The paper aims to provide a “holistic approach” to investigate the ecological factors driving the primary succession on glacier forelands. On a case study in the Italian Alps, the authors tested the effects of a set of variables deriving from the previous literature in order to analyze their effects on plant cover and plant species composition.

The paper is interesting for the holistic approach, which takes into accounts the different “spheres”, and for the nice literature review. Data are well collected and the analytic methods are sound. The paper addresses scientific questions that are within the scopes of BG

.

However, some points need to be taken into account before publication. Sometimes the paper seems to be somewhat in the middle between a review and a research paper. The choice of the variables is somewhat constrained by their use in previous paper, while, in my opinion, the authors should have made their own choice. As a consequence, the hypotheses tested are a little bit trivial, and thus the paper does not tell anything really new.

The main problem, in my opinion, is that a true holistic approach cannot exclude completely the biotic factors as drivers of the succession. Although the role of facilitation, competition etc have been part of the theory of primary succession since its beginning, no biotic variable is considered as potential explanatory variables. Many papers, included some considered in the literature review (e.g. Losapio et al. 2021) showed that interspecific interactions are a strong driver, as facilitation in the early phases and as competitive exclusion in the latter ones. It would have been nice to take into account such topic, at least in the discussion. Also propagule availability (under the form of distance from potential sources) could have been taken into account, as well as the microclimatic effect of ice on the earliest phases. I acknowledge that it is impossible to take into account everything, but given the “holistic” emphasis, a broader consideration is expected. Also a comparison with other forelands described in literature could be useful, as the importance of some of the considered variables (e.g. temperature of the growing season) can be appreciated comparing areas in different bioclimatic contexts.

Sometimes the discussion of the drivers is not very convincing: from one side, it does not provide ecological hypothesis for the role of a variable (for example “south-eastness”- see below in the specific points); from the other, it provides explications that sound a little bit too stretched. For example, to explain the role of slope (component 5) it is said that its influence could be linked to the consequent soil properties, which lead to a lower C:N ratio (line 425): but soil N content is one of the variables included in the independent component 4, so I would expect a connection between the two.

The first paragraph of the conclusions should be placed at the end of the discussion, as a separated paragraph dealing with the potential effects of climate change. The conclusions should not treat a topic that has not been treated elsewhere.

I suggest to strengthen the paper with a more robust description of the observed succession and its comparison with the numerous case studies occurring in literature: does the succession imply addition and persistence or replacement? Can we hypothesize from such features a role for biotic vs abiotic drivers? Which variables must be taken into account and/or discussed?

Specific points

1. 73-75 sentence unclear
2. 79 Why giving only data for July? Yearly mean rainfall is important, as well as temperature at least for th whole growing (or snow-free) season

Line 124. I would not use the term “climax” for stages that are max 200 years old. Succession requires a much longer time span to reach such stage (if it does)

1. 179 and following Data for the pedosphere are derived from Landolt’s indices, which means that they derive from plant species occurrence. Even if they are correlated with soil analyses performed on a small subsample, they are not very appropriate as explanatory variables for plant species composition, as they are not independent. I don’t understand why the interesting soil analyses weren’t performed in all the plots and used as independent explanatory variables.
2. 200 and following. I agree that the human impact is of great importance for vegetation development. I wonder if data collected only for the present time are appropriate for explaining the development of the succession. Are there historical data about the past load of livestock somehow available, to check how representative is the present situation?
3. 295. How can bee that PCA variables 3, 4 and 5 show an increasing explained variance?
4. 302. If RC2 is linked to solar radiation, what could be the ecological meaning of “south-eastness” of RC3? In the discussion a possible explanation for this variable should be provided, particularly because in the conclusion, its influence on vegetation cover is reported to be the main difference between vegetation cover and species richness.

Line 411. I would not say that this hypothesis is not supported, as variable 1 is by far the most important. The fact that variable 1 includes other factors than those cited in the hypothesis is not meaningful: by definition, factors included in the same PCA axis cannot be disentangled, so the really significant ones could be just one or another of them, or all of them. So the hypothesis is very likely supported: the factors that were supposed to be the most significant are among those mainly contributing to the main variance. It’s up to the authors’ knowledge discuss which of them could or could not be important. For example, altitude and terrain age are correlated, as usual on glacier foreland: is the altitudinal interval big enough to represent an important factor? A comparison with similar intervals outside the LIA moraines could provide some insights.

Maybe it’s just me, but I don’t find the figure 4 very clear

Line 431. I would not say that grazing “slowed down” the development of vegetation cover we see an effect on plant cover and diversity but it is unclear its role from the point of view of the succession.

Line 442. Replace “individuum” with “individual”

General comments

The paper, after a necessary and important literature review, applies a multidisciplinary approach for studying primary succession along a glacier foreland on European Alps. The aims of the paper fit well with the scopes of the journal.

The review effort is a very important part of this paper, and has a very international interest and application.

The other part of the work (the application of the multidisciplinary approach to a case study) is, in my opinion, less “holistic” than expected for different reasons:

• it is a single case study, while it was already pointed out in literature that now, for having a new, innovative view of proglacial habitat (“holistic”?), is necessary to have a synthesis of a wider spectrum of case studies. And it is evident that some variables, at small scale, could not have a great importance, but, at bigger scale, are decisive (like lithology). De facto, you compared two glacier foreland (not three; see comment below) of the same glacial site. Some things, that you could not consider at small scale on a single site, should be considered in the discussion.
• among explanatory variables (mainly) only physical variables have been considered, excluding biological variables like, for example, arthropods successions (see comments below) or plant interactions (see comment from the other reviewer)

This considerations do not give less importance to the work, that is very interesting, but suggest to valorise the fact that a very detailed work on a single case study has be done focusing, with a great detail, on physical explanatory variables. In my opinion, it could not be considered “holistic”, it would be an error.

Specific considerations and technical corrections:

Line 19: “proglacial areas …. undergo considerable enlargement and structural changes”: this sentence should be explained. The enlargement is clear, Is less clear what do you mean with “structural Changes”

Line 23: “which has been supported by a large number of studies”. I think you should add some example of studies

Line 29: after “as a result” insert a comma

Line 30: after “processes” insert a comma

Line 61: I would correct “Our objectives were: (1) We conducted…(2) We investigated..”  in “Our objectives were: (1) to conduct…(2) to investigate.. etc”

Line 67: if your objective is also to test hypothesis

Line 63: “three proglacial areas”: from the map I see only two proglacial succession. I checked Knoflach et al. (2021) and I have seen that for the third proglacial area only lateral moraine has been sampled: in my opinion you could not consider this sampling on the third glacier foreland as a sampling of a proglacial succession. In addition, you compared proglacial areas of the same site, thus, they should be considered as replicates

Line 63: I think it should be useful to add the successional steps reported by Knoflach et al. (2021) in the Fig. 2

Lines 64-69: “to test the following hypotheses: i) Many of the known potential explanatory variables are correlated….. ii) The most important explanatory variables for vegetation cover development include …. iii) Disturbances such as geomorphic disturbance and grazing/trampling reduce ….. iv) We expected that there are no single potential explanatory variables, and we will provide a better understanding of primary succession for prediction of future development.”: the hypothesis iv) should be written in the same format of the others “no single potential expl. Variables are expected….”

Line 67: better to explicit what do you mean with “climatic variables”

Line 74: it is not clear in this sentence if vegetation survey itself was performed by  Knoflach et al. (2021) or only plot identification. It should be clearer

Line 93: repetition of “for primary succession”

Lines 101-102 “we excluded variables only mentioned once or twice (e.g., wind exposure, snow depth, or soil type), except they could be relevant due to climate change”: It is not clear this criterion, since I would have said that snow depth could be strongly related to climate change

Line 102: Premise: since it is not explained, I interpreted “soil type” as “lithology”. Considering a single site with quite uniform rock composition, you didn’t considered the importance of soil type on succession; this is reasonable. However, limestone and crystalline succession are very different and this should not be ignored. You said that soil type was mentioned only once or twice: this could be done to the fact that there are very few works, unfortunately, that compare successions on different soil types. You should specify that it is not important in a homogenous site, but important at larger scale

Line 104: I would add a reference to the 31 explanatory variable list : (Tab1)?. It could be confusing to report in the text also the 39 variables found in literature, especially if you put in the text at first the table with only the 31 selected explanatory variables. Since, in another point, you report the 26 explanatory variables selected for PCA, the risk is that it become very confusing. Maybe, fig. 3 should be removed and the same information should be added in Tab1?

Line 118: I would change the title of the paragraph “3.2 Dependent variables: Vegetation indicators (Biosphere)” in “3.2 Dependent variables: Vegetation sampling (Biosphere)”

Lines 119-120: “The vegetation surveys in the three glacier forelands of Fürkele-, Zufall-, and Langenferner were carried out in July/August of 2019 and 2020 (n = 65)” if vegetation was sampled by Knoflach et al. (2021) I would add reference

Lines 122-124: “According to the change in species composition along the chronosequence, Knoflach et al.

(2021) discriminated four successional stages: (i) a pioneer stage, (ii) an early successional stage, (iii) a late successional stage with snowbed and grassland communities, and (iv) a climax stage with dwarf shrub—” I would make clear in the Fig. 2.c the four successional stages

Line 124: it is not a “climax” if the terrain deglaciated only 200 years ago

Line 126: you did not consider any variables linked to arthropod succession. I think that in a “holistic approach” this component should not be ignored along a glacier foreland: it is known, especially in pioneer stages, the importance of arthropods as colonizer, even before plants appears. Then, their importance as disperser and pollinators could not be ignored. In general, biosphere influences biosphere during succession and this point is not considered in the paper, that is mainly focused in considering the impact of (mainly) physical factors on vegetation. Thus, I would not have used the term “holistic”.

Line 150: “of these ice-dammed lakes.(Fig. 1b)Further” delete dot before brackets, move it after brackets

Lines 149-151: are you sure that the succession restarted from zero? Organic matter should be present in soil after glacier lake outburst floods. I think you should better contextualized this point: have you checked the organic matter deposited and the grain size distribution?

Lines 152-153: “The parameter ‘distance to the glacier front’ was determined as the shortest distance from every single study plot to the glacier tongue using the ‘near’ function in ArcGIS 10.6”. I would specify the year you are considering, even if it is guessable

Line 157: insert dot after the reference

Line 158: “The distinction between no snow and snow cover was defined by a threshold of 5 mm snow water equivalent.” Specify on which basis do you fix this threshold. Is it trustable?

Line 161: “TWI”: since it is the first time TWI appears in the text I would explicit it: "Topographic wetness index (TWI)"

Lines 183-185:  why didn't you do soil analyzes of all the points to get direct values ​​of some soil variables?

Line 185 “for a subsample of the 65 study plots (n = 15).” How did you select this subsample? Which samples are they?

Lines 196-97: “Finally, the estimated cover of coarse-grained material (scree cover) in the field was

used as an additional independent variable (scree cover).” I would specify “for each plot”

line 200: the current signs of livestock grazing does not consider the effective influence in the past. If it is not possible to have information about past usage, it is better to clarify this point…

Line 253: I would add “The most frequently analysed vegetation-related, dependent variables (biosphere)”

Line 258: add “:” after “variables”

Table 3: why sometimes you use the name “RC1 etc” extrapolated from PCA and sometimes you use the full name of the variables?

Line 409-413: “In our study, we have now demonstrated that – contrary to our initial expectation – a series of

other variables correlates with our hypothesised three variables, jointly described by the components RC1 (‘elevation and time’), RC2 (‘solar radiation’), RC3 (‘south-eastness’), and RC5 ‘(low inclination’). Thus, hypothesis (ii) that the most important explanatory variables for vegetation cover development include years since deglaciation, elevation, and climatic variables, cannot be confirmed.”: demonstrating that many variables are correlated also with years since deglaciation, elevation, and climatic variables does not means that they are less important. You should comment these result, even in relation to NMDS results, were there is clearly a pattern related to years since deglaciation

Line 424: delete “a”

Line 440: “grazing and/or trampling showed no significant correlation” could be related to the fact that you did not considered grazing during all the period you considered (from LIA to now)