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

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

Predicting NEE (Ricciuto et al)

Climate Drivers of Carbon Flux

•Temperature•Precipitation•Radiation

•[CO2]

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

Optimization

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HOURLY

IAV

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

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)

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1997 1998 1999 2000 2001 2002 2003 2004

Year

gC m-2 yr-1

Flux towers

FIA model

ABL Budget

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