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Exploration of Model Structure
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Forest Carbon Exchange
Photosynthesis
6CO2 + 6H2O + energy
C6H12O6 + 6O2
Respiration
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Terms
• NPP - net primary production
• GPP – gross primary
production
• R – respiration
– RA – autotrophic respiration
– RH – heterotrophic respiration
– RE – ecosystem respiration
• RE = RA + RH
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Terms
• GPP = NPP – RA
• NEP – Net Ecosystem
Production
• NEP = GPP – RE
+NEP – system gaining C
- NEP – system losing C
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Net Ecosystem Productivity (NEP)
• NEP reflects the annual change in carbon
storage (vegetation + above and belowground
carbon + soil C) and indicates whether an
ecosystem functions as a source or sink for
atmospheric CO2.
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How do we estimate GPP?
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Leaf Photosynthesis
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Individual Leaf Photosynthesis:
Light response curve
`
8
Net Photosynthesis (A)
(µmol CO2 m-2s-1)
Amax
6
Φ
4
A = Amax(1-(1-Rd/Amax)(1-(PAR/LCP)))
2
Φ = Amax / LCP(1-(Rd / Amax)(log(1 - Rd / Amax)
LCP
0
Rd
0
500
1000
1500
PAR (µmol m-2s-1)
2000
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Photosynthesis - Leaf photosynthesis
• Photosynthetic performance – linked to
leaf traits and environment:
–
–
–
–
–
–
atmospheric CO2
nutrition
water
temperature
leaf age
phenology
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Leaf Photosynthesis
Response to Environment
6
Upper canopy
"sun leaves"
-2 -1
A (µmol m s )
5
4
Lower canopy
"shade leaves"
3
2
1
0
-1
0
200
400
600
800
1000
1200
1400
PAR (µmol m -2s -1)
23 year-old Pinus taeda plantation
1600
1800
2000
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Leaf Photosynthesis
Response to Environment
12
[CO 2 ] = 570 ppm
8
-2
-1
A (µmol m s )
10
6
4
2
[CO 2 ] = 370 ppm
0
0
200
400
600
800
1000
1200
1400
-2 -1
PAR (µmol m s )
23 year-old Pinus taeda plantation
1600
1800
2000
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Leaf Photosynthesis
A
Response to Environment
30 year-old Pinus strobus plantation
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Canopy Photosynthesis
• Scale from the leaf to canopy level. How?
• Canopy level photosynthesis – express as GPP
(Mg carbon ha-1 yr-1 or g carbon m-2 s-1)
• Understand the variation in leaf traits and
environment within the canopy
• Different approaches
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A
Canopy Photosynthesis (GPP)
Absorbed PAR
A
PAR
A
PAR
PAR
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GPP and Absorbed PAR
• GPP proportional to
absorbed PAR
• Express light use efficiency
as α.
GPP
• Slope of this relationship is
canopy light use efficiency
• α has units of (Mgdry mass
MJ-1, Mgcarbon MJ-1)
Absorbed
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Modeling Canopy Photosynthesis
(GPP)
GPP = Σ APAR * α
–α - maximum quantum efficiency
–APAR – absorbed PAR by the canopy
• α is a function of climatic
and site factors
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GPP (Mg ha-1)
Modeling Canopy Photosynthesis
(GPP)
α max
(CO2)
(soil moisture)
(D, temperature)
Absorbed PAR (MJ ha-1)
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Modeling Canopy Photosynthesis
(GPP)
GPP = Σ APAR * α * {ƒ(H2O) ƒ(D) ƒ(T) ƒ(CO2)}
– α - maximum quantum efficiency
– APAR – absorbed PAR by the canopy
– ƒ(i) modifying factors – range from 0 to 1
• Soil water deficits
• Vapor pressure deficits (D)
• Temperature (T)
• Atmosphere [CO2]
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Terms:
•
•
•
•
•
•
•
Net photosynthesis (A) - carbon fixation
Autotrophic respiration (RA) - oxidation of organic substances to CO2
and H2O providing carbon skeletons and energy (ATP, NADPH) that
supports the growth and maintenance of tissues.
Heterotrophic respiration (RH)- process of oxidation of organic matter
(decomposition) in the forest floor (foliage, stems, branches,
reproductive structures) or in the soil (mycorrhizae, fine and coarse root
turnover).
Carbon allocation - the distribution of carbohydrates formed during
photosynthesis to component parts of plants (foliage, branches, stems,
fine and coarse roots, and storage) and respiration.
Gross primary production (GPP) - total amount of carbon fixed by the
canopy during photosynthesis.
Net primary production (NPP) - the amount of carbon accumulated by
a tree over a specified time interval, the difference between GPP and
autotrophic respiration: NPP = GPP - RA.
Net ecosystem productivity (NEP) - net flux of CO2 to or from a forest
ecosystem, the difference between carbon fixation and respiratory
losses- NEP = GPP - (RA+RH).
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Terms:
•
•
•
•
•
•
Carbon Use Efficiency (CUE) – Ratio of net production to production
plus RA or CUE=NPP/GPP.
Photosynthetically Active Radiation (PAR) – part of the
electromagnetic spectrum (400 to 700 nm) that drives photosynthesis
APAR – fraction of incident PAR absorbed by the canopy
Leaf Area Index (LAI) – total leaf area per unit area of ground
DBH – tree diameter at 1.4m from ground (diameter at breast height)
Specific Leaf Area (SLA) – leaf area per unit leaf mass