What We Know and What We Do Not Know about Dragon Trees?

Round 1

Reviewer 1 Report

The authors have written an exhaustive review on dragon trees, focusing on species with an arborescent form and excretion of the famous red resin, dragons blood. Relevant literature on the topics specified by the authors is comprehensively covered. As a background, an overview of the evolution, taxonomy and distribution of the dragon trees is provided, followed by a rather detailed description of the anatomical and morphological features, growth, ontogeny, and water relations. The part on the resin and ethnobotany has a historical emphasis, and studies on the chemical composition and pharmacological activities are only briefly described, apparently because these aspects are better covered by journals in other disciplines, and have been recently reviewed. The last, and in my opinion the most important part of the manuscript focuses on ecological aspects, threats to the current populations and conservation status of these species.

The strength of the manuscript is in its broad coverage and comprehensiveness. It is also well written, although the style of writing varies somewhat among the chapters. There is a lot of information in the manuscript, and at times, one gets the feeling of an excessive amount of details being described and listed one by one, which makes the text less readable. This concerns mainly the parts on anatomy and morphology, growth and physiology. However, there are no major weaknesses, and the the few minor issues should be relatively easily addressed. 

For revision, I would suggest that the authors would go carefully through the manuscript, particularly the parts with detailed information mentioned above, and try to increase the readability, where possible. In some cases, an introductory sentence explaining the following paragraph might suffice and elsewhere, efforts to make a synthesis of a longish list of features or values for a particular parameter would be helpful. One should also consider the use of short tables instead of listing numerical values in the text.

More specific comments:

Line 108: so base on

Line 127: Base on

Line 129: This facts

Lines 130-131: show to horizontal ...

Lines 136-137: Not quite sure whether listing of these names is actually necessary at all, since they are not further discussed. The whole paragraph could as well be shortened to a couple of sentences.

Line 154: CP not explained, better to write chloroplast(ic).

Line 180: grows naturally grows

Line 232: "D. species", used here and a couple of times later, should be written as elsewhere in the manuscript for consistency (e.g., Dracaena species).

Lines 233 - 345: Due to many detailed description of various features, this part is not so easy to follow, and would benefit from illustration of some additional features as in Fig. 2b. That might also help to shorten the text slightly.

Line 355: Gibberella

Lines 347 - 352: The resin can also be induced chemically (Ding et al. 2018, PLoS ONE 13 (12): e0209258, and references therein).

Line 389: Table 1 could include the duration of the stage as described in lines 454-458.

Lines 394-396: D. cinnabari as among the "slowest growing tree species in the world": I don't have any reason to believe that this would not be so, but the claim is based on individuals growing at one site, without irrigation, and no information is given here on the environmental conditions. There are a lot of places in the world where trees located in harsh conditions would grow very, very slowly. Without data of other tree species, growing in comparable environmental conditions, this statement is a bit too strong to my taste. Perhaps you could rephrase this or provide some further evidence to support the claim.

Lines 454 - 458: This could go to Table 1, as mentioned.

Line 466: This chapter is not really about physiology, but rather on water relations. This is naturally due to the lack of basic physiological studies, e.g., on photosynthesis, metabolism, nitrogen assimilation, mineral nutrition, etc. I suggest that the title is changed to better describe the contents, and the lack of physiological studies emphasised in the last part on future research needed.

Lines 467 - 556: I suggest condensing this part and focusing on the most essential. 

Line 495: The Figure 3 is not necessarily needed at all. I would prefer having a photo or two on morphological features and omitting this.

Lines 559 - 560: Dragon's blood is produced also by Calamus rotang (e.g., ref 142).

Line 569: To be exact, the compounds reported in refs 108 and 135 should here be called carotenoids (a broader chemical group that includes carotenes); carotenes do not contain oxygen. One of the sources used "carotene" in the text, which is not correct.

Line 592: spectroscopic

Linse 945 - 985: Lack of studies on basic plant physiology should be mentioned, particularly ecophysiology (also with respect to the current threats and conservation management). This is now only briefly mentioned in the context of secondary growth. For the future studies on the chemistry of the resin, I woud suggest that the lack of studies on genetic and environmental factors affecting the production and composition of the resin would be more important than further characterization of minor components. Furthermore, there is no reason to suggest these particular hyphenated approaches. Personally, I would not start doing this MALDI-based or even worse, with LC-NMR. There are very good reasons why LC-NMR is not commonly used for these kind of purposes.

The review was exhaustive, but one important reference is missing: there is now the first draft genome of Dracaena (Ding et al, mentioned above). You could also check Zhu et al 2016, Scientific Reports 6: 38315, although that is not so much in the focus of this review.

Finally, I would like to thank the authors, the manuscript was a pleasure to read although not flowing so nicely in some parts. I enjoyed particularly the decision to include historical background in a bit more detailed manner. This is not that common in these kind of reviews, but here, it fits nicely because of the cultural relevance.

Reviewer 2 Report

This is an interesting review about what is known of Dragon trees. Its importance goes beyond economics due to its singular habit and botanical characteristics as well as cultural value. It was also a very rich reading covering different subjects (sections) and the review of previous literature is quite adequate.

I am not sure if the unpublished data referred along the manuscript should be mentioned before its publication.

It is important for the readers to be able to find your most important findings along the manuscript and in the conclusions.

I did not spend an inordinate amount of time in proof reading or correcting grammar.

Specific comments are listed below according sections or in general.

Title

I would suggest to change the title to a sentence instead of question type

Introduction - Dragon trees, tertiary relicts in current reality

When referring to the “oldest ecosystems in the world” it is not clear what do you mean, which or where?

Also, the designation “group of species” is not quite clear and is further on used along the text.

I would recommend using also the common names, namely those from the countries where the species still exist and from the countries where these species are from.

It is also refereed the “red resin” as technical and scientific designation but along the text it is rather used “Dragon’s blood”. This should be pointed at some point and why the preference for using such “common name”.

Evolution, taxonomy and distribution

Line 100: “genus Dracaena species”?

Line 118: “type species”?

Line 120: Is this an important differentiation detail, it should be better explained/mentioned or placed in some other text part. It seems lost here.

Line 123-124: Requires reference on DNA analysis

Line 126-127: Is not quite clear.

Line 138: “polinomorphs”? Is this terminology correct!? Palynomorphs would be the most usual!? Verify.

Line 139: “from Miocene from Anatolia”?

Line 143-144: Verb tense. Is or will be ready in the next future?

Line 145-148: Are these two sentences refereeing to the same author?

Line 152-153: Why is this mentioned and related to this paragraph? Please explain.

Line 161-162: This information should be better introduced and related with the review.

Line 180: “grows” is repeated

Line 202: The uncertainty of the existence of the species could not be cleared?

Anatomy and morphology

Line 217: Depends or varies?

Line 234-269: References to species in brackets is not the uniform

Line 330: Here it is mentioned the third stage. However, what about its explanation and the other two stages.

Line 340-341: “This means”, what is your certain? Is it an hypothesis?

Line 346: Please consider changing this title and uniform it along the manuscript, explaining the most accurate term.

Line 350: “D. cochinchinensis” could be mistaken with Dalbergia?

Line 354: If Figure d is showing the chips form the it should be inserted an symbol in the respective figure.

Line 366: Consider rewriting the Figure caption, avoiding “indicated” and “Abbreviations”, use brackets, instead. Please identify the tick-walled fibres.

Growth, age estimation

It should be better stressed out that there are growth stages and growth phases.

Table 1: I do not see the need of a table for this information. It would be more useful to use a schematic figure, for exampling improving the one from reference 75 and showing the leaf rosettes, etc.

Line 399: “accelerates”, nothing is refereed previously.
Growth rate section could be converted to a Table version.
Please explain the standard criteria for height and other characteristics measurements.
Lifespan of ontogenetic cycle content is different from the previous section ontogenetic cycle section? In what terms!? Consider discussing it together. When reading it seems a little bite repeated.
Line 436: “Icod” should be better introduced.
Line 439-440: This is valid for all parameters in general so this could be pointed elsewhere.
Line 447-451: If this is related to the previous work it should be mentioned. Also, what do you mean by highland and montane locality. Clarify.
Line 452-453: This paragraph seems lost, there is no discussion about it at all. Rephrase.
Line 455: These descriptions could be placed early or here in the text and Table 1 should be avoided.

Physiology
The most remote species was mentioned before, it could be more useful in introduction.
Figure 3 it is not necessary and it is not quite understandable it would be more useful to explain it as text.

Line 534-535: Why mentioning this here?
Resin and other ethnobotanical use

The subsections could be renamed and reorganized it is sometimes misleading.
Line 571-578: Some references are not consistent with the list, namely Gupta et al is 105 and 151 and Sun et al is not 111.

The historical section could be more useful if presented firstly.

Line 592: “A Fourier…” sentence does not make sense here, why referring this?

The management and harvesting of dragon’s blood resin section uses “resin” term but in 6.3. section it was not used.

Line 608-610: is quite repeated. Remove.
Line 628: Figure 1 does not refer to harvesting.
Line 630: “other species of dragon trees” – so the main source is D. cinnabari (Line 561) and this could be highlighted here.
“Other ethnobotanical use of dragon trees” This could be renamed or the text rewrite to avoid repeated information.
Line 645-647: Is there any idea/records of the period in civil years?

The references for countries are not inserted for each artefact. Why such a detail “drums” and “smaller drums” when values are not shown, this is relative with no reference value.

Ecology, physiognomy, and composition of plant communities with dragon trees
Consider renaming this section, is not quite clear and for example no life-form schematic is shown
Geographical groups could be represented in Figure 1.
Why the term “chapter”?
Line 678-679: this number and information only appear in section 8 for all the other species. Please rewrite.
Line 687-693: It would more uniform to refer Cape Verde Island here, too as it is done for the other species. The association “Fico gnaphalocarpae-Acacion caboverdeanae” should not be in brackets, for the same reason.
Line 696: Which mountains? Line 793-794 there is a mention, is it the same?
I am not sure if this section should be one section or placed in 2.3.
Line 713-719: The description of the species according geographical group could be presented at the beginning of the section.
This section would benefit if it was replaced by a Table, showing values and associations allowing comparison and easy reading.

Populations, threat, nature conservation
Within this section the author references are more highlighted than the species or characteristics, which is not according the manuscript style till here. Please make it uniform.
Line 804: it was only here that Yemen was mentioned, this should be at first appearance.
Line 810: “D. kaweesakii” was also firstly mentioned here.
The term population is not related to the number of individuals in some cases, line 809 – “Height populations”.
If trees per Km squared is also shown in brackets it would be more readable too.
Line 820: “The least abundant” seems to belong to previous section or maybe it could also be referred early.

Threats to populations and their conservation status section could be renamed for example to “Threats and conservation status”
Line 854: Please introduce/explain the “bottleneck effect”
Line 914: it would be better to rename the geographical groups by the geographical region instead of numbers it was easier to relate with its distribution and so on.
In section 8 it seems there are some details already presented in the manuscript it could be useful a careful reading and rewrite avoiding repeating.

Rescue programs section does not refer to any institution of project reference, are there any? Also, some information could be placed in the future perspectives, for example.

Conclusions and future research needed should be separated since correspond to the main goals of this review. This section is not quite clear a more concise text for both sections would improve the general views of the authors.
Line 956: “few”? Two would be more accurate. Rewrite.
I would recommend inserting a Table with the main characteristics of Dragoon species that are reviewed in this manuscript with the respective references, as it common in some published revisions.

Definitively, inserting this type of Tables is very important for the next readers and future works allowing easy read regarding some data, material and tree's characterization, instead of going back to the original studies.