Comparing CLM and CLM-ED as a basis for representing carbon cycling dynamics in a Central Amazonian forest

Friday, 19 December 2014
Ryan G Knox1, Jennifer A. Holm1, Charles D Koven1, William J Riley1, Jeffrey Q Chambers1,2, Rosie Fisher3, Stefan Muszala3 and Niro Higuchi4, (1)Lawrence Berkeley National Laboratory, Berkeley, CA, United States, (2)University of California Berkeley, Berkeley, CA, United States, (3)National Center for Atmospheric Research, Boulder, CO, United States, (4)Instituto Nacional de Pesquisas da Amazônia, Departamento de Silvicultura Tropical, Manejo Florestal, Manaus AM, Brazil
Old-growth tropical forests are responsible for a potentially large portion of the terrestrial carbon sink, although the underlying control mechanisms of that sink, has large uncertainties. The quantification of the tropical forest carbon sink is a grand challenge of measurement scale. Therefore, there is a strong emphasis on incorporating improved vegetation structure and compositional representativeness in land-surface modeling. Vegetation demography, plant competition, mechanistic mortality, disturbance cycling, and plant functional traits strongly control carbon dynamics and energy budgets of the Earth’s surface. Size and age structured scaling processes have not been represented in the widely used Community Land Model (CLM) until the recent inclusion of the Ecosystem Demography (ED) model into CLM 4.5, i.e., CLM-ED. The goal of this study was to compare how CLM-ED captured tropical carbon cycling dynamics compared to CLM and 16 years of field measurements from a central Amazonian forest. We evaluated critical carbon flux processes (Mg C ha-1 yr-1) such as net ecosystem exchange (NEE), net primary production (NPP), and autotrophic respiration (AR), and additional representations of growth and maintenance in CLM and CLM-ED. For a central Amazonian forest CLM estimated GPP from 2000-2012, with transient CO2 concentrations, to be 31.5 Mg C ha-1 yr-1 and was similar to field measurements, while initial evaluation of the newly developed CLM-ED estimated GPP to be substantially higher. The introduction of the size and age structure of ED to the CLM framework enables a finer granularity of state information in the canopy and new ways to represent canopy physics. Therefore alternative physics processes were compared, including those that are highly resolved at the cohort scale (i.e. plant groups) to those at the highly parameterized community scale (i.e. the plant canopy as a whole). CLM serves as the land-model component for nearly 40% of the Earth System Models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5). The incorporation of vegetation size and age structure into CLM-ED aims to improve the representation of ecosystem processes that govern structure, flux of carbon, and potential biomass accumulation.