Secrets: Simulated carbon fluxes from a mixed coniferous/deciduous Belgian forest

A new carbon/water balance process model, known as SECRETS, was used to simulate carbon fluxes for a mixed coniferous/deciduous forest in northern Belgium (51 degrees 18'N, 4 degrees 31'E). This model simulates carbon (C) and water fluxes from individual or multiple forest patches, sequentially (area weighted), that may vary in structure and species composition. In this contribution, simulations of C fluxes for 1997 are presented for a hypothetical forest comprised of three patches: a Scots pine (Pinus sylvestris L.) stand without understory (assumed 50% of the forest), a Molinia caerulea L. grassland (25% of the forest), and a Scots pine stand with a Molinia understory (also 25% of the forest ecosystem). Simulated gross primary productivity (GPP) and net canopy assimilation (NETp) for this forest ecosystem were 19.7 and 5.9 Mg C ha(-1) a(-1), respectively. Net canopy assimilation is defined here as GPP minus maintenance respiration and minus fine root exudates. Autotrophic respiration was nearly 70% of GPP. Simulated total ecosystem respiration was 13.2 Mg C ha(-1) a(-1) and, therefore, net ecosystem exchange (NEE) for 1997 for base simulations was close to 3.0 Mg C ha(-1) a(-1). However, the interval of uncertainty (IOU) in NEE - based on analyses of the random error in canopy architecture, leaf area index, and/or basic gas exchange characteristics - ranged from 2.2 to 3.9 Mg C ha(-1) a(-1). As such, it was concluded from the model simulations that this three-patches forest ecosystem was a slight sink for carbon during the 1997 simulation year. Eddy flux measurements made during the same period above the entire forest, comprised of many different forest patches, confirmed these simulated values.