Chapter 5 Carbon Input to Terrestrial Ecosystems

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Chapter 5Carbon Input to Terrestrial Ecosystems

• Principles of
• Terrestrial Ecosytem Ecology
• Chapin, Matson and Mooney
• Continued

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Ways to manipulate plant productivity
Photosynthetic efficiencies a) Mode of
photosynthetic pathway (C3,
C4, CAM) Why, as ecosystem ecologists,
do we care?
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Ways to manipulate plant productivity
Photosynthetic efficiencies a) Mode of
photosynthetic pathway (C3,
C4, CAM) Why, as ecosystem ecologists, do
we care? Under identical climate,
topography, parent material and timewe can
alter carbon fixation (GPP) by 50!
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• Hows your short-term retention?
• The main enzyme used in primitive
  photosynthesis
• is called
• RIBISCO
• PEP Carboxylase
• CAM
• d) Preparation H

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RIBISCO is the most abundant protein on earth
it makes up 50 of leaf protein.
Rubisco.  Each holoenzyme is composed of 8 large
(blue light blue) and 8 small (red orange)
subunits.  The yellow loops indicate the
positions of the active-site.
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• The problems with the primitive photosynthetic
• enzyme include ALL except
• It uses nitrogen in its structure, and nitrogen
• can be limiting.
• It does not have a high affinity for carbon
• dioxide, so plants need lots of it
• It reverses itself and photo-oxidizes carbon
• compounds at high temperatures.
• d) All of the above are true

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Light reaction dark reaction
Fig. 4.3
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The superiority of the C4 (pep-carboxylase) Invo
lves all except a) has a very high affinity
for carbon dioxide. b) surrounds the C3
photosynthetic pathway and keeps it in a
high carbon dioxide environment. c)
does not exhibit photo-oxidationhas no
affinity for the oxygen molecule. d) requires
far less energy to build and maintain.
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• C4 adaptation to photorespiration
• CO2 ? 4-C compound in mesophyll cells (before
  Calvin cycle)
• 4-C cmpd exported to bundle sheath cells CO2
  released Calvin cycle continues

Aber-Melillo
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C4 photosynthetic pathway Has an enzyme (PEP
Carboxylase) that functions to bind with carbon
dioxide even at very low concentrations. This
enzyme releases carbon dioxide to C3 system
Which is protected inside of the leaf. Great
idea! Whats the catch?
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C4
C3
J. Ehleringer
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Different photosynthetic pathways have Different
efficiencies e.g., grams carbon fixed/per
unit resource Resource a) water
b) light
c) nitrogen Do
efficiencies help us explain global and local
patterns?
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C4 plants have greater affinity for carbon
dioxide per unit of enzyme. Enzymes can
therefore be reduced, cutting nitrogen
cost. This same affinity for carbon dioxide
allows stomatal openings to be closed a higher
percentage of time. Water use efficiency is
therefore much higher.
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The light-use efficiencies of C3/C4 plants are
intrinsically different
photorespiration is temperature sensitive
a temperature - dependent tradeoff is created
J. Ehleringer
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• We should expect C4 photosynthesis
• to be most dominant in
• The tropics
• The semiarid subtropics
• The boreal forest
• The arctic and alpine tundra

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Map of C3 and C4 grasses All forests are C3
(Arctic and alpine tundra are 100 C3)
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The quantum yield model of C4 distribution
J. Ehleringer
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So, what are effects of global climate change?
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The carbon isotope signature in the soil tells
you the C3-C4 ratio
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Western Regional Climate Center, wrcc_at_dri.edu
Period of Record 1/ 1/1918 to 12/31/2005
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Aboveground production
The sensitivity of ecosystems to water
additions is related to its photosynthetic
pathway! (Huxman et al. Nature 2004)
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C3 C4 CAM
Fig. 4.5
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C3 C4 CAM
Fig. 4.5
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CAM plants succulents (cacti) epiphytes
(plants that live in trees) few aquatic plants
(!) huh?
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CAM a second adaptation to low C02 or arid
conditions (Crassulacean acid metabolism) Night
stomata are open, CO2 incorporated into organic
acids and stored in mesophyll cells Day stomata
are closed, organic acids release CO2, light
reactions drive Calvin cycle CAM plants are found
where ever carbon dioxide is severely limited.
Surprisingly, this occurs in lakes as well as in
arid environments!!! (but is rare).
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What kind of photosynthesis is used for our
crops.and where do they grow?
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(No Transcript)
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How much water does it cost to make a gram of
plant material? C3 450-950 g H20 C4
250-350 g H20 CAM 18-125 g H20 (the winner!)
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If C4 plants are better adapted for hot
environments, why then are the tropics dominated
by C3 plants? (the only C4s tend to occur in
disturbed or early successional communities.
Climax vegetation is all C3.)
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The light-use efficiencies of C3/C4 plants are
intrinsically different
photorespiration is temperature sensitive
To get to the canopy, you must survive the
understory! cooler, dark favors C3
a temperature - dependent tradeoff is created
J. Ehleringer
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Ways to manipulate Plant Production
Photosynthetic efficiencies a) Mode of
photosynthetic pathway (C3,
C4, CAM) b) concentration of enzymes
1. Low-light specialists
2. High-light specialists c) Leaf
geometry
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Chapins Law Organisms allocate resources to
attempt to optimize the benefit/cost
ratio. allocate resources to MINIMIZE the
extent to which one factor can limit a critical
activity.
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Basic principle of environmental control

• Equalize physical and biochemical limitations of
  photosynthesis
• Physical limitation delivery of CO2 to leaf
• Biochemical limitation carboxylation rate
• Light limitation
• Enzyme limitation

(text, pg 105)