International audience ; Within the framework of the EU ECOCRAFT (European collaboration on CO$_2$ responses applied to forests and trees), we developed a stand scale process model to simulate short-term carbon (C) and water fluxes from a mixed coniferous/deciduous forest in Northern Belgium (51o31$'$ N, 4o22$'$ E). The model, termed SECRETS, is a sequential, multi-species and multiple layer simulator that uses process modules adapted from several sources. Namely, we adapted BIOMASS (maintenance respiration and water balance), and coded the sun/shade model (photosynthesis; modified for forest species), and the GRASSLAND DYNAMICS (soil carbon and nitrogen) models. In this contribution we simulate carbon fluxes for a 70-year-old Scots pine (Pinus sylvestris L.) stand and we introduce an approach to characterize uncertainty in the model outputs. Simulated, annual gross primary productivity (GPP) for 1997 and 1998 was 1 965 and 1 888 g C m$^