Atmospheric Analysis Lecture 6

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)

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 Radiation Balance for a Plant Canopy

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

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 opening depends on light intensity, moisture availability, temperature, humidity and internal CO2 concentration

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)

Plant canopies elevate the position of the active 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 radiation due 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 by which the leaf temperature exceeds the air temperature during 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)

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

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

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

The short-term influence of increased CO2 concentration. Also: Stomatal conductance tends to decrease (enough CO2), leading to increased water 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 prairie grassland What causes the Midday 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) = K0e-kLAI 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) accumulated from the top of the canopy to the level in question (‘A1(z)’ in Oke)

Q* influences the temperature and humidity Structure within a canopy

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

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

Effect of LAI on Pc

Effect of Respiration Parameter on Pc

Effect of Extinction Coefficient, k, on Pc

Soil respiration measurements

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

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

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

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

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

Source: Dr. Larry Flanagan

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