## 3 Results
### Calibration and Sensitivity Analysis of Growth Dynamics Model  
According to the retrieved AGB, upward translocation of biomass to the above-ground organs was observed at the end of April, then the AGB reached the peak value around August and finally declined to the minimum around November. To reflect the typical growth pattern and make the model more suitable for simulating the growth cycle of _P. australis_ in the HWNR, the following calibration of the key parameters was performed:  
1. The dates of the phenological points were calibrated based on the preliminary value obtained by the empirical relationships in Section 2.3.2. Most phenological points were moved backward to match the actual growth cycle of _P. australis_ in the HWNR. The specific time of some representative phenological events during the growth cycle of _P. australis_ is shown in Figure 4.
2. The constant of the nutrient availability and the biomass transfer rate from fertile leaf, sterile leaf, and peduncle to new rhizomes were slightly increased, and the respiration rate of above-ground organs was reduced in our model, to achieve higher peak biomass, as shown by the retrieved results.
3. The mortality rates of the above-ground organs at the onset of senescence, especially those of shoots and flowers, were increased after \(t_{s}\) to match the occurrence of abrupt biomass loss at that stage. The seasonal variation in the predicted AGB of _P. australis_ was compared with the average value of the retrieved data across the study area (Figure 5a). For all retrieved data  
Figure 4: Phenological events as implemented in the _P. australis_ model.  
Figure 3: Map of retrieved above-ground biomass (AGB) of _P. australis_ in three representative months: (a) June, (b) July, and (c) November.  
points, the model predictions agreed satisfactorily (R\({}^{2}\) = 0.9678) and captured the general growth pattern.  
For instance, a mildly increasing biomass growth rate in the early-growing season, followed by a drastically increasing biomass growth rate during the mid-growing season, the decline in biomass due to senescence at the end of the growing season, the time of peak biomass, etc., were all successfully reproduced by our model prediction, which was consistent with the growth pattern of _P. australis_ reported in the literature [14]. Figure 5b shows the seasonal variation in the BGB of _P. australis_, which was in line with the typical growth pattern of the below-ground organs--e.g., the reduction in the BGB during the early growing season due to the supply of stored material to shoot growth--and the increase in the BGB after \(t_{p}\) (new rhizomes start forming) because of the downward translocation of material from photosynthesis and shoot dry matter, etc.[14]. Additionally, the BGB at the end of the growth season was slightly higher than that at the beginning, which can be attributed to the need of nutrient storage and preparation for growth in the following year [17].  
Through sensitivity analysis of the key modeling parameters, the growth dynamics model was found to be most sensitive to maximum specific net daily photosynthesis rate, specific mortality rate of shoots, fraction of shoot transfer to rhizome, constant of the nutrient availability, and fraction of shoot biomass for elongation. Table 6 shows the sensitivity of the model output on key modelling parameters for the study site. For instance, a 50% increase in the maximum specific net daily photosynthesis rate resulted in a 56.35% increase in the AGB at \(t_{s}\) (the date when shoots start senescence), and a 50% decrease resulted in a 58.01% decrease in the AGB at \(t_{s}\). By contrast, a 50% increase or decrease in the other four parameters led to much lesser effects on the AGB at \(t_{s}\). In addition, the outcome of the model was also affected marginally by some other parameters such as respiration rates, mortality rates, and fractions of transfer, etc.  
Figure 5: Seasonal biomass variation in _P. australis_: (**a**) in the above-ground organs; (**b**) in the below-ground organs.