Linking CO2 exchange with water/energy fluxes and remote sensing as a tool for partitioning and scaling-up flux tower measurements

Adequate gap filling of the flux tower CO2, water, and energy exchange measurements in terrestrial ecosystems, partitioning of the CO2 fluxes (FCO2) into photosynthesis and respiration components, evaluation of the water-use efficiency characteristics, and up-scaling of tower observations to larger spatial areas is often significantly complicated by the gaps in tower measurements of latent heat (LE)/evapotranspiration (Et). While the overall percentage of gaps in LE records is comparable with gaps in CO2 fluxes, the number of days when they are observed is often significantly higher, making it difficult to evaluate evapotranspiration and water-use efficiency at the daily and higher time scales. In addition, defensible algorithms of partitioning FCO2 into photosynthesis (Pg) and ecosystem respiration (Re) components require explicit accounting of the effects of water stress on ecosystem CO2 exchange to avoid overestimation of the Re (and consequently, the Pg). The most promising of currently available approaches for scaling up local flux tower measurements to larger geographical areas include relating them to remotely sensed (e.g., spectral vegetation indices) and on-site (soil data, relief, C3/C4 composition, etc.) characteristics (Zhang et al. 2011). Remotely sensed data, such as the normalized difference vegetation index (NDVI) are usually available as 8-day or 7-day composites, requiring calculation of the tower-based characteristics at the corresponding time step. We describe a method for post-processing the standard outputs of flux-tower measurements (e.g., represented by the 30-min Level 2 Ameriflux format) to calculate the daily and weekly aggregated values of gross photosynthesis, ecosystem respiration, net ecosystem production and other characteristics, which combines: (1) gap-filling of the latent heat measurements at the 30-min scale using the modified Penman-Monteith equation (Jarvis and McNaughton 1986) which allows daily evapotranspiration (Et) estimates; (2) incorporation of the vapor pressure deficit (VPD) control into the algorithm of partitioning diurnal FCO2 into Pg and Re components allowing physiologically-based gap-filling of diurnal CO2 fluxes and quantification of the daily totals of photosynthesis and respiration (Gilmanov et al. 2013); (3) evaluation of the stand level water-use efficiency coefficients (such as Pg/Et ratio, g CO2/kg H2O) in the seasonal and annual time scales, and (4) identification of the relationships of the weekly aggregated Pg, Re, Et, etc. characteristics to remotely sensed data (e.g., 250 m NDVI composites based on 8-day MODIS or 7-day eMODIS data, where available), opening the opportunities for up-scaling of the tower-site measurements to larger spatial scales. Application of the method is illustrated with 30-min data sets from a number of grassland, shrubland, and cropland eddy covariance tower sites in North America, Europe, and Asia.

References

Gilmanov, T.G., Wylie, B.K., Tieszen, L.L., Meyers, T.P., Baron, V.S., Bernacchi, C.J., Billesbach, D.P., Burba, G.G., Fischer, M.L., Glenn, A.J., Hanan, N.P., Hatfield, J.L., Heuer, M.W., Hollinger, S.E., Howard, D.M., Matamala, R., Prueger, J.H., Tenuta, M., Young, D.G., 2013. CO2 uptake and ecophysiological parameters of the grain crops of midcontinent North America: Estimates from flux tower measurements. Agric. Ecosyst. Environment 164, 162-175.

Jarvis, P.G., McNaughton, K.G., 1986. Stomatal control of transpiration: Scaling up from leaf to region. Adv. Ecol. Res. 15, 1-49.

Zhang, L., Wylie, B.K., Ji, L., Gilmanov, T.G., Tieszen, L.L., Howard, D.M., 2011. Upscaling carbon fluxes over the Great Plains grasslands: Sinks and sources. J. Geophys. Res. 116, doi:10.1029/2010JG001504.