Can Larix sp. Mill. Provide Suitable Habitats for Insects and Lichens Associated with Stems of Picea abies (L.) H. Karst. in Northern Europe?

Round 1

Reviewer 1 Report

The authors have conducted a study that assesses the suitability of introduced larch species for creating habitat for insects and lichen associated with Norway spruce. This study is relevant, interesting, and of suitable scope for Diversity, but could use some edits to make the story a bit clearer and more impactful. Overall, I commend the authors on a good first draft of this manuscript and offer some specific comments to help further improve the manuscript:

Lines 29-31: Do you have a citation to support this? I’m not questioning the accuracy of the sentence, but by saying “expected to have profound consequences” sounds like research has been conducted that indicates so.

Line 34: More vulnerable to what if it’s already being affected by drought and pests and pathogens?

Line 36: “In north temperate and boreal European forests, the most economically, ecologically, and spatially important and abundant…”

Line 42: “…and the mountain ranges of central Europe.”

Line 56: pronounced

Lines 57-58: “…vulnerability of P. abies to abiotic and biotic damage requires special attention”

Line 75: Can you speak to the successional dynamics between Larix and Picea? Is larch an early successional species there that flourishes after a disturbance and shades young spruce that eventually outgrow the larch?

Lines 78-79: “Several Larix species were cultivated in forest stands (L. leptolepis, L. decidua and L. polonica), but L. decidua ssp. Polonica Ostenf et. Syrach showed the best growth rate…” Was this part of this study, another study, or just something that forest managers tried?

Lines 84-86: “A large-scale planting of introduced tree species instead of native species requires evidence-based evaluation [30]. In addition to productivity and adaptability or resilience to climate change, other factors, such as impacts on native flora and fauna communities, should be considered.”

Line 88: “such as fungi, lichens, and insects.” No need to say “e.g.” when you have “such as”

Line 89: What do you mean by “as biodiversity elements”?

Line 100: Change “showed” to “demonstrated”

Figure 1. In the description, italicize Larix. Were the Larix stands always on the south side and spruce stands on the north side of the site or does the coloration not indicate where the stands were in relation to one another?

Lines 116-119: “ Identification of larch species is problematic due to frequent hybridization [50-51], so, in this study, they are referred to as Larix sp. At each site, there was one P. abies and one Larix sp. stand, which were within a radius of 200 m, so they were within the same geographical area and had similar climatic conditions.” Was the topography fairly consistent in these areas too so you wouldn’t have differences in aspect or slope?

Line 148: “…Larix sp. trees were the prevailing species at the site…”

Line 162: Both traps were placed at the same height above the ground? Why was this one height used?

Line 169: Why were insects only identified to a “higher level?” What is this level? Order? Family?

Line 172: tree stems

Lines 189-190: “…, but to the family or genus level for other specimens.” What would be causing confusion here?

Lines 199-200: So, the lichen specimens were identified to species level? If that is not correct, please elaborate more about the level at which you identified the lichen specimens.

Line 210: “differed among different tree species.”

Line 214: if you did not identify all insects to species, you can’t say that there were 76 different species. You identified 76 different insect taxa.

Line 315: “The other most common lichen species…”

Line 318: Is there a way to identify these unknown lichen specimens, especially the specimens that have a fairly high relative abundance (e.g., Unidentified sp. 1)? At least to the family/genus level? The manuscript might benefit from having a panel of images of some of the most common insect and lichen taxa that were recorded during the study.

Lines 419-439: Can you relate the similarities and differences between xylophagous insects on larch and spruce to evolutionary history? For example, compared to the lichens and surface dwelling insects, woodboring beetles have to overcome plant defenses to successful penetrate the bark and feed on the phloem to create galleries and exit holes. There must be a more intimate relationship between the insects and their hosts, which may also drive some differences that you found.

Lines 451-453: Do you have some data from older, more dense stands for comparison?

Lines 471-476: But there will still be some woodborer species that cannot make the jump between the host species and may be lost if spruce mortality increases.

General comment: It would be interesting to compare the diversity of taxa in these mixed stands to the diversity of insects and lichens in a pure stand of spruce to see if there is a difference. Perhaps the mixed stands make it more difficult for spruce specialists to thrive, thus most of the species are more generalist species that can survive on either larch or spruce? What spruce specialist species will truly be lost if spruce is functionally eliminated from its native range?

Author Response

Dear Reviewer,

Thank you for your detailed comments.

Comments indicated in lines: 34, 36, 42, 56, 57-58, 78-79, 84-86, 88,89, 100, 148,172, 189-190, 199-200, 210, 214, 315 were corrected and marked with “track changes” at the main text.

Comment:

Lines 29-31: Do you have a citation to support this? I’m not questioning the accuracy of the sentence, but by saying “expected to have profound consequences” sounds like research has been conducted that indicates so.

Response:

Citation was added to the text.

Comment:

Line 75: Can you speak to the successional dynamics between Larix and Picea? Is larch an early successional species there that flourishes after a disturbance and shades young spruce that eventually outgrow the larch?

Response:

The following information was added to the text: “Larix is an early successional tree species and after disturbances such as large-scale windfall, it is able to establish on Picea sites [35]. Larix is much more light demanding tree species as compared to P. abies. Besides, it requires large openings for regeneration and juvenile growth, while P. abies can regenerate in much smaller gaps or under the canopy [36]” (Lines 78-82).

Our comment: observations show that after windstorms, forest fires or other natural disturbances in Lithuania, larch stands do not recover by themself, including those areas where spruce was growing before the disturbance.

Comment:

Figure 1. … Were the Larix stands always on the south side and spruce stands on the north side of the site or does the coloration not indicate where the stands were in relation to one another?

Response:

The coloration does not indicate where the stands were in relation to one another.

Comment:

Lines 116-119: “ Identification of larch species is problematic due to frequent hybridization [50-51], so, in this study, they are referred to as Larix sp. At each site, there was one P. abies and one Larix sp. stand, which were within a radius of 200 m, so they were within the same geographical area and had similar climatic conditions.” Was the topography fairly consistent in these areas too so you wouldn’t have differences in aspect or slope?

Response:

Yes, the topography was fairly consistent in the sites. The explanation on the site topography was added at the text (Lines 126-127).

Comment:

Line 162: Both traps were placed at the same height above the ground? Why was this one height used?

Response:

Yes, both traps were placed at the same height of ca. 1.5 m above the ground to prevent the interference from grasses and shrubs. This was specified in the text (Lines 168-170).

Comment:

Line 169: Why were insects only identified to a “higher level?” What is this level? Order? Family?

Response:

The following was added to the text: “Many insects were identified to a species level, while others to the order, family or genus level. Several insects remained unidentified, which was largely because these were missing body parts or were heavily covered by glue from sticky traps, thereby making reliable identification impossible.” (Lines 177-182).

Comment:

Line 318: Is there a way to identify these unknown lichen specimens, especially the specimens that have a fairly high relative abundance (e.g., Unidentified sp. 1)? At least to the family/genus level? The manuscript might benefit from having a panel of images of some of the most common insect and lichen taxa that were recorded during the study.

Response:

Unfortunately, we do not have the possibility to identify unidentified lichen species, because the samples taken during the study have lost their characteristic morphological features, and also, we do not have photos of appropriate quality. We agree that such information would be useful.

Comment:

Lines 419-439: Can you relate the similarities and differences between xylophagous insects on larch and spruce to evolutionary history? For example, compared to the lichens and surface dwelling insects, woodboring beetles have to overcome plant defenses to successful penetrate the bark and feed on the phloem to create galleries and exit holes. There must be a more intimate relationship between the insects and their hosts, which may also drive some differences that you found.

Response:

The following information was added to the text: “Therefore, slight differences in the composition of xylophagous insects between Larix sp. and P. abies trees can probably be explained by certain host specificity. Xylophagous beetles colonizing fresh wood or dying trees need to overcome the tree resistance in the form of chemical barriers [123], and therefore, are much more host-adapted than those of later decomposition stages [102]. Interestingly, Muller et al. [124] showed a low rankings of Larix decidua as the hosts, which was due to a generally lower number of herbivorous species, including saproxylic beetles, colonizing this tree species as compared to other coniferous tree species such as e.g., P. abies” (Lines 457-466).

Comment:

Lines 451-453: Do you have some data from older, more dense stands for comparison?

Response:

Unfortunately, we do not have such data for comparison.

Comment:

Lines 471-476: But there will still be some woodborer species that cannot make the jump between the host species and may be lost if spruce mortality increases.

Response:

The following information was added to the text: “However, the possibility should not be excluded that some wood boring insect species will not be able to jump between the host tree species and may be lost if the mortality of P. abies would drastically increase in the future.” (Lines 503-506).

General comment:

It would be interesting to compare the diversity of taxa in these mixed stands to the diversity of insects and lichens in a pure stand of spruce to see if there is a difference. Perhaps the mixed stands make it more difficult for spruce specialists to thrive, thus most of the species are more generalist species that can survive on either larch or spruce? What spruce specialist species will truly be lost if spruce is functionally eliminated from its native range?

Response:

We acknowledge this observation and agree that this would be an interesting comparison, which we will consider in our future studies.

Reviewer 2 Report

The aim of this study is to determine if Latrix sp. could provide suitable habitats for Picea abies associated organism under of climate change.  

For this, different sampling, and statistical methods (Ordination, Anova) are applied to compare the insect and lichens community structure of the two tree species. Sampling, analysis and the presenting of results are done in a solid and convincing way. The introduction and discussion are a bit too wordy in my opinion but still in an acceptable limit.

I have doubts that the study really can answer whether Latrix sp could really provide a suitable habitat for Picea abies under a changed climate. Why should insect and lichen species not respond to climate independently of its host tree? It is not conclusive to me assuming that climate will change the occurrence of the host trees present but not change the biology/ecology of organisms associated to a specific host tree.  In short, that Latrix can provide suitable habitats for Picea associated organism nowadays does not necessarily mean it will under changes climate conditions. I think that should be discussed by the authors. 

My main concern is the missing species ID (identification) for parts of the sample. 26 out 96 most abundant species sampled using stick traps are not identified to species level. Some of them just to family level.  I did not check it for the whole table S1 but for some samples (e.g. S1, S3, S5, S10, L2, L3) those taxa without species ID are with >50% the far most abundant species. Similar, only 6 of 14 taxa of insects detected in bark sheets or by exit holes were identified to species level. I would not feel safe to conclude about insect habitat those kind of data. Being an entomologist by myself (but with little knowledge on dead wood insects in particular) I know how tricky it is to get species ID from studies like this (e.g. having many larvae sampled). But I really would like to encourage the authors to use the barcode approach. I would suggest barcoding subsamples of those taxa to get information on species ID and hopefully an impression how many species are hidden in the taxa assigned as "family sp."  If the barcode approach is not possible to the authors, they should explain why and explain the limitation/restriction of the dataset in the discussion. I do not doubt that community ecology can provide useful insights in ecological processes (as habitat choice in dead wood insects) but in the end of the day it is the species that acts/reacts/respond. Thus, the study would improve and getting more substantial having more taxa identified to species level. I also was wondering how many experts for insects and lichens were contacted to get the difficult taxa identified (according to M&M and acknowledgment: none).

Minor comments:

-      Line 360: rewrite “tree species-species insect species”, for example with host tree specific insect species.

-      Line 348: I am sure there more relevant/general references than 69-70   

Author Response

Dear Reviewer,

Thank you for your comments.

Comments indicated in Lines 348 and 360 were accepted, corrected, and marked with track changes at the text.

Comment:

I have doubts that the study really can answer whether Latrix sp could really provide a suitable habitat for Picea abies under a changed climate. Why should insect and lichen species not respond to climate independently of its host tree? It is not conclusive to me assuming that climate will change the occurrence of the host trees present but not change the biology/ecology of organisms associated to a specific host tree.  In short, that Latrix can provide suitable habitats for Picea associated organism nowadays does not necessarily mean it will under changes climate conditions. I think that should be discussed by the authors.

Response:

It is possible that insect and lichen species would respond to climate change independently, but these species would still need a habitat. In case P. abies would disappear in the area, these insect and lichen species, especially more specifically associated with P. abies would need to find a new habitat or disappear.

However, we agree that several insect species associated with P. abies will not be able to adapt to Larix sp. On the other hand, insects are known to adapt to changing conditions faster than trees in the course of evolution and might inhabit other tree species such as e.g., P. sylvestris and not Larix trees.

The explanations were added in the discussion Lines 451-458 (marked with track changes).

Comment:

My main concern is the missing species ID (identification) for parts of the sample. 26 out 96 most abundant species sampled using stick traps are not identified to species level. Some of them just to family level.  I did not check it for the whole table S1 but for some samples (e.g. S1, S3, S5, S10, L2, L3) those taxa without species ID are with >50% the far most abundant species. Similar, only 6 of 14 taxa of insects detected in bark sheets or by exit holes were identified to species level. I would not feel safe to conclude about insect habitat those kind of data. Being an entomologist by myself (but with little knowledge on dead wood insects in particular) I know how tricky it is to get species ID from studies like this (e.g. having many larvae sampled). But I really would like to encourage the authors to use the barcode approach. I would suggest barcoding subsamples of those taxa to get information on species ID and hopefully an impression how many species are hidden in the taxa assigned as "family sp."  If the barcode approach is not possible to the authors, they should explain why and explain the limitation/restriction of the dataset in the discussion. I do not doubt that community ecology can provide useful insights in ecological processes (as habitat choice in dead wood insects) but in the end of the day it is the species that acts/reacts/respond. Thus, the study would improve and getting more substantial having more taxa identified to species level. I also was wondering how many experts for insects and lichens were contacted to get the difficult taxa identified (according to M&M and acknowledgment: none).

Response:

Yes, among your mentioned >50% dominating insects, which were identified only to a family level was Ichneumonidae and Eucnemidae in particular sites. Ichneomonidae in our study was not target insects as they are not tree steam colonizers. Therefore, they were important group as a possible parasites of wood colonizing insects acting in the community as a whole. Their identification to a family level provides such important information, when the prey was missed (not trapped by the sticky traps). Other example for Eucnemidae: small beetles of this family were difficult to distinguish when they have the body damages during the trapping using sticky traps (the main drawback) and the species-specific morphological signs as e.g., the form of antennae, became unclear. To avoid identification errors, those insects were identified to a family level. We consider that insect identification to the family or genus level provided information on their lifestyle and possible habitats.

We expanded the description of insect identification in M&M, Lines: 175-179.

We agree that the exit hole and bark sheet methods not always lead to successful insect identification to the species level but provide the information on the level of tree colonization/damage. In our study, we presented results about the dead spruces and larches, and analyzed, which tree species was more vulnerable/attractive for wood borers.

We agree that DNA sequencing is generally considered as a reliable, cost-effective, and easy molecular identification tool with a wide applicability across metazoan taxa. However, DNA sequencing may also suffer from a number of potential limitations when used for the identification of insects. The recent speciation, the prevalence of paraphyly and the regular interspecific hybridization in many insect taxa, as well as their often poorly established taxonomy and their high degree of infection by endosymbiotic bacteria such as Wolbachia may all negatively affect the performance of insect DNA sequencing. Even more importantly, the reliability of insect DNA sequencing may be questioned because insects include >1,000,000 described species and probably millions of still undescribed taxa.

The following was added to discussion: “However, to increase the accuracy of species identification additional methods such as DNA sequencing may be needed as for several insects trapped the species identity could not be established using morphological methods (Table 3).” (Lines 374-377).

Regarding experts: one of the best experts on identification of forest insect species in Lithuania is among the authors of the presented manuscript. However, when there was doubt about some species, we consulted with experts at the National Zoological Museum, which was now included in the acknowledgments Lines: 522-523. Besides, as a result of collaboration between our group and an expert at the National Zoological Museum, a new insect species was described for Lithuania (Lynikienė, J.; Tamutis, V.; Gedminas, A.; Marčiulynas, A.; Menkis, A. First Report of the Larch Longhorn (Tetropium gabrieli Weise, Coleoptera: Cerambycidae: Spondylidinae) on Larix spp. in Lithuania. Insects 2021, 12, 911. https://doi.org/10.3390/insects12100911

Round 2

Reviewer 2 Report

The authors corrected and improved the manuscript according my comments.  I think the overemphasize the pitfalls of the barcoding approach in their comments but the added a note to this approach in the revised version.