Controls of carbon budgets in terrestrial ecosystems

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• Controls of carbon budgets in terrestrial
  ecosystems
• Does carbon storage in terrestrial ecosystems
  really depend on temperature?
• What factors do we need to consider when we
  assess vegetationatmosphere coupling in climate
  change scenarios?
• How well do coupled climatevegetation models
  simulate the response of forest CO2 fluxes and
  carbon sequestration to variations in
  temperature?
• What controls the long-term storage of organic
  carbon in boreal soils ? current content
  200400 ppm of CO2 all soils, gt 700 ppm
• Steven C. Wofsy, Harvard University
• Presented at ICDC7 Boulder, CO, 29 Sep 2005

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• Land carbon storage depends on the balance
  between the input of carbon as Net Primary
  Productivity (NPP), and the loss of carbon as
  heterotrophic (soil) respiration (Rh).
• NPP modeled as dependent on light, nutrients,
  precipitation
• Rh modeled as a function of T. Is this right?

Daily Mean Ecosystem Respiration (mmole/m2/s) vs
T (C) Photosynthesis vs sunlight
Harvard Forest
Exp fit
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Modeling long-term Net Ecosystem Carbon
Production (NEP)(1st-order)
tree
solar
Atmosphere
CO2, H2O
Leaves (P, R6)
H2O, nutrients
Org C
CO2
Boles (R5)
Org C
Roots (R4)
Soils R1 R2 R3
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Do climateecosystem models capture the
temperature dependence of carbon storage? Or the
long-term trends in C storage? Example 1 a
typical mid-latitude forest (Harvard Forest,
Central New England agricultural use 1750-1850).
60-80 year old mixed deciduous forest, with CO2
fluxes to/from the atmosphere measured every half
hour, for 15 years (1991-2005) (S. Wofsy, J. W.
Munger, B. Daube, M. Goulden, C. Barford, S.
Urbanski, many others), plus a soil warming
manipulation (J. Melillo, K. Nadelhofer, P.
Studler).
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Errors in both seasons are due to T driving
respiration
Model (IBIS) vs. Observed C balance at a
transition deciduous forest.
emission
uptake
phenology
R vs T fundamental to climate changethe mean
is right, but the model is never right in any
month and the CO2 FluxT feedback is wrong!
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Modeled and observed respiration at Harvard Forest
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Overall, warming treatment did not significantly
change soil respiration either with or without
clipping. (Luo et al., 2001)
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Annual change () in soil respiration due to 5o
soil warming over a thirteen-year period at
Harvard Forest Melillo, Nadelhofer, Studler, et
al. Marine Biological Laboratory, Woods Hole,
MA.
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Does this extra N increase enhance tree growth?
Annual change () in N mineralization due to soil
warming over a thirteen-year period at Harvard
Forest Melillo, Nadelhofer, Studler, et al.
Marine Biological Laboratory, Woods Hole, MA.
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• Accel. G, R
• Accel. NEE
• Higher LUE

0
NEE
Harvard Forest
AmeriFlux Data 1991--2004
-1
-2
NEE (tonC/ha/yr)
-3
AGWB
incr. rate net uptake
-4
1992
1994
1996
1998
2000
2002
2004
Long-term changes at Harvard Forest J. W. Munger,
S. Urbanski, S. C. Wofsy et al.
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Example 2 Climate and C at a Boreal Forest
(NOBS) flux site, Thompson, MB
PEAT
45 cover
Jan
Snow cover Temperature (rapid warming)
1970
2000
1900
2000
Jul
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Uptake emission
Temperatures warm up, ecosystem switches
source?sink
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NEE C balance at a bog JunSep
Temper1ature, C
R (gC m-2 day-1)
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Uptake emission
Water balance (temperature) explain the
transition source?sink
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Correlation DT, D soil moisture
indexCCSM1-Carbon Control Simulation
DJF
JJA
slide courtesy Inez Fung I. Fung, S. Doney, et
al.
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high T, high ppt
Low T, low ppt
Recent climate variations in central Canada have
been colddry and warmwet
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Holdridge Life Zones potential vegetation
Mean T, Precip, and E/P control vegetation cover
warmer-drier leads to strong degradation in the
tropics.
.25
125
Data courtesy of D. Skole
1.0
P
1.5
E/P
1500
T
6
8.0
8000
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Holdridge life zones (Holdridge 1967)
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SummaryHow well do coupled climatevegetation
models simulate the response of forest CO2 fluxes
and carbon sequestration to warming? Many models
over-estimate the sensitivity of ecosystem C
storage to T. What controls the long-term
storage of organic carbon in boreal soils ?
current content 200400 ppm of CO2 all soils,
gt 700 ppm equivalent. Hydrological balance in
concert with T dominate if changed climate has
warmer? wetter covariance, boreal lands may
actually increase C storage in a warmer world.
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What did we learn?Coupled climate vegetation
modeling must carefully and critically examine
predictions for climate change and ecosystem
response in terms of the key parameters
(regional T Precipitation Human impacts
(ignition, agriculture, air pollution) and their
covariances .Consider advancing beyond the
traditional focus on global mean T. These are
all 1st order factors.