This is a protocol to evaluate gross primary productivity (GPP) of a forest stand based on the measurements of tree’s sap flow (SF), ¹³C derived water use efficiency (WUE), and meteorological (met) data. GPP was calculated from WUE and stomatal conductance (g_s), the later obtained from SF up-scaled from sampled trees to stand level on a daily time-scale and met data. WUE is obtained from ¹³C measurements in dated tree-ring wood and/or foliage samples. This protocol is based on the recently published study of Klein et al., 2016.