This is a very nice and elegant study and the geographical link makes the manuscript suitable for publication in BG in the editor's opinion, even if the journal does rarely pubhlish genetic studies.

Here are some minor technical revision that the editor asks the authors to correct before final submission to BG

Page 2 – line 10: followings: => following:
Page 2 – line 11: Surtsey has => Surtsey had [It is customary to quote own results in past tense]
Page 2 – line 19: and this appears => and this appeared
Page 3 – line 3: of the frigid North Atlantic Ocean => of the North Atlantic Ocean [this was an unnecessary word]
Page 3 – line 5: just 3 nautical miles => [Give this distance in km, please. Nautical miles is not a SI unit and most people are not familiar with them]
Page 3 – line 17: The study species, Honckenya peploides, was first recorded on the island in 1967, 26 individuals (Fridriksson and Johnsen, 1968). The plants survived the first winter and the species has since grown on and dominated the vegetation of sandy habitats on the island to this day (Fridriksson, 1970; Magnússon et al., 2009 and 2014). => The study species, Honckenya peploides, was first recorded on the island in 1967, 26 individuals (Fridriksson and Johnsen, 1968) and the plants survived the first winter (Fridriksson, 1970). [The flow of the story was interrupted by jumping back and forth in time – better to tell the colonization story first and then state what is the status of the species today...]
Page 3 – line 23: total of 548 H. peploides individuals were recorded indicating an increase in seed producing plants on the island. => total of 548 H. peploides individuals were recorded indicating the first episode of seeding of the first colonizers on the island [is this what you were trying to say?]
Page 3 – line 24: ... on the island. => H. peploides has since become a key species in plant communities of most sandy habitats on the island (Magnússon et al., 2009 and 2014). [it has actually lost to other species in sandy habitats where N availability has been increased by seagull activity – so I suggest that you say „most sandy habitats“]
Page 4 – line 8: with ecosystems in higher succession stages => with ecosystems of higher successional stages [or: higher successional seres]
Page 4 – line 9: these islands provide, => such areas provide, [you were not only talking about islands earlier!]
Page 5 – line 5: with decrease in island size => with decreasing island size [??]
Page 5 – line 8: topographic capacity => topographic heterogeneity [?? – I don‘t remember Whittaker using the term „topographic capacity“??]
Page 6 – line 7: In the following study => In the present study
Page 7 – line 5: under study Honckenya => under study, Honckenya


Page 7 – line 11 m above sea level (masl) => [you don‘t need to define m a.s.l. specifically. But to avoid using the a.s.l. I suggest that you just write it out as you do the first time, and after that you just say „m“ => please remove „masl“ elsewhere.]
Page 7 – line 15: volcanic tuft => volcanic tuff [I believe that the more correct term is „tuff“, even if „tuft“ occurs infrequently in written English...]
Page 8 – line 10: Five locations are on Surtsey: => Five locations were on Surtsey: [please use past tense when you discuss your own study and findings – correct this also elsewhere in the text in this sub-chapter!]
Page 16 – line 16: can be seen => could be seen [please use past tense when you present and discuss your own results]
Page 17 – line 4: A similar pattern... => [It is not considered a good style to start a paragraph with such a link to what was said earlier... If it is a direct continuum of what was said above this should not be a new paragraph... Rephrase or state „similar to what“...]
Page 17 – line 6: (results available on request) => [The authors should consider to submit this data as „supporting material“ when submitting the final version of the manuscript – and then you can refer to that here]

Page 17 – line 7: shows => showed [please go through your WHOLE Results chapter and correct to past tense where needed]. The editor gives up correcting this everywhere from now – but check also earlier sections.

Page 18 – line 5: present study is from Surtsey. => present study was from
Surtsey. [please go through your WHOLE Discussion chapter and correct to past tense where needed]. When you quote the results of the present study you should keep using past tense – only when you quote published results you can use present tense...

Page 19 – line 9: to dispersal in this plant => to dispersal for this plant
Page 20 – line 9: may have also been => may also have been
Page 20 – line 18: that facilitates gene flow between populations. => [??? I don‘t understand this last statment – skip? – how does introgressive hybridization lead to more gene flow between local populations (of the same species at a place like Surtsey)? I understand this as you are referring to some process leading to gene flow – but not just that introgressive hybridization would likely increase the total genetic variability within a species as a whole ???]
Page 21 – line 18: (Branta bernicla hrota) => (Branta bernicla ssp. hrota)

Finally, the editor asks the authors to make one last check that all quoted references are found in the reference list and vice versa...

The handling editor thanks the authors for changing the manuscript according to the comments made earlier. He has no further suggestions and now accepts the manuscript for publication in Biogeosciences.
Your
Bjanri D. Sigurdsson