Chapter 5 Carbon Input to Terrestrial Ecosystems

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Chapter 5Carbon Input to Terrestrial Ecosystems

• Part II Mechanisms
• Chapin, Matson, Mooney
• Principles of Terrestrial Ecosystem Ecology

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Carbon inputs to ecosystems

• Process photosynthesis
• Importance
• Energy that drives all biotic processes
• Accounts for half of organic matter on Earth

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Gross Primary Production(GPP)

• Net photosynthesis at the ecosystem scale

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GPP is C input to the ecosystem
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Photosynthesis

• Levels of control
• Controls in individual leaves
• Control by canopy processes
• Controlling factors
• Direct controls light, CO2
• Indirect controls water, nutrients

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Over long time scales (a year) indirect controls
predominate
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Two major sets of reactions

• Light-harvesting reactions
• Converts light into chemical energy
• Carbon fixation reactions
• Uses chemical energy to convert CO2 into sugars

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Rubisco can gain or lose carbon

• Carboxylase
• Reacts with CO2 to produce sugars
• Leads to carbon gain
• Oxygenase
• Reacts with oxygenase to convert sugars to CO2
• Respires 20-40 of fixed carbon
• Photo-protection mechanism

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3 photosynthetic pathways

• C3 photosynthesis
• 2 other pathways (see textbook)

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Basic principle of environmental control

• Equalize physical and biochemical limitations of
  photosynthesis
• Physical limitation diffusion of CO2 to leaf
• Boundary layer
• Stomatal opening
• Biochemical limitation carboxylation rate
• Light limitation
• Enzyme limitation

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CO2 response curve of photosynthesis
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Light response curve of photosynthesis
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Light use efficiency

• Efficiency of using light to fix carbon
• Same thing as light response curve
• Nearly constant in C3 plants at low light (about
  6)
• i.e., linear portion of light response curve is
  same in all plants
• Known as quantum yield of photosynthesis

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Causes of light variation in ecosystems
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Mechanisms of adjusting to variation in light

• Light response curve (almost instantaneous)
• Acclimation (physiological adjustment)
• Sun leaves (produce new leaves)
• More cell layers
• Higher photosynthetic capacity
• Shade leaves
• More light-harvesting pigments
• Adaptation (genetic changes)
• Mechanisms same as for acclimation

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Mechanisms of adjusting to variation in light

• Other neat tricks
• Maximize leaf area
• More leaves
• Thin leaves (shade) or cylindrical leaves (sun)
• Leaf angle
• Leaf movements
• Efficient use of sun flecks

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Multiple species increase range of light levels
over which light use efficiency remains constant
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Leaf area determines light environment

• Light declines exponentially within canopy

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Species that are adapted to growing at reduced
light intensities, which are referred to as shade
tolerant species (e.g. sugar maple, hemlock,
beech), generally have lower compensation points
and levels of light saturation than shade
intolerant species like the aspens and many pines.
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Shade intolerant species saturate at relatively
high levels of photon flux density, while shade
tolerant species saturate at relatively low
levels of photon flux density.
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Vegetation maintains relatively constant LUE

• Leaf level regulation
• Balance biochemical and physical limitations to
  photosynthesis
• Canopy level regulation
• Maintain highest Ps capacity at top of canopy
• Shed leaves that dont maintain positive carbon
  balance

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Some plants alter photosynthetic capacity in
response to changes in CO2
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Vegetation adjusts photosynthetic capacity and
leaf area to balance availability of soil
resources

• Adjustment of photosynthetic capacity to soil
  resources
• Adjustment of stomatal conductance
• Adjustment of leaf area
• Change in species composition

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Leaf nitrogen determines photosynthetic capacity
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Stomatal conductance adjusts to match
photosynthetic capacity (or vice versa)
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Leaf longevity is a major factor determining
photosynthetic capacity Inevitable tradeoff
between photosynthesis and leaf
longevity Long-lived leaves contain lots of
non-photosynthetic compounds Herbivore
protection Desiccation resistant
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Suite of traits that influence carbon gain

• Growth rate
• Depends on availability of soil resources
• Leaf longevity
• Leaf nitrogen concentration
• Photosynthetic capacity
• Inevitable tradeoff between leaf longevity and
  photosynthesis

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Fast-growing plants have high photosynthetic rates
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SLA is a good predictor of photosynthetic capacity
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Water limitation

• Short-term response reduce stomatal conductance
• Long-term response reduce leaf area
• Maintains high LUE

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Leaf Water Potential and Photosynthesis
Rates of photosynthesis can be decreased as
moisture stress causes stomata to close. Thus,
drought is another environmental constraint on
the trees carbon balance.
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The summer of 2003 in Europe was one of the
driest, hottest years on record. Studies based
on micrometeorological techniques showed that GPP
was substantially reduced across Europe during
this period (Ciais et al., Nature, 2005).
GPP Black line no drought Green line
drought Grey area summertime
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Water use efficiency

• Highest in dry environments
• Tradeoff between efficiency and capacity
• Mechanism longer path length for CO2 than for
  water
• Change in stomatal conductance has larger effect
  on water loss than on CO2 gain
• Higher photosynthetic capacity than expected for
  stomatal conductance

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Temperature adaptation of photosynthesis Increase
d photosyn. capacity in cold environ. High ET in
warm wet environments Small leaf size in warm
dry environments
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Response to pollutants

• Damages photosynthetic machinery
• Reduces photosynthetic capacity
• Plants reduce stomatal conductance

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Canopy controls over GPP

• Most leaf-level controls still function in entire
  canopies
• Leaves at top of canopy carry out most
  photosynthesis
• Receive most light
• Youngest, most N-rich leaves

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Canopy effects on environment reinforce
maximization of carbon gain at the top of the
canopy High light Ambient CO2 (declines within
canopy) High boundary layer conductance (wind)
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Canopy processes increase range of light
intensities over which LUE is constant
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Carbon gain estimated from satellites
NDVI is useful because it correlates with the
amount of light energy absorbed by photosynthetic
tissues.
The so-called greening index, but it doesnt
involve green at all.
(NIR-VIS)
NDVI
(NIRVIS)
NDVI Normalized difference vegetation index NIR
Near-infrared radiation VIS Visible radiation
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Relationship of NDVI to ecosystem carbon
gain Data collected from a wide range of
ecosystems Since the relation bet. measured and
satellite estimates is decent, this means
satellites can provide accurate estimates of GPP.
(Measured light absorbed)
(Satellite estimate)
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NASAs Moderate-resolution Imaging
Spectroradiometer (MODIS) allows scientists to
gauge our planet's metabolism on an almost daily
basis. This composite image over the continental
United States, acquired during the period March
26April 10, 2000, shows regions where plants
were more or less productivei.e., where they
inhaled carbon dioxide and then used the carbon
from photosynthesis to build new plant
structures.
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An example of using satellite data to estimate
GPP at a large spatial scale (Bunn Goetz, Earth
Interactions, 2006)
This figure shows the spatial distribution of
significant trends in gross photosynthesis (GPP)
between May August of 1982 2003. Areas with
more photosynthesis are shown in red and yellow,
and browning areas are shown in shades of blue.
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Most (70) of the worlds ecosystems have open
canopies
Why does this matter? Because GPP correlates
highly with LAI. Note the tropical forest
closed canopies, and very productive. GPP is
determined primarily by LAI and the length of the
growing season.
This estimate is based on satellite data.
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Main points about photosynthesis

• Balance biochemical and physical limitations
• Match photosynthetic potential to soil resources
• Adjust leaf area to maintain constant LUE

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Major controls over GPP

• Quantity of leaf area
• May be reduced by herbivores and pathogens
• Length of photosynthetic season
• Photosynthetic rate of individual leaves
• Photosynthetic capacity
• Environmental stress that alters stomatal
  conductance

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Carbon and Energy Flows are Equivalent