A new version of the CABLE land surface model (Subversion revision r4601) incorporating land use and land cover change, woody vegetation demography, and a novel optimisation-based approach to plant coordination of photosynthesis
Authors: Haverd Vanessa , Smith Benjamin , Nieradzik Lars , Briggs Peter R. , Woodgate William , Trudinger Cathy M. , Canadell Josep G. , Cuntz Matthias 
Published: July 2018

Abstract. The Community Atmosphere–Biosphere Land Exchange model (CABLE) is a land
surface model (LSM) that can be applied stand-alone and provides the
land surface–atmosphere exchange within the Australian Community Climate
and Earth System Simulator (ACCESS). We describe new developments that extend
the applicability of CABLE for regional and global carbon–climate
simulations, accounting for vegetation responses 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 coordination hypothesis and 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 at standard temperature
(Jmax, 0∕Vcmax, 0) is at odds with this hypothesis; we implement
an alternative algorithm for dynamic optimisation of this ratio such that
coordination is achieved as an outcome of fitness maximisation. The results have
significant implications for the magnitude of the simulated CO2
fertilisation effect on photosynthesis in comparison to alternative estimates
and observational proxies. These new developments enhance CABLE's capability for use within an Earth
system model and in stand-alone applications to attribute trends and
variability in the terrestrial carbon cycle to regions, processes and
drivers. Model evaluation shows that the new model version satisfies several
key observational constraints: (i) trend and interannual
variations in the global land carbon sink, including sensitivities of
interannual variations to global precipitation and temperature anomalies;
(ii) centennial trends in global GPP; (iii) coordination of Rubisco-limited
and electron-transport-limited photosynthesis; (iv) spatial distributions of
global ET, GPP, biomass and soil carbon; and (v) age-dependent rates of
biomass accumulation in boreal, temperate and tropical secondary forests. CABLE simulations agree with recent independent assessments of the global
land–atmosphere flux partition that use a combination of atmospheric
inversions and bottom-up constraints. In particular, there is agreement that
the strong CO2-driven sink in the tropics is largely cancelled by net
deforestation and forest degradation emissions, leaving the Northern
Hemisphere (NH) extratropics as the dominant contributor to the net land
sink.