  CABLE is a land surface model (LSM) that can be applied stand-alone, as well
as providing for land surface-atmosphere exchange within the Australian
Community Climate and Earth System Simulator (ACCESS). We describe critical new
developments that extend the applicability of CABLE for regional and global
carbon-climate simulations, accounting for vegetation response to biophysical
and anthropogenic forcings. A land-use and land-cover change module, driven by
gross land-use transitions and wood harvest area was implemented, tailored to
the needs of the Coupled Model Intercomparison Project-6 (CMIP6). Novel aspects
include the treatment of secondary woody vegetation, which benefits from a
tight coupling between the land-use module and the Population Orders Physiology
(POP) module for woody demography and disturbance-mediated landscape
heterogeneity. Land-use transitions and harvest associated with secondary
forest tiles modify the annually-resolved patch age distribution within
secondary-vegetated tiles, in turn affecting biomass accumulation and turnover
rates and hence the magnitude of the secondary forest sink. Additionally, we
implemented a novel approach to constrain modelled GPP consistent with the
Co-ordination Hypothesis, predicted by evolutionary theory, which suggests that
electron transport and Rubisco-limited rates adjust seasonally and across
biomes to be co-limiting. We show that the default prior assumption - common to
CABLE and other LSMs - of a fixed ratio of electron transport to carboxylation
capacity is at odds with this hypothesis, and implement an alternative
algorithm for dynamic optimisation of this ratio, such that co-ordination is
achieved as an outcome of fitness maximisation. Results have significant
implications the magnitude of the simulated CO2 fertilisation effect on
photosynthesis in comparison to alternative estimates and observational
proxies.
