1. Atmospheric
Analysis
Lecture 6

2. Energy balance of soil-plant-air system
Q* = QH + QE + QG +QS + QP + QA
QS - physical storage change due
- absorption or release of heat from air,
soil or plant biomass
QP - biochemical energy storage due to
photosynthesis
QA - horizontal sensible and latent heat
transport (later)

3. Water balance of soil-plant-air system
p = E + r + S
S – net water storage of air, soil and plants
(internal and external)
Photosynthesis, P:
6CO2 + 6H2O + sunlight  C6H12O6 + 6O2
Respiration, R:
C6H12O6 + 6O2  6CO2 + 6H2O + energy

4. Surface
Radiation
Balance
for a
Plant
Canopy

5. Heat Storage by Photosynthesis
The net rate of CO2 assimilation (kgm-2s-1)
P = P – R
Heat storage by net photosynthesis is, therefore:
QP =  P
where  is the heat of assimilation of carbon (Jkg-1)
Values are very small compared to other fluxes
- up to ~10 Wm2 during the day
- about -3 Wm2 during the night

6. Transpiration through stomata
increases the QE flux
prevents overheating
induces moisture and nutrient transport
Stomata
- open during the day for gas exchange
- closed at night
- stomata open when there is enough
light, and appropriate levels of moisture,
temperature, humidity and internal CO2
concentration
m long, 0-10 m wide
stomata mm-2

7. Stomate (wheat) Degree of opening depends on light intensity, moisture
availability,
temperature, humidity
and internal CO2
concentration

9. Stand Architecture and the Active Surface
Position of active surface lies at the zero plane
displacement: d  2/3 h
Modified logarithmic wind profile equation:
uz = (u*/k) ln (z-d/z0)
For simplicity, energy exchange is considered at
a plane at the top of the system (‘big leaf’ approach)

10. Plant canopies
elevate the
position of the
active surface

11. Wavelength Dependence of Leaves
Leaves absorb photosynthetically-active radiation (PAR)
effectively for carbon assimilation
Better absorption in blue and red bands than in the green band
Leaves reflect and transmit near infra-red radiation (NIR)
This helps limit heating
Leaves are very efficient emitters of longwave radiation
due to their high water content (absorb L too)
This helps the leaves shed heat effectively

13. Leaf Radiation Balance
Q*leaf = [(Kin(t) + Kin(b))(1--)]+
[(Lin(t)-Lout(t))+(Lin(b)-Lout(b))]
= K*(t)+K*(b)+L*(t)+L*(b)
= K*leaf + L*leaf

14. Leaf Energy Balance Q*leaf = (QH(t)+QH(b))+(QE(t)+QE(b))
= QH(leaf) + QE(leaf)

15. Sensible Heat Flux and Leaf Temperature
QH = Ca (T0-Ta)/rb
- rb is the diffusive resistance the laminar sublayer
rb value higher for larger leaves as laminar layer grows
higher resistance during calm conditions
T0 = Ta + rb/Ca (Q*leaf – QE leaf)
Air temperature is important for leaf temperature
Leaf may be warmer or cooler than the air
If rb is large, Q*leaf – QE leaf determine T0-Ta
Hot, dry environments: plants develop small leaves, with
high albedo,or orient leaves vertically near solar noon
-Very cold environments: leaves grow close to ground,
have large rb, and, in the arctic, touch the warmer ground

16. Number of degrees by
which the leaf temperature
exceeds the air temperature
during the daytime

17. Evapotranspiration from a Leaf
Depends on vapour pressure deficit and diffusive resistance
of the laminar sublayer
E = (*v(To) - va)/ (rb + rst)
- rst is a variable stomatal resistance
at the canopy level, we can think of a canopy resistance,
vdd/E, which varies with rst/LAI and an aerodynamic
resistance describing the role of turbulence in evaporation
Carbon flux from a Leaf
Fc = (ca - ci)/ (rb + rst)

19. Carbon dioxide must travel from atmosphere,
through mesophyll to chloroplasts
cuticle
upper epidermis
palisade mesophyll
spongy mesophyll
lower epidermis

22. Leaf-level net photosynthesis modelling
(Thornley and Johnson, 1990)

23. Photosynthesis vs. Elevation
Miconia sp.
1450 masl
Net photosynthesis (molm-2s-1)
2150 masl
PAR (molm-2s-1)

25. The short-term
influence of
increased
CO2 concentration.
Also:
Stomatal
conductance
tends to decrease
(enough CO2),
leading
to increased
water use
efficiency

26. Plant Canopies and Carbon Dioxide Flux
At night: - flux directed from canopy to the atmosphere
- respiration from leaves, plant roots, soil
Daytime: - CO2 assimilation rate exceeds respiration rate
Seasonal Variation in Temperate Environments
Spring: Assimilation increases with leaf area index and
increasing solar radiation availability/day length
Midsummer: Fc drops despite sun, due to soil moisture
depletion – flux higher in morning
Winter: Small, negative flux

27. Vertical flux of
carbon dioxide
(FC) over a prairie
grassland
What causes the
Midday minimum in
August?

28. Notice how low the CO2 concentration was in 1969 !

29. Canopy Radiation Budget
- Incident light greatest at crown and decreases
logarithmically with depth in the canopy
- Approximated by Beer’s Law for canopy extinction
K(z) = K0e-kLAI
k is a canopy-specific extinction coefficient ( )
(‘a’ in Oke)
LAI is the leaf area index (m2 leaf m-2 ground) accumulated
from the top of the canopy to the level in question
(‘A1(z)’ in Oke)

30. Q* influences
the temperature
and humidity
Structure within
a canopy

32. cloud cover
Energy balance
over an English
barley field
QE dominated in
dissipating radiative
surplus
Leaf temperature
remained cool
due to evaporation
Decreasing light
intensity or increasing
water stress
Dew present

34. Net canopy photosynthesis (Pc)
Charles-Edwards (1986)

36. Effect of LAI on Pc

37. Effect of Respiration Parameter on Pc

38. Effect of Extinction Coefficient, k, on Pc

39. Soil respiration measurements

40. There is a much easier way to
assess productivity…
A micrometeorological solution:
Eddy correlation

42. NEE = A + R A = Gross Photosynthesis (-) R = Total Ecosystem
Respiration (+)

43. Night-time NEE = Total Ecosystem Respiration
Mer Bleue Bog,
Eastern Ontario
NEE (mol CO2m-2s-1)
Soil Temperature at 5cm depth (C)

44. Daytime NEE Gross Photosynthesis – Total Ecosystem Respiration
NEE (mol CO2m-2s-1)
Photosynthetically-active radiation (molm-2s-1)

47. Fluxnet-Canada Carbon Flux Stations
Balsam fir
Coastal conifers
Eastern peatland
Western peatland
Southern boreal conifers and hardwoods
Boreal mixedwood

51. Source: Dr. Larry Flanagan

52. Footprint: Area affecting measurements at the tower varies with
wind speed, wind direction, roughness, stability etc.
The black area has the greatest
influence on tower measurements