Remote sensing reveals fire-driven facilitation of a C4 rhizomatous alien grass on a small Mediterranean volcanic island
Abstract. Volcanic islands are special ecosystems for studying biogeographical and evolutionary processes. Occasional disturbance events, such as eruptions, tsunami or big fires, can represent major drivers of such processes leading to biotic sterilisation or major changes in island biotas. In this study, through remotely sensed data, we investigated the intensity and the extent of a large fire event that occurred on the small volcanic island of Stromboli (Aeolian archipelago, Italy) on 25–26 May 2022, to assess the short-term effect of fire damages on local plant communities. For this purpose, two different spectrally sensitive indices, i. e. the differential Normalised Burned Index (dNBR) and the Normalised Difference Vegetation Index (NDVI), were used. The dNBR was also used to quantify the extent of early-stage vegetation recovery, dominated by Saccharum biflorum Forssk. (Poaceae), a rhizomatous C4 perennial grass of paleotropical origin. The burned area was estimated to have an extension of around 337.83 ha, corresponding to 27.7 % of the island surface and to 49.8 % of Stromboli’s vegetated area. On the one hand, this event considerably damaged the native plant communities, hosting many species of high biogeographic interest. On the other hand, Saccharum biflorum clearly benefited from arson. In fact, it showed a very high vegetative performance after burning, being able to exert unchallenged dominance in the early stages of the post-fire succession, reaching within a few months stem density values that are only slightly lower than those of the unburned stands. Our results confirm the complex and probably synergic impact of different human disturbances (recurrent fires, introduction of invasive alien plants) on the structure and the functioning of natural ecosystems on small volcanic islands. The natural dynamics of such ecosystems is dependent on the complex relation between successional processes and the intensity and frequency of natural or anthropogenic disturbance, which can regulate mid- and long-term response of Saccharum. In fact, although the expansion of Saccharum proves to be surprisingly fast, its decline may also be relatively rapid as well, if local vegetation is no more affected by fire. After the abandonment of the agricultural practices in the highest portion of the island, the rewilding process could lead to the replacement of the large beds dominated by this invasive grass by native woody vegetation within a few decades.
Riccardo Guarino et al.
Status: open (until 08 Jul 2023)
- RC1: 'Comment on bg-2023-19', Anonymous Referee #1, 13 Apr 2023 reply
Riccardo Guarino et al.
Riccardo Guarino et al.
Viewed (geographical distribution)
The papers objective is to test the power of relatively simple and straightforward indices (dNBR and NDVI) to map fire intensity, extent and the subsequent vegetation recovery of a recent fire on a Mediterranean volcanic island. The vegetation recovery seems to be dominated by an alien grass species. The title fits this objective and the topic fits the Journals Special issue: "The role of fire in the Earth system: understanding interactions with the land, atmosphere, and society". The language in the study is mainly fluent and precise, however some parts would benefit from rephrasing (specified in the "Specific comments” section). The authors use two in theory well fitting indices for their approach (NDVI and dNBR) to map the extent and to a certain degree the fire intensity. Both indices are explained and used properly. The study is able to show, that the investigated fire boosted the distribution of the invasive alien species S. biflorum, outcompeting native vegetation in the time directly after the fire.
However the structure should be streamlined: parts that are now in the method section rather belong in the introduction, parts that are now in the results belong in the methods part. The mentioned ground data should be explained further. Additionally, the figures / maps need to be reworked. Overall, the study investigates an interesting event and circumstance, but lacks depth, which should be added. It is also not entirely clear to me what exactly is the focus. Is it the fact that you could successfully map fire intensity and extent plus the vegetation recovery or the fact, that S.biflorum is the one species to rapidly and successfully grow on the burned areas. In the methods/ result section, you mainly talk about the mapping, how you created your damage/recovery maps and only very briefly touch on the fact, that S. biflorum could be observed as the dominant species and did fieldwork regarding its ability to colonize the burnt area. This lead me to believe, that the mapping with Satellite imagery is the main focus. However, in the discussion you mainly talk about the S. biflorum rather than your mapping. Some general suggestions for improvement are already given, more in depth comments can be found in the next section. The abstract summarizes the content of the paper well. The Keywords include “field monitoring”, however, the description of the field methods is lacking and should be talked about and explained in more detail, especially if “field monitoring” is used as a keyword.
The introduction overall manages to set the scene for the investigation. Some minor changes I would suggest:
1.1 Is the species Saccharum biflorum an alien or also an invasive alien species? In your short summary you call it invasive alien species, throughout the text you often only lable it as alien.
1.2 I suggest to add the part on S. biflorum (Line 167- 182) from the methods here rather than having it in the methods.
2.1 Sensitivity of the red-edge portion of the electromagnetic spectrum to variations in vegetation cover/health has been discussed and accepted, have you tried e.g. also NDRE rather than NDVI for more precise results? Why did you exactly choose the NDVI?
2.2 Study area section has some unneccessary information (e.g. when the first weather station was installed and when the newer one took over e.g. around lines 92 ff). However, a better overview map over the study area is missing. You reference different landmarks on the island a lot, but give no map including any labeling.
2.3 For the NBR you describe which bands are used to calculate it, for the NDVI this information is missing.
2.4 Maybe include the NDVI maps in the supplements or leave out the NDVI entirely if you do not show any results on it and rather mention, that you tested it but th dNBR worked considerably better. The whole explanation icluding the formula is not needed in my opinion if you don't show any results (at least in the supplements) on it.
2.5 Line 167- 182 rather belong in introduction than in methods. By this time, it should be established, why the species S.biflorum is of interest.
2.6 For me, a map of the location of the carried out fieldwork is missing. Overall, the part about the evaluation of stem density (Line 182-187) was not clear for me: did you collect this data? If you collected the data for this study, I would expect more information on the field sampling (transects? plots? distance between the unburned/burned patches which are compared? How will you compare them? When did you sample it? Location?). You mention the time of field sampling only in the description of figure 3.
Overall, the method part would benefit from a table including what data was used (which satellite images from when/drone images/field data?), what analyses where done with it (dNBR, NDVI...) and for what porpuse (map fire intensity/extent/recovery..). Right now, there is not all the data mentioned in the method section for which results are showin/described. Additionally, more information on the fieldwork and the location of the sampling sites would be beneficial.
3.1 Line 189 f: The part about the fixed threshold should be moved to the methods and explained better. Why exactly this threshold? What made you decide on it ? References? Your own assessment?
3.2 Line 200-206: Rather belongs in methods
3.3 Line 209 f: You describe using Satellite images from August 15th-17th to calculate a new dNBR- those images are not mentioned before in the methods.
3.4 Line 227f : You describe the usage of Satellite images from September 2022 (which again are not mentioned before in the methods)
3.5 Line 232: How did you verify the patches being S.biflorum? Visually?
Overall, the results seem to be a little shallow. I would expect some more analyses investigating a potential relationship between fire intensity and recovery for example. Also the vegetation before the fire compared with the vegetation after the fire (based on NDVI / NDRE maps) could be interesting in terms of vegetation recovery. Additionally, the ground data mentioned in the methods are only plotted, but not analysed/no analyses shown (testing for significant differences eg?). It is not clear to me, to which extent the field work is connected to your remote sensing analyses.
Additionally, in the results are often parts mixed in that belong in the methods. Reread and streamline the part with that in mind, that in the results section only the results derived from analyses described in the methods should be presented.
4.1 Lines 234-240: Move explaining/justifying your method to another part of the discussion. Start with discussing your results in context of other literature.
The discussion focuses on the role of S. biflorum on vegetation recovery. However, the results focus on mapping fire intensity and extent and map recovery. Your field data is simply plotted, no analyses for this data is shown and the field data simply provides proof, that S. biflorum grows quickly after fire in the affected areas. Apart from the short part in the beginning, you don't talk about your methods for mapping anymore. I would expect some comparison to other literature/studies also mapping fire/fire intensities (e.g. Weiser et al. (2021) (also on an oceanic island) or Gibson et al. (2020)) additionally to the in depth discussion about the recovery dominated by S. biflorum.
5.1 Line 293 -298 : You introduce new information, which maybe fits better in the discussion. In the conclusion I expect to find the main takeaway message from the study, not an entirely new idea (S. biflorum being desirable to provide erosion protection).
The maps need to be reworked. Basic necessities for maps are missing (no direction (north arrow), no legend, no scale etc). Additionally, there is too much unnecessary ocean/land mass that is of no interest.
Suggestion: Include an overview map to show the location of the island, and than zoom in and show only the parts of interest (the northeastern part of the island). As those map are the heart of your study right now, they should be much more informative and better.
Again, no north arrow, no scale, no legend. Additionally, to be able to compare, the zoomed in drone image without the overlapping dNBR results added would make the reader able to really compare the damage map with the drone image.
Line 64: unprecise phrasing. It sounds like the Island of Stromboli is the volcano, but as I understand, the island is called "Stromboli" and the volcano on it is Mount Stromboli.
Line 102: mentioning of holm-oak, but missing the scientific name (quercus ilex)
Line 147 f: Rephrase, unprecise phrasing, verify, if "whose" is the right word to use here.
150 f: not good English, rephrase
Line 151: capitalize the first letters for Normalised Difference Vegetation Index (as you did for the NBR).
Line 191: you talk about, how close the fire gets to the inhabited area according to figure 1. However, Figure 2 would be more of a fitting example, as it is easier to see.
Line 484: spaces are missing in-between words
Gibson, R., Danaher, T., Hehir, W., & Collins, L. (2020). A remote sensing approach to mapping fire severity in south-eastern Australia using sentinel 2 and random forest. Remote Sensing of Environment, 240, 111702.
Weiser, F., Sauer, A., Gettueva, D., Field, R., Irl, S. D., Vetaas, O., ... & Beierkuhnlein, C. (2021). Impacts of forest fire on understory species diversity in Canary pine ecosystems on the island of La Palma. Forests, 12(12), 163