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Next week’s assignment:1) Using clumping indexes, LAI and values for a conifer stand (Loblolly pine forest, Duke Univ.) and for a Eucalyptus plantation (New Zealand), calculate their Monthly GPP (potential GPP).
- Loblolly pine: = 2.37 gC MJ-1 APAR
- Eucalyptus plantation: = 3.85 gC MJ-1 APAR
2) Assuming that all of the above parameters vary by plus or minus 20%, calculate how Annual GPP would be affected for each forest type.
GP
P
-20% +20%LAI
, Clumping =constant
GP
P
-20% +20%Clumping
, LAI =constant
GP
P
-20% +20%
Clumping, LAI =constantG
PP
-20% +20%LAI
, Clumping =constant
GP
P
-20% +20%Clumping
, LAI =constant
GP
P
-20% +20%
Clumping, LAI =constant
Lobl
olly
pin
e
Euc
alyp
tus
Monthly GPP
0
200
400
600
800
1000
1200
1 2 3 4 5 6 7 8 9 10 11 12
month
GPP
, gC
m-2
mon
th-1
Pine
Euca
1) Using clumping indexes, LAI and values for a conifer stand (Loblolly pine forest, Duke Univ.) and for a Eucalyptus plantation (New Zealand), calculate their Monthly GPP (potential GPP).
- Loblolly pine: = 2.37 gC MJ-1 APAR ; - Eucalyptus plantation: = 3.85 gC MJ-1 APAR
Pine & Euca
4800
5000
5200
5400
5600
5800
LAI-20% BASE LAI+20%
GPP
, gC
m-2
yea
r-1
Pine & Euca
4800
5000
5200
5400
5600
5800
Ω-20% BASE Ω+20%
GPP
, gC
m-2
yea
r-1
Pine & Euca
4200
4600
5000
5400
5800
6200
6600
α-20% BASE α+20%
GPP
, gC
m-2
yea
r-1
Actual GPPs
Pine = 2500 gC m-2 year-1
Euca = 3300 gC m-2 year-1
2) Assuming that all of the above parameters vary by plus or minus 20%, calculate how Annual GPP would be affected for each forest type.
GPP = {f(D)f(T)f()
f(CO2)}*APAR
“Productivity” equation
Constraints to photosynthesis
= Aleaf / PAR
Maximum potential photosynthesis rateCanopy quantum efficiency
Aleaf = ca (1- ci/ca) * gleaf
Light supply and light capture
CO2 moves from the air to the leaf to the chloroplast by diffusion (but really CO2 moves both directions)
At the same time, H2O vapor moves out of the leaf by diffusion (but really H2O vapor moves both directions)
Ca = external CO2 concentration
(note that this leaf has stomata only on the “abaxial” or bottom side. Some leaves also have stomata on the adaxial, or upper surface. Leaves with stomata on both sides are called “amphistomatous”)
Ci = internal CO2 concentration. This value can be measured (indirectly) with common gas exchange instruments
Some definitions ….
CO2 diffuses into leaves, moving “down” a concentration gradient
Ca = 370-400 ppm?
Typical CO2 concentration of a C3 plant at midday is about 270-300 ppm
The CO2 concentration at the site of fixation approaches “zero”
The diffusive movement of CO2 into and out of a leaf can be described by Fick’s Law:
Net flux = concentration * conductance
(a membrane or barrier with a “conductance” to substance “x” = gx)
Net flux of “x” = Fx
[xo] = concentration of “x” on the “outside” of “barrier”
[xi] = concentration of “x” on the “inside” of the “barrier”
Fx = ([xo] – [xi]) * gx
• Conductance is a PROPERTY of leaf, kind of analogous to its “porosity” to CO2 or H2O vapor. It is NOT a “rate”!!!
Conductance is the inverse of resistance. Both quantities are commonly used. The symbol “g” is commonly used for conductance, “r” for resistance
gH2O = conductance to water vapor
gCO2 = conductance to CO2
gs = stomatal conductance (usually to water vapor)
gl = total leaf conductance (usually to water vapor)
The units used for conductance and resistance can be very confusing -
Applying Fick’s Law to carbon assimilation :
Net C assimilation = (ca-ci) * gleaf
Or: Aleaf = ca (1- ci/ca) * gleaf
(Norman 1982; Franks & Farquhar 1999)
Factors affecting net assimilation (A) and stomatal conductance (gleaf):
• Vapor pressure deficit, D (that is related to the humidity of the air)
• Soil Moisture, • Temperature, T
Aleaf = ca (1- ci/ca) * gleaf
f(D, )f(T)
Factors affecting net assimilation (A) and stomatal conductance (gleaf):
• Vapor pressure deficit, D (that is related to the humidity of the air)
• Soil Moisture, • Temperature, Temperature, TT
Aleaf = ccaa (1- (1- ccii/c/caa)) * gleaf
f(D, )f(T)
Humidity and vapor pressure deficit
The portion of total air pressurethat is due to water vapor is
water vapor pressure (ea)
measured in kPa
When air has no extra capacityfor holding water, the vapor pressure
is termed:saturation vapor pressure
(es, units kPa)
Saturation vapor pressure is mostly a function of air temperature
When air temperature falls withouta change in water content, the
point of condensation is called thedew point temperature
Relative Humidityis the ratio between actual
vapor pressure (ea)and saturation vapor pressure (es)
RH = ea/es
Vapor Pressure Deficit (D)is the difference between saturation
vapor pressure (es)and actual vapor pressure (ea)
D = es - ea
Stomata respond to the vapor pressure deficit between leaf and air (D). Stomata
generally close as D increases and the response is often depicted as a nonlinear
decline in gs with increasing D.
(Breda et al. 2006) (Oren et al. 1999)
Sto
mat
a (c
anop
y) c
ondu
ctan
ce
D (kPa)
Rel
ativ
e co
nduc
tanc
eg l
eaf/g
leaf
-max
imum
D (kPa)
LnD (Vapor pressure deficit)
Vapor pressure deficit, D (kPa)
0
1
0
1
Relative conductancegleaf/gleaf-maximum
1
0
532 4
Relative conductancegleaf/gleaf-maximum
gleaf/gleaf-maximum= -0.6 LnD +1
0.6
gleaf/gleaf-maximum= 1
(Oren et al. 1999)
Stomata respond to the vapor pressure deficit between leaf and air (D). Stomata generally close as D increases and the
response is often depicted as a nonlinear decline in gs with increasing D.
If D <1, then gleaf/gleaf-max = 1 Aleaf/Aleaf-max = 1 / max = 1
If D > 1, then gleaf/gleaf-max= -0.6 LnD +1 Aleaf/Aleaf-max < 1 / max < 1
GPP = {f(D)f(f(TT)f()f() ) f(COf(CO22))}*APARAPAR = Aleaf/PAR
Aleaf = ca (1- ci/ca) * gleaf
Stomata respond to changes in soil moisture ( ). During water
shortage, when drops below ca. 0.2, gleaf declines gradually
down to very low values
Soil moisture, (m3 m-3)
0.1 0.30.2 0.4
Modified after Breda et al. (2006)
Soil moisture, (m3 m-3)
0
1
0.1 0.50.30.2 0.4
gleaf/gleaf-maximum = s +bRelative conductancegleaf/gleaf-maximum
gleaf/gleaf-maximum = 1
s
0
1
0.1 0.50.30.2 0.4
0.2
0.08
Soil moisture, (m3 m-3)
Relative conductancegleaf/gleaf-maximum
If > 0.2, then gleaf/gleaf-max = ? Aleaf/Aleaf-max = ? / max = ?
If < 0.2, then gleaf/gleaf-max= ? Aleaf/Aleaf-max < ? / max < ?
GPP = {f(f(DD)f()f(TT)f(CO)f(CO22))f()}*APARAPAR
= Aleaf/PARAleaf = ca (1- ci/ca) * gleaf
Stomata respond to changes in soil moisture ( ). During water shortage, when drops below ca. 0.2, gleaf declines gradually down
to very low values
Factors affecting net assimilation (A) and stomatal conductance (gleaf):
• Vapor pressure deficit, Vapor pressure deficit, DD (that is related to (that is related to the humidity of the air)the humidity of the air)
• Soil Moisture, Soil Moisture, • Temperature, T
Aleaf = ccaa (1- ci/ca) * * ggleafleaf
f(D, )f(T)
Temperature effect on Ci/Ca and on net assimilation
Ci : Typical CO2 concentration is about 270-300 ppm
Ca = external CO2 concentration (Ca = 380-400 ppm?)
Temperature (C)
0
A/Amax
Ci/Ca
5 3020 40
Temperature (C)
0
0.6
5 3020 40
1
Warren and Dreyer (2006)
If T <20C or T> 30 C, then ci/ca = ? Aleaf/Aleaf-max = ? / max = ?
If 20 C<T <30C, then ci/ca = ? Aleaf/Aleaf-max = ? / max = ?
GPP = {f(f(DD))f(T)f(CO2)f(f())}*APARAPAR
= Aleaf/PARAleaf = ca (1- ci/ca) * gleaf
ci/ca respond to changes in temperature (T). Under low or high T, ci/ca increases gradually
to high values
Final assignment:
Just calculate GPP and have fun experimenting !
GPP = {f(D)f(T)f() f(CO2)}*APAR
ReferencesBreda N. et al. 2006. Temperate forest trees and stands under severe drought: a review. Annals of Forest Science. 63:625-644.
Dye, P.J. et al. 2004. Verification of 3-PG growth and water-use predictions in twelve Eucalyptus plantation stands in Zululand, South Africa. For. Ecol. Management. 193:197–218
Franks PJ, Farquhar GD. 1999. A relationship between humidity response, growth form and photosynthetic operating point in C3 plants. Plant, Cell Environment 22:1337–1349.
Norman J. M. 1982. Simulation of microclimates, in Biometeorology in integrated pest management, edited by J. L. Hatfield and I. J. Thomason, p. 65-99, Academic, New York.
Oren R. et al. 1999. Survey and synthesis of intra- and interspecific variation in stomatal sensitivity to vapour pressure deficit. Plant, Cell and Environment 22: 1515-1526
Waring W.H. and S.W. Running 1998. Forest ecosystem analysis at multiple scales. 2nd Ed. Academic press. San Diego, CA 370p.
Warren C.R. and E. Dreyer. 2006. Temperature response of photosynthesis and internal conductance to CO2: results from two independent approaches. Journal of Experimental Botany 57:3057-3067.
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