.9.15)), and is 490 J mol ^{1} K ^{1} for (Bonan et al. 2011, Lombardozzi et al. 2015). Because as implemented here varies with , uses the same temperature parameters as . For C_{4} plants,
(2.9.12)¶
with , K^{1} K, K^{1}, and K. Additionally,
(2.9.13)¶
with , K^{1} and K, and
(2.9.14)¶
with .
Parameter 
(J mol^{1}) 
(J mol^{1}) 

72000 
200000 

50000 
200000 

72000 
200000 

46390 
150650 

79430 
– 

36380 
– 

37830 
– 
In the model, acclimation is implemented as in Kattge and Knorr (2007). In this parameterization, and vary with the plant growth temperature. This is achieved by allowing to vary with growth temperature according to
(2.9.15)¶
The effect is to cause the temperature optimum of and to increase with warmer temperatures. Additionally, the ratio at 25 ^{o}C decreases with growth temperature as
(2.9.16)¶
In these acclimation functions, is the 10day mean air temperature (K) and is the freezing point of water (K). For lack of data, acclimates similar to . Acclimation is restricted over the temperature range ^{o}C and ^{o}C.
2.9.5. Canopy scaling¶
When LUNA is on, the for sun leaves is scaled to the shaded leaves , , , and scale similarly.
(2.9.17)¶
Where and are the leaftocanopy scaling coefficients of the twostream radiation model, calculated as
(2.9.18)¶
k_{n,ext} is the extinction coefficient for N through the canopy (0.3). k_{b,ext} is the direct beam extinction coefficient calculated in the surface albedo routine, and is the fraction of sunlit leaves, both derived from Chapter 2.3.
When LUNA is off, scaling defaults to the mechanism used in CLM4.5.
2.9.6. Numerical implementation¶
The CO_{2} partial pressure at the leaf surface, (Pa), and the vapor pressure at the leaf surface, (Pa), needed for the stomatal resistance model in equation (2.9.1), and the internal leaf CO_{2} partial pressure (Pa), needed for the photosynthesis model in equations (2.9.2)(2.9.4), are calculated assuming there is negligible capacity to store CO_{2} and water vapor at the leaf surface so that
(2.9.19)¶
and the transpiration fluxes are related as
(2.9.20)¶
where is leaf boundary layer resistance (s m^{2} mol^{1}) (section 2.5.3), the terms 1.4 and 1.6 are the ratios of diffusivity of CO_{2} to H_{2}O for the leaf boundary layer resistance and stomatal resistance, , is the atmospheric CO_{2} partial pressure (Pa) calculated from CO_{2} concentration (ppmv), is the saturation vapor pressure (Pa) evaluated at the leaf temperature , and is the vapor pressure of air (Pa). The vapor pressure of air in the plant canopy (Pa) is determined from
(2.9.21)¶
where is the specific humidity of canopy air (kg kg^{1}, section 2.5.3). Equations and are solved for and
(2.9.22)¶
(2.9.23)¶
Substitution of equation (2.9.23) into equation (2.9.1) gives an expression for stomatal resistance ( ) as a function of photosynthesis ( ), given here in terms of conductance with and
(2.9.24)¶
where
(2.9.25)¶
and
(2.9.26)¶
Stomatal conductance, as solved by equation (2.9.24) (mol m ^{2} s ^{1}), is the larger of the two roots that satisfy the quadratic equation. Values for are given by
(2.9.27)¶
The equations for , , , and are solved iteratively until converges. Sun et al. (2012) pointed out that the CLM4 numerical approach does not always converge. Therefore, the model uses a hybrid algorithm that combines the secant method and Brent’s method to solve for . The equation set is solved separately for sunlit ( , ) and shaded ( , ) leaves.