Chapter 6 Terrestrial Production Processes

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Chapter 6Terrestrial Production Processes

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

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Review

• What controls photosynthesis at the canopy scale?
• What controls water and energy exchange by an
  ecosystem?
• How do ecosystems influence runoff from
  watersheds?
• What is NPP, and what controls differences in NPP
  among ecosystems?

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Major controls over GPP

• Quantity of leaf area
• Length of photosynthetic season
• Photosynthetic rate of individual leaves
• Photosynthetic capacity
• Environmental stress that alters stomatal
  conductance

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Atmospheric evidence of large carbon exchanges
by the biosphere
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Overview of ecosystem carbon cycle Plants play
major role C inputs to ecosystem (GPP) C
transfers to soil (litterfall) C losses to
atmosphere (respiration)
Net primary production
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Net primary production (NPP)

• NPP GPP - Respiration

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What are the components of respiration?
R R R R
plant growth maint
ion

• Respiration provides energy for essential plant
  processes
• Plants photosynthesize in order to fuel
  respiration
• At the cellular level, the same biochemical
  processes operate for each of the respiration
  components
• ATP NAD(P)H are produced from the breakdown of
  glucose, and provide energy to run cellular
  metabolism and synthesize new biomass

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Growth respiration

• Growth respiration C respired to build new
  biomass
• C is incorporated into the biomass
• C oxidized to CO2 to produce ATP to drive the
  biosynthetic reactions
• Amount of C needed for ATP can be calculated
  because the biosynthetic pathways are known

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Carbon costs of a leaf
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Growth Respiration

• C costs vary 3-fold for major classes of
  compounds (e.g. cellulose, proteins, nucleic
  acids, lipids, etc.)
• Most plant tissues have both expensive and
  cheap compounds, so overall cost of different
  tissues is not very different
• On average, growth respiration is 25 of the C
  incorporated into new tissues

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Carbon cost of growth is similar among species
and plant parts
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Respiratory cost of maintaining ion gradients
Probably correlates with NPP
R R R R
plant growth maint
ion
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What is maintenance respiration?
R R R R
plant growth maint
ion

• Respiration associated with repair
• Proteins
• Membranes
• Other stuff

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What controls maintenance respiration?

• Plant chemistry
• especially protein content
• Environment
• especially temperature and drought

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How variable is the cost of maintenance
respiration?

• We dont know BUT
• Respiration seems to be a constant fraction of
  GPP
• when ecosystems are compared

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NPP is about half of GPP
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How variable is the partitioning to plant
respiration?

• Seasonally?
• Almost certainly
• Higher when temperatures are warm
• Continues even at times of no growth
• Interannually?
• Probably
• Among ecosystems?
• No strong evidence for ecosystem differences

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What is NPP?
Components of NPP of NPP New plant
biomass 40-70 Leaves and reproductive parts
(fine litterfall) 10-30 Apical stem
growth 0-10 Secondary stem growth 0-30 New
roots 30-40 Root secretions 20-40 Root
exudates 10-30 Root transfers to
mycorrhizae 10-30 Losses to herbivores,
mortality, and fire 1-40 Volatile
emissions 0-5
What do we usually measure?? Litterfall Sometime
s stem growth
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What do we really care about?

• Biomass increment and carbon storage
• Energy available to other trophic levels
• Energy transfer to mycorrhizae (maybe)
• Volatile emissions (maybe not)
• Root exudates (probably not)

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NPP Who is in charge?Photosynthesis, NPP, or
respiration?

• NPP GPP - Respiration
• Control probably depends on time scale

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Physiological controls over NPP

• Driven by plant demand for C
• Resources available for growth (sink strength)
• Governed largely by soil resources
• Climate influences NPP by determining soil
  resources
• Growth demands for C determine photosynthetic
  rate and respiration rate

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Climate influences NPP in complex ways Direct
effects on growth short-term temporal
variation Effects on species composition
spatial variation (which determines growth
potential) (takes time to adjust to climate)
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Plants allocate most growth to tissues that
maximize capture of limiting resources

• Allocate to roots when dry nor nutrient poor
• Allocate to leaves when light is limiting
• Constantly adjust allocation
• Prevents overwhelming limitation by any one
  resource
• Tends to make plants limited by multiple resources

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Why is NPP often limited by multiple resources?

• Adjustment of allocation to prevent overwhelming
  limitation by one resource
• Environment changes seasonally and from year to
  year
• Plants can increase supply of limiting resources
• Different resources limit different species

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Storage buffers plants from variation in resource
supply

• Plants acquire most resources when they are
  abundant
• Plants use resources when needed for growth
• Plants follow economic rules similar to those
  of business firms

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Biomass increment depends on tissue gain and
tissue loss

• Why do plants lose tissues they work so hard to
  produce
• (i.e., why do I have to rake my yard?)

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Advantages of tissue loss

• Re-allocate resources to explore new territory
• Shed tissues when cost exceeds gain
• Shed parasites, herbivores, and pathogens
• Sometimes cant help it
• Windstorms, fires, etc.

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Turnover is faster in high-resource environments
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Global patterns of NPP vary with
climate Increases with ppt (up to max at 2
m/yr) Increases exponentially with
temperature High variance due to variation in
soils, etc.
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Ecosystems exhibit predictable relationship with
climate
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Vegetation composition determines growth potential
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Land dominates the global carbon cycle over
short time scales

• Oceans account for 70 of earths surface
• gt99 of biomass is on land
• 60 of NPP occurs on land

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Biomass is greatest in tropical and temperate
forests
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Half of global biomass and a third of global NPP
is in tropical forests
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NPP varies 14-fold among biomes
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NPP per unit leaf area and time is almost
constant across biomes
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Disturbance and succession are major causes of
variation in NPP within a biome
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What is Net Ecosystem Carbon Balance (NECB)?

• Rate of carbon accumulation in an ecosystem
• Balance between carbon inputs and outputs
• Small difference between two large fluxes

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Net Ecosystem Carbon Balance (NECB)

• Rate of carbon accumulation in an ecosystem

NEP GPP - (R R F
F )
plant heterotr disturb leach
NEP GPP - (R F F
)
ecosyst disturb leach
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Net Ecosystem Production (NEP)

• Difference between GPP and Respiration

NEP GPP - (R R )
plant heterotr
NEP GPP - R
ecosyst
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NEP is the balance between two large fluxes GPP
and ecosystem respiration
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Net ecosystem exchange

• Net carbon uptake (or loss) by ecosystem
• NEE GPP - Ecosystem respiration

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Land-Atmosphere exchange

• Climate feedbacks

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Biome differences in NEE reflect large net carbon
loss by respiration at high latitudes
Valentini
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Why is NEE positive in most ecosystems?

• Maybe all ecosystems accumulate C between
  disturbances
• Maybe bias in site selection
• Researchers prefer productive sites?
• Maybe carbon loss by leaching is significant
• Maybe terrestrial biosphere is gaining carbon
• due to elevated CO2 and N deposition

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Regional variation in NECB

• Represents net carbon storage in ecosystem
• Negative when disturbance frequent
• Positive during recovery from disturbance
• Often very small relative to inputs and outputs

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Atmospheric evidence of large carbon exchanges
by the biosphere
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