Biogeosciences Discuss., 10, 3039-3077, 2013
© Author(s) 2013. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG.
Evaluating the agreement between measurements and models of net ecosystem exchange at different times and time scales using wavelet coherence: an example using data from the North American Carbon Program Site-Level Interim Synthesis
P. C. Stoy1, M. Dietze2, A. D. Richardson3, R. Vargas4, A. G. Barr5, R. S. Anderson6, M. A. Arain7, I. T. Baker8, T. A. Black9, J. M. Chen10, R. B. Cook11, C. M. Gough12, R. F. Grant13, D. Y. Hollinger14, R. C. Izaurralde15, C. J. Kucharik16, P. Lafleur17, B. E. Law18, S. Liu19, E. Lokupitiya20, Y. Luo21, J. W. Munger22, C. Peng23, B. Poulter24, D. T. Price25, D. M. Ricciuto11, W. J. Riley26, A. K. Sahoo27, K. Schaefer28, C. R. Schwalm29, H. Tian30, H. Verbeeck31, and E. Weng32
1Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, MT 59717, USA
2Department of Earth and Environment, Boston University, Boston, MA 02215, USA
3Department of Organismic & Evolutionary Biology, Harvard University, Cambridge, MA, USA
4Department of Plant and Soil Sciences, Delaware Environmental Institute, University of Delaware, Newark, DE 19717, USA
5Climate Research Division, Atmospheric Science and Technology Directorate, Saskatoon, SK S7N 3H5, Canada
6Numerical Terradynamic Simulation Group, University of Montana, Missoula, MT 59812, USA
7School of Geography and Earth Sciences and McMaster Centre for Climate Change, McMaster University, Hamilton, ON L8S 4K1, Canada
8Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523, USA
9Faculty of Land and Food Systems, University of British Columbia, Vancouver, BC V6T 1Z4, Canada
10Department of Geography and Program in Planning, University of Toronto, Toronto, ON M5S 3G3, Canada
11Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
12Department of Biology, Virginia Commonwealth University, Richmond, VA 23284, USA
13Department of Renewable Resources, University of Alberta, Edmonton, AB T6G 2E3, Canada
14Northern Research Station, USDA Forest Service, Durham, NH 03824, USA
15Pacific Northwest National Laboratory and University of Maryland, College Park, MD 20740, USA
16Department of Agronomy & Nelson Institute Center for Sustainability and the Global Environment, University of Wisconsin – Madison, Madison, WI 53706, USA
17Department of Geography, Trent University, Peterborough, ON K9J 7B8, Canada
18Department of Forest Ecosystems and Society, Oregon State University, Corvallis, OR 97331, USA
19US Geological Survey (USGS) Earth Resources Observation and Science (EROS) Center, Sioux Falls, SD 57198, USA
20Department of Zoology, POB 1490, University of Colombo, Colombo 03, Sri Lanka
21Department of Botany and Microbiology, University of Oklahoma, Norman, OK 73019, USA
22School of Engineering and Applied Sciences and Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA
23Department of Biology Sciences, University of Quebec at Montreal, Montreal, QC H3C 3P8, Canada
24Laboratoire des Sciences du Climat et de l'Environnement, Gif-sur-Yvette, France
25Northern Forestry Centre, Canadian Forest Service, Edmonton, AB T6H 3S5, Canada
26Climate and Carbon Sciences, Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
27Department of Civil and Environmental Engineering, Princeton University, Princeton NJ 08544, USA
28National Snow and Ice Data Center (NSIDC), University of Colorado, Boulder, CO 80309, USA
29School of Earth Science and Environmental Sustainability, Northern Arizona University, Flagstaff, Arizona 86001, USA
30School of Forestry and Wildlife Sciences, Auburn University, Auburn, AL 36849, USA
31Laboratory of Plant Ecology, Ghent University, 9000 Ghent, Belgium
32Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544, USA

Abstract. Earth system processes exhibit complex patterns across time, as do the models that seek to replicate these processes. Model output may or may not be significantly related to observations at different times and on different frequencies. Conventional model diagnostics provide an aggregate view of model-data agreement, but usually do not identify the time and frequency patterns of model misfit, leaving unclear the steps required to improve model response to environmental drivers that vary on characteristic frequencies. Wavelet coherence can quantify the times and frequencies at which models and measurements are significantly different. We applied wavelet coherence to interpret the predictions of twenty ecosystem models from the North American Carbon Program (NACP) Site-Level Interim Synthesis when confronted with eddy covariance-measured net ecosystem exchange (NEE) from ten ecosystems with multiple years of available data. Models were grouped into classes with similar approaches for incorporating phenology, the calculation of NEE, and the inclusion of foliar nitrogen (N). Models with prescribed, rather than prognostic, phenology often fit NEE observations better on annual to interannual time scales in grassland, wetland and agricultural ecosystems. Models that calculate NEE as net primary productivity (NPP) minus heterotrophic respiration (HR) rather than gross ecosystem productivity (GPP) minus ecosystem respiration (ER) fit better on annual time scales in grassland and wetland ecosystems, but models that calculate NEE as GPP – ER were superior on monthly to seasonal time scales in two coniferous forests. Models that incorporated foliar nitrogen (N) data were successful at capturing NEE variability on interannual (multiple year) time scales at Howland Forest, Maine. Combined with previous findings, our results suggest that the mechanisms driving daily and annual NEE variability tend to be correctly simulated, but the magnitude of these fluxes is often erroneous, suggesting that model parameterization must be improved. Few NACP models correctly predicted fluxes on seasonal and interannual time scales where spectral energy in NEE observations tends to be low, but where phenological events, multi-year oscillations in climatological drivers, and ecosystem succession are known to be important for determining ecosystem function. Mechanistic improvements to models must be made to replicate observed NEE variability on seasonal and interannual time scales.

Citation: Stoy, P. C., Dietze, M., Richardson, A. D., Vargas, R., Barr, A. G., Anderson, R. S., Arain, M. A., Baker, I. T., Black, T. A., Chen, J. M., Cook, R. B., Gough, C. M., Grant, R. F., Hollinger, D. Y., Izaurralde, R. C., Kucharik, C. J., Lafleur, P., Law, B. E., Liu, S., Lokupitiya, E., Luo, Y., Munger, J. W., Peng, C., Poulter, B., Price, D. T., Ricciuto, D. M., Riley, W. J., Sahoo, A. K., Schaefer, K., Schwalm, C. R., Tian, H., Verbeeck, H., and Weng, E.: Evaluating the agreement between measurements and models of net ecosystem exchange at different times and time scales using wavelet coherence: an example using data from the North American Carbon Program Site-Level Interim Synthesis, Biogeosciences Discuss., 10, 3039-3077, doi:10.5194/bgd-10-3039-2013, 2013.
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