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. GPP -20% +20% LAI , Clumping =constant GPP -20% +20% Clumping , LAI =constant GPP -20% +20%  Clumping, LAI =constant GPP -20% +20% LAI , Clumping =constant GPP -20% +20% Clumping , LAI =constant GPP -20% +20% Clumping, LAI =constant  Loblolly pine Eucalyptus

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

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  = A leaf / PAR Maximum potential photosynthesis rate Canopy quantum efficiency A leaf = c a (1- c i /c a ) * g leaf Light supply and light capture

CO 2 moves from the air to the leaf to the chloroplast by diffusion (but really CO2 moves both directions) At the same time, H 2 O vapor moves out of the leaf by diffusion (but really H2O vapor moves both directions)

C a = external CO 2 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”) C i = internal CO 2 concentration. This value can be measured (indirectly) with common gas exchange instruments Some definitions ….

CO 2 diffuses into leaves, moving “down” a concentration gradient C a = ppm? Typical CO 2 concentration of a C3 plant at midday is about ppm The CO 2 concentration at the site of fixation approaches “zero”

The diffusive movement of CO 2 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” = g x ) Net flux of “x” = F x [x o ] = concentration of “x” on the “outside” of “barrier” [x i ] = concentration of “x” on the “inside” of the “barrier” Fx = ([x o ] – [x i ]) * g x

Conductance is a PROPERTY of leaf, kind of analogous to its “porosity” to CO 2 or H 2 O 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 g H2O = conductance to water vapor g CO2 = conductance to CO2 g s = stomatal conductance (usually to water vapor) g l = 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 = (c a -c i ) * g leaf Or: A leaf = c a (1- c i /c a ) * g leaf (Norman 1982; Franks & Farquhar 1999)

Factors affecting net assimilation (A) and stomatal conductance (g leaf ): Vapor pressure deficit, D (that is related to the humidity of the air) Soil Moisture,  Temperature, T A leaf = c a (1- c i /c a ) * g leaf f(D,  ) f(T)

f(D,  ) f(T)

Humidity and vapor pressure deficit The portion of total air pressure that is due to water vapor is water vapor pressure (e a ) measured in kPa

When air has no extra capacity for holding water, the vapor pressure is termed: saturation vapor pressure (e s, units kPa) Saturation vapor pressure is mostly a function of air temperature When air temperature falls without a change in water content, the point of condensation is called the dew point temperature

Relative Humidity is the ratio between actual vapor pressure (e a ) and saturation vapor pressure (e s ) RH = e a /e s Vapor Pressure Deficit (D) is the difference between saturation vapor pressure (e s ) and actual vapor pressure (e a ) D = e s - e a

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 g s with increasing D. (Breda et al. 2006) (Oren et al. 1999) Stomata (canopy) conductance D (kPa) Relative conductance g leaf /g leaf-maximum D (kPa)

LnD (Vapor pressure deficit) Vapor pressure deficit, D (kPa) Relative conductance g leaf /g leaf-maximum Relative conductance g leaf /g leaf-maximum g leaf /g leaf-maximum = -0.6 LnD g leaf /g leaf-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 g s with increasing D. If D <1, then g leaf /g leaf-max = 1  A leaf /A leaf-max = 1   /  max = 1 If D > 1, then g leaf /g leaf-max = -0.6 LnD +1  A leaf /A leaf-max < 1   /  max < 1

Stomata respond to changes in soil moisture (  ). During water shortage, when  drops below ca. 0.2, g leaf declines gradually down to very low values Soil moisture,  (m 3 m -3 ) Modified after Breda et al. (2006)

Soil moisture,  (m 3 m -3 ) g leaf /g leaf-maximum = s  +b Relative conductance g leaf /g leaf-maximum g leaf /g leaf-maximum = 1 s Soil moisture,  (m 3 m -3 ) Relative conductance g leaf /g leaf-maximum

If  > 0.2, then g leaf /g leaf-max = ?  A leaf /A leaf-max = ?   /  max = ? If  < 0.2, then g leaf /g leaf-max = ?  A leaf /A leaf-max < ?   /  max < ? Stomata respond to changes in soil moisture (  ). During water shortage, when  drops below ca. 0.2, g leaf declines gradually down to very low values

f(D,  ) f(T)

Temperature effect on C i /C a and on net assimilation C i : Typical CO 2 concentration is about ppm C a = external CO 2 concentration (C a = ppm?)

Temperature (  C) 0 A/A max Ci/CaCi/Ca Temperature (  C) Warren and Dreyer (2006)

If T 30  C, then c i /c a = ?  A leaf /A leaf-max = ?   /  max = ? If 20  C<T <30  C, then ci/ca = ?  A leaf /A leaf-max = ?   /  max = ? c i /c a respond to changes in temperature (T). Under low or high T, c i /c a increases gradually to high values

Final assignment: Just calculate GPP and have fun experimenting ! GPP =  {f(D)f(T)f(  ) f(CO2)}*  APAR

References Breda N. et al Temperate forest trees and stands under severe drought: a review. Annals of Forest Science. 63: Dye, P.J. et al 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 A relationship between humidity response, growth form and photosynthetic operating point in C3 plants. Plant, Cell Environment 22:1337–1349. Norman J. M Simulation of microclimates, in Biometeorology in integrated pest management, edited by J. L. Hatfield and I. J. Thomason, p , Academic, New York. Oren R. et al Survey and synthesis of intra- and interspecific variation in stomatal sensitivity to vapour pressure deficit. Plant, Cell and Environment 22: Waring W.H. and S.W. Running Forest ecosystem analysis at multiple scales. 2 nd Ed. Academic press. San Diego, CA 370p. Warren C.R. and E. Dreyer Temperature response of photosynthesis and internal conductance to CO2: results from two independent approaches. Journal of Experimental Botany 57: