1. A challenge to the flux-tower upscaling hypothesis? A multi-tower comparison from the Chequamegon Ecosystem-Atmosphere Study K.J. Davis 1, D.R. Ricciuto 1, B.D. Cook 2, M.P. Butler 1, A.R. Desai 1, W. Wang 1, C. Yi 3, P.S. Bakwin 4, P.V. Bolstad 2, J. Martin 2, E. Carey 2, D.S. Mackay 5, B.E. Ewers 6, J. Chen 7, A. Noormets 7, F.A. Heinsch 8, A.S. Denning 9, R. Teclaw 10 1 Penn State, 2 U.Minnesota, 3 U.Colorado, 4 NOAA-CMDL, 5 U.Buffalo, 6 U.Wyoming, 7 U.Toledo, 8 U.Montana, 9 Colorado State, 10 USDA Forest Service With support from: DoE Terrestrial Carbon Processes Program, DoE National Institutes for Global Environmental Change, NOAA, NSF Division of Environmental Biology

2. Motivation What is and what governs ecosystem- atmosphere exchange of CO 2 on spatial scales of geopolitical and bioclimatological relevance?

4. Outline What is the “flux tower upscaling hypothesis?” Method: How do we test this hypothesis? Results: Flux magnitudes and flux variability. Simultaneous up-scaling and down- scaling.

5. Flux tower upscaling hypothesis Fluxes of CO 2 (NEE, R, GEP) = f (climate variables, ecosystem characteristics) Climate and ecosystem variables can be mapped, functions determined, fluxes interpolated and integrated across space. NEE = net ecosystem-atmosphere exchange, R = ecosystem respiration, GEP = gross ecosystem productivity, NEP = net ecosystem productivity NEE = R – GEP = - NEP

6. Flux tower upscaling hypothesis Flux: R, NEE, GEP Climate variables (x, y) Flux = ax + by + c, interpolate fluxes over ~ (1000 km) 2 Each point ~ (1 km) 2 Segregate further by ecosystem characteristics? Stand type (conifer, deciduous, grass, crop) Stand age (young, mature, old)

7. Flux tower upscaling hypothesis Plots at towers Within stand : biometric data, chamber fluxes Tower sites Stand : Eddy covariance flux towers representing key biomes and climate regions Continent : Map biomes and climate, model fluxes Upper Midwest, N. America

8. Flux tower upscaling hypothesis with simultaneous constraints Within stand : biometric data, chamber fluxes Stand : Eddy covariance flux towers Forest : Clusters of flux towers WLEF tower Continent : Map biomes and climate, model fluxes Region : Map ecosystem variables, model fluxes N. Wisconsin [CO 2 ] N. American [CO 2 ]

9. Chequamegon Ecosystem- Atmosphere Study (ChEAS) region

10. Testing the upscaling hypothesis Flux: R, NEE, GEP Climate variables (x, y) ChEAS

11. Testing the upscaling hypothesis: Regional clusters of flux towers Can fluxes be up-scaled from stand to forest or region? Clusters can isolate the role of ecosystem characteristics via identical climate across sites. What must be measured and mapped for flux upscaling?

12. Photo credit: UND Citation crew, COBRA WLEF tall tower (447m) CO 2 flux measurements at: 30, 122 and 396 m CO 2 mixing ratio measurements at: 11, 30, 76, 122, 244 and 396 m Forest-scale evaluation of the upscaling hypothesis: WLEF flux tower

13. ChEAS vegetation

14. ChEAS flux tower array Forest-scale flux: WLEF tower,1997-present Dominant stand types and flux towers: Northern AspenForestedConifer hardwoodwetland young old mature Willow Creek(UMBS)Lost CreekChen B 2000-present1999-present2001-present2002-present Bolstad et al, in press Cook et al, in prep Chen A 2002–present Sylvania 2002-present Desai et al, in prep Desai et al, B52D-04Chen mobile 20032002 Yi et al, 00 Berger et al, 01 Davis et al, 03 Ricciuto et al, B51 Mackay et al, 02 Mackay et al, H29 Ewers et al, 02 Ewers et al, H30

15. ChEAS upscaling test results 1.Climate alone does not explain ChEAS CO 2 fluxes. 2.The WLEF region is a source of CO 2 to the atmosphere. drying wetlands? disturbance/management?

17. ChEAS upscaling test results 1.Climate alone does not explain ChEAS CO 2 fluxes. 2.The WLEF footprint is a source of CO 2 to the atmosphere. drying wetlands? disturbance/management? 3.WLEF fluxes cannot be explained as a linear combination of Lost Creek and Willow Creek fluxes. aspen? conifers? WLEF footprint dissimilar?

18. NEE (gC m -2 ) Respiration (gC m -2 ) Photosynthesis (gC m -2 ) WLEF 1997 27991964 WLEF 1998 48986938 WLEF 1999 1001054954 WLEF 2000 741005931 WLEF 2001 1411067926 WLEF average 781021942 Willow Creek 2000 -3477621109 Willow Creek 2001 -108741849 Willow Creek 2002 -4376481085 Willow Creek average -2977171014 Lost creek 2001 1759758 Lost Creek 2002 -58631689 Lost Creek average -30695724 NEE and gross fluxes at ChEAS sites: 1997-2002

20. ChEAS upscaling test results 1.Climate alone does not explain ChEAS CO 2 fluxes. 2.The WLEF footprint is a source of CO 2 to the atmosphere. drying wetlands? disturbance/management? 3.WLEF fluxes cannot be explained as a linear combination of Lost Creek and Willow Creek fluxes. aspen? conifers? WLEF footprint dissimilar? systematic errors that differ among flux towers? 4.Soil + leaf + stem respiration is similar in aspen and northern hardwoods in the Willow Creek area. WLEF high respiration rate due to coarse woody debris?

21. Chamber respiration fluxes Table 4. Estimated annual respiration for the whole ecosystems and components, 1999-2002. All rates are reported in Mg C ha -1 yr -1. Bolstad et al, in press. Forest type and respiration (soil + leaf + stem) 1999200020012002 Northern Hardwoods 11.5511.9212.7110.89 Mature Aspen 13.5713.9614.6912.95 Intermediate Aspen 9.9310.2410.769.49

22. ChEAS upscaling test results 1.Climate alone does not explain ChEAS CO 2 fluxes. 2.The WLEF footprint is a source of CO 2 to the atmosphere. drying wetlands? disturbance/management? 3.WLEF fluxes cannot be explained as a linear combination of Lost Creek and Willow Creek fluxes. aspen? conifers? WLEF footprint dissimilar? systematic errors that differ among flux towers? 4.Soil + leaf + stem respiration is similar in aspen and northern hardwoods in the Willow Creek area. WLEF high respiration rate due to coarse woody debris? Chamber R >> W Creek R implies error? 5.Sylvania (old growth) fluxes differ from Willow Creek (mature) fluxes as expected due to stand age (similar GEP, old R > mature R). But soil respiration from chambers contradicts this result.

23. Sylvania – Willow Creek flux tower comparison GEP, old growth (red) vs. mature (blue) forest R, old growth (red) vs. mature (blue)

24. Summary Simple tower upscaling hypothesis, WLEF = a*W Creek + b*L Creek, fails. Means of reconciliation is not clear. Upscaling the magnitude of R, GEP, NEE is challenging.

25. Motivation II What is and what governs the interannual variability in ecosystem-atmosphere exchange of CO 2 on spatial scales of geopolitical and bioclimatological relevance?

26. Interannual variability in the rate of accumulation of atmospheric CO 2

27. Flux tower upscaling hypothesis II – interannual variability  flux) = flux – mean flux Climate variables (x, y)  (flux) = ax + by + c, interpolate interannual variability in fluxes over ~ (1000 km) 2 Each point ~ (1 km) 2 Ecosystem fluxes respond similarly to climate variability across a wide range of forest types and ages(?)

28. Testing the interannual variability upscaling hypothesis Flux tower clusters deployed for multiple years test the hypothesis that various forest stands respond similarly to climate variability.

29. Interannual variability upscaling results 1.ChEAS annual fluxes (R, GEP, NEE) are moderately coherent across ChEAS sites, 2000-2001. (Caterpillars, not climate?). 2.ChEAS chamber and tower R fluxes show similar variability, 2001-2002, across sites. (2001 high flux, 2002 low flux).  (WLEF) = a*  (W Creek) + b*  (L Creek)? 3. Continental scale fluxes are very coherent, spring 1998, and linked to [CO 2 ]! (Butler et al, this session) An extreme climatic event.

30. Joint constraints! Complementary methods

31. Flux tower upscaling hypothesis with simultaneous constraints Within stand : biometric data, chamber fluxes Stand : Eddy covariance flux towers Forest : Clusters of flux towers WLEF tower Continent : Map biomes and climate, model fluxes Region : Map ecosystem variables, model fluxes N. Wisconsin [CO 2 ] N. American [CO 2 ]

32. ChEAS regional flux experiment domain = LI-820 sampling from 75m above ground on a communications tower. = 40m Sylvania flux tower with high-quality standard gases. = 447m WLEF tower. LI-820, CMDL in situ and flask measurements.

33. Potential VTT network: Selection of new sites to be based on optimization study, Skidmore et al, and plans for a Midwest regional intensive

34. Complementary nature of inversion downscaling and flux tower upscaling Inversion downscalingFlux tower upscaling Excellent spatial Intrinsically local integration measurements. Strong constraint on Difficult to upscale flux flux magnitudemagnitudes. Variability easier. Poor temporal Excellent temporal resolution resolution Limited mechanisticStrong mechanistic understanding.understanding

35. Conclusions It is relatively difficult to upscale stand level fluxes to a region. Upscaling interannual variability may be more tractable than absolute flux magnitudes. Clustered flux towers provide upscaling methods testbeds. Flux tower up-scaling and inversion down- scaling are very complementary.