Photosynthesis of Vascular Plants
413
20.3.1.2 Energy Balance and Leaf Temperatures
The temperature of leaves within the canopy is a function of incident
shortwave radiation, incident longwave radiation from other foliage, the soil
surface and the sky, latent heat loss due to transpiration, and the emission
of longwave radiation by the foliage (Norman 1978). Longwave radiation
reaching leaves from the sky, the soil surface, and from surrounding foliage
is computed using the same geometry as employed for diffuse shortwave
radiation. Soil surface temperature is an input parameter and sky temperature is calculated from air temperature and vapor pressure (Brutsaert 1975).
In dense canopies, such as considered in Caldwell et al. (1986), the longwave
exchange is primarily between foliage elements and the process becomes an
iterative in which leaf temperatures in each layer are affected by and have
influence on temperatures of leaves in same and neighboring layers (Fig.
20.1). Additionally, because transpiration is a function of stomatal aperture
(see next section), transpiration and the latent loss of heat is influenced by
leaf temperature and such relationships must be added to the iterative
calculation process. Probably the least developed aspects of the energy balance calculations involve the canopy boundary layer and turbulence within
the stand since only a simply logarithmic attenuation of horizontal momentum measured above the canopy is assumed.
20.3.1.3 Leaf Conductance and Photosynthesis
Leaf stomatal conductance is calculated in response to incident shortwave
radiation within 400-700 nm (PFD), predawn water potential, leaf temperature, and vapor pressure difference between leaf and air. The functional
relationships between environmental parameters and stomatal conductance
were first assembled from general concepts of stomatal function of plants
(e.g., Schulze and Hall 1982) and then adapted to the species of concern by
means of gas exchange cuvette measurements in different canopy layers.
The rate of transpiration is then calculated from stomatal conductance, the
boundary layer resistance (as affected by wind), and the vapor pressure
difference between leaf and air.
The photosynthesis sub model is a modified version of earlier models
developed by Tenhunen et al. (1976, 1980) and is similar to that described
in Tenhunen et al. (1987). Input variables are total photon flux density
(400-700 nm), leaf temperature, leaf conductance, and ambient CO 2 partial
pressure. The COz-saturated photosynthetic rate is calculated as a function
of leaf temperature and incident PFD (Tenhunen et al. 1976). The carboxylation efficiency, the initial slope of the relationship between photosynthesis,
and leaf internal CO2 partial pressure are computed as a function of maximum photosynthetic capacity, PFD, and leaf temperature at 21% O 2 . The
leaf photosynthetic response to internal CO2 is nonlinear, as described by
413
20.3.1.2 Energy Balance and Leaf Temperatures
The temperature of leaves within the canopy is a function of incident
shortwave radiation, incident longwave radiation from other foliage, the soil
surface and the sky, latent heat loss due to transpiration, and the emission
of longwave radiation by the foliage (Norman 1978). Longwave radiation
reaching leaves from the sky, the soil surface, and from surrounding foliage
is computed using the same geometry as employed for diffuse shortwave
radiation. Soil surface temperature is an input parameter and sky temperature is calculated from air temperature and vapor pressure (Brutsaert 1975).
In dense canopies, such as considered in Caldwell et al. (1986), the longwave
exchange is primarily between foliage elements and the process becomes an
iterative in which leaf temperatures in each layer are affected by and have
influence on temperatures of leaves in same and neighboring layers (Fig.
20.1). Additionally, because transpiration is a function of stomatal aperture
(see next section), transpiration and the latent loss of heat is influenced by
leaf temperature and such relationships must be added to the iterative
calculation process. Probably the least developed aspects of the energy balance calculations involve the canopy boundary layer and turbulence within
the stand since only a simply logarithmic attenuation of horizontal momentum measured above the canopy is assumed.
20.3.1.3 Leaf Conductance and Photosynthesis
Leaf stomatal conductance is calculated in response to incident shortwave
radiation within 400-700 nm (PFD), predawn water potential, leaf temperature, and vapor pressure difference between leaf and air. The functional
relationships between environmental parameters and stomatal conductance
were first assembled from general concepts of stomatal function of plants
(e.g., Schulze and Hall 1982) and then adapted to the species of concern by
means of gas exchange cuvette measurements in different canopy layers.
The rate of transpiration is then calculated from stomatal conductance, the
boundary layer resistance (as affected by wind), and the vapor pressure
difference between leaf and air.
The photosynthesis sub model is a modified version of earlier models
developed by Tenhunen et al. (1976, 1980) and is similar to that described
in Tenhunen et al. (1987). Input variables are total photon flux density
(400-700 nm), leaf temperature, leaf conductance, and ambient CO 2 partial
pressure. The COz-saturated photosynthetic rate is calculated as a function
of leaf temperature and incident PFD (Tenhunen et al. 1976). The carboxylation efficiency, the initial slope of the relationship between photosynthesis,
and leaf internal CO2 partial pressure are computed as a function of maximum photosynthetic capacity, PFD, and leaf temperature at 21% O 2 . The
leaf photosynthetic response to internal CO2 is nonlinear, as described by