|This is a very interesting and challenging research topic that has the potential to open up new avenues for the accurate reconstruction of past vegetation and biomass patterns globally. By using an extensive dataset from standardised pollen trapping experiments around Europe, the manuscript attempts to provide thresholds for the local presence of a number of key tree species in the vegetation history of the region. By focussing on pollen accumulation rates, the authors provide the first continental-scale dataset that is relevant to reconstructing past changes in biomass, tree migration rates, palaeo-population dynamics and so on. The results could even be used to help anticipate the effect of climate change on pollen production in the future, which could have implications for allergy sufferers in the region, forest health and conservation. The manuscript includes some very involved and novel graphical presentations in R and much relevant literature. |
Unfortunately the manuscript is not yet ready for publication, in my opinion. The shortcomings are detailed below and are all quite easy to solve with some moderate revision of the text, particularly the Introduction and Methods, which currently lack coherent links to the study aims. I really hope the authors will make the effort to modify their manuscript, as this work is certainly novel and has great potential to be a turning point in palaeoecological and biogeographical research.
There is a surprising number of grammatical mistakes given the authors involved. Some of these mistakes are identified in detailed comments, below, but thorough proof reading would be helpful before resubmission.
The Introduction would benefit from tighter structuring and should emphasise the broader context for the research (i.e. beyond palynology - see suggestions in the first paragraph above). Perhaps breaking it into subsections, each dealing with one of the aims, would make the text more focussed, e.g. Relationships between PARs, climate and vegetation cover; The importance of long-distance transported pollen; Applicability of PARs to palaeovegetation reconstruction. At the moment, the Introduction seems to be largely directed at Aim 1, whereas Aims 2 and 3 are not adequately introduced. This leaves the reader wondering why certain methods of analysis were chosen later in the paper. The current text could be shortened in places without loss of information. Taphonomic processes that affect lake sediment pollen should be introduced in the section relating to fossil PARs, not at the end of the Discussion (Section 4.4), as these are critical to interpretation.
In the Methods, it would also be good to split sections 2.3 and 2.4 into 3 sections dealing with each of the 3 aims. Section 2.4 mentions selecting fossil sites within each trapping region, but were any criteria used to make the selection? Were records filtered according to taxonomic resolution or the number of radiocarbon dates? As PARs are highly dependent on accurate chronologies, some mention of chronological control should be made in this section. Table 1 should include the number of dates contributing to each site’s chronology. Section 2.4 describes a statistical clustering technique that is poorly explained. Please give some more details to explain how the method works and why it is the most appropriate option for addressing Aim 3. A number of new methods are introduced in the Results and Discussion sections (see below) – these should first appear in the Methods.
In Results, the “3% wide bin” approach requires more explanation and justification. It seems to be an arbitrary solution to finding a trend in the data, rather than based on any objective criteria. Please try to avoid this perception by making it clearer how and why this approach was taken. It would be best to add this to the Methods, rather than introducing new data treatments (i.e. the binning approach and regression analysis) here in Results.
The authors claim that a threshold of 30 grains/cm2/yr indicates long-distance transport (LDT), but Fig. 4 shows that only Picea and Quercus had LDT components below the error bars for traps within the species’ geographic ranges. This is important to note because researchers could apply the threshold and mistakenly reach the conclusion that it denotes the absence of the species (a key question in island biogeography and post-glacial tree migration). This makes Fig. 5 potentially misleading. Maybe a better approach is to consider different thresholds for each species – the PAR threshold of 30 grains/cm2/yr might be appropriate for Fagus, but not for other taxa (Fig. 4b). The authors should also explain why 200 km is an appropriate distance when much better results are obtained at greater distances.
Section 3.4 includes a t-test for differences between fossil and trap PARs, but it is unclear why this was done and which aim it addresses.
Section 3.5 includes detailed descriptions of 8 different taxa. These are very long and include too much interpretation for a Results section. Consider removing these or reducing the descriptions in the manuscript to a single sentence per taxon and put the longer interpretations (existing text) in the supplement. The ad hoc exclusion of Betula data from Turkey and Georgia seems difficult to justify (see specific comments) and requires some explanation in the text.
The Discussion seems to say that taphonomic processes can be disregarded in comparing trap and fossil pollen. It is hard to see how this can be claimed without a full explanation of those processes. The authors link pollen production to primary productivity gradients (latitude), yet do not consider how these gradients might have changed during the Holocene due to millennial-scale climatic variations. This makes the comparison between fossil and trap data quite complex and these complexities should be assessed in the Discussion. The section about long-distance transport could also consider how elevation might affect trap results – a trap placed in Fagus forest will presumably catch more pollen than a trap on a treeless mountaintop, even though both traps are within Fagus’s geographic range. The use of a Gaussian Plume dispersal model is mentioned for the first time in the Discussion, but should be introduced in Methods.
Section 4.3 has lots of potential to explain the importance of linking modern and fossil PARs, but gets very detailed very quickly, making it difficult to see the overall picture. Please try to broaden the scope here and use the site-specific details to support your argument. Help the reader understand why it’s important that no modern analogues for early-Holocene Corylus PARs exist in Europe, for example.
The limitations and problems section (4.4) is well thought through and contributes substantially to the paper’s scientific value. However, it leaves the reader perplexed as to what are the strongest points of the analysis. Which results/outcomes of the paper can we regard as being the most robust?
Specific line-by-line comments:
Page 2, line 1: Consider putting a comma after “modern” and replacing “diagrams” with “assemblages and the reconstruction of past vegetation communities in space and time” to expand the scope of the paper from a purely palynological one.
2: Replace “Such” with “Modern” [to avoid confusion]
4: “European latitudes” sounds strange as the same latitudes are found in N America and E Asia. Consider “Europe” instead
7: Replace “are still collecting” with “still collect”
9: “Comparisons… show comparable values” sounds strange – consider “similar values”
10: Replace “fossils” with “fossil”; this sentence is hard to understand – are the fossil sites located further south and downhill compared to the trap sites, or vice versa? What is meant by “similar high values” in this context. Please rephrase more simply
11: The sentence “While modern… do not occur” is unclear. Do you mean that, for some taxa, PARs in the past were much higher than those recorded in the traps?
12: Replace “PAR’s” with “PARs” and “publically” with “publicly”
13: Replace “serves improving” with “serves to improve” or simply “aids”
19: I suggest adding a statement before the opening sentence that highlights the relevance of pollen analysis. This would provide a broader context for the paper and might attract non-palynological readers!
21: Comma after “tree-line”
22: “procuring” should be “producing”
Page 3, line 1: place commas around the phase “or… period of time”; also note that “is better” should be “are better” to agree with rates
3: It would be useful to point out what makes this paper so groundbreaking, as the sentence seems lost without such elaboration
8: Inconsistent use of “PAR” vs “PARs” (cf. line 6, this page); change “sediments” to “sediment”
14: Comma needed after citation
18: Replace “numerable” with “numerous”
19: Replace “comparably” with “comparatively”
21: “were based” should be “was based” to agree with construction.
26: “of the previous, as well as the year of flowering” – it is unclear whether this means the previous topic (tree biomass) or the previous year. Consider “of the year of flowering and previous year”
27: The question posed here does not arise from the previous statements. You state that pollen deposition rates represent absolute tree abundance, but then say that interannual PAR variations are determined by weather, so it is unclear why climate (which is different to weather) or site conditions (whatever that means) would raise questions. Please rephrase this to help readers follow your arguments
28: The sentence “Comparing… suggest…” would make more sense with “A comparison of… suggesting”; this sentence might be better placed before the question above to provide context
30: What is the basis for interpreting the PAR:weather relationship as reflecting primary productivity of the tree? Isn’t flowering (and pollen production) specific to the phenology of each species and may have many different weather triggers according to each species? See Autio and Hicks 2003 https://doi.org/10.1080/00173130310017409. Your primary productivity theory cannot explain masting or trees that flower in response to stress. Please rephrase this sentence and the following one and include some references in support of your claims. What’s the basis of the CO2 argument?
32: Delete “Already” [awkward] or replace with “As early as the 1940s”
34: Place a comma either side of “however”
35: Do you mean “then” rather than “than”? How is the climatic interpretation here different to what Davis and Deevey proposed?
Page 4, line 1: The “initial question” has not been introduced previously. Please elaborate on this and explain to the reader why it is important to determine the long-distance component more accurately, including references (e.g. Markgraf 1980, Grana 19, 127-146). It’s a very important question for island palaeoecology, where the presence of absence of pollen can often be used to decide if a species if native or exotic (e.g. http://doi.org/10.1111/j.1365-2699.2008.02012.x, and http://doi.org/10.1126/science.1163454). Splitting the long-distance aspect off into a standalone paragraph might be a good idea.
7: Delete “as references helping” [unnecessary]
8: Comma needed after “Finland”
9: Change “applied” to “applied to”
16: Add comma after citation
32: This aim seems fine, but the preceding introduction adds a lot more variables, such as site condition, CO2 fertilization, weather of the flowering season etc. Perhaps you could explain why only climate and forest cover are retained in the aims?
Page 5, line 3: The 3rd aim does not seem to arise so easily from the introduction. Could you perhaps provide some more context in the introduction to tell readers why this aim is important and necessary?
8: Replace “the” with “their” or simply delete; I suggest putting the names of the trap regions in this section in capital letters, e.g. “North Boreal” instead of “north boreal” to make them stand out more
Page 6, line 21: Comma after “overview”
29: Replace “In consequence” with “Consequently,”; replace “might overgrow or cover” [future tense] with “might have overgrown or covered” [past tense]
Page 7, line 5: Provide a citation to Tauber’s paper where these components are described.
29: Add a comma after “PAR”
32: “in the pollen type described above” – perhaps “in each of the pollen types listed above” would be clearer
34: “these taxa” – do you mean the taxa not suitable for comparison, or the taxa that were suitable? Unclear. It is also unclear how the pollen traps were placed at exactly 200 km from the edge of the plant distribution limits. Do you mean >200 km? Or within 200 km?
Page 8, line 2: Maybe “target taxa” instead of “taxa considered”
4: Why was log10 PAR used instead of PAR? Justify
8: Replace “Per” with “For”
10: Add comma after “comparison”
12: “logged PAR” – do you mean “log-transformed PAR”? It is unclear how this sentence compares traps and fossil data – it seems to only deal with trap data – please expand.
15: “at level” – replace with “at the level”. The description of the methods here is wordy but does not really convey why one-dimensional clustering was the most appropriate method and what statistical criteria were used to form the clusters. More detail about the method would be useful. “The classes produced were used to facilitate the comparison between trap and fossil data and to match the trap values with analogous situations in the past. The aim of this comparison was to find traps with similarly high values for individual taxa that compared to the highest average fossil PAR” – these sentences are quite wordy and seem to be saying “These classes helped us compare trap and fossil data and to link high trap PARs with high fossil PARs of the past”. Please explain why only high values were considered meaningful for comparison.
19: The grammar of this sentence needs attention
23: supplementary material?
27: How does a “mean trap assemblage” differ from a “trap location”?
28-30: These sentences describing climate and elevation might be more appropriate for the Study Area section (2.1)
Page 9, line 2: Comma after “environments”
3: Comma after “type”
6: Explain briefly what makes these differences noticeable
10: Delete “Nevertheless”; hyphenate “log-transformed”; the text here refers to “total PAR” but the figure referred to (Fig. 2) only displays “tree PAR” – please indicate where the total PAR data can be found
12: How much variance did elevation explain on its own? This information is missing from Table S4
Page 10, Fig. 2: This is a very comprehensive figure! However, the absence of data for the Georgian sites should be explained somewhere in the text. Vegetation data were collected for the Georgian pollen traps in Filipova-Marinova et al. 2010 (https://doi.org/10.1007/s00334-010-0257-z ).
Page 11, line 1: “affect” rather than “effect”. It’s unclear what relationship is meant here – clarify. Also, consider “tree species” to replace the potentially confusing wording “different trees”.
4: Comma after “grasses”. Why were grasses included here and not in the previous analysis? This is not mentioned in the Methods and should be explained somewhere.
5: Hyphenate “log-transformed”
8: Replace “pattern, the” with “pattern, so the”
9: Replace “are” with “is” to agree with “distribution”; same problem in next line
15: What is meant by the “3% wide bins”? This seems to be a methodological decision that needs further explanation. How was the 3% value determined? Why not use a more ‘usual’ value like the lower quartile? What is the regression model referred to here? The sentence here is very difficult to understand and should be rewritten more clearly.
18: Is “PAR” singular or plural in this sentence? Perhaps using PAR for singular and PARs for plural would avoid confusion?
19: What is meant by “distribution limit” – the geographic distribution or distribution of points on the plot?
21: Comma after “taxa”. Regression analysis should be mentioned in the Methods rather than Results. In this section, it would be useful to mention which taxa show a significant difference between LDT pollen and local pollen (within the species’ range).
26: Delete “here”
29-33: Where are these results presented?
Page 12, line 1: Replace “Minimal” with “Minimum”
2: Commas after “Cyperaceae” and “particular”. Please provide a supplementary figure that shows the distributions being described in this paragraph, or consider omitting this information about frequency distributions as its relevance is unclear.
3: It’s hard to understand what is meant by “fossil PARs show a local maximum in the frequency of low values, which does not occur in the traps”, especially since there is no associated figure.
5: Commas after “types” and “these”. It is unclear why this analysis was done or what the “pairs” are comprised of – please explain.
6: Comma after “comparison”
Page 13, line 3: Comma after “PAR”
5: What are “maximum averages”? Do you mean average maxima? Hyphenate “site-by-site”
7: Why are results for all these 8 taxa included, while others are in supplementary material? How were the 8 taxa chosen and are they all important? Also replace “description” with “descriptions”.
8: Missing word – “supplementary material” or use “supplement”
12: Delete “the” before “different”
15: Comma after “populations”
17: Comma after “Sumava”
Page 15, line 4: Explain why the Georgian and Turkish traps were excluded (this information is in the responses to reviewers, but not in the text). It’s hard to understand why these countries were excluded because of the presence of other species of Betula, when the same approach was not taken to Pinus, Fraxinus, Fagus, Carpinus, Quercus – all of which have different species in the Caucasus and Anatolia. The map, Fig. 6, clearly shows Betula pendula’s range overlapping with the Georgian trap locations.
Page 16, line 4: There are at 6 species of Corylus in the Caucasus and C. colurna occurs in both Georgia and Turkey, so perhaps add “…and other species” here.
5: What do you mean by “as discussed in the main text”?
8: Replace “small” with “low”
Page 17, line 7: Italics for Picea abies
Page 19, line 10: Add “The” before “highest”. Change “Balkan” to “Balkans”
12: “seem too high” – this is interpretation and misplaced in the Results section
Page 20, line 4: Lagodekhi misspelled
5: Semicolon after “Georgia”
6: The highest…
Page 21, line 4: This is the first time taphonomy is mentioned in the paper, which seems a significant oversight.
7: Add comma after “locations”
11: “On the regional scale PAR” – replace with “On a regional scale, PAR”
13: If latitude influences primary productivity (and thereby pollen production), then surely elevation has a similar effect?
15-16: This seems to be saying the same thing as lines 8-11. Or do you mean average PAR, or regional PAR here?
Page 22, line 2: How do the PARs for local vs long-distance presence from that study compare to the thresholds in this paper?
8: Replace “larger” with “higher” – and explain how the fall speed influences pollen thresholds. It’s hard to tell whether the data support the authors’ claims about thresholds here as the Fagus results (Fig. 4) do not cover the same distance range as the Corylus results. Consider revising this statement.
18-20: Remind the reader in a few words why fossil data from these areas were considered unreliable.
21: Split “for the”
22: Is PAR plural here?
24: Comma after “percentages”
Page 23, line 2: Please provide a source for the statement that nitrogen increases pollen production in other tree species independently of changes in forest composition. Please provide details of the CO2 experiment – were the levels of CO2 comparable to the current climate, i.e. is the recent increase in atmospheric CO2 sufficient to explain the recent increase in pine pollen in the Brandenburg forests?
12: Make “percentage” plural
23: Comma after “sites”
Page 24, line 1: “in boreal region result above” – seems to be a word missing here
3: This section (4.4) has many sentences starting with “Nevertheless, Although, Also, Despite” – this stream of contradictions makes the argument seem disjointed.
6: What are some examples of these modern processes? This statement assumes that these are common knowledge.
8: Replace “stringer” with “stronger”
13: How the “best available” sites were chosen should be elaborated in the Methods
15: Comma after “available”
20: Replace “high” with “large”
25: In this context it might be worth referring to Tauber’s experiments with roofed and unroofed traps, where the roofed traps would have presumably avoided any direct pollen fall.
28: Comma after “dataset”
29: Replace “is” with “are”
Page 25, line 7: Double check Conclusions once other changes have been made (also Abstract)
7: One instance of “that” needs to be deleted
S1 and S2 – the axes are labelled as % while the data seem to be proportions
S3 Carpinus – caption is missing (a). For part (d), the maximum for site “Sum” is not highlighted (intermediate values highlighted instead)
S3 Fraxinus – “demining tree” perhaps should be “demanding tree”