Prediction of leaf area dynamics by maximizing the Net Carbon Profit

Leaf area dynamics are often prescribed in terrestrial biosphere models (TBMs) or based on
predefined carbon allocation rules and plant functional types. However, reliance on observational
data hampers predictions under future scenarios, as leaf area dynamics and allocation patterns
may change due to feedbacks with soil and atmosphere. Therefore, dynamical modelling of leaf
area in TBMs based on fundamental principles could greatly improve our ability to better
understand and predict vegetation response to environmental change.
The Vegetation Optimality Model (VOM, Schymanski et al., 2009) uses an optimality principle based
on the maximization of the Net Carbon Profit (NCP) to predict vegetation properties such as root
distributions, photosynthetic capacity and vegetation cover at the daily time scale, as well as water
and CO₂ exchange at the hourly scale. The NCP is defined as the difference between the total
CO₂ assimilated by photosynthesis and the carbon costs for construction and maintenance of the
light and water harvesting plant organs. In a previous study (Nijzink et al. 2021), we found that the
VOM systematically overestimated wet season light absorption and CO₂ uptake along the North
Australian Tropical Transect (NATT), suggesting that the original big-leaf approach may be missing
self-shading effects at high leaf area index (LAI) values. Therefore, we extended the VOM to
explicitly consider light absorption as a function of the LAI, and dynamically optimize LAI while
considering the carbon costs and benefits of maintaining leaf area. The model was extended step-
wise while its predictions were compared to measurements at five flux tower sites along the NATT,
with a strong precipitation gradient from north to south.
Here we present the insights gained from this process, including the importance of considering
sunlit and shaded leaf area fractions, and separate optimization of photosynthetic capacity for
each. In a first step, dynamical leaf area was introduced in the VOM without considering shading,
which led to a relatively high CO₂-assimilation. Nevertheless, including shaded and sunlit leaf
fractions in the big leaf approach of the VOM was not sufficient, as in nature, shaded leaves in the
lower canopy have lower photosynthetic capacities than the mostly sunlit upper canopy leaves.
For this reason, a separate optimization of photosynthetic capacities, in order to maximize the
NCP, was included for shaded and sunlit leaves. Eventually, we will compare the modelled leaf
area dynamics and fluxes with remotely sensed LAI and locally measured fluxes at the different
flux tower sites along the NATT.

References
Nijzink, R. C., Beringer, J., Hutley, L. B., and Schymanski, S. J.:, 2021. Does maximization of net
carbon profit enable the prediction of vegetation behaviour in savanna sites along a precipitation
gradient?, Hydrol. Earth Syst. Sci. Discuss. [preprint], https://doi.org/10.5194/hess-2021-265,
accepted
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