PE EIE[R-Rg RESEARCH ON - HJ Andrews Experimental Forest

where T = leaf temperature, ° K
S = solar radiation, gcal cm-2 min
Pn = net photosynthesis rate ,
mg CO 2 (g dry wt)-1 hr 1
(3i = parameters to be estimate d
Equation 7 was derived from equation 8 and
equation 9 :
P(L)=a+b 1nS (8)
which had been used to fit photosynthesis dat a
from several herbaceous species (Blackman an d
Rutter 1946, Blackman and Wilson 1951) .
Equation 8 was coupled with equation 9 suggested
by data of Pisek and Winkler (1958) ,
Krueger and Ferrell (1965), and others :
G(T) = a + bT + cT2 (9 )
Data from two species of oak were obtained
from infrared gas analysis and wer e
used to estimate the parameters of equation 7
by stepwise multiple regression analysis . Som e
of the terms of equation 7 were nonsignificant,
and were thus discarded . The final
model was :
Pn =Qo +(3 1 T+(3 2 In S+(3 5 T In S (10 )
Having estimated the parameters of equation
10, the model was used to predict photo -
synthesis of oak in the field . Their model gav e
fair to good agreement with subsequentl y
measured net photosynthesis .
In terms of the criteria discussed above ,
equation 10 is generally inadequate . It doe s
allow for interaction between two variables but
fails to take into consideration other factor s
that affect photosynthetic rate (stomatal behavior,
micrometeorological conditions, plan t
nutrition) . The model does satisfy the requirement
that the Pn model be solved as a functio n
of systems variables, that is, Pn = f [S, T] .
In terms of the other criteria, the model fall s
short of having a great deal of theoretica l
validity in that the function is one of convenience
rather than having general physicalchemical
meaning . It is unnecessary to discuss
in detail Botkin's model with respect to the
other criteria . Like most photosynthesis models,
it is specified at the leaf level, where th e
inputs to the leaf are measured, and the
relations between the leaf subsystems are
inferred .
While his model has many failings, it doe s
have one virtue. If one were interested in
comparing Pn = f[T,L] in two species under
identical conditions, this model may be adequate,
given that some other factor, e .g . ,
stomatal resistance, is not limiting . The fact
that the parameters of the model were estimated
by least-squares allows the use o f
statistical tests useful in comparing such data .
Energy Budget Mode l
Idso and Baker (1968) based their model of
photosynthesis and their earlier model (Ids o
and Baker 1967) on that of Gates (1965 )
which is an energy budget model where precis e
measurement of incoming radiant energy is
equated with outgoing energy . Thus, at equilibrium,
the amount of energy leaving a leaf i s
equal to that coming in . Gates ' (1965) leaf
energy budget model is :
where
(1 +r) (S + s) (Rg' +R a)
as 2 +a t 2 (11 )
-e taTZ ± C±LE= 0
a s = mean total absorbance of plant to
sunlight and skyligh t
r = reflectance of underlying ground
or plane surface to sunlight and
skylight, S and s
at = absorbance of plant to thermal
radiation, Rg and Ra
e t = emissivity of plant to thermal
radiatio n
Tl = leaf temperature ° K
C = energy gained or lost by convection
cal cm-2 min- 1
L = latent heat of evaporation ca l
cm-2 miri- l
E = transpiration rate of leaf gm cal - 2
min- 1
S, s, Rg, Ra = various short and lon g
wave radiation inputs ca l
cm-2 min 1
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