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 USA Ankur R Desai, Benjamin N Sulman University of Wisconsin-Madison D. Scott Mackay State 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 [CO 2 ]

Climate Drivers of Carbon Flux Temperature-> Phenology Precipitation-> Drought Radiation -> Light Quality [CO 2 ] -> Acclimation Interannual ^

Interannual NEE at ChEAS

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 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

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 HOURLY NOLEAF IAV NOLEAF HOURLY LEAF IAV LEAF

Interannual HOURLY NOLEAF IAV NOLEAF HOURLY LEAF IAV LEAF

NEE-Leaf

GPP ER Leaf

Precipitation and Water Table Sulman et al (in prep) Biogeosciences - see Ameriflux poster

Shrub Wetland Flux Response

Three Wetlands

Regional NEE See NACP poster in Feb.

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