This MS provides a literature review on the mapping of biological soil crusts and presents the author's perspectives on future developments. The study of biological soil crust mapping is an intriguing and essential research direction. While there are existing studies on mapping, a comprehensive synthesis of these efforts is lacking, making the author's research significant. Although the descriptive aspects of the MS are well-done, it lacks theoretical depth and the author's viewpoints. Therefore, major revisions are necessary before considering publication. Specific issues include:

In section 2.2, the limitations of this model should be addressed, particularly regarding monitoring areas, as no model can feasibly cover anywhere.

In section 2.3, the influence of human activities on the growth of biological crusts in many areas, such as afforestation, should be considered. This may not necessarily be related to local climatic conditions and should be included in section 2.3.

In section 3.1, it is recommended to incorporate research hotspot maps or tables, categorizing cited and uncited literature and research areas. This provides readers with an intuitive understanding of the research hotspots and identifies regions where research has not yet been initiated.

Lines 218-222: Inconsistencies can be circled in the fig and analyzed to determine why they are higher in Rodriguez Caballero et al. (2018).

Section 3.3, the first question is why these influencing factors should be selected. The second issue is that the theoretical expression lacks depth, and some visual or intuitive numbers should be given. The entire 3.3 section only has two numbers, which is regrettable. The third issue is that this paragraph lacks a summary and the author's viewpoint, not just descriptive language. The fourth question, the author uses two paragraphs to describe. The first paragraph describes the impact of climate on biocrust, while the second paragraph mainly describes the impact of soil on biocrust. Additionally, the author adds a sentence about the impact of human activities. Is this structure reasonable.

Line 248: Confirm if it should be "20,000 years."

Lines 312-315: Clarify the relationship between high-resolution imagery and the database.

Lines 347-349: Provide examples or precedents for this point. If none exist, explain the scientific basis for this method.

The conclusion lacks an overall summary of the entire article. The author is encouraged to provide a concluding paragraph that synthesizes the key findings and insights.

Additionally, review the entire manuscript for grammar, capitalization, and singular/plural form correctness.

This manuscript provides a summary of existing knowledge about biocrust distribution, identified factors that relate to the distribution, gaps in global distribution knowledge, and proposed tools to expand the knowledge. I proposed that potential reorganization of the paper and attention to topic sentences and grouping similar information together will help the reader efficiently find the most useful components of the paper.

L34. suggest update to “continuous biotic complexes” to preface the next part of the sentence

L35. This seems to narrow the focus from all photosynthetic organisms that live at the soil surface (e.g. mosses found when glaciers retreat that are early successional stages) to just arid and semiarid. Later (L 212) they do talk about middle latitudes and polar regions.

L45. Elbert is a modeled amount rather than an observed amount, I suggest revising to “biocrusts were estimated to contribute 15%...”

L53. What is a carbon and nitrogen mechanism?

L68. Please revise the topic sentence to clarify the scope of the paragraph for the reader.

L104. Revise to “focus”

L185. The shift from statements to question was a little bit confusing – should this be a separate paragraph and leave the rest of the paragraph to summarizing the information from these different countries?

L200. These sentence likewise feels like a different topic – the previous part of that paragraph are defining the snapshots of distribution information and then this sentences talks about changes with future scenarios; may be worth moving it into in a separate paragraph, perhaps after the paragraph about factors that influence biocrust distribution (228).

L227. I generally wonder if the order of this manuscript can be revised for clarity – on line 278, the authors say that traditionally, biocrust distribution methods were based on observational and controlled experiments, so this summary of factors that determine distribution – were these based on those three methods (spectral, vegetation dynamics, geospatial)? Or on the observational/experimental? If so, it may make more sense to summarize what is already known from traditional methods before discussing what new information can be gathered from these remote sensing-assisted options. If the structure is not changed, the authors should however be really clear about methods that were used in different parts of the paper so the reader can clearly discern what is the gap in knowledge so that the author’s proposed next steps are in context of the broader field.

L228. I suggest being consistent – either have questions as section headers throughout, or remove the occasional instances.

L228. I assume this refers to total precipitation because later the authors discuss seasonality/frequency. It will help the reader if the metric is described explicitly and specifically up front.

L235. In what situations did small rain events benefit biocrusts most? That will set up the reader for the contrast with the moss die-off in the Colorado Plateau.

L240. I think that fog has also been ascribed to biocrust in the Columbia Basin, WA/OR USA. Check papers from the McCune Lab.

L243. This transition to temperature is confusing because no where else in the paragraph was temperature mentioned. Please make a separate paragraph and flesh out the impacts of temperature and the interaction with moisture.

L246. What is the relevant information for the reader from the Ferrenberg study that relates to the rest of this paragraph?

L247. Again, this sentence would better serve the reader if the “state of knowledge” section about precipitation patterns and biocrust distribution were separate from “how climate has or will change and effects on biocrusts” paragraph or paragraphs.

L262. This discussion of disturbance should be a separate paragraph from the one discussing soil parent material and characteristics to help the reader clearly follow the key information.

L270. This sentence doesn’t make sense next to the intensification of human disturbance topic. Also, please summarize the key information from the Bowker 2016 publication for the reader.

L277. Please revise this sentence for clarity: “… and thus biocrust distrubion gradually becomes a hot spot sinc ethe turn of the century” – is this “hot spot” refering to the importance for dryland ecosystems? Is it refering to the number of publications?

L284. Revise to include verb. “We suggest that a global effort should build a standardized and specialized… “ or something like that.

L288. I think that this paragraph is trying to set up the difference between traditional methods which are compiling available information from the literature with building a database with instructions for new observations that would ensure that the same data items and inclusion criteria are added. Adding a topic sentence that sets out explicitly the purpose of this paragraph rather than initially discussing the recommendation may help clarify.

L319. PROVAV_LC100 database not discussed in text while other databases in that figure were discussed in text.

L369. Revise topic sentence to include climatic characteristics in addition to spatial characteristics for this paragraph.

L380. Please revise – biocrusts are not an organizing principle, they were previously described as continuous complexes.

L382. Suggest remove the study “this study summarized” sentence unless the authors next these conclusions into the broader literature.

Thank you for your work and I hope my review is helpful to maximize the impact of your research. - Eva Stricker

General Comments 

This MS presents a review of the current knowledge of biocrust distribution, and although the authors have cited the appropriate papers and have accurately identified the current state of the research, the MS requires reorganization and a cohesive synthesis of the current state of knowledge. I have addressed potential ways of doing this in the specific comments below. Also, as the third referee, I read the responses of the others and agreed with their comments. I hope that my comments, in addition to theirs, are helpful during the revision of this MS. 

Specific Comments 

Section 3, titled “what have we known”,  should be rephrased to a statement rather than a question. I suggest “Current State of Knowledge” or something similar. 

In the first paragraph of section 3, citations should reference the work of the authors you mentioned so the reader can find previous studies on biocrust distribution. In this section, it would also be helpful to have a map of all the places where biocrust distribution has been measured. Most studies provide lat and long values, which can easily be mapped and is a good way to show the gaps in our knowledge. It should also be mentioned that although biocrust distribution has been heavily focused on arid and semi-arid regions, several arid and semi-arid regions have received very little attention (especially the Global South). With a map, this can easily be explained in the text. 

Additionally, Figure 2 (the author framework) does not provide much new scientific information other than demonstrating which authors are the primary experts in the field. It would be more informational to show a schematic with hubs at the most researched locations (I expect China and Utah would dominate), again this would highlight the gaps in the research that need to be filled. 

In section 3.1, this information may also be displayed on the map I mentioned previously, or in a table. It is difficult, as the reader, to understand how all the values relate to each other and to place them in a geographic context. Also, within the table, it would be good to specify the scale of the data for each study to provide greater context when comparing it against other studies. 

In section 3.3, there should be a connection between the importance of precipitation with the previously mentioned studies of biocrust distribution. This can be done again, by using a map. The authors can make a map, using publicly available data, showing the global precipitation patterns next to the models of the global biocrust distribution. The same can be done with temperature. L262: there is an abrupt shift from soil variables to fire which does impact biocrust distribution, but this should be a separate section, perhaps with other anthropogenic impacts on biocrust distribution. 

I think section 3 would work better if it just included a review of local scale and global scale studies. Then there should be an additional section 4 which reviews the climatic, abiotic (i.e. soil texture), and disturbance (natural and anthropogenic) effects on biocrust distribution. 

I enjoyed section 4 and like your proposed methods of solving some of the issues with measuring biocrust distribution. This information would also be helpful to have in a table like you did with Table 1, highlighting the advantages and disadvantages. However, in agreement with the other referees, there should be a concluding paragraph summarizing the goals and results of this paper. 

Technical Corrections 

L49: typo in the citation “(KrÖPfl et al., 2022)”

L174-178: check the phrasing of this sentence, it should be two separate sentences 

L185, 228: avoid using questions in the text, simply state the results 

L262: there is an abrupt shift from soil variables to fire

L267: Brianne should be Palmer et al. 2020

L274: Please summarize Bowker 2016 then cite correctly

L289: check the grammar 

L290-292: rephrase this sentence, the grammar is off