Dear Reviewer,

Thank you for your positive review and for your constructive comments. We will certainly take into considerations your comments when we revise the manuscript. While waiting for other review comments, we take this opportunity to clarify some points.

1. The exclusive focus of the analysis on fluxes, relegating the changes in stocks to the Supplement, may allow misinterpretation and misappropriation of the findings to justify further intensive management policies.

The objective of our study is “to investigate the medium to long-term evolution of the forest C sink, as affected by the complex interactions between climatic variables and forest ecosystems”, focusing on the methodological aspects. In this sense, the continuation of forest management (BAU) was chosen just to test our method, but this is not a policy scenario. We will further clarify this aspect when we revise the manuscript. However, we take this opportunity to highlight that the CBM model used within our modelling framework does conserve mass, thus the sum of the fluxes is equal to the sum of the stock changes. Therefore, we think that there is no need to address both in the main text: the evolution of biomass C stocks is reported in the supplementary information (see Fig 13S).

1. This concern has two components – the change in stocks themselves under different scenarios, and the dynamics of heterotrophic respiration (Rh). I suggest moving figures 5S and 6S to the main body, and discussing the interaction of fluxes, pools and management all together.

Figures 5S and 6S report the relative stock change applied to conifers and broadleaves respectively, as derived from the combination between climate simulations and LPJ-GUESS and used as input for CBM (for this reason this additional information was added as supplementary material), to calibrate the growth functions against climate change. Losses from fires are included in the DGVM simulations but not harvest. Both harvest and fires are included in the CBM simulations. The effect of management on C stocks is reported in figure 13S, under the reference scenario, therefore excluding climate change and it is discussed on the main text (i.e., L 544-546, 555 – 557, 647-650). We understand the point highlighted by the reviewer, however, since we did not consider different management scenarios (because we did not assess policy scenarios linked to various management strategies), we mostly focused our discussion on the fluxes.

1. While the short-term flux dynamics certainly will reflect the developmental stage they are currently in, the harvest intensity must be balanced with the long-term NEP. Maximizing NEP does not maximize the climate mitigation potential of forests.

We recall again the fact that, within the present study, we did not aim to provide any policy scenario analysis, therefore we never stated that we should “maximize NEP”. However, we also notice that (i) a high NEP is an indication that the forest operates as a strong C sink (at least excluding the possible impact of natural disturbances), and (ii) to maximize the overall contribution of the forest sector to climate change mitigation, we need to maximize the “net sector productivity”, including NEP and the net contribution of HWP emissions (which were not considered within our study). Both these factors are clearly linked to management practices. Other studies have previously used the CBM to conduct scenario analyses of changes in harvest rates in different regions and have demonstrated that harvest rates do affect future NEP (see for example, Pilli et al., 2013, Pilli et al., 2017, Jevšenak et al., 2020).

1. It would be appropriate to acknowledge that the depiction of Rh in LPJ-GUESS does not reflect the latest understanding that Rh can be partly decoupled from NPP (https://doi.org/10.1029/2020GL092366), and that management-related disturbances can stimulate Rh for years to decades (https://doi.org/10.1016/j.foreco.2015.05.019; https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2010JG001495; https://www.nature.com/articles/d41586-019-01026-8). These factors likely contribute to Rh being underestimated in the LPJ-GUESS simulations.

In our study we used only net growth changes from LPJ-GUESS – as affected by climate change and fires – and we do not use Rh from that model in this analysis. In addition, we used “soc2005” LPG-GUESS simulations that uses fixed year-2005 land use and other human. Whether or not LPJ-GUESS represents Rh properly does not at all affect the outcomes of our study. In fact, the main point of the reviewer – i.e. that disturbances, including harvesting, can affect Rh for years to decades - is well represented in the CBM-CFS3.  That is why NEP changes over time across the scenarios.  Moreover, Rh in CBM-CFS3 is temperature dependent (and the temperature is varying within our simulation) – and thus is can and does vary independent of NPP and it is not assumed to be a fixed proportion of NPP.

1. I understand that a rigorous evaluation of these aspects is not feasible, but adding a paragraph to summarize remaining unknowns about soil C dynamics is appropriate, in my opinion. This section could also include references to the effect of nutrient availability (including deposition) on productivity, carbon allocation and the dynamics between plants and rhizosymbionts. There is growing evidence that these relationships are currently changing and may affect the growth and fitness of organisms involved, including changing the functional balance of soil microbial communities (leading to higher Rh).

Thanks for your suggestion, we will add a paragraph to mention remaining uncertainties in these models. However, we recall that, in this case, soil C dynamic is represented in the CBM. Other studies have previously assessed the uncertainty of these parameters, within the CBM (see for example, Smyth et al., 2009; Hararuk et al., 2017; Blujdea et al., 2021)

1. While the use of wood in various products was not a factor in the current analysis, it may be appropriate to acknowledge that recent assessments of the substitution benefits of forest products conclude that these have likely been overestimated (http://dx.doi.org/10.1088/1748-9326/ab1e95, https://doi.org/10.1038/s41598-020-77527-8).

We already highlighted within our conclusions, that “the additional mitigation potential provided from carbon storage in harvested wood products and material and energy substitution were not considered in our study” (L. 684-686). Taking into account the reviewer’s suggestions, we can further emphasize this point in other sections, but discussing whether or not substitution benefits are over or underestimated in the literature is beyond the scope of this paper

1. Finally, while the paper is overall well written and easy to follow, there are a number of typographical errors (duplication of words and punctuation marks, and minor grammatical errors) that are easy to fix using the spell checker.

Many thanks for highlighting this point, we will carefully review the text.

Additional References

Blujdea, V. N., Viskari, T., Kulmala, L., Gârbacea, G., Dutcă, I., Miclăuș, M., ... & Liski, J. (2021). Silvicultural interventions drive the changes in soil organic carbon in Romanian forests according to two model simulations. Forests, 12(6), 795.

Hararuk, O., Shaw, C., & Kurz, W. A. (2017). Constraining the organic matter decay parameters in the CBM-CFS3 using Canadian National Forest Inventory data and a Bayesian inversion technique. Ecological Modelling, 364, 1-12.

Jevšenak, J., Klopčič, M., & Mali, B. (2020). The effect of harvesting on national forest carbon sinks up to 2050 simulated by the CBM-CFS3 model: a case study from Slovenia. Forests, 11(10), 1090.

Pilli, R., Grassi, G., Kurz, W. A., Fiorese, G., & Cescatti, A. (2017). The European forest sector: past and future carbon budget and fluxes under different management scenarios. Biogeosciences, 14(9), 2387-2405.

Pilli, R., Grassi, G., Kurz, W. A., Smyth, C. E., & Blujdea, V. (2013). Application of the CBM-CFS3 model to estimate Italy's forest carbon budget, 1995–2020. Ecological Modelling, 266, 144-171.

Smyth, C. E., Trofymow, J. A., Kurz, W. A., & CIDET Working Group. (2009). Decreasing uncertainty in CBM-CFS3 estimates of forest soil C sources and sinks through use of long-term data from the Canadian Intersite Decomposition Experiment.

Dear Reviewer,

Thank you for your positive review and for your useful and constructive suggestions. We will certainly take into considerations your comments when we revise the manuscript. Meantime, we take this opportunity to clarify some points.

1. …. And here is my only criticism: in the paper the authors draw policy conclusions like the study “may constitute a first benchmark to set up specific management strategies”. Due to the limitations of the modelling approach and the rather crude assumptions on the reference scenario conclusions on needed responses to revert the declining trend should not be drawn. As the authors mention in their response to Anonymous Referee #1 “the continuation of forest management (BAU) was chosen just to test our method, but this is not a policy scenario”. In that sense the manuscript should more carefully draw conclusions on how to respond to the scenario results. Instead, the authors could provide requirements for making the results more policy relevant, e.g. by a more policy-oriented scenario design, sensitivity analyses regrading forest management options etc.

We understand the point highlighted by the Reviewer and, taking also into consideration the comments provided from Ref#1, we will certainly recall, both in the abstract and in our conclusions, the fact that our study does not aim to analyze a policy scenario. We also understand that some of our conclusions could be interpreted in such a way. For this reason, following your suggestion, we aim to rephrase the last statement of our conclusions (in particular between L. 699 and 702), highlighting  that our methodological framework may help other studies to be more policy relevant. In this sense, further studies should certainly include, (i) a sensitivity analyses on different forest management options, and the consequent effects on the overall harvest levels, (ii) an assessment of the direct effect of these removals on the HWP net C sink and, possibly, (iii) even a first assessment of the possible indirect substitution benefits.

1. Lines 205 ff: This explains… This is not clear to me: please more explicitly explain the geographical differentiation in the RC 2000-2015

Figure 1 reports, on the upper panels, the geographical distribution of the average Net Growth estimated by CBM within the historical period 2000 – 2015 for broadleaved species, on the left side, and conifers, on the right side. Since the net growth represents the net biomass increment before losses from disturbances, these values are directly proportional to the Net Annual Increment (NAI) reported from NFI data. Despite the methodological differences between various European countries (Tomter et al., 2016), the NAI reported from Mediterranean countries and Northern European countries is generally lower than the NAI reported from central European countries (see for example Table 3.1-1, Lanz and Marchetti, 2020). Since the same NFI data are mostly used as input to initialize the CBM model, these differences – between the NAI of various regions - explains the lower Net Growth generally estimated for Mediterranean and Northern European countries and the higher values estimated for Central European regions. We will clarify this sentence within the revised version of the manuscript.

1. Line 213 ff: This is… You attribute the reduction of NG “partially” to “ageing”. I think this needs to be verified and better supported by data. Also the “rejuvenation” might reduce NG if it moves the biomass stock of a stand below the maximum increment. That there is instead a saturation effect can be observed from Figure 13S. I suggest adding a sentence on these dynamics and refer to this figure at this point as the biomass stock/increment relations and dynamics cannot be observed from Fig 1.

We understand your point and we may certainly add a further reference to Figure 13S, after L 215, highlighting the saturation effect as can be observed from this figure.

1. Lines 245 - 280 Fig 1 - 2: The labels of what the panels show are very small. I suggest adding “upper panel right:…” etc. to the figure caption for better readability

Thanks for your suggestion, we will increase the size of the labels and expand the caption.

1. Lines 289-291: At the European… The sentence is hard to understand. I suggest splitting it into two for more comprehensiveness.

Thanks for your suggestion, the sentence will be rephrased as:  "At the European level, however, within the period 2016 – 2100 the overall NEP decreases from about 1.3 t C ha-1 yr-1 within the historical period to 0.97 t C ha-1 yr-1 in 2100 (i.e., -28%, see Figure 3 panel A). This is due to the larger share of forest land distributed in central and north European countries, where NEP is stable or decreasing, compared with the Mediterranean regions, where it is generally increasing."

1. Section 3.4. “Comparison with other studies, limitations, and uncertainty of the present study” is very long. It addresses different aspects that should rather be separated for more readability and clearer structure. Suggestions for additional sections are: Comparison of the reference period with other data sources, Assessing impacts of climate change, Assessing underlying trends of growth dynamics, Limitations of the model, …

Thanks for your suggestion, the content of this section will be distributed in different subparagraph within the revised version of the manuscript.

1. Lines 692-695: Due to the… The expected ranges for RCP 2.6 and 6.0 for 2050 are not clear from this sentence. Please revise it, e.g., by using the formulation in the abstract where it is much clearer.

Thanks for your suggestion, the sentence will be rephrased as: "Due to the uncertainty about the future evolution of environmental variables and the relative impact of these variables on forest growth and mortality, in 2050 the EU 27+UK forest net C sink may range from about -100 Mt CO2e yr-1 under RCP 2.6 and 6.0 to about -400 Mt CO2e yr-1 and –300 Mt CO2e yr-1, under RCP 2.6 and 6.0, respectively."

 Many thanks also for your additional technical corrections, we will address them within the revised version of the manuscript.

 Additional References

Tomter, S. M., Kuliešis, A., & Gschwantner, T. (2016). Annual volume increment of the European forests—description and evaluation of the national methods used. Annals of Forest Science, 73(4), 849-856.

Lanz, A, Marchetti, M. (2020). Criterion 3: Maintenance and Encouragement of Productive Functions of Forests (Wood and Non-Wood). In FOREST EUROPE, 2020: State of Europe’s Forests 2020.