A new version of the CABLE land surface model (Subversion revision r4546), incorporating
land use and land cover change, woody vegetation demography and a novel optimisation-based
approach to plant coordination of electron transport and carboxylation capacity-limited
photosynthesis
Authors: Haverd Vanessa , Smith Benjamin , Nieradzik Lars , Briggs Peter R. , Woodgate William , Trudinger Cathy M. , Canadell Josep G. 
Published: December 2017

Abstract. The Community Atmosphere-Biosphere Land Exchange model (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 at standard temperature (Jmax,0/Vcmax,0) 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. These new developments convert CABLE to a state-of-the-art terrestrial biosphere model 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, including (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) co-ordination 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) extra-tropics as the dominant contributor to the net land sink.