Comparing Different Model Structures for Carbon Allocation in the Community Land Model (CLM)

Abstract:

Quantifying the intensity of feedback mechanisms between terrestrial ecosystems and climate is a central challenge for understanding the global carbon cycle. Part of this challenge includes understanding how climate affects not only NPP, but also C allocation in different plant tissues (leaves, stem and roots) which determines the C residence time. For instance, C could be sequestered over longer time periods if changes in climate increase allocation to long-lived plant tissue (e.g. woody components) with respect to short-lived tissues (e.g. leaves).

Networks of eddy covariance towers like AmeriFlux provide the infrastructure necessary to study relationships between ecosystem processes and climate forcing. We ran the Community Land Model (CLM) for six temperate forests in North America (AmeriFlux sites) using different model structures for the C allocation module: i) standard carbon allocation module in CLM, which allocates C to the stem and leaves as a dynamic function of NPP and with fixed coefficients for the rest of parameters; ii) alternative C allocation module, which allocates C to the root and stem as a dynamic function of NPP and with fixed coefficients for the rest of parameters; and iii) alternative C allocation module with fixed coefficients for all the parameters. We compare C allocation patterns and climate sensitivities betwen the different model structures and available observations for the sites. We suggest some future approaches to reduce model uncertainty in the current scheme for C allocation in CLM and its climate sensitivity.