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)

Energy balance of soil-plant-air system

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

Surface RadiationBalance for aPlant Canopy

Heat Storage by Photosynthesis

The net rate of CO2 assimilation (kgm-2s-1)

P = P – R

Heat storage by net photosynthesis is, therefore:

QP = P

where is the heat of assimilation of carbon (Jkg-1)

Values are very small compared to other fluxes - up to ~10 Wm2 during the day- about -3 Wm2 during the night

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- 10-30 m long, 0-10 m wide- 50-500 stomata mm-2

Stomate (wheat)

Degree of openingdepends on lightintensity, moistureavailability, temperature, humidity and internal CO2 concentration

Stand Architecture and the Active Surface

Position of active surface lies at the zero planedisplacement: 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)

Plant canopieselevate the position of theactive surface

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 radiationdue to their high water content (absorb L too)This helps the leaves shed heat effectively

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

Leaf Energy Balance

Q*leaf = (QH(t)+QH(b))+(QE(t)+QE(b))= QH(leaf) + QE(leaf)

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

Number of degrees bywhich the leaf temperatureexceeds the air temperatureduring the daytime

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)

cuticle

palisade mesophyll

spongy mesophyll

lower epidermis

upper epidermis

Carbon dioxide must travel from atmosphere,through mesophyll to chloroplasts

Photosynthesis Measurement

max

max

PPAR

PPARPn

Leaf-level net photosynthesis modelling

(Thornley and Johnson, 1990)

-2

0

2

4

6

8

10

12

14

16

0 500 1000 1500 2000 2500

PAR (molm-2s-1)

Net

ph

oto

syn

thes

is (m

ol

m-2s

-1)

Photosynthesis vs. Elevation

1450 masl

2150 masl

Miconia sp.

-2

0

2

4

6

8

10

12

14

0 50 100 150 200 250

Modelled LMCF

Observed LMCF

Modelled UMCF

Observed UMCF

-4

0

4

8

12

16

20

0 600 1200 1800 2400

Modelled LMCF

Observed LMCF

Modelled UMCF

Observed UMCF

a.

b.

PAR (µmolm-2s-1)

PAR (µmolm-2s-1)

Pn

µmol CO2m2s1

Pn

µmol CO2m2s1

LMCF

RMSE = 1.10 µmolm-2s-1

R2 = 0.77; N=820

UMCF

RMS E = 1.69 µmolm-2s-1

R2 = 0.55; N=500

-5

0

5

10

15

20

25

0 500 1000 1500 2000 2500

2xCO2

Ambient

Mod 2xCO2

Mod Ambient

-5

0

5

10

15

20

25

0 500 1000 1500 2000 2500

2xCO2

Ambient

Mod 2xCO2

Mod Ambient

Pn = 0.0364 (PAR) - 0.2994

R2 = 0.88; 30<PAR<350

Pn = 0.0442 (PAR) - 0.3025

R2 = 0.93; 7<PAR<570

-5

0

5

10

15

20

25

0 200 400 600

Ambient

2xCO2

Linear (Ambient)

Linear (2xCO2)

-5

0

5

10

15

20

25

0 400 800 1200

2xCO2

Ambient

Mod 2xCO2

Mod Ambient

-5

0

5

10

15

20

25

0 500 1000 1500 2000 2500

2xCO2

Mod 2xCO2

Ambient

Mod Ambient

-5

0

5

10

15

20

25

0 500 1000 1500 2000 2500

2xCO2

Ambient

Mod 2xCO2

Mod Ambient

Anthurium sp.

All generaPsychotria sp.

Clusia sp.Miconia sp.

Cecropia sp.

Pn

molm2·s

PAR (mol·m-2·s-1)

PAR (mol·m-2·s-1)PAR (mol·m-2·s-1)

PAR (mol·m-2·s-1)PAR (mol·m-2·s-1)

PAR (mol·m-2·s-1)

Pn

Pn Pn

PnPn

The short-terminfluence of increasedCO2 concentration.

Also:Stomatal conductancetends to decrease(enough CO2), leading to increasedwater use efficiency

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

Vertical flux of carbon dioxide(FC) over a prairiegrassland

What causes theMidday minimum in August?

Notice how low the CO2 concentration was in 1969 !

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) = K0e-kLAI

k is a canopy-specific extinction coefficient (0.4-0.9)(‘a’ in Oke)

LAI is the leaf area index (m2 leaf m-2 ground) accumulatedfrom the top of the canopy to the level in question(‘A1(z)’ in Oke)

Q* influencesthe temperatureand humidityStructure withina canopy

Leaf temperatureremained cooldue to evaporation

Decreasing lightintensity or increasing water stress

Dew present

Energy balanceover an Englishbarley field

cloud cover

QE dominated indissipating radiativesurplus

LAIPPARk

ePPARP

LAIk

c

max

)(max ]1[

Net canopy photosynthesis (Pc)

Charles-Edwards (1986)

Photosynthetically-active radiation(“direct” portion,0.3-0.4 CI, 0400h-1200h)

0400-0500h 0500-0600h 0600-0700h 0700-0800h

0800-0900h 0900-1000h 1000-1100h 1100-1200h

-70

-60

-50

-40

-30

-20

-10

0

10

20

0 1 2 3 4 5 6 7 8

August

November

Rel

ativ

e pr

od

ucti

vity

(%

)

Leaf Area Index (LAI)

Effect of LAI on Pc

-60-50-40-30-20-10

010203040

0 0.2 0.4 0.6 0.8 1

August (1400m)November (1400m)August (1600m)November (1600m)

Rel

ativ

e pr

od

ucti

vity

(%

)

Canopy leaf respiration rate (molCm-2s-1)

Effect of Respiration Parameter on Pc

-25

-20

-15

-10

-5

0

5

10

0.2 0.4 0.6 0.8 1

August

November

Rel

ativ

e pr

od

ucti

vity

(%

)

Extinction coefficient, k

Effect of Extinction Coefficient, k, on Pc

Soil respiration measurements

There is a much easier way toassess productivity…

A micrometeorological solution:Eddy correlation

NEE = A + R

A = Gross Photosynthesis (-)

R = Total EcosystemRespiration (+)

NEE = 0.1223 (soil temp) - 0.0525

R2 = 0.2477

-10

-8

-6

-4

-2

0

2

4

6

8

10

0 5 10 15 20 25 30

Night-time NEE = Total Ecosystem RespirationN

EE

(m

ol C

O2

m-2s

-1)

Soil Temperature at 5cm depth (C)

Mer Bleue Bog,Eastern Ontario

-12

-8

-4

0

4

8

0 1000 2000

Daytime NEE Gross Photosynthesis – Total Ecosystem Respiration

NE

E (m

ol C

O2

m-2s

-1)

Photosynthetically-active radiation (molm-2s-1)

Fluxnet-Canada Carbon Flux Stations

Coastal conifers

Southern boreal conifers and hardwoods Boreal

mixedwood

Balsam fir

Eastern peatlandWestern

peatland

CO2 richDry, Cool

Low CO2

Humid, Warm

Source: Dr. Larry Flanagan

c

Distance

Profile View

c

Top View

Footprint: Area affecting measurements at the tower varies withwind speed, wind direction, roughness, stability etc.

The black area has the greatestinfluence on tower measurements