Net primary productivity (NPP) of forests changes with the age. The relationship between NPP and age is crucial for quantifying the carbon sink of forests. The study investigates the species-specific relationships between NPP and forest age over subtropical China on the basis of different sources of field data. Overall, this manuscript is well-written. The topic is interesting. After some modifications, this manuscript is publishable.

In the calculation of dB, biomass in two different years is required. For the SPPCB dataset, are there biomass values in two different years available?

Are the values of variable C in equations (2) and (3) the same? Lf is the turnover of leaves per year. Equation (1) assumes that the NPP allocated into leaves is equal to the turnover of leaves for evergreen forests. It means that the foliage carbon does not change annually. This is true for mature evergreen forests. For young evergreen forests, this assumption is questionable to some extent. The foliage carbon of young evergreen forests increases year by year. The NPP allocated into leaves is larger than the turnover loss.

Leaf area index changes seasonally. Is the annual maximum of LAI used in Equation (3)? Please clarify.

As shown in Table 2, the foliage turnover ratio is smaller than 1.0 for all species. Are these species all evergreen?

The period from 1901 to 1985 was used for the spin-up of the initial model parameters. What do you mean? Do model parameters change with time? It is better to change “the initial model parameters” into “carbon pools”. What is the role of the BEPS model? It is not clear how the spin-up was implemented.

Accurately establishing the relationship between Net Primary Productivity (NPP) and forest age is a crucial prerequisite for precisely simulating ecosystem carbon sequestration capacity. This study presents the first attempt to establish this relationship at the species scale, with a specific focus on improving NPP prediction accuracy for mature forests. Validation demonstrated that the NPP-age relationship based on the species scale effectively enhanced the accuracy of aboveground biomass (AGB) estimates simulated by the ecological model. This research holds significant scientific merit. The paper addresses an appropriate topic, features a rigorous experimental design, provides thorough argumentation, and maintains a well-structured presentation. It is recommended for acceptance after minor revisions.

Specific Suggestions:

1. Figure 1a: Since the variable represented is categorical, the use of a color gradient is not recommended.
2. AGB Validation Data: The specifics of the AGB data used for validation need further clarification. This includes details on how the data was acquired and its spatial scale. Furthermore, regarding the InTEC model simulation, the grid size employed must be explicitly stated. This is particularly important given the vastly differing spatial resolutions of the model input data (Table 3). Clarify how these data were harmonized to a common scale for simulation. Additionally, address whether scale discrepancies exist between the model-simulated AGB and the ground-collected AGB, and if so, how these were reconciled.
3. Lines 174-177: The content in these lines would be more suitably placed within the figure caption.
4. Lines 225-230: Several citations must be supplemented in this section.
5. Line 247 ("three previously built NPP–age curves for entire China"): Clarify the meaning of "three NPP-age curves". If this refers to curves for different forest cover types mentioned later, note that more than three types appear to exist. Additionally, it is recommended that the Methods section briefly describe how these prior China-wide NPP-age curves were constructed and explain the methodology used for their effective comparison with the relationship proposed in this study.