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Impacts of leaf phenology and water table on interannual variability of carbon fluxes in subboreal uplands and wetlands Implications for regional fluxes in the upper Midwest
USAAnkur R Desai, Benjamin N SulmanUniversity of Wisconsin-Madison
D. Scott MackayState University of New York-Buffalo
Ameriflux/ChEAS PIs
Ameriflux Meeting 2008
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Motivation
• Interannual variation (IAV) in carbon fluxes from land to atmosphere is significant at most flux sites
• Key to understanding how climate affects ecosystems comes from modeling IAV
• IAV (years-decade) is currently poorly modeled, while hourly, seasonal, and even successional (century) are better
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Predicting NEE (Ricciuto et al)
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Climate Drivers of Carbon Flux
•Temperature•Precipitation•Radiation
•[CO2]
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Climate Drivers of Carbon Flux
•Temperature -> Phenology
•Precipitation -> Drought•Radiation -> Light Quality
•[CO2] -> Acclimation
Interannual
^
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Interannual NEE at ChEAS
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Questions• What controls IAV of NEE in subboreal uplands?– Hypothesis: Phenology -> Growing season start, end, or length affects GPP
– Piao et al (2008) -> Autumn warming and Rh
• What controls IAV of NEE in subboreal wetlands?– Hypothesis: Phenology + Water table affects ER– Ise et al (2008) -> Decomposition and moisture
• What controls IAV of regional NEE in subboreal North America?
• Can a very simple model be constructed to explain IAV?– Can we make do a parameter opimization more attuned to IAV?
– Hypothesis: MCMC overfits to hourly data
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Optimization
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HOURLY
IAV
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Phenology
• Five sites with 5-8 years of data– 1 regional (LEF), 1 wetland (LCR), 3 uplands (SYL, UMB, WCR)
– Assimilate 1st 4 years of data
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Phenology Model• Twice daily model, annually resetting pools• Driven by PAR, Air and Soil T, VPD• LUE based GPP model f(PAR,T,VPD)• Three respiration pools f(Air T, Soil T, GPP)
• Model 1. NOLEAF– Constant leaf on and leaf off days
• Model 2. LEAF (Phenology)– Sigmoidal Threshold GDD (base 10) function for leaf on
– Sigmoidal Threshold Daily Mean Soil Temp function for leaf off
• 17 parameters, 3 are fixed– Output: NEE, ER, GPP, LAI
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Hourly
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HOURLY NOLEAF
IAV NOLEAF
HOURLY LEAF
IAV LEAF
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Interannual
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HOURLY NOLEAF
IAV NOLEAF
HOURLY LEAF
IAV LEAF
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NEE-Leaf
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GPP ER Leaf
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Precipitation and Water Table
• Sulman et al (in prep) Biogeosciences - see Ameriflux poster
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Shrub Wetland Flux Response
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Three Wetlands
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Three Wetlands
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Regional NEE
• See NACP poster in Feb.Annual flux (NEE)
-250
-200
-150
-100
-50
0
1997 1998 1999 2000 2001 2002 2003 2004
Year
gC m-2 yr-1
Flux towers
FIA model
ABL Budget
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Conclusions• Autumn soil temperature appears to be a major control on interannual variability in subboreal upper Midwest USA flux tower site annual NEE– Due mainly to effect of growing season length for GPP
– Only detectable using a modified MCMC cost function that penalizes for poor fit to cumulative annual NEE
• Additionally, growing season average water table strongly affects ER in wetlands– GPP effect in both uplands and wetlands -> related to precipitation deficits?
• Regional NEE is messy
• Thanks: DOE NICCR, DOE TCP, NASA CC, NOAA CPO, USDA/USFS NRS, NSF, UW Foundation, ChEAS