Authors comment:

We are grateful for the careful consideration of our paper by the referee, and have considered all the points; our responses are given below.

Referee #1 comments:

Yamulki et al. present a four-year study quantifying the effects of clearfell harvesting on important soil greenhouse gas fluxes. Overall, the study is well designed and executed and descriptions of methodology and interpretation of results are thorough. Studies monitoring GHG fluxes across ecosystem and biomes are imperative to assessing present and future carbon source-sink dynamics following disturbance. However, I suggest the authors consider several comments below before a decision on acceptance. These concerns primarily relate to justification of scientific significance and clarification of how GWP results complement GHG flux analysis.

General Comments:

Justification of scientific significance:

The authors argue that there is inconsistency across the literature regarding the effect of clearfelling on soil GHGs. They present an extensive literature review of these inconsistencies, making up a bulk of the introduction. However, by the discussion and conclusion sections the authors have not specifically addressed how this study has helped to resolve the inconsistency issue, creating some disconnection between the knowledge gap that was set up in the intro and the interpretation of results. A meta-analytical or synthesis framework would be more appropriate to address this issue of inconsistency, which is not the goal of this contribution. Therefore, the authors should consider revising this initial pitch laid out in the introduction and focus more on knowledge gaps that this study directly addresses in order to avoid overstating or misrepresenting the scientific significance.

Authors comment:

We have carefully considered this point. We reviewed the literature to point out the variation in effects of clearfelling on GHG balances that have been found, and to outline the key factors that contribute to the variation. Our purpose was not to emphasise that there are ‘inconsistencies’ (we only used the phrase ‘there is little consistent information’ – which we have now reworded), but that the effect is (understandably) site specific and depends on many soil and climatic factor. We have shortened our introduction and reference less literature giving only the necessary examples to illustrate the factors and effects observed. 

We disagree that we’ve overstated or misrepresented the scientific significance of our study. We specifically addressed in our “Discussion” all the key soil factors that affect GHG balance due to felling (e.g. temperature, moisture, pH, bulk density, roots, monitoring period length and methodology) that we mentioned in the “Introduction”. We showed how our results compared with the relevant literature, and did not extrapolate wider, but only report and discuss how these measurements have helped towards the characterisation of the GHG balance of this country & climate specific forestry system (upland Sitka spruce plantations in the British Isles).

Referee #1 comment on “Clarification of how GWP results complement GHG flux analysis”:

The authors do a thorough job interpreting results from each GHG flux, but there is a lack of synthesis to highlight the most important results and the broader implications of those results. Specifically, while I think the addition of GWP is an intriguing and powerful part of the analysis, the significance, relevance and context isn’t fully developed relative to the GHG flux analysis. Authors state they measured GWP to assess the total GHG budget following clearfelling and alluded to the importance of including emissions from clearfell management in IPCC reports, for which it currently does not. However, they do not follow-through with how these results could be contextualized in this applied context and what conclusions can be made about “predicting rate and duration of changes in GHG balance by clearfelling..”, as they stated in the introduction. Without such discussion, the calculation of GWP seems out of place and incomplete.

Authors comment:

With respect, we disagree with this view, and note that a similar criticism is not made by referee 2. As indicated above we believe we have shown and discussed the most important findings throughout this study in the Discussion, Conclusion and in the paper Abstract sections. However, we have made these clearer with rearrangements to the discussion about N2O as suggested by the referee, thank you. We’ve shown (and quantified) how fluxes of CO2, CH4 and N2O changed through the 4 years monitoring period as summarised in Fig 7 (will be Fig 6) and their main soil drivers.

With regards to referee’s comment on how GWP results complement GHG flux analysis, we think there is a misunderstanding of what was implied by the term GWP in the Discussion section 4.4. We used it here only as a way to compare the relative importance of the different GHG fluxes, and to calculate the total GHG emissions in ‘CO2 equivalents’ (as very commonly done). This follows the recommendations given in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), with CH4 having a 34 and N2O a 298 times greater GWP than CO2 on a per mass basis over a time horizon of 100 years. We have rearranged this section to show the summary of the GHG balance first, then compare the GWP-weighted fluxes later, to avoid any misinterpretation of this simple (and common) analysis and used the term ‘CO2 equivalents’ when describing the fluxes in Table 5 (will be table 4).

Referee #1 “Specific (line-by-line) comments” (RC1) and authors comments (AC):

Introduction:

RC1, Line 57-79: This paragraph is very lengthy and could be reduced to fewer examples of inconsistencies in the literature.

AC: We agree with referee 1. We’ve now reduced the paragraph to streamline it and include only a few recent relevant literature examples.

RC1, Line 81-82: There is very little explanation of why long-term studies of soil GHG are important, or could potentially clarify the inconsistency problem.

AC: Thank you - we have rephrased this to make the point more clearly that longer-term studies are necessary to characterise the changes that occur over time (periods of years) since harvesting disturbance, which are not captured by short, post-felling period studies.

RC1, Line 83: “life-cycle analysis” needs to be defined/clarified.

AC: Thank you - we have rephrased this to clarify that we meant the complete forest growth cycle (i.e. not meaning LCA as is used in other disciplines).

Methods:

RC1, Figure 1: Include a scale for map.

AC: Thanks for pointing this out. Figure 1 is revised to include the scale and scale bar. 

RC1, Line 102: Specify distance between A and B sites.

AC: The distance between areas A and B was approx. 2.5 km and is now included in the text.

RC1, Figure 2: Consider moving this Figure to an appendix

AC: Thanks for the suggestion. we’ve moved the Figure to Supplementary info as requested.

RC1, Line 178: If soil parameters were only taken once, differences between felled and unfelled could also be site-level variation. Authors should acknowledge this limitation.

AC: We acknowledge the referee’s point that soil parameters were only measured on one occasion post-felling and were not sampled prior to felling, so could be due to site differences, not felling. We have noted this limitation in the results and discussion where e.g. bulk density comparisons are made as possible factors affecting altered GHG fluxes.

RC1, Table 1: Consider moving this table to an appendix.

AC: Thanks for the suggestion; we’ve moved this Table to Supplementary info as requested.

Results:

RC1, Table 2: Consider adding significance levels to this table.

AC: Thanks for the suggestion. As requested, we’ve added that the significance p-values for the mean differences were all <  than 0.01.

RC1, Figure 4: This graph is clunky and challenging to read. Consider substituting for a line graph.

AC: Thanks for the suggestion; we’ve changed Figure 4 (will be Figure 3) to line graph as requested.

RC1, Table 3: Consider condensing this table down to the most important output and the rest could go in the appendix.

AC: Thanks for the suggestion. We agree with Referee 1 and have rearranged the Table and therefore we prefer to leave it in the main paper for the reader’s benefit.

Discussion/conclusion:

RC1, Line 443-473: The main take-away from the N2O results are not made clear in this section. In previous paragraphs, authors led with a concise summary of how CO2 and CH4 were overall affected by clearfelling, but that type of synthesis is lacking in this section.

AC: Thanks for the suggestion. We’ve now highlighted the main effect of felling on N2O flux at the beginning of the section, by rearranging existing text.

RC1, Line 487-489: The sentence starting with “Over the 3 years since felling…” is the most important and concise synthesis of the GWP results from all GHG fluxes, consider emphasizing this point in the conclusions and placing this sentence towards the beginning of the paragraph.

AC: Thanks for the suggestion. We agree with the referee 1 about the importance of this statement; however, this is already emphasized at the end of the conclusion as a closing statement indicating an overall reduction of 45% on a CO2e basis; now further defined as GWP between brackets.

RC1, Line 503-506:  Authors stated large discrepancies (3 fold differences) between GHG flux estimates and the previously published EC ecosystem respiration measurements at their site, calling to question the accuracy of scaling to large earth-system calculations of GWP, which rely on accurate absolute flux measurements. While it is important authors acknowledged this limitation here, they should also consider including this potential source of error directly in their discussion of GWP.

AC: With regards to the GWP question, please refer to our earlier response to the “Clarification of how GWP results complement GHG flux analysis” question above.  The expression of GWP is only to enable all measured GHGs to be summed into total flux in CO2 equivalents. 

The differences between gas fluxes from the soil measured in this study and total ecosystem fluxes we reported should not be looked upon as a source of error. The differences arise from completely different temporal and spatial sampling as we noted in section 4.4. where we highlighted key possible reasons, including brash and belowground respiration of the colonising vegetation, which  were not included due to the limitations of our commonly used small flux chamber methodology. We also discussed how the GHG balance would have changed if it was possible to also measure CH4 and N2O fluxes at a much larger scale from brash and soil below.

Authors comment (AC)

We are grateful for the careful consideration of our paper by the referee, and have considered all the points; our responses are given below.

Referee #2 comments (RC2):

This is a very clear and well written paper on the impact of clearfelling of adjacent Sitka Spruce stand on GHG fluxes. As the authors say, there are not many studies on this topic, and IPCC inventory currently does not include the impact GHG emissions from felling and thinning operations. So, this paper will add considerably to the literature.

The authors could only investigate GHG fluxes from areas were brash mats were cleared. To measure brash as well would have required two eddy covariance towers (in site A and the other in site B), and I suppose this was not an option.

Their CO2 methodology is not the standard gas flow through system (IRGA), but instead the authors opted to use the static chamber, taking 4 samples over the chamber closure period, and analyses these gas samples also for CO2. They provided a convincing comparison of both systems, and demonstrated the differences in soil respiration rate measurements were very small. I certainly agree, that the large heterogeneity of the forest floor would have not enabled a direct comparison of CO2, N2O and CH4 fluxes, and that using the IRGA outside the static chamber would have been probably more uncertain than providing data from the SAME chamber for all 3 GHG.

AC: We thank the referee for the positive comments.

RC2, Line 136-137: Could you add how many samples had to be discarded

AC: Thanks for the enquiry. Overall 18 samples were rejected, 9 of which were during a snow fall period in January 2016. This has been added to the paper as requested.

RC2, Line 141: delete ‘also’

AC: Thank you - corrected

RC2, Figures and tables, were appropriate: It would be clearer if you mention that there are 8 chambers per site

AC: Thanks for the suggestion. We’ve added this to Figure 3 (will be Figure 2).

RC2, Table 4: there are two overlapping values, last column, 1st row.

AC: This is a misplaced line number – corrected.

RC2, Line 182: At what temperature did you dry the soils?

AC: The soil was dried at 105 °C until constant weight. This has now been indicated in section 2.4 of paper.

RC2, Line 183: ‘Soil tree live and dead fine root biomass’ The ‘Soil tree’ part of the sentence is very confusing. Do you mean to say: ‘Live and dead fine root biomass’?

AC: Thank you – corrected as suggested.

RC2, Line 235: Table 2: ‘area A (mature spruce stand) and area B (clearfell area after year 1)’ It would be clearer if you write: ‘area A (mature spruce stand, remaining ) and area B (a mature spruce stand clearfelled after year 1)’

AC: We agree with the referee and we’ve now changed the table as suggested.

RC2, Line 256: ‘(1.6 t ha-1 cf. 4.9 t ha-1 )’ what does cf stand for?

AC: ‘cf.’ is a widely used abbreviation meaning ‘compare with’, but for clarity we have put ‘compare with’ on first use, and await advice from the editor over its use. 

RC2, Line 262: a fullstop is missing.

AC: Thanks for the correction.

RC2, Line 291 Table 4: There are two overlapping values in Row 1, Column 7 

AC: see above – misplaced line number

RC2, Line 372/373: Q10 comparison with Siberian Larch and UK Oak forests. Would it not be more meaningful to compare your data with CO2 Q10 values from temperate climate Sitka spruce plantations?

AC: Thanks for the suggestion. We’ve revised the sentence to compare the results with example reference from the UK (Fenn, K.M., Malhi, Y. and Morecroft, M.D.: Soil CO2 efflux in a temperate deciduous forest: environmental drivers and component contributions. Soil Biol. Biochem., 42, 1685-1693, 2010).

RC2, Line 415: ‘ reflect a time lag in the microbial community change’ fair enough, but should you not also mention that fungal decomposition also causes this time lag. Can you provide a reference?

AC: Thanks for the suggestion. We’ve revised the sentence as suggested and will add a reference e.g. (Glassman, S.I., Weihe, C., Li, J., Albright, M.B., Looby, C.I., Martiny, A.C., Treseder, K.K., Allison, S.D. and Martiny, J.B.:  Decomposition responses to climate depend on microbial community composition. Proc. Natl. Acad. Sci. U.S.A., 115, 11994-11999, 2018).