Simulated soil CO2 efflux and net ecosystem exchange in a 70-year-old belgian Scots pine stand using the process model SECRETS

Abstract

Within the framework of the EU ECOCRAFT (European collaboration on CO2 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 Northem Belgium (51°31′ N, 4°22' 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), land 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 1965 and 1 888 g C m-2, respectively. Soil respiration was 25% (495 g C m-2 a-1) and 27%(505 g Cm-2 a -1)of the GPP in 1997 and 1998, respectively, in this slow growing Scots pine stand. Heterotrophic respiration (RH) accounted for, roughly, 32% of the total soil C efflux for both years. Simulated daily fluxes for net ecosystem exchange (NEE) suggested C uptake throughout most, but not all, of the spring and summer, but net release during mid-autumn to early winter periods for both years. Our base estimates of NEE ranged from 385 g C m2 a-1 in 1997 to 310 g C m-2 a -1 in 1998. However, the uncertainty in NEE varied from 167 to 509 g C m-2 a-1 and 138 to 392 g C m-2 a-1 in 1997 and 1998, respectively. Thus, this stand may be accumulating C at a rate of 138 to 509 g C m-2a -1 depending on the assumed stand and site characteristics, tree physiology, and local variation in weather.