Nitrate and Ammonium Deposition in the Midwestern Fragmented Forest
Round 1
Reviewer 1 Report
The manuscript "Nitrate and Ammonia Deposition in the Midwestern Fragmented Forest" is evaluates transects of throughfall deposition of atmospheric nitrogen in three forest patches in Michigan. The experimental design is sound, the paper is well written, and I believe that the conclusions and discussion are sound. I really appreciate the clear hypotheses stated at in the introduction and using them to clearly structure the manuscript.
My comments are relatively minor, with one being a something that would make the manuscript more personally relevant to me.
1) I think it would be useful to have a few sentences describing the difference in deposition collected in the three open collectors at each site. I think it would be interesting to know whether the values were consistent across the sites and thus there was consistent atmospheric mixing. If so, I think that this would strengthen the story of implications of the canopy.
2) Depending on how the final formatting of the document is, I would consider changing Figure 2 to two rows of three figures similar to Figure 3. Whether it be this or the opposite, I would recommend matching the format and order of graphs between the two figures so that they are directly comparable.
3) I think within the discussion it might be worth mentioning differences in potential stemflow as part of the mechanisms for the differences among plots (https://www.mdpi.com/1999-4907/13/10/1644). You mention it as a pathway of N deposition in the introduction, but don't discuss it directly in the discussion. I imagine that you don't have direct measurements from each species, but maybe there is data available or observational data on how it may differ among species.
4) This is the one that is more of a personal interest and thus you are welcome to ignore as it is a little tangential to your work. I worked regularly with comparisons between modeled deposition and measurements to better understand the accuracy of the models in different areas of the US. This may not work since you don't have a full year of measurements, but I would be interested in knowing how the measured deposition compares to the most recent modeled data available for the area (https://www.epa.gov/cmaq/equates). Again, this would be challenging given the variation in landcover etc, but at least having a sentence or two saying that scaling our deposition to an annual values gives a range of deposition between X-Y kg/ha/yr, while modeled deposition (or measured from local NADP sites: https://nadp.slh.wisc.edu/maps-data/ntn-interactive-map/) says that deposition is around Z kg/ha/yr. Having these comparisons is useful for us when trying to update model parameters and understand where additional measurement sites are needed.
minor edit: I would combine the two sentences in lines 52-54 to increase clarity. It reads weird to describe the wet deposition process and then just say 'dry deposition' but then you kind of describe it in the next sentence.
minor edit 2: You use 'various' twice in the same sentence. Just a personal preference.
Author Response
Editor and Reviewer Comments:
We thank the reviewer for the assessment of our manuscript. We have revised the text in accordance with their suggestion for the published version of the manuscript
The manuscript "Nitrate and Ammonia Deposition in the Midwestern Fragmented Forest" is evaluates transects of throughfall deposition of atmospheric nitrogen in three forest patches in Michigan. The experimental design is sound, the paper is well written, and I believe that the conclusions and discussion are sound. I really appreciate the clear hypotheses stated at in the introduction and using them to clearly structure the manuscript.
My comments are relatively minor, with one being a something that would make the manuscript more personally relevant to me.
I think it would be useful to have a few sentences describing the difference in deposition collected in the three open collectors at each site. I think it would be interesting to know whether the values were consistent across the sites and thus there was consistent atmospheric mixing. If so, I think that this would strengthen the story of implications of the canopy.
Response: We have modified the text to now describe observed variability in open rainfall chemistry among the three sites (lines 266-269). P values will be added to line text to show significance.
Depending on how the final formatting of the document is, I would consider changing Figure 2 to two rows of three figures similar to Figure 3. Whether it be this or the opposite, I would recommend matching the format and order of graphs between the two figures so that they are directly comparable.
Response: Thank you for the suggestion on matching formats for Figures 2 and 3. We have changed the layout of Figure 3 so that it now matches that of Figure 2.
I think within the discussion it might be worth mentioning differences in potential stemflow as part of the mechanisms for the differences among plots (https://www.mdpi.com/1999-4907/13/10/1644). You mention it as a pathway of N deposition in the introduction, but don't discuss it directly in the discussion. I imagine that you don't have direct measurements from each species, but maybe there is data available or observational data on how it may differ among species.
Response: To maintain our primary focus on throughfall, we have removed the previous reference to stemflow in the text. We have removed the stemflow mentioned in the text around the calculation of the flux-based enrichment ratio this correction helps with the streamline description of how E is calculated.
This is the one that is more of a personal interest and thus you are welcome to ignore as it is a little tangential to your work. I worked regularly with comparisons between modeled deposition and measurements to better understand the accuracy of the models in different areas of the US. This may not work since you don't have a full year of measurements, but I would be interested in knowing how the measured deposition compares to the most recent modeled data available for the area (https://www.epa.gov/cmaq/equates). Again, this would be challenging given the variation in landcover etc, but at least having a sentence or two saying that scaling our deposition to an annual values gives a range of deposition between X-Y kg/ha/yr, while modeled deposition (or measured from local NADP sites: https://nadp.slh.wisc.edu/maps-data/ntn-interactive-map/) says that deposition is around Z kg/ha/yr. Having these comparisons is useful for us when trying to update model parameters and understand where additional measurement sites are needed.
Response: We appreciate the reviewer’s feedback, and we also see the utility of incorporating data from NADP to develop regional deposition estimates. Due to our current dataset, which is based on a limited number of observations made during the growing season, it would be beyond the scope of our study to extrapolate our findings to annual deposition estimates. We will be considering this suggestion for a future research project.
minor edit: I would combine the two sentences in lines 52-54 to increase clarity. It reads weird to describe the wet deposition process and then just say 'dry deposition' but then you kind of describe it in the next sentence.
Response: We have modified the text by combining lines 52-54 to increase clarity as suggested.
minor edit 2: You use 'various' twice in the same sentence. Just a personal preference.
Response: The suggestion for the change of “various” in the manuscript can be seen on lines 59 and 62
Author Response File: 
Author Response.docx
Reviewer 2 Report
The paper by Rivera-Cubero et al. uses an observational/laboratory approach in southern Michigan to explore the issue of nitrogen deposition in modified human landscapes that includes woodlots separated by suburbs, roads, and agricultural fields. They take measurements of throughfall along transects within each of three plots and just outside, which enables them to determine the effect of the canopy on throughfall, as well as dissipation from edge and effect of different tree species.
I thank the authors for a well written and clear article. The statistics are all nicely presented. I would change the title from “Ammonia” to “Ammonium”, as the N deposition is composed of nitrate and ammonium, even if ammonia is the source of some of the ammonium. The only caveat for this study is that the sample size is very small, consisting of only three plots. It strikes me that a more robust statistical analysis would have several plots with specific tree species (e.g. perhaps 5 plots each in which there is dominance of basswood, red oak, and sugar maple, respectively), and maybe several plots dominated by agriculture (or even split between crops and dairy farms) and others dominated by roads. I do not know the exact number of replicas, but that can be determined by the statistics. Of course, requiring this would really negate the entire study, so I am not suggesting that, but rather would like to see the authors address this issue. Otherwise I just have some minor comments.
1. Figure 2: Just add a little more explanation to the caption as to what the connecting lines mean. I figured it out, but am used to the “a”, “b” labeling to distinguish significant differences between groups. Also I think that “TF” should be added to the concentrations (a, b) as well.
2. Figure 3: Some of the figures are a bit blurry and difficult to read. I could not distinguish the symbols for Hudson and Loti on this figure.
3. Figure 4: Please add statistical analysis to this figure as in Figure 2. Also, please include a similar figure to show the NH4+ data for sugar maple, since that showed significance.
4. Line 256: There is no table 4.
5. Line 305: What is the quantitative justification for using the word “more” important here?
6. Lines 314-316: I do not understand how these numbers relate to the whiskers in Figure 2, which should represent the high and low ranges.
7. Table 3: Define BA and TPA
8. Lines 441-442: Curious if the wind speed and direction data are not already available from the nearby weather station that was used for precipitation, or is the variation so high that you would need to literally set up wind sensors at the edge of each plot? If those data are available, perhaps they can be used now rather than for a future study.
9. Is there a way to separate out the effect of atmospheric deposition vs fertilizer runoff? That would be a really useful addition to the discussion.
Author Response
Editor and Reviewer Comments:
We thank the reviewer for the assessment of out manuscript. We have revised the text in accordance with their suggestion for the published version of the manuscript
Reviewer 2
The paper by Rivera-Cubero et al. uses an observational/laboratory approach in southern Michigan to explore the issue of nitrogen deposition in modified human landscapes that includes woodlots separated by suburbs, roads, and agricultural fields. They take measurements of throughfall along transects within each of three plots and just outside, which enables them to determine the effect of the canopy on throughfall, as well as dissipation from edge and effect of different tree species.
I thank the authors for a well written and clear article. The statistics are all nicely presented. I would change the title from “Ammonia” to “Ammonium”, as the N deposition is composed of nitrate and ammonium, even if ammonia is the source of some of the ammonium. The only caveat for this study is that the sample size is very small, consisting of only three plots. It strikes me that a more robust statistical analysis would have several plots with specific tree species (e.g. perhaps 5 plots each in which there is dominance of basswood, red oak, and sugar maple, respectively), and maybe several plots dominated by agriculture (or even split between crops and dairy farms) and others dominated by roads. I do not know the exact number of replicas, but that can be determined by the statistics. Of course, requiring this would really negate the entire study, so I am not suggesting that, but rather would like to see the authors address this issue. Otherwise I just have some minor comments.
Response: This suggestion will be taken in consideration for a future study with the addition of more plots. The site were selected in accordance to surrounding land use.
- Figure 2: Just add a little more explanation to the caption as to what the connecting lines mean. I figured it out, but am used to the “a”, “b” labeling to distinguish significant differences between groups. Also I think that “TF” should be added to the concentrations (a, b) as we
Response: The line were meant to show the significance between sites.
- Figure 3: Some of the figures are a bit blurry and difficult to read. I could not distinguish the symbols for Hudson and Loti on this figure.
Response: The figure was updated since as you clearly stated it was difficult to read the new figure can be found in line text 283.
- Figure 4: Please add statistical analysis to this figure as in Figure 2. Also, please include a similar figure to show the NH4+ data for sugar maple, since that showed significance.
Response. Figure 2 statistical analysis was on section 3.1 of the results as well the sugar maple data was added on a figure 3 which showed a correlation with the collectors and the trees.
- Line 256: There is no table 4.
Reviewer: The correction was made since there is no table 4.
Line 305: What is the quantitative justification for using the word “more” important here?
Response: The quantitative justification of more was changed to additional the correction was made on line text 326.
- Lines 314-316: I do not understand how these numbers relate to the whiskers in Figure 2, which should represent the high and low ranges.
Response: The values were in accordance to what the literature mentioned a median value less than 1 indicated a sink of NO3 in incoming precipitation.
- Table 3: Define BA and TPA
Response: BA and TPA were defined in line text 411-412.
- Lines 441-442: Curious if the wind speed and direction data are not already available from the nearby weather station that was used for precipitation, or is the variation so high that you would need to literally set up wind sensors at the edge of each plot? If those data are available, perhaps they can be used now rather than for a future study.
Response: The wind speed and direction data were not considered for the purpose of the hydrologic study. The data was available on the database of the MSU Hort weather station the suggestions will be taken into consideration for a future study.
- Is there a way to separate out the effect of atmospheric deposition vs fertilizer runoff? That would be a useful addition to the discussion.
Reviewer: This would likely be another idea for a future project making that correlation would be outside of the scope of the study.
Author Response File: Author Response.docx
Reviewer 3 Report
See attached file
Author Response
Editor and Reviewer Comments:
We thank the reviewer for the assessment of our manuscript. We have revised the text in accordance with their suggestion for the published version of the manuscript.
Review of Nitrate and Ammonia Deposition in the Midwestern Fragmented Forest
Manuscript Number: FORESTS 2203687
General Comments
Acid rain and atmospheric N deposition are no longer in the research spotlight, however that does not mean that the issue has gone away. N deposition rates have declined, but hotspots occur, and legacies persist. This paper takes an interesting twist on the topic by examining forests that are fragmented and are in the located in populated areas where local N inputs are likely to enhance N deposition and alter N cycling and forest processes.
This is a well-designed study, and the methods are appropriate to the work. Generally, the paper is well written, however there are many places where the writing can be simplified and tightened (examples given in the specific comments). I think that the authors could really help the readers by naming the sites by land use rather than site names, which requires the reader, who will be unfamiliar with these sites, to memorize the descriptions as they attempt to understand the results.
Response: The names of the sites were changed throughout the rest of the manuscript to facilitate the reader to learn about the descriptions and interpret better the results.
I’m not convinced that the authors can conclude that species composition is more a controlling factor than species in determining N dynamics. In the paper (347-348), arguments are presented giving reasons why basswood and maple may leak more N, but there are no direct data in this paper, and although there were statistical differences by species, there were also significant differences by land use. At best it can be suggested, but to argue that this study “demonstrated that TF N concentrations and fluxes were more strongly influenced by the internal characteristics of the woodlots, such as forest structure and species composition, than by the surrounding land use” is an overstatement.
Response: The authors agree that stating demonstrated in the conclusion is making an overstatement. Changes were made to illustrate to the reader that is a possible suggestion on why we state how it was influenced by the internal characteristics of the woodlots.
The discussion on deer seems tangential to this paper. Most of the LAI in the forest interior is likely to be due to the canopy, not seedlings or small saplings. Furthermore, deer might explain the forest composition and species trajectory, but that does not inform what is directly controlling the N fluxes.
Response: Thank you for the comment on the deer information. We understand that it is outside the scope of this study. Therefore we have deleted this portion of the discussion
Specific Comments
Line Comment
42 Delete “all”
Response: The word all was deleted at line text 42.
Fig 1. The map of MI (A) is elongated N-S. Maps should also have scales and compass arrows.
Response: Figure 1 map of MI was modified and we added the scale and compass arrow
Table 1. This will need to be much larger in the published version.
Response: Table 1 was updated with the correct font and a made larger for the published version
Table2. Precipitation can simply be represented as cm – delete “depth”; “mm/hrs” should be “mm h-1”. Use SIU. Event number is irrelevant. It might be better to prepare this as a figure, with dates on the x-axis and the weather data on the y-axis. That would reduce the redundancy of the site names, and make it easier to see the range of sampled conditions.
Response: The suggested corrections were made and the redundancy of the site names was modified as described above.
103 I’d recommend renaming the sites into names useful for the reader – e.g. highway, mixed landscape, and agricultural. It is hard for a reader to constantly try to remember which site has what attributes. In the end, we care about the land uses and effects on N cycling, not on site names.
Response: We have renamed the sites so that the new names reflect their respective
surroundings and land uses, improving the readability of the text. The modification can be seen in line text 112, 115, and 118.
160 This calculation is quite standard. Stating that concentrations were converted to total mass input is sufficient; the equation is unnecessary.
Response: Correction were made on line 160 there was a typo on that sentence.
Fig. 2. Increase the size of the figure.
Response: The size of the figure was increased for the purpose of the published version as well as fixing the names of the sites.
236 This paragraph can be simplified and condensed. With 6 variables and 3 land uses, it is very challenging to keep track of all the differences. Renaming the sites by land use will help. And, just point out differences.
Response. The paragraph was modified with the updated names for the sites throughout the manuscript text.
329 It is not necessary to point out specifically who conducted a study, or that a study was done. Simply state the findings and include a reference. For example, this sentence “In a study by Lodhi and Ruess [35], basswood had twice the total leaf N concentration of red oak and sugar maple, and rates of leaf nitrate reductase activity were more than 30 times higher in basswood compared to red oak and sugar maple” could be simplified to “Basswood has twice the total leaf N concentration and 30 times higher leaf nitrate reductase activity than red oak and sugar maple [35].” This simplification can be employed throughout the manuscript.
Response: Thank you for the suggestion, the simplification was done on line 329.
341 “on the other hand” Avoid using clichés.
Response: Correction was made on line 341
349 “was the fact that” is wordy and should be avoided.
Response: Correction was made on line 349
355 “depth” means amount.
Response: Correction was made on line 355
366 The information about deer is interesting, but not important in this study. You are documenting species composition effects on N dep, not on stand history.
Response: As noted above we deleted the portion of the discussion where it had the information about deer.
380-395 These literature comparisons are not particularly useful. Fluxes and enrichment will be dependent on emissions, deposition, weather patterns, as well as other factors. Comparing rates in MI with rates in Iran provides no insight to findings here.
Response: We agree that the comparison of the rates in Iran has no insight into the findings of our study. We proceeded to remove the study of Salehi and remain with Limpert and Siegert since our numbers are in accordance with their findings and it was also conducted in the eastern US.
404 Similar to my previous comment (line 329), citing citation numbers is an odd way to cite the literature.
Response: Corrections we made in line text 329 and now 404.
408 “only” is in the wrong place in this sentence.
Response: Correction was made on line 408
420 What is TPA? Trees per acre? It is not defined. In the text, you refer to stems per acre.
Response: Correction was made on line 420. TPA stands for trees per acre and the information was added to the manuscript.
424 This sentence can be deleted – get right to the point.
Response: Correction was made in line text 424.
435 And most of the dry periods were quite short. How do your periods compare to those in other studies?
446 This sentence is not needed.
Response: Correction was made on line 446
447 I’m not sure that this study can determine which factor is more important, unless a multi-factorial statistical approach is taken. The mixed land use site (Hudson) had ER values very highly significantly different from the other sites, suggesting that this combination of land use activities alters enrichment.
It might also be possible that at this relatively small scale, air mixing is sufficient to mask differences among the land uses. Perhaps at a regional scale (Midwestern agric, large city suburban forests, etc.) differences might be detected.
Response: As we noted above, we realize we can’t for certain and we qualify the language.
461 a word is missing in this sentence
Response: Correction was made on line 461 as suggested by the reviewer.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
I think the updates look good.
Author Response
Thanks you for your time to review the manuscript
Reviewer 2 Report
The authors did not really address my points 1, 2, 3, or 6 at all. I would still like to see an improved figure caption for figure 2 to better explain the lines. Figure 3 still blurry and cannot distinguish the symbols. Not significance added to Figure 4. Why do sugar maple as scatter plot? I still don't understand the numbers with respect to the whisker plots.
Author Response
The authors did not really address my points 1, 2, 3, or 6 at all. I would still like to see an improved figure caption for figure 2 to better explain the lines.
Response: The caption for figure 2 was revised to clearly described that the lines were drawn between sites that were significant between each other.
Figure 3 still blurry and cannot distinguish the symbols.
Response: The figure was revised and made clear visually for the readers.
Not significance added to Figure 4.
Response: We could not add significance to figure 4 as we did in figure 2 because we did not conduct a pairwise comparison for the species effect. Species effects were analyzed as a covariate in our mixed-effect models. We expanded the caption to make it clear that the figure only visualizes species effects that were significant in our mixed effect model. Detailed results from the mixed effect model are described in the text on lines 309-311.
Why do sugar maple as scatter plot? I still don't understand the numbers with respect to the whisker plots.
Response: We used box-whiskers plots to visualize the effect of basswood and red oak because they occurred infrequently as scattered individuals. We could not plot it as a scattered plot because most collectors would have 0 values. In contrast, sugar maple was widespread across all of our collectors so we could use a scatter plot to plot NH4+ concentrations against sugar maple abundance. We added text to the revised manuscript to explain this on lines 309-311.
Reviewer 3 Report
Review of Nitrate and Ammonia Deposition in the Midwestern Fragmented Forest
Manuscript Number: FORESTS 2203687
Review 2 - revised manuscript
119 Figure 1. The map is still stretched north to south, and is thus not proportionally correct.
326 convert mg/L to mg l-1 (SI units)
397 “In comparison, [38] reported a 55 % decline in TF fluxes of NO3- and NH4+ along a 60 m transect from the edge to the interior of a Pine Forest in the south of Gotenborg, on the Swedish West coast.”
This is an odd sentence construction (see previous comments). Calling out a study by citation number is not the norm (unless Forests has now moved in this direction). More importantly, just state the information and then provide the reference. For example:
“In comparison, there was a 55 % decline in TF fluxes of NO3- and NH4+ along a 60 m transect from the edge to the interior of a Pine Forest in southern Sweden [38].”
That the study was conducted on the west coast or south of Göteborg is irrelevant.
425 “For the study,” delete this.
373 as pointed out previously, “only” is in the wrong place. As it stands, this sentence states that the only thing that occurs in the mixed land use site is that it has 368 stems per acre. “only” should come after “had” Fix here and in other places in the manuscript.
373 Why report in acres (not SIU)?
411 “To establish this relationship, we looked at the basal area (BA) and Trees per hectare (TPA) from the three sites.” Not sure what this means at the end of this paragraph.
423 “The antecedent dry periods from our study are short but are in alignment with those found in the literature [56] but not for [53].” This is a confusing sentence? What is in alignment with what?
Author Response
Review 2 - revised manuscript
119 Figure 1. The map is still stretched north to south, and is thus not proportionally correct.
Response: Figure 1 has been updated
326 convert mg/L to mg l-1 (SI units)
Response: Correction was made throughout the text with the SI unit suggestion
397 “In comparison, [38] reported a 55 % decline in TF fluxes of NO3- and NH4+ along a 60 m transect from the edge to the interior of a Pine Forest in the south of Gotenborg, on the Swedish West coast.”
This is an odd sentence construction (see previous comments). Calling out a study by citation number is not the norm (unless Forests has now moved in this direction). More importantly, just state the information and then provide the reference. For example:
“In comparison, there was a 55 % decline in TF fluxes of NO3- and NH4+ along a 60 m transect from the edge to the interior of a Pine Forest in southern Sweden [38].”
That the study was conducted on the west coast or south of Göteborg is irrelevant.
Response: Lines text 397 was updated with the reviewer suggestion
425 “For the study,” delete this.
Response: For the study was removed from like text 425
373 as pointed out previously, “only” is in the wrong place. As it stands, this sentence states that the only thing that occurs in the mixed land use site is that it has 368 stems per acre. “only” should come after “had” Fix here and in other places in the manuscript.
Response; Suggestion was taken on consideration and updated for better syntax of the manuscript.
373 Why report in acres (not SIU)?
Response: This was a typo error in the manuscript correction was made on line text 373.
411 “To establish this relationship, we looked at the basal area (BA) and Trees per hectare (TPA) from the three sites.” Not sure what this means at the end of this paragraph.
Response: Correction was made on line 411 we used BA and TPH previously (TPA) to look at the vegetative structure at the edge of the sites.
423 “The antecedent dry periods from our study are short but are in alignment with those found in the literature [56] but not for [53].” This is a confusing sentence? What is in alignment with what?
Response: The authors agree the word alignment was vague and changed were made in the manuscript on the suggested line text.
Round 3
Reviewer 2 Report
I am ok with final set of revisions.
Reviewer 3 Report
This paper is now ready for acceptance. Those studying N deposition and forest fragmentation will find the information useful.
Author Response
Thank you for your time and suggestions for the manuscript final edits
