The subject of this study is highly relevant and timely. Hydrological aspects of forestation have received less attention compared to carbon dynamics and albedo change, making this research particularly valuable. The study presents insightful results that advance the conversation on the role of forestation in climate change mitigation. The abstract and introduction are engaging and well-crafted, and the methods section appears sound. However, the quality of the manuscript declines significantly in the later sections. My primary concern is with the presentation of the results, while the discussion and conclusion sections also lack strength. Further details are provided in the following.

Abstract: I am wondering, if the higher water demand of forests compared to grass- or shrubland isn’t partly compensated by the improved storage of water in forest ecosystems? Here are a few thoughts (that might also help the discussion section):

1.    Forest soils often have a higher organic matter content and more complex structure than grassland or shrubland soils. This can increase soil moisture retention and water infiltration rates.

2.    Forests have deep and extensive root systems that help capture and store water deep in the soil. This can be especially beneficial during dry periods, as the roots can access stored water.

3.    The forest floor's layer of leaf litter and organic matter can help retain moisture and slow down evaporation, contributing to the overall water storage capacity.

4.    The shade provided by forests can reduce evaporation rates compared to open grasslands or shrublands, allowing water to remain in the ecosystem for longer periods.

Introduction: Very well-written. The introduction nicely summarizes the current state of research and the research gaps that the authors aim to fill with this study.

l. 88 and 97 although most people reading this journal will know the definition of albedo and latent heat, I would add a short explanation (e.g., in parenthesis) as they are very important in the context of your study.

Methods: I am not an expert for the methods used in this study, but the approach appears solid to me.

l. 160-162 you could also mention that you didn’t account for any other disturbances (fire is not the only disturbance with global importance for climate regulating services)

Figure 1: The figure is hard to read. Consider to move B below A and increase the fond sizes.

Results: In this section I see room for improvement. First, the result section should just focus on the outcome of the study. Currently, the results also contain parts that should be moved into the introduction, methods, or discussion sections. Second, the results provide lots of details, but a general overview is missing in the sections. I am wondering if this isn’t too much detail as it distracts from the main message, but this is also related to my first point. Third, the figures are not well-designed and should be revised. 

l. 222 It is just a stylistic note, but I would hope for a better start into the results section. First it would be nice to get an overview, and from there you can continue with the details.

l. 249 and elsewhere: Not sure, if “experiments” is a good term here. I would rather think about a manipulation or warming experiment. Maybe “simulation experiment” or just “simulation” would be clearer.

Figs. 2B, 3, 7, 8: The legends and the letters insight the figures are very small. They don’t have an x-axis label. Without reading the caption it isn’t clear why there are these two dots per scenario what the letters mean. Overall, I don’t feel these figures are well-designed. Think about improving them or changing them fundamentally. Moreover, I found it surprising to see the results of t-tests here. Those do not appear in the methods section. I am also wondering if t-tests make sense for this kind of analysis as it isn’t based on a sample, is it?

l. 271-274 methods?

l.292 – 308 It is quite unusual to provide this information in the results section

Figs. 5 and 6 also here the fond sizes are quite small. Why are the colors reversed in 6B?

l. 324 – 327 discussion?

l. 331-332 introduction/methods?

l. 376-380 discussion?

l. 383-390 discussion?

l. 419-441 discussion?

l. 447-455 discussion?

Fig. 9 increase fond size; the x-axis is missing in B.

Discussion and Conclusions: Both sections do not go far beyond the results of the study. The authors start with some interesting implications in l.573, but already end this discussion in l. 579. The conclusions are otherwise basically just a summary of the study. I am also missing a discussion about the increase of natural disturbances as well as water and heat stress under climate change which may greatly dampen the potential of afforestation efforts to mitigate climate change.

l. 478 – 481 what about other disturbances?

This is an important and timely study and is well suited for publication in Biogeosciences. The authors examine the impacts of global-scale forestation scenarios in the CESM climate model. The novelty in the paper comes both from the assessment of forestation scenarios and from the focus on hydrological impacts.  An important policy-relevant finding is that afforestation of tropical grasslands and savannah can decrease water availability. This adds further evidence highlighting the negative impacts of afforestation of tropical grassland and savannah ecosystems.

The paper is well written. The model experiments ad results are clearly described. I suggest publication after the authors have considered these minor comments.

Minor comments

Line 185:  Do you report the difference in global forest area under the different scenarios displayed in Figure 1? If not, I think this would be useful. How does this area compare with the previous studies reported in line 70-85?

Line 220: Bala et al. (2007) report simulated impacts of altered ET due to land-cover change. It might be useful to mention this study.

Line 295: Hua et al. (2023) examine impacts of deforestation on cloud cover in the CMIP6 models. It might be interesting to compare the response simulated by CESM. Xu et al. (2022) used satellite observations to examine the impacts of forests on clouds.

Line 303: What “effects” are you referring to here? Can you please clarify.

Line 304: Is this correct “where lifting of air masses occurs due to strong latent heating”? Can you provide a reference to support this statement?

Line 375: It would be useful to discuss these results in context of Luo et al. (2022) who report the impacts of large-scale deforestation on precipitation in CMIP6 models. Also recent study of De Hertog et al. (2024).

Line 510: A few recent studies have assessed the elevated CO2 concentrations on ET and impacts on climate (e.g., Sampaio et al., 2021). They might be useful to mention here. Suggest rewording “CO2 levels” to “CO2 concentrations”.

References

Bala et al., Combined climate and carbon-cycle effects of large-scale deforestation, PNAS, 2007. https://doi.org/10.1073/pnas.0608998104

De Hertog, S. J., Lopez-Fabara, C. E., van der Ent, R., Keune, J., Miralles, D. G., Portmann, R., Schemm, S., Havermann, F., Guo, S., Luo, F., Manola, I., Lejeune, Q., Pongratz, J., Schleussner, C.-F., Seneviratne, S. I., and Thiery, W.: Effects of idealized land cover and land management changes on the atmospheric water cycle, Earth Syst. Dynam., 15, 265–291, https://doi.org/10.5194/esd-15-265-2024, 2024.

Hua et al 2023 Environ. Res. Lett. 18 094047 DOI 10.1088/1748-9326/acf232

Luo et al., The Biophysical Impacts of Deforestation on Precipitation: Results from the CMIP6 Model Intercomparison, J. Climate, 2022 https://doi.org/10.1175/JCLI-D-21-0689.1

Sampaio, G., Shimizu, M. H., Guimarães-Júnior, C. A., Alexandre, F., Guatura, M., Cardoso, M., Domingues, T. F., Rammig, A., von Randow, C., Rezende, L. F. C., and Lapola, D. M.: CO₂ physiological effect can cause rainfall decrease as strong as large-scale deforestation in the Amazon, Biogeosciences, 18, 2511–2525, https://doi.org/10.5194/bg-18-2511-2021, 2021.

Xu, R., Li, Y., Teuling, A.J. et al. Contrasting impacts of forests on cloud cover based on satellite observations. Nat Commun 13, 670 (2022). https://doi.org/10.1038/s41467-022-28161-7