Comparing eddy covariance estimates of gross primary production to estimates from stem sap flux and phloem d13C across sites.

Gross primary production (GPP) describes ecosystem-scale canopy photosynthesis and provides the foundation of the ecosystem carbon budget. It is often derived from eddy covariance data based on models of the component processes. At several sites in Sweden and the Czech Republic, we have quantitatively tested these GPP estimates against independent empirical data based on stem-scale measurements of xylem water flux and intrinsic water-use efficiency (iWUE), where iWUE is estimated from the stable isotope composition of phloem contents. With one exception, these comparisons have agreed well in the middle of the growing season. On the other hand, at several sites, the methods showed distinct discrepancies either at the beginning or the end of the growing season. We discuss possible causes of these seasonal discrepancies ,including the decoupling of phloem contents from gas-exchange, the scaling of sap flux, mesophyll conductance, decoupling of air masses above and below the canopy, and the inference of GPP from eddy covariance data. Quantitative tests of these methods against independent data will be critical as our need to quantify carbon sources and sinks continues to grow.