Using the Net Carbon Profit for optimizing vegetation properties along a precipitation gradient

An important aspect in catchment and ecosystem complexity consists of the vegetation component, which is highly dynamic. Here, we hypothesize that vegetation properties are optimized in order to maximize the total carbon taken up by photosynthesis minus all the carbon costs, i.e. the Net Carbon Profit. The Vegetation Optimality Model (VOM) schematizes the vegetation as two big leaves, one for annual grasses and one for perennial trees. Several vegetation properties, such as rooting depths and percentage vegetation cover, are optimized to maximize the long-term Net Carbon Profit.. Here, the VOM was applied to multiple study sides along the North Australian Tropical Transect ranging from tropical savanna to semi-arid conditions. No form of calibration was applied and no site-specific vegetation properties were prescribed in the model. Simulated time series of evaporate-transpiration and gross primary production were compared with observations from eddy covariance towers along the transect. All scientific steps from model parametrization and data handling to the generation of final figures are recorded by the tool Renku (renkulab.io) in order to assure reproducibility and repeatability. The model captured the dynamics of the fluxes relatively well in comparison with previous modelling efforts (Whitley et al., 2016). The seasonal amplitudes were mainly reproduced, but an off-set between modelled and observed assimilation remained in the transition from the wet to dry season across sites. Moreover, assimilation was increasingly overestimated for drier sites, whereas modelled evaporation remained closer to the observations. Vegetation cover was also strongly overestimated by the model in comparison with remotely sensed vegetation cover, pointing at issues considering the leaf area dynamics in the model. References Schymanski, S.J., Sivapalan, M., Roderick, M.L., Hutley, L.B., Beringer, J., 2009. An optimality‐based model of the dynamic feedbacks between natural vegetation and the water balance. Water Resources Research 45. https://doi.org/10.1029/2008WR006841 Whitley, R., Beringer, J., Hutley, L.B., Abramowitz, G., De Kauwe, M.G., Duursma, R., Evans, B., Haverd, V., Li, L., Ryu, Y., Smith, B., Wang, Y.-P., Williams, M., Yu, Q., 2016. A model inter-comparison study to examine limiting factors in modelling Australian tropical savannas. Biogeosciences 13, 3245–3265. https://doi.org/10.5194/bg-13-3245-2016Funded by the Luxembourg National Research Fund, ATTRACT programme (A16/SR/11254288