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Terrestrial Ecosystem Carbon Exchange
Dr. Creighton M. Litton Department of Natural Resources and Environmental Management
University of Hawai‘i at Mānoa
• Objectives – Terrestrial Ecosystem Carbon Cycling
• Carbon Input (GPP) • Autotrophic respiration (R) • Net primary production (NPP) • Net ecosystem production (NEP) • Net Ecosystem Carbon Balance (NECB) • Belowground C flux (TBCA) • C allocation & global patterns
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• Why should we care about C cycling? – C is the energy currency of all ecosystems
• Plant (autotrophic) production is the base of almost all food/energy pyramids
• Central to all ecosystem goods & services
– Plant C cycling, to a large extent, controls atmospheric CO2 concentrations (& climate)
• ~4x as much C stored in terrestrial ecosystems as the atmosphere
• Forests account for ~50% of global terrestrial biomass and ~35% of global terrestrial productivity
– Plant-derived C fundamental to soil processes • Belowground resources are a primary control over all
ecosystem processes
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• Terrestrial metabolism: the “breathing” of Earth
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• C enters via photosynthesis – Gross Primary Production (GPP)
• Net photosynthesis (Gross photo - foliage R during the day)
1. Accumulates in ecosystems (C sequestration) as: (a) plant biomass; (b) SOM & microbial biomass; or (c) animal biomass
2. Returned to the atmosphere via (a) respiration (R; autotrophic or heterotrophic); (b) VOC emissions; or (c) disturbance
3. Transferred laterally to another ecosystem
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• How do you measure GPP? – Measure photosynthesis of every leaf in the canopy?
– Measure a few leaves and scale to the canopy?
→ →
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• How do you measure GPP? – Modeling studies
• LAI estimates from remote sensing or field studies
• APAR or FPAR • LUE from existing studies • Plug it all into a TEMs or DGVM
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• How do you measure GPP? – Eddy flux / covariance
• CO2 sensor above the canopy – Vertical flux of CO2 is a function
of the covariance of wind velocity and gas concentration
• Really measure Net Ecosystem Exchange (NEE) – NEE = GPP - Recosystem
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• How do you measure GPP? – Sum of individual
components • Need measurements of all
individual components • Only ~30-40 studies globally
Litton et al. (2007)
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• What controls GPP? – Within a given set of biotic &
environmental conditions: • Leaf area
– LAI (leaf area / unit ground area; m2 m-2)
• Growing season length • Nutrient availability (esp. N) • Temperature, light, & CO2
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• How is GPP distributed globally across biomes?
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Biomes GPP
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• Net primary production (NPP) – Net annual C gain (or
loss) by plants
– NPP = GPP – Rplant
– ANPP, ANPPwood, ANPPfoliage, BNPP, TNPP, etc.
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• What controls NPP globally?
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Running et al. (2004)
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• How is NPP distributed globally across biomes?
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Biomes NPP
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• How is NPP distributed globally across biomes?
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Global distribution of terrestrial biomes and their total carbon in plant biomassa.
Biome
Area (106 km2) Total C pool (Pg C)
Total NPP(Pg C yr-1)
Tropical forests 17.5 340 21.9Temperate forests 10.4 139 8.1Boreal forests 13.7 57 2.6Mediterranean shrublands 2.8 17 1.4Tropical savannas andgrasslands
27.6 7914.9
Temperate grasslands 15.0 6 5.6Deserts 27.7 10 3.5Arctic tundra 5.6 2 0.5Crops 13.5 4 4.1Ice 15.5Total 149.3 652 62.6
a Data from [Roy, 2001 #3858]. Biomass is expressed in units of carbon,assuming that plant biomass is 50% carbon.• Tropical forests are ~12% of land area, but account for
~50% of global biomass and ~35% of global NPP
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• Rplant = Rgrowth + Rmaint + Rion
– What respires? • All living biomass, all the time
– Why does living biomass respire? • Grow new biomass and maintain existing biomass • Provides energy for essential metabolic processes
– Mitochondrial oxidation of CHO’s to make ATP
– Not “wasted” C
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• Rgrowth (growth/construction) – Total C cost = C in new
biomass + C used to generate that biomass
– Varies widely by compound • Function of concentration & cost • Protein rich (leaves), structural
(wood), and defense
– How do you measure Rgrowth? • ~25% x NPP
– Total C cost = ~1.23g CHOs per 1 g of biomass produced
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• Rmaint (maintenance of existing biomass) – Repair of non-growing tissues
• Protein turnover (~85%)
• Membrane lipids • Rion (transport across membranes)
– ≥ ½ of Rtotal
– How do you measure Rmaint? • Strongly correlated with temperature and N content
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• Rmaint – Rm = R0(Q10)(T/10)
– Rm = 0.0106 x N content
Ryan et al. (2004)
Tw (°C)
Curtis et al. (2005)
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• NPP = GPP - Rplant
• Typically measured on annual time scales • Units of biomass or C / unit area / unit time
• g C m-2 yr-1
• How do you measure NPP? • Remember that we typically get GPP by measuring all the components, including NPP
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• Measuring NPP • NPP = ΔBiomass
• Biomass from allometric equations
• Need to account for biomass increment and loss because plant tissue is continually shed
• NPP = (ΔLeaf Bio. + Leaf Litter) + (ΔWood Bio. + Wood Litter) + (ΔRoot Bio. + Root Litter) • Litterfall quantified with littertraps
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Litton & Kauffman (2008)
Metrosideros polymorpha
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• Net ecosystem production (NEP) – Net annual C gain (or
loss) by an ecosystem
– NEP = GPP – Recosystem
– NEP = NPP – Rhetero
• Same as NEE???
– What are we missing?
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• Net Ecosystem Carbon Balance (NECB) – Net annual C gain (or
loss) by an ecosystem over long time scales
– NECB = GPP - Recosystem - Fdisturb - Fleach - Femissions
• Information most useful for C sequestration estimates
• Both natural & anthropogenic disturbances
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