Linking an ecosystem demography model to CLM as a basis for representing carbon cycling dynamics in tropical forests
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. Therefore, there is a strong emphasis on incorporating improved vegetation structure and compositional representativeness in land-surface modeling. Vegetation demography, plant competition among tree size classes, mechanistic mortality, and plant functional traits strongly control carbon dynamics and energy budgets of the Earth’s surface. These 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, ED2, and 16 years of field measurements from a central Amazonian forest, as well as sites in Peru and Panama. We have incorporated new diagnostics to evaluate variables such as diameter growth increment and mortality rate by diameter size class (cm) and plant functional type (PFT).
We evaluated critical carbon fluxes (Mg C ha-1 yr-1) such as net primary production (NPP), and biomass stocks (Mg ha-1) by size class in CLM, CLM-ED, and ED2. For a central Amazonian forest CLM-ED largely over-estimated biomass in the >100 cm size class by 90 Mg ha-1, but closely matched the total aboveground biomass compared to field data (357 vs. 312 Mg ha-1 respectively). CLM-ED improves upon the large tropical biomass observed in CLM4.5, but more diagnostics are need to evaluate the bias towards large trees in CLM-ED. ED2 underestimated the aboveground biomass stock (i.e., 231 Mg ha-1) for the same forest, and exhibits fast growth and mortality rates leading to an over prediction of the Central Amazon carbon sink. The NPP flux in CLM-ED was erratic and highly variable over time, ranging from -5 to 20 Mg C ha-1 yr-1, while observed NPP averaged ~9.0 Mg C ha-1 yr-1. This large range in NPP and bias of large trees in CLM-ED could be attributed to misrepresentation of hydraulic stress in CLM-ED. 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), and reducing uncertainty in CLM can help improve estimates of the terrestrial carbon sink. 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.