General comments

This study updates an existing ecosystems model (TEM 5.0) to account for mosses ­- including moss photosynthesis and respiration, and the influence of the moss layer on soil temperature, moisture and ecosystem N dynamics. The updated model (TEM-Moss) is then used to simulate future carbon dynamics for northern high latitudes, and by comparing the TEM-Moss simulations to those from TEM 5.0, the authors aim to understand the role of mosses in determining the future carbon balance of the region.

This is an important topic – forecasting northern high latitude C dynamics is critical for understanding global change, and mosses are an important component of northern vegetation. Attempting to understand the role of mosses on such a broad scale is novel; there has been some work incorporating the thermal properties of mosses in land-surface models, but I’m not aware of any similar analyses at this scale. It’s an ambitious study and in general the manuscript is well structured and logically presented.

My main criticism is around how the TEM-Model is calibrated and validated, and whether the comparison to TEM 5.0 is valid. It may be that I haven’t understood the methods fully, but it seems TEM-Moss is based on ecosystem-level calibrations of the ‘moss parameters’, but TEM 5.0 is not based on representative ecosystem level calibrations. If this is the case, it doesn’t make sense to compare the performance of the two models. It also means that the calibrated ‘moss parameters’ will be compensating for un-calibrated ‘non-moss parameters’ i.e. the optimal moss parameters for an ecosystem will likely reflect differences in the properties of the higher plant vegetation which have not been captured by the ‘default’ version of TEM 5.0.

In conclusion, I think the aims of the study are worthwhile, and the general approach to update TEM 5.0 is valid, but a more robust model analysis is needed.

Specific comments

I’ve made line by line comments below which I hope will be helpful in revising the paper.

Line 41: Define northern high latitudes and the types of ecosystems that are included in the study.

Response:  Thanks for your comments and suggestions.  We changed the sentence to “Northern high latitude ecosystems, which refers to the land ecosystems (>45 ºN) in northern temperate, boreal, grassland and tundra regions”.

Line 43. Add some text to highlight the uncertainty around the 1024 Pg figure.

Response: We revised the sentence as “contain as much as 1024 Pg soil organic carbon from 0 to 3 m depth”.

Line 44-47. “This large amount of carbon is potentially responsive to ongoing global warming”. The references supporting this statement are quite old, please cite some more recent literature (e.g. Burke et al., 2017, Koven et al., 2015, Comyn-Platt et al., 2018)

Response:  Following suggestions, we updated the references.

Line 154: Please provide more detail on the function f(NA).

Response:  We added “which is a scalar function that depends on monthly N available for incorporation into plant production of new tissue” to describe f(NA).

Line 238: “higher plants” rather than “higher vegetations”.

Response:  We revised it.

Line 238: Did you use a single set of default parameters for the standard TEM model? I’m not sure I follow the reasoning here. Zha and Zhuang 2018 is an arctic study, yet you are using data from temperate forests and grasslands to calibrate TEM-Moss. Did you use the same set of default parameters across all sites? And did you use any other site-level information – apart from the NEP data – when calibrating the model?

Response: For TEM 5.0 simulations, we used different sets of default parameters for each vegetation type.  Zha and Zhuang (2018) focused on the same region, but we parameterized that TEM version with site level information.  In this study, we used site level data to parameterize TEM_Moss, but use the default parameterization of TEM 5.0 to compare with TEM-Moss simulations.  Site-level parameterization was conducted based NEP data in addition to site level vegetation and soil information. Some site level data of NEP were used for model validation.  Additionally, soil temperature and moisture at validation sites were also evaluated.

Line 247: I don’t fully understand how the posterior parameter distributions were generated. As I understand it, the SCE algorithm provides a point-estimate for each parameter, then you treat the 50 independent point estimates as samples from a posterior parameter distribution? Is this correct? Please provide some clarification on this in the text. Please also update the legend in figure 4 – what probabilities do the boxes and tails represent?

Response: Yes, the posterior parameter distribution is just the distribution for the 50 independent point estimates. We added the explanation to boxes and whiskers into the figure caption “Boxes represent the range between the first quartile and the third quartile of the parameter values, the red line within box represents the second quartile or the mean of the values. The bottom and top whiskers represent minimum and maximum parameter values, respectively.”

Line 250: Zhuang 2010 is a study from the Tibetan plateau, and Zhuang 2015 is northern high latitude wetlands. How do you justify using (I assume calibrated?) parameters from these studies to model C and N dynamics at temperate forest and grassland sites?

Response: The correct citation is Zhuang et al. (2003).

Zhuang, Q., A. D. McGuire, J. M. Melillo, J. S. Clein, R. J. Dargaville, D. W. Kicklighter, R. B. Myneni, J. Dong, V. E. Romanovsky, J. Harden, J. E. Hobbie (2003) Carbon cycling in extratropical terrestrial ecosystems of the Northern Hemisphere during the 20th Century: A modeling analysis of the influences of soil thermal dynamics, Tellus, 55B, 751-776, 2003

Line 266: please explain in more detail how the six site-level calibrations for TEM-Moss are applied to the pixel by pixel simulation. Is this on the basis of vegetation class?

Response:  Yes.  We added a sentence “With six site-level calibrated parameters, TEM-Moss is applied to the region pixel by pixel based on vegetation distribution data.”.

Line 289: If I understand correctly, TEM-Moss uses calibrated parameters for the representative ecosystems, but TEM 5.0 uses a single set of default parameters. If this is the case, it is not surprising that TEM-Moss performs better than TEM 5.0 in the validation exercise.

Response:  TEM 5.0 also used the calibrated parameters for representative ecosystems and extrapolated to the region based on the same set of data of vegetation distribution.

Line 359: The number for RH for TEM 5.0 is not correct, and the figure reference should be figure 11b.

Response:  We corrected the number. But the figure is figure 11a.

Line 412-414: These figures for the moss percentage contribution to NPP seem very high. 20 % of NPP may be realistic for boreal forest (note the Turetsky study is 20% of aboveground NPP, which is probably < 10 % of total NPP) but your study covers the entire northern latitudes from 45oN. Is a moss contribution of >25 % of 21st century NPP really plausible? I would want to see a much more thorough discussion of this, with references to observed data from a wider range of representative ecosystems.

Response:  Thanks for the comments. Yes, Turetsky et  al. (2010) suggested an average contribution of 20% of aboveground NPP from moss in boreal forests. Frolking et al. (1996) even reported a contribution of 38.4% to total NPP by moss at a boreal forest site. These estimates are for the historical periods and our estimates of 17.6% of NPP in the 20th century is at the lower end of their estimates. Our estimates of 28.8% and 27.6% in the 21st century under the RCP 2.6 410 and RCP 8.5 scenarios, respectively, are still similar to the range of existing estimates for the historical period. 

Turetsky et al. (2010) conducted a long-term data analysis through literature synthesis, representing a good knowledge about moss contribution to both wetlands and upland ecosystems in Alaska.  They found that mosses contributed 48% and 20% of wetland and upland productivity, respectively.  In this revision, we revised the sentence to “This is comparable with the results reported by a synthesis study, indicating an average contribution of 20% of aboveground NPP from moss in upland boreal forests and the contribution is 48% in wetlands ecosystems.”

Line 440: Changing vegetation is a key limitation, I recommend adding some more discussion here on the likely changes in moss abundance as climate warms, e.g. with respect to changing temperature/ hydrology/ shading by vascular plants.

Response:  Thanks for the suggestion.  We added a few references to explicitly discuss the potential the impacts of moss distribution and abundance on carbon budget in the region. In this revision, we also added the following to further discuss the impacts of vegetation including mosses on carbon dynamics in the region. “A long-term warming experiments along natural climatic gradients, ranging from Swedish subarctic birch forest and subarctic/subalpine tundra to Alaskan arctic tussock tundra concluded that both diversity and abundance of mosses are likely to decrease under arctic climate warming (Long et al. 2012).  Similarly, total moss cover declined in both heath and mesic meadow under experimental long-term warming (by 1.5–3 °C), driven by general declines in many species (Alatalo et al., 2020).   Due to global warming, significant losses in moss diversity are expected in boreal forests and alpine biomes, leading to changes in ecosystem structure and function, nutrient cycling, and carbon balance (He et al., 2015). “

General comments

This study updates an existing ecosystems model (TEM 5.0) to account for mosses ­- including moss photosynthesis and respiration, and the influence of the moss layer on soil temperature, moisture and ecosystem N dynamics. The updated model (TEM-Moss) is then used to simulate future carbon dynamics for northern high latitudes, and by comparing the TEM-Moss simulations to those from TEM 5.0, the authors aim to understand the role of mosses in determining the future carbon balance of the region.

This is an important topic – forecasting northern high latitude C dynamics is critical for understanding global change, and mosses are an important component of northern vegetation. Attempting to understand the role of mosses on such a broad scale is novel; there has been some work incorporating the thermal properties of mosses in land-surface models, but I’m not aware of any similar analyses at this scale. It’s an ambitious study and in general the manuscript is well structured and logically presented.

My main criticism is around how the TEM-Model is calibrated and validated, and whether the comparison to TEM 5.0 is valid. It may be that I haven’t understood the methods fully, but it seems TEM-Moss is based on ecosystem-level calibrations of the ‘moss parameters’, but TEM 5.0 is not based on representative ecosystem level calibrations. If this is the case, it doesn’t make sense to compare the performance of the two models. It also means that the calibrated ‘moss parameters’ will be compensating for un-calibrated ‘non-moss parameters’ i.e. the optimal moss parameters for an ecosystem will likely reflect differences in the properties of the higher plant vegetation which have not been captured by the ‘default’ version of TEM 5.0.

In conclusion, I think the aims of the study are worthwhile, and the general approach to update TEM 5.0 is valid, but a more robust model analysis is needed.

Specific comments

I’ve made line by line comments below which I hope will be helpful in revising the paper.

Line 41: Define northern high latitudes and the types of ecosystems that are included in the study.

Response:  Thanks for your comments and suggestions.  We changed the sentence to “Northern high latitude ecosystems, which refers to the land ecosystems (>45 ºN) in northern temperate, boreal, grassland and tundra regions”.

Line 43. Add some text to highlight the uncertainty around the 1024 Pg figure.

Response: We revised the sentence as “contain as much as 1024 Pg soil organic carbon from 0 to 3 m depth”.

Line 44-47. “This large amount of carbon is potentially responsive to ongoing global warming”. The references supporting this statement are quite old, please cite some more recent literature (e.g. Burke et al., 2017, Koven et al., 2015, Comyn-Platt et al., 2018)

Response:  Following suggestions, we updated the references.

Line 154: Please provide more detail on the function f(NA).

Response:  We added “which is a scalar function that depends on monthly N available for incorporation into plant production of new tissue” to describe f(NA).

Line 238: “higher plants” rather than “higher vegetations”.

Response:  We revised it.

Line 238: Did you use a single set of default parameters for the standard TEM model? I’m not sure I follow the reasoning here. Zha and Zhuang 2018 is an arctic study, yet you are using data from temperate forests and grasslands to calibrate TEM-Moss. Did you use the same set of default parameters across all sites? And did you use any other site-level information – apart from the NEP data – when calibrating the model?

Response: For TEM 5.0 simulations, we used different sets of default parameters for each vegetation type.  Zha and Zhuang (2018) focused on the same region, but we parameterized that TEM version with site level information.  In this study, we used site level data to parameterize TEM_Moss, but use the default parameterization of TEM 5.0 to compare with TEM-Moss simulations.  Site-level parameterization was conducted based NEP data in addition to site level vegetation and soil information. Some site level data of NEP were used for model validation.  Additionally, soil temperature and moisture at validation sites were also evaluated.

Line 247: I don’t fully understand how the posterior parameter distributions were generated. As I understand it, the SCE algorithm provides a point-estimate for each parameter, then you treat the 50 independent point estimates as samples from a posterior parameter distribution? Is this correct? Please provide some clarification on this in the text. Please also update the legend in figure 4 – what probabilities do the boxes and tails represent?

Response: Yes, the posterior parameter distribution is just the distribution for the 50 independent point estimates. We added the explanation to boxes and whiskers into the figure caption “Boxes represent the range between the first quartile and the third quartile of the parameter values, the red line within box represents the second quartile or the mean of the values. The bottom and top whiskers represent minimum and maximum parameter values, respectively.”

Line 250: Zhuang 2010 is a study from the Tibetan plateau, and Zhuang 2015 is northern high latitude wetlands. How do you justify using (I assume calibrated?) parameters from these studies to model C and N dynamics at temperate forest and grassland sites?

Response: The correct citation is Zhuang et al. (2003).

Zhuang, Q., A. D. McGuire, J. M. Melillo, J. S. Clein, R. J. Dargaville, D. W. Kicklighter, R. B. Myneni, J. Dong, V. E. Romanovsky, J. Harden, J. E. Hobbie (2003) Carbon cycling in extratropical terrestrial ecosystems of the Northern Hemisphere during the 20th Century: A modeling analysis of the influences of soil thermal dynamics, Tellus, 55B, 751-776, 2003

Line 266: please explain in more detail how the six site-level calibrations for TEM-Moss are applied to the pixel by pixel simulation. Is this on the basis of vegetation class?

Response:  Yes.  We added a sentence “With six site-level calibrated parameters, TEM-Moss is applied to the region pixel by pixel based on vegetation distribution data.”.

Line 289: If I understand correctly, TEM-Moss uses calibrated parameters for the representative ecosystems, but TEM 5.0 uses a single set of default parameters. If this is the case, it is not surprising that TEM-Moss performs better than TEM 5.0 in the validation exercise.

Response:  TEM 5.0 also used the calibrated parameters for representative ecosystems and extrapolated to the region based on the same set of data of vegetation distribution.

Line 359: The number for RH for TEM 5.0 is not correct, and the figure reference should be figure 11b.

Response:  We corrected the number. But the figure is figure 11a.

Line 412-414: These figures for the moss percentage contribution to NPP seem very high. 20 % of NPP may be realistic for boreal forest (note the Turetsky study is 20% of aboveground NPP, which is probably < 10 % of total NPP) but your study covers the entire northern latitudes from 45oN. Is a moss contribution of >25 % of 21st century NPP really plausible? I would want to see a much more thorough discussion of this, with references to observed data from a wider range of representative ecosystems.

Response:  Thanks for the comments. Yes, Turetsky et  al. (2010) suggested an average contribution of 20% of aboveground NPP from moss in boreal forests. Frolking et al. (1996) even reported a contribution of 38.4% to total NPP by moss at a boreal forest site. These estimates are for the historical periods and our estimates of 17.6% of NPP in the 20th century is at the lower end of their estimates. Our estimates of 28.8% and 27.6% in the 21st century under the RCP 2.6 410 and RCP 8.5 scenarios, respectively, are still similar to the range of existing estimates for the historical period. 

Turetsky et al. (2010) conducted a long-term data analysis through literature synthesis, representing a good knowledge about moss contribution to both wetlands and upland ecosystems in Alaska.  They found that mosses contributed 48% and 20% of wetland and upland productivity, respectively.  In this revision, we revised the sentence to “This is comparable with the results reported by a synthesis study, indicating an average contribution of 20% of aboveground NPP from moss in upland boreal forests and the contribution is 48% in wetlands ecosystems.”

Line 440: Changing vegetation is a key limitation, I recommend adding some more discussion here on the likely changes in moss abundance as climate warms, e.g. with respect to changing temperature/ hydrology/ shading by vascular plants.

Response:  Thanks for the suggestion.  We added a few references to explicitly discuss the potential the impacts of moss distribution and abundance on carbon budget in the region. In this revision, we also added the following to further discuss the impacts of vegetation including mosses on carbon dynamics in the region. “A long-term warming experiments along natural climatic gradients, ranging from Swedish subarctic birch forest and subarctic/subalpine tundra to Alaskan arctic tussock tundra concluded that both diversity and abundance of mosses are likely to decrease under arctic climate warming (Long et al. 2012).  Similarly, total moss cover declined in both heath and mesic meadow under experimental long-term warming (by 1.5–3 °C), driven by general declines in many species (Alatalo et al., 2020).   Due to global warming, significant losses in moss diversity are expected in boreal forests and alpine biomes, leading to changes in ecosystem structure and function, nutrient cycling, and carbon balance (He et al., 2015). “

General comments

This study updates an existing ecosystems model (TEM 5.0) to account for mosses ­- including moss photosynthesis and respiration, and the influence of the moss layer on soil temperature, moisture and ecosystem N dynamics. The updated model (TEM-Moss) is then used to simulate future carbon dynamics for northern high latitudes, and by comparing the TEM-Moss simulations to those from TEM 5.0, the authors aim to understand the role of mosses in determining the future carbon balance of the region.

This is an important topic – forecasting northern high latitude C dynamics is critical for understanding global change, and mosses are an important component of northern vegetation. Attempting to understand the role of mosses on such a broad scale is novel; there has been some work incorporating the thermal properties of mosses in land-surface models, but I’m not aware of any similar analyses at this scale. It’s an ambitious study and in general the manuscript is well structured and logically presented.

My main criticism is around how the TEM-Model is calibrated and validated, and whether the comparison to TEM 5.0 is valid. It may be that I haven’t understood the methods fully, but it seems TEM-Moss is based on ecosystem-level calibrations of the ‘moss parameters’, but TEM 5.0 is not based on representative ecosystem level calibrations. If this is the case, it doesn’t make sense to compare the performance of the two models. It also means that the calibrated ‘moss parameters’ will be compensating for un-calibrated ‘non-moss parameters’ i.e. the optimal moss parameters for an ecosystem will likely reflect differences in the properties of the higher plant vegetation which have not been captured by the ‘default’ version of TEM 5.0.

In conclusion, I think the aims of the study are worthwhile, and the general approach to update TEM 5.0 is valid, but a more robust model analysis is needed.

Specific comments

I’ve made line by line comments below which I hope will be helpful in revising the paper.

Line 41: Define northern high latitudes and the types of ecosystems that are included in the study.

Response:  Thanks for your comments and suggestions.  We changed the sentence to “Northern high latitude ecosystems, which refers to the land ecosystems (>45 ºN) in northern temperate, boreal, grassland and tundra regions”.

Line 43. Add some text to highlight the uncertainty around the 1024 Pg figure.

Response: We revised the sentence as “contain as much as 1024 Pg soil organic carbon from 0 to 3 m depth”.

Line 44-47. “This large amount of carbon is potentially responsive to ongoing global warming”. The references supporting this statement are quite old, please cite some more recent literature (e.g. Burke et al., 2017, Koven et al., 2015, Comyn-Platt et al., 2018)

Response:  Following suggestions, we updated the references.

Line 154: Please provide more detail on the function f(NA).

Response:  We added “which is a scalar function that depends on monthly N available for incorporation into plant production of new tissue” to describe f(NA).

Line 238: “higher plants” rather than “higher vegetations”.

Response:  We revised it.

Line 238: Did you use a single set of default parameters for the standard TEM model? I’m not sure I follow the reasoning here. Zha and Zhuang 2018 is an arctic study, yet you are using data from temperate forests and grasslands to calibrate TEM-Moss. Did you use the same set of default parameters across all sites? And did you use any other site-level information – apart from the NEP data – when calibrating the model?

Response: For TEM 5.0 simulations, we used different sets of default parameters for each vegetation type.  Zha and Zhuang (2018) focused on the same region, but we parameterized that TEM version with site level information.  In this study, we used site level data to parameterize TEM_Moss, but use the default parameterization of TEM 5.0 to compare with TEM-Moss simulations.  Site-level parameterization was conducted based NEP data in addition to site level vegetation and soil information. Some site level data of NEP were used for model validation.  Additionally, soil temperature and moisture at validation sites were also evaluated.

Line 247: I don’t fully understand how the posterior parameter distributions were generated. As I understand it, the SCE algorithm provides a point-estimate for each parameter, then you treat the 50 independent point estimates as samples from a posterior parameter distribution? Is this correct? Please provide some clarification on this in the text. Please also update the legend in figure 4 – what probabilities do the boxes and tails represent?

Response: Yes, the posterior parameter distribution is just the distribution for the 50 independent point estimates. We added the explanation to boxes and whiskers into the figure caption “Boxes represent the range between the first quartile and the third quartile of the parameter values, the red line within box represents the second quartile or the mean of the values. The bottom and top whiskers represent minimum and maximum parameter values, respectively.”

Line 250: Zhuang 2010 is a study from the Tibetan plateau, and Zhuang 2015 is northern high latitude wetlands. How do you justify using (I assume calibrated?) parameters from these studies to model C and N dynamics at temperate forest and grassland sites?

Response: The correct citation is Zhuang et al. (2003).

Zhuang, Q., A. D. McGuire, J. M. Melillo, J. S. Clein, R. J. Dargaville, D. W. Kicklighter, R. B. Myneni, J. Dong, V. E. Romanovsky, J. Harden, J. E. Hobbie (2003) Carbon cycling in extratropical terrestrial ecosystems of the Northern Hemisphere during the 20th Century: A modeling analysis of the influences of soil thermal dynamics, Tellus, 55B, 751-776, 2003

Line 266: please explain in more detail how the six site-level calibrations for TEM-Moss are applied to the pixel by pixel simulation. Is this on the basis of vegetation class?

Response:  Yes.  We added a sentence “With six site-level calibrated parameters, TEM-Moss is applied to the region pixel by pixel based on vegetation distribution data.”.

Line 289: If I understand correctly, TEM-Moss uses calibrated parameters for the representative ecosystems, but TEM 5.0 uses a single set of default parameters. If this is the case, it is not surprising that TEM-Moss performs better than TEM 5.0 in the validation exercise.

Response:  TEM 5.0 also used the calibrated parameters for representative ecosystems and extrapolated to the region based on the same set of data of vegetation distribution.

Line 359: The number for RH for TEM 5.0 is not correct, and the figure reference should be figure 11b.

Response:  We corrected the number. But the figure is figure 11a.

Line 412-414: These figures for the moss percentage contribution to NPP seem very high. 20 % of NPP may be realistic for boreal forest (note the Turetsky study is 20% of aboveground NPP, which is probably < 10 % of total NPP) but your study covers the entire northern latitudes from 45oN. Is a moss contribution of >25 % of 21st century NPP really plausible? I would want to see a much more thorough discussion of this, with references to observed data from a wider range of representative ecosystems.

Response:  Thanks for the comments. Yes, Turetsky et  al. (2010) suggested an average contribution of 20% of aboveground NPP from moss in boreal forests. Frolking et al. (1996) even reported a contribution of 38.4% to total NPP by moss at a boreal forest site. These estimates are for the historical periods and our estimates of 17.6% of NPP in the 20th century is at the lower end of their estimates. Our estimates of 28.8% and 27.6% in the 21st century under the RCP 2.6 410 and RCP 8.5 scenarios, respectively, are still similar to the range of existing estimates for the historical period. 

Turetsky et al. (2010) conducted a long-term data analysis through literature synthesis, representing a good knowledge about moss contribution to both wetlands and upland ecosystems in Alaska.  They found that mosses contributed 48% and 20% of wetland and upland productivity, respectively.  In this revision, we revised the sentence to “This is comparable with the results reported by a synthesis study, indicating an average contribution of 20% of aboveground NPP from moss in upland boreal forests and the contribution is 48% in wetlands ecosystems.”

Line 440: Changing vegetation is a key limitation, I recommend adding some more discussion here on the likely changes in moss abundance as climate warms, e.g. with respect to changing temperature/ hydrology/ shading by vascular plants.

Response:  Thanks for the suggestion.  We added a few references to explicitly discuss the potential the impacts of moss distribution and abundance on carbon budget in the region. In this revision, we also added the following to further discuss the impacts of vegetation including mosses on carbon dynamics in the region. “A long-term warming experiments along natural climatic gradients, ranging from Swedish subarctic birch forest and subarctic/subalpine tundra to Alaskan arctic tussock tundra concluded that both diversity and abundance of mosses are likely to decrease under arctic climate warming (Long et al. 2012).  Similarly, total moss cover declined in both heath and mesic meadow under experimental long-term warming (by 1.5–3 °C), driven by general declines in many species (Alatalo et al., 2020).   Due to global warming, significant losses in moss diversity are expected in boreal forests and alpine biomes, leading to changes in ecosystem structure and function, nutrient cycling, and carbon balance (He et al., 2015). “

General comments

This study addresses a critical gap in the Terrestrial Ecosystem Model by including mosses as a plant type at northern latitudes (>45 degrees). The authors do a good job of establishing the key role of moss in these ecosystems and show an improvement in model-data assimilation from the previous iteration of the model through the inclusion of mosses. In general, this is an important contribution to improving these models. However, some non-trivial caveats may have large effects on the future carbon storage potential of high latitude ecosystems, particularly the expected decreases in moss biomass with climate warming.

Specific comments:

1. Throughout the manuscript, I would suggest replacing “higher plants” with a more appropriate terminology, such as “vascular plants”. For a discussion on why (and how) to implement this change, please see McDaniel 2021 in New Phytologist (Title: Bryophytes are not early diverging land plants)

Response:  Thanks for the suggestions and comments.  In this revision, we replaced all “higher plants” with “vascular plants”.

1. In order to support the claim that this paper quantifies the interaction between vascular plants and mosses as mentioned in the abstract and introduction, I would like to see a more explicit explanation of how that interaction was included within the model.

Response:  Thanks for the comments.  In TEM_Moss, we have explicitly considered moss effects on soil thermal dynamics, soil water and soil moisture, and nutrient conditions in boreal ecosystems.  Mosses compete with vascular plants for water and nutrient (nitrogen) in the modeling system.  Three kinds of effects are described below:  

• Moss effects on soil thermal dynamics: In TEM 5.0, a moss plus fibric soil organic layer is considered and specified with respect to thickness through site-level parameterization. Here moss layer thickness was explicitly considered for each pixel.  
• Moss effects on water balance: In TEM 5.0, water balance is modeled as the difference between precipitation, vascular plant evapotranspiration, runoff, and percolation. In TEM_Moss, water loss through moss is considered, and soil water content is thus affected by both vascular plants and mosses.   See equations 17 and 18 in the text.
• Nutrient feedbacks, in TEM 5.0, N balance is modeled without considering moss N uptake. The change rate of soil organic N is modeled as the difference between the vascular plant N uptake and net N mineralization rate.  In TEM_Moss, N uptake is modeled as:

                                                                                                      Nuptake = Nuptake_v + Nuptakem (16)

Thus, the change rate of soil organic N is affected and the N feedbacks to C cycling is affected by considering moss N uptake.

1. In the discussion, the authors say their simulation confirms that that mosses and vascular plants respond similarly to climate change in terms of productivity. He et al. 2016 (Title: Will bryophytes survive in a warming world?) finds an expected divergence between vascular plants and bryophytes in response to climate change, as do many experimental manipulations (see below). It seems like the authors set up the model to have vascular plants and mosses respond similarly, rather than the model proving that they do? It may also be worth considering the higher CO2 concentration at the moss carpet—is it appropriate to use mean atmospheric CO2 concentration?

Response:  Thanks for the comments.  While there were a number of studies suggesting that the bryophyte act differently from vascular plants in terms of photosynthesis, nutrient uptake, and carbon allocation, and fundamental plant physiology, the algorithms of these processes of moss are not ready to be implemented in modeling activities.  Here we made a number of assumptions in our Method section to model moss productivity and nutrient uptake. To further address your concerns, we added a few sentences to discuss this limitation in Discussion section.  Regarding your comments on moss responses to climate change, we added “Future moss dynamics will also impact carbon dynamics in this region. For instance, a long-term warming experiments along natural climatic gradients, ranging from Swedish subarctic birch forest and subarctic/subalpine tundra to Alaskan arctic tussock tundra concluded that both diversity and abundance of mosses are likely to decrease under arctic climate warming (Long et al. 2012).  Similarly, total moss cover declined in both heath and mesic meadow under experimental long-term warming (by 1.5–3 °C), driven by general declines in many species (Alatalo et al., 2020).   Due to global warming, significant losses in moss diversity are expected in boreal forests and alpine biomes, leading to changes in ecosystem structure and function, nutrient cycling, and carbon balance (He et al., 2015).”

Additionally, we have to acknowledge that our modeling can not reveal moss physiology and associated carbon cycling processes, rather we strive to incorporate the knowledges into modeling to quantify carbon consequences. It is still difficult to quantify the level of CO2 concentration near /inside of mosses clusters so as to have more accurate quantification of CO2 impacts on moss productivity.

1. I think the potentially large decreases in moss biomass expected with warming are a non-trivial concern for future carbon storage expectations found in this model. I would recommend papers such as Elmendorf et al. 2012, Lang et al. 2012, and Alatalo et al. 2020 as sources on changes in moss biomass in response to simulated warming.

Response:  Thanks for the comments.  We cited these references to discuss the potential changes of moss diversity and abundance and their effects on ecosystem structure and functioning and carbon dynamics. “Future moss dynamics will also impact carbon dynamics in this region. For instance, a long-term warming experiments along natural climatic gradients, ranging from Swedish subarctic birch forest and subarctic/subalpine tundra to Alaskan arctic tussock tundra concluded that both diversity and abundance of mosses are likely to decrease under arctic climate warming (Long et al. 2012).  Similarly, total moss cover declined in both heath and mesic meadow under experimental long-term warming (by 1.5–3 °C), driven by general declines in many species (Alatalo et al., 2020).   Due to global warming, significant losses in moss diversity are expected in boreal forests and alpine biomes, leading to changes in ecosystem structure and function, nutrient cycling, and carbon balance (He et al., 2015).”

1. Soil uptake is only one pathway for mosses to access N. Studies have shown that they receive nitrogen from associations with nitrogen fixers (see Bay et al 2013 and Berg et al 2013 for examples in various types of host mosses). Mosses can translocate N from within the senescent moss body to incorporate new growth (Aldous 2002). Mosses also acquire nitrogen via deposition. The cited studies (Ayres et al. 2006 and Fritz et al. 2014) show that mosses can acquire nitrogen from soil (a previously unexpected N source due to the lack of roots and vasculature), but in Ayres et al. mosses incorporated more nitrogen via wet deposition.

Response:  We recognize the limitation of current understanding of N uptake and its algorithms in our current modeling.  While these N uptake pathways are potentially important to moss productivity, the data and knowledges are not sufficient to allow us represent these processes in the model.  In this revision, we cited these studies to discuss future efforts to improve moss N uptake representations in modeling.  We added this following to Discussion “First, due to the limited understanding of moss photosynthesis (He et al., 2015) and various moss N uptake pathways (e.g., Bay et al 2013; Berg et al 2013), a few important assumptions have been made in our modeling.  For instance, we assume that mosses behave similarly to vascular plants regarding photosynthesis and soil N uptake is the only pathway for mosses without considering N uptake through N fixers and atmospheric wet N deposition (Ayres et al. 2006).” 

Technical corrections

L22 ""which do not" instead of "without" moss.

Response: Changed.

L27 "nutrient" should be nutrients.

Response: Changed.

L41 “hold” instead of occupy, perhaps?

Response: Changed.

L59 Rephrase for clarity

Response: Rephrased the sentence as “However, the role of boreal forests in carbon sink or source activities has not been clear due to a number of model limitations”.

L69 "nutrient" should be nutrients.

Response: Changed.

L81 Since the degree to which mosses facilitate nitrogen fixation is not well-studied across the broad array of host mosses, rephrase to say “because of their associations with microbial nitrogen fixers” or similar.

Response: Changed.

L83 “of” not “on”

Response: Changed.

L84 “being” recognized.

Response: Changed.

L90 “exceeding” instead of “exceed”.

Response: Changed.

L98 “higher plants” <- but also, see comment above.

Response: Changed.

L103 Rephrase—perhaps exclude interaction?

Response: Excluded the word “interaction”.

L210: This sounds like a great feature of the model!

Response: Thanks.

L307-308 Very cool result. I think this is a major take-away of this study.

Response: Thanks.

L403 Please refer to a table or figure here to direct audience to that finding.

Response: We did not compare modeled NPP with observations, thus we deleted “Thus, with incorporation of moss into our models, NPP estimation in our model is improved.” in this revision.

L422-424 Past tense for past estimates?

Response: Changed.

L458 “which have their own functional traits” I would like to see a couple key traits enumerated—perhaps differing levels of insulation provided for soil, perhaps different associated microbiomes? Whichever may be most relevant to the assumptions within the model. Also remove next sentence that starts “In our model,…”.

Response: We revised the sentence to “Different kinds of mosses may provide different levels of insulation for soil, resulting in different soil thermal conditions that affect microbial activities.”.

Figure 1: Since Moss as a category was added in this model, perhaps the Moss boxes should also be green? I would find that helpful in interpreting the figure.

Response: Changed the color of Moss boxes to green.

Figure 3: Include a map as an inset or separate figure to show the location of these sites. I was surprised to see that half were on the southern end of area included in TEM_Moss, would you expect this to impact your results in any way?

Response: Added a map to show the sites for calibration. The locations for sites won’t influence the calibration results.

Follow-up question: Why was 45 degrees N selected as the cut-off point? This includes temperate, boreal, and Arctic ecosystems--though the introduction and discussion seem tailored more to the Arctic and boreal ecosystems.

Response: For model comparison convenience, we generally treat 45 ºN above region as pan arctic.

General comments

This study addresses a critical gap in the Terrestrial Ecosystem Model by including mosses as a plant type at northern latitudes (>45 degrees). The authors do a good job of establishing the key role of moss in these ecosystems and show an improvement in model-data assimilation from the previous iteration of the model through the inclusion of mosses. In general, this is an important contribution to improving these models. However, some non-trivial caveats may have large effects on the future carbon storage potential of high latitude ecosystems, particularly the expected decreases in moss biomass with climate warming.

Specific comments:

1. Throughout the manuscript, I would suggest replacing “higher plants” with a more appropriate terminology, such as “vascular plants”. For a discussion on why (and how) to implement this change, please see McDaniel 2021 in New Phytologist (Title: Bryophytes are not early diverging land plants)

Response:  Thanks for the suggestions and comments.  In this revision, we replaced all “higher plants” with “vascular plants”.

2. In order to support the claim that this paper quantifies the interaction between vascular plants and mosses as mentioned in the abstract and introduction, I would like to see a more explicit explanation of how that interaction was included within the model.

Response:  Thanks for the comments.  In TEM_Moss, we have explicitly considered moss effects on soil thermal dynamics, soil water and soil moisture, and nutrient conditions in boreal ecosystems.  Mosses compete with vascular plants for water and nutrient (nitrogen) in the modeling system.  Three kinds of effects are described below:  

• Moss effects on soil thermal dynamics: In TEM 5.0, a moss plus fibric soil organic layer is considered and specified with respect to thickness through site-level parameterization. Here moss layer thickness was explicitly considered for each pixel.  
• Moss effects on water balance: In TEM 5.0, water balance is modeled as the difference between precipitation, vascular plant evapotranspiration, runoff, and percolation. In TEM_Moss, water loss through moss is considered, and soil water content is thus affected by both vascular plants and mosses.   See equations 17 and 18 in the text.
• Nutrient feedbacks, in TEM 5.0, N balance is modeled without considering moss N uptake. The change rate of soil organic N is modeled as the difference between the vascular plant N uptake and net N mineralization rate.  In TEM_Moss, N uptake is modeled as:

                                                                                                      Nuptake = Nuptake_v + Nuptakem (16)

Thus, the change rate of soil organic N is affected and the N feedbacks to C cycling is affected by considering moss N uptake.

3. In the discussion, the authors say their simulation confirms that that mosses and vascular plants respond similarly to climate change in terms of productivity. He et al. 2016 (Title: Will bryophytes survive in a warming world?) finds an expected divergence between vascular plants and bryophytes in response to climate change, as do many experimental manipulations (see below). It seems like the authors set up the model to have vascular plants and mosses respond similarly, rather than the model proving that they do? It may also be worth considering the higher CO2 concentration at the moss carpet—is it appropriate to use mean atmospheric CO2 concentration?

Response:  Thanks for the comments.  While there were a number of studies suggesting that the bryophyte act differently from vascular plants in terms of photosynthesis, nutrient uptake, and carbon allocation, and fundamental plant physiology, the algorithms of these processes of moss are not ready to be implemented in modeling activities.  Here we made a number of assumptions in our Method section to model moss productivity and nutrient uptake. To further address your concerns, we added a few sentences to discuss this limitation in Discussion section.  Regarding your comments on moss responses to climate change, we added “Future moss dynamics will also impact carbon dynamics in this region. For instance, a long-term warming experiments along natural climatic gradients, ranging from Swedish subarctic birch forest and subarctic/subalpine tundra to Alaskan arctic tussock tundra concluded that both diversity and abundance of mosses are likely to decrease under arctic climate warming (Long et al. 2012).  Similarly, total moss cover declined in both heath and mesic meadow under experimental long-term warming (by 1.5–3 °C), driven by general declines in many species (Alatalo et al., 2020).   Due to global warming, significant losses in moss diversity are expected in boreal forests and alpine biomes, leading to changes in ecosystem structure and function, nutrient cycling, and carbon balance (He et al., 2015).”

Additionally, we have to acknowledge that our modeling can not reveal moss physiology and associated carbon cycling processes, rather we strive to incorporate the knowledges into modeling to quantify carbon consequences. It is still difficult to quantify the level of CO2 concentration near /inside of mosses clusters so as to have more accurate quantification of CO2 impacts on moss productivity.

4. I think the potentially large decreases in moss biomass expected with warming are a non-trivial concern for future carbon storage expectations found in this model. I would recommend papers such as Elmendorf et al. 2012, Lang et al. 2012, and Alatalo et al. 2020 as sources on changes in moss biomass in response to simulated warming.

Response:  Thanks for the comments.  We cited these references to discuss the potential changes of moss diversity and abundance and their effects on ecosystem structure and functioning and carbon dynamics. “Future moss dynamics will also impact carbon dynamics in this region. For instance, a long-term warming experiments along natural climatic gradients, ranging from Swedish subarctic birch forest and subarctic/subalpine tundra to Alaskan arctic tussock tundra concluded that both diversity and abundance of mosses are likely to decrease under arctic climate warming (Long et al. 2012).  Similarly, total moss cover declined in both heath and mesic meadow under experimental long-term warming (by 1.5–3 °C), driven by general declines in many species (Alatalo et al., 2020).   Due to global warming, significant losses in moss diversity are expected in boreal forests and alpine biomes, leading to changes in ecosystem structure and function, nutrient cycling, and carbon balance (He et al., 2015).”

5. Soil uptake is only one pathway for mosses to access N. Studies have shown that they receive nitrogen from associations with nitrogen fixers (see Bay et al 2013 and Berg et al 2013 for examples in various types of host mosses). Mosses can translocate N from within the senescent moss body to incorporate new growth (Aldous 2002). Mosses also acquire nitrogen via deposition. The cited studies (Ayres et al. 2006 and Fritz et al. 2014) show that mosses can acquire nitrogen from soil (a previously unexpected N source due to the lack of roots and vasculature), but in Ayres et al. mosses incorporated more nitrogen via wet deposition.

Response:  We recognize the limitation of current understanding of N uptake and its algorithms in our current modeling.  While these N uptake pathways are potentially important to moss productivity, the data and knowledges are not sufficient to allow us represent these processes in the model.  In this revision, we cited these studies to discuss future efforts to improve moss N uptake representations in modeling.  We added this following to Discussion “First, due to the limited understanding of moss photosynthesis (He et al., 2015) and various moss N uptake pathways (e.g., Bay et al 2013; Berg et al 2013), a few important assumptions have been made in our modeling.  For instance, we assume that mosses behave similarly to vascular plants regarding photosynthesis and soil N uptake is the only pathway for mosses without considering N uptake through N fixers and atmospheric wet N deposition (Ayres et al. 2006).” 

Technical corrections:

L22 ""which do not" instead of "without" moss.

Response: Changed.

L27 "nutrient" should be nutrients.

Response: Changed.

L41 “hold” instead of occupy, perhaps?

Response: Changed.

L59 Rephrase for clarity

Response: Rephrased the sentence as “However, the role of boreal forests in carbon sink or source activities has not been clear due to a number of model limitations”.

L69 "nutrient" should be nutrients.

Response: Changed.

L81 Since the degree to which mosses facilitate nitrogen fixation is not well-studied across the broad array of host mosses, rephrase to say “because of their associations with microbial nitrogen fixers” or similar.

Response: Changed.

L83 “of” not “on”

Response: Changed.

L84 “being” recognized.

Response: Changed.

L90 “exceeding” instead of “exceed”.

Response: Changed.

L98 “higher plants” <- but also, see comment above.

Response: Changed.

L103 Rephrase—perhaps exclude interaction?

Response: Excluded the word “interaction”.

L210: This sounds like a great feature of the model!

Response: Thanks.

L307-308 Very cool result. I think this is a major take-away of this study.

Response: Thanks.

L403 Please refer to a table or figure here to direct audience to that finding.

Response: We did not compare modeled NPP with observations, thus we deleted “Thus, with incorporation of moss into our models, NPP estimation in our model is improved.” in this revision.

L422-424 Past tense for past estimates?

Response: Changed.

L458 “which have their own functional traits” I would like to see a couple key traits enumerated—perhaps differing levels of insulation provided for soil, perhaps different associated microbiomes? Whichever may be most relevant to the assumptions within the model. Also remove next sentence that starts “In our model,…”.

Response: We revised the sentence to “Different kinds of mosses may provide different levels of insulation for soil, resulting in different soil thermal conditions that affect microbial activities.”.

Figure 1: Since Moss as a category was added in this model, perhaps the Moss boxes should also be green? I would find that helpful in interpreting the figure.

Response: Changed the color of Moss boxes to green.

Figure 3: Include a map as an inset or separate figure to show the location of these sites. I was surprised to see that half were on the southern end of area included in TEM_Moss, would you expect this to impact your results in any way?

Response: Added a map to show the sites for calibration. The locations for sites won’t influence the calibration results.

Follow-up question: Why was 45 degrees N selected as the cut-off point? This includes temperate, boreal, and Arctic ecosystems--though the introduction and discussion seem tailored more to the Arctic and boreal ecosystems.

Response: For model comparison convenience, we generally treat 45 ºN above region as pan arctic.