Prediction of leaf area dynamics by maximizing the Net Carbon Profit
Creators: name: Nijzink, Remko C.
name: Schymanski, Stanislaus J.
Publisher: name: Zenodo
Year: 2022
Subjects: vegetation optimality
leaf dynamics
Net Carbon Profit

description: Leaf area dynamics are often prescribed in terrestrial biosphere models (TBMs) or based on<br> predefined carbon allocation rules and plant functional types. However, reliance on observational<br> data hampers predictions under future scenarios, as leaf area dynamics and allocation patterns<br> may change due to feedbacks with soil and atmosphere. Therefore, dynamical modelling of leaf<br> area in TBMs based on fundamental principles could greatly improve our ability to better<br> understand and predict vegetation response to environmental change.<br> The Vegetation Optimality Model (VOM, Schymanski et al., 2009) uses an optimality principle based<br> on the maximization of the Net Carbon Profit (NCP) to predict vegetation properties such as root<br> distributions, photosynthetic capacity and vegetation cover at the daily time scale, as well as water<br> and CO<sub>2 </sub>exchange at the hourly scale. The NCP is defined as the difference between the total<br> CO<sub>2 </sub>assimilated by photosynthesis and the carbon costs for construction and maintenance of the<br> light and water harvesting plant organs. In a previous study (Nijzink et al. 2021), we found that the<br> VOM systematically overestimated wet season light absorption and CO<sub>2</sub> uptake along the North<br> Australian Tropical Transect (NATT), suggesting that the original big-leaf approach may be missing<br> self-shading effects at high leaf area index (LAI) values. Therefore, we extended the VOM to<br> explicitly consider light absorption as a function of the LAI, and dynamically optimize LAI while<br> considering the carbon costs and benefits of maintaining leaf area. The model was extended step-<br> wise while its predictions were compared to measurements at five flux tower sites along the NATT,<br> with a strong precipitation gradient from north to south.<br> Here we present the insights gained from this process, including the importance of considering<br> sunlit and shaded leaf area fractions, and separate optimization of photosynthetic capacity for<br> each. In a first step, dynamical leaf area was introduced in the VOM without considering shading,<br> which led to a relatively high CO<sub>2</sub>-assimilation. Nevertheless, including shaded and sunlit leaf<br> fractions in the big leaf approach of the VOM was not sufficient, as in nature, shaded leaves in the<br> lower canopy have lower photosynthetic capacities than the mostly sunlit upper canopy leaves.<br> For this reason, a separate optimization of photosynthetic capacities, in order to maximize the<br> NCP, was included for shaded and sunlit leaves. Eventually, we will compare the modelled leaf<br> area dynamics and fluxes with remotely sensed LAI and locally measured fluxes at the different<br> flux tower sites along the NATT. <strong>References</strong><br> Nijzink, R. C., Beringer, J., Hutley, L. B., and Schymanski, S. J.:, 2021. Does maximization of net<br> carbon profit enable the prediction of vegetation behaviour in savanna sites along a precipitation<br> gradient?, Hydrol. Earth Syst. Sci. Discuss. [preprint], https://doi.org/10.5194/hess-2021-265,<br> accepted<br> Schymanski, S.J., Sivapalan, M., Roderick, M.L., Hutley, L.B., Beringer, J., 2009. An optimality‐based<br> model of the dynamic feedbacks between natural vegetation and the water balance. Water<br> Resources Research 45. https://doi.org/10.1029/2008WR006841
description: Funded by the Luxembourg National Research Fund, ATTRACT programme (A16/SR/11254288)