Impact of Invasive Tree Species on Natural Regeneration Species Composition, Diversity, and Density

Round 1

Reviewer 1 Report

Review: general

This paper addresses a novel, interesting, and timely question: What is the impact of the three most frequent invasive tree species in Europe on natural regeneration diversity, species composition, and density? The authors hypothesized that invaded forests would differ in terms of species composition, have lower taxonomic, functional and phylogenetic diversity of natural regeneration, and have lower densities of native tree species.

The introduction does a good job justifying and stating the research objectives. The last sentence of the introduction, “Due to well-recognized patterns of invasive tree species natural regeneration in the study plots, we decided to not analyze them again”, however, is confusing. If invasive species regeneration is excluded from the paper, why do the names of invasive species appear in the ordination diagram (Figure 2)? It would seem invasive species regeneration was included.

The methods section has some weak spots. The choice of DECORANA as an ordination method is to be questioned, as many of those working with community data advocate non-metric multi-dimensional scaling (NMDS) as a better method than DECORANA. The authors need to justify their choice. Moreover, the authors never tell us exactly what units were ordinated and what attributes (species presence, species abundance? What units of abundance?) were used to ordinate them.

The results section could be improved by making general patterns more explicit. For example, see the comment below about lines 246-247.

The first paragraph of the discussion identifies two issues causing low differences in species composition. In both cases the issue is identified but the consequences are not made explicit.  Why is it a problem if vegetation varied continuously or failed to precisely match know community types? Does this mean greater residual variation and thus less chance of detecting difference? Explanation is needed. The second issue is of greater concern. If plots in stands with uninvaded canopies had invasive regeneration, perhaps invasive regeneration should have been use as a predictor variable.

 

Specific comments

Line 26. “types, however,” should be “types; however”

Lines 28-31. This should be written with greater clarity.

Lines 62-64. Either not clear or contradictory. Does this mean that invasive plant regeneration was not studied in this paper? The methods seem to indicate that all regenerating trees and shrubs were included in the sampling, and the ordination includes invasive species names.

Line 68. “18th, 19th and 17th”. This should be progressive, no? “17^th, 18^th, 19^th”

Line 88.  Exactly what is meant by “regularly arranged”?

Lines 89-90.  How were the stands (i.e., “blocks”) chosen? What were the criteria?

Line 92.  “middle” is vague. Do you mean “center”?

Line 108. What is meant by “dominant”? Was some specific abundance threshold applied? Was it simply the most abundant species? If so, how was abundance assessed?

Lines 116-117. “The distinguished forest types represent a semi-paired design, as invaded and non-116 invaded types occupying similar habitats.”  I don’t understand this. Moreover, it is not a sentence.

Line 123.  Table 1. What are the gray shaded areas? If they are meant to highlight alternate classes, the first class should be gray. Right now the first two classes are both unshaded.

Line 129.  Including both native and non-native species?

Lines 142-151. As NMDS is widely regarded as a better ordination procedure, you should include a justification for the use of DECORANA. Also, what units were ordinated (plots? Stands?) and what attributes (species presence, species abundance? If the latter, what units of abundance?) were used to ordinate them.

Line 145. “analyzes” to “analyses”

Line 153-155. Were these indices calculated using all species – native and invasive – or just native?

Line 196. The sentence: “This analysis allows to describe the dissimilarity among groups…” is missing something, likely “us” after “allows”.

Lines 202-203. What is the basis for the conclusion that DCA 1 is a soil fertility axis. Canopy interception seems to be a better correlate here. In fact soil factors seem to relate better to axis 2.

Line 215-217.  What is this ordination based on? Legend should give sample size, entities ordinated and attributes used. Explain what species names represent (are these centroid locations??)  What were invasive species include (last sentence of introduction says they were not included).

Lines 224-225. “Invaded and non-invaded stands differed in species richness the most in P. sylvestris plantations – invaded..”  This is awkwardly written. Could say “differed most strongly”.

Line 227.  What exactly does “they” refer to?

Lines 227-228. “Similarly, forests with Q. rubra had 7.4±0.8 species, while Q. petraea had 9.4±0.7.”  Were these two forest types significantly different in this variable?

Lines 246-247. “Analyses of regeneration densities for particular tree species revealed that most of the dominant or co-dominant tree species reached higher densities in their native optimum forest types than in invaded forests.”   In this section a general statement is made using a qualitative term “most” and then examples are given. It would be much better to give a general quantitative comparison. Instead of “most”, give the actual proportion of dominant or co-dominant tree species that reached higher densities in their native optimum forest types than in invaded forests.

Lines 266-268. “Therefore, our study design covered a continuum of plots from invaded to non-invaded, as well as light availability and soil fertility gradients.” More explanation is needed to explain why this is a problem. Light and soils were treated as continuous variables in the analysis; perhaps canopy species composition should have been also been treated as a continuous variable instead of as discrete types.

Lines 268-270. “A more important factor is spread of invasive tree species natural regeneration to the non-invaded plots. High densities and biomass of invasive tree species [50,51], recorded also in plots with non-invaded canopies, contributed to the biotic homogenization of regeneration layers [84,85].” If density of invasive regen might have influenced native regen, then perhaps invasive regen should have been used as a predictor variable in the analysis.

Line 306. Subheading is grammatically incorrect. Use “regenerate”.

 

 

 

 

Author Response

Dear Reviewer,

we are thankful for thorough and valuable commenst. In the attached file we listed our point-by-point response to each remark

Best regards

Marcin Dyderski

Reviewer 2 Report

The manuscript of Dyderski and Jagodziński is the outlet of a nice study, and most of all, a well-thought and well-written paper. I liked that the authors have already answered most of the potential questions raised while reading the manuscript. The analyses are complex, they present from different perspectives and in a comprehensive way the faith of seedlings and saplings of woody species in invaded and non-invaded forest stands.

 

I have only a few questions and comments.

1. Study design: your design was clearly invented to study the effects of decreasing propagule pressure on invasive tree species’ natural regeneration, thus show a transition from the ”core” invaded sites, through “ecotones”, towards “clear” sites. I think you should analyse your data accordingly: not just invaded and non-invaded, but with a “transient” category as well. Or if you want to stick to your ideas, then I would drop the transient plots form the dataset. This sampling design contribute significantly to your main conclusion: “there is continuity in species composition of the native tree’s natural regeneration”. I suggest to include a series of analyses in the appendix without the “transient” plots.
2. I miss the values of light availability data from spring, as seeds have chances to germinate before the canopy closes. I expect different values of light availability from those measured during summer: e.g. Pinus has evergreen leaves, older forest stands might have denser branch architecture etc. It would probably be much more important than light availability during summer. Besides, the seedlings and saplings were counted in different year than the measurements of light. I know the authors don’t have the right measurements, but please, at least discuss these aspects.
3. It is not clearly presented, how the invasive stands were formed. The case of Prunus serotine is straightforward, but Robinia and Quercus rubra show spontaneous invasions or they were planted in these sites? It they were planted initially, it would be nice to discuss the differences between a native (P. sylvestris) and alien species planted in these forests.
4. The authors published several paper on natural regeneration of invasive species. I think the density of invasive saplings might influence the faith of native seedlings too. So, my question is, have the authors considered to include the density of dominant invasive’ sapling density as covariate in the analyses?
5. Robinia, but also some of the native species (e.g. Fagus) have prolific clonal growth. How could you differentiate between seedlings (germinated from seeds) and clonal shoots? No word was mentioned about the clonality of these tree species.

Minor comments:

L. 58. Please list here similar studies which deal with the regeneration of native species influenced by alien tree species to be able to integrate your work in the specific context.

L. 58. “natural regeneration diversity, species composition and density”. I know your refer to woody species’ regeneration and not of all plants’, but you have to make it clear.

L. 62-64: I prefer that these results to be presented briefly before hypotheses, because they are needed to understand your results.

L. 73. At first mention please indicate the whole name of the species: Pinus sylvestris

L. 67-77. The species are strangely presented, picking seemingly random information about the species. I suggest to stick to their behaviour in invaded range and show that they are indeed functionally different. And present information that are useful for understanding your results.

L. 91. Established

Throughout the text: I am not a native English speaker, but I think that the correct form would be “tree species’ natural regeneration”, with apostrophe.

Table 1: what is “DIFN”, explain abbreviation

Fig. 3. MNTD: Are there all “a” letters? This is the case described in lines 178-181?

Author Response

Dear Reviewer,

we are thankful for thorough and valuable commenst. In the attached file we listed our point-by-point response to each remark

Best regards

Marcin Dyderski

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Dear Authors,

Thank you for your input, I am fully satisfied with your answers to my comments, and I like the improved version of the manuscript.

I have only three minor-minor comment:

l. 96: surroundeed, delete an “e"

l. 100: established, not stablished

l 310-311: I don’t understand this sentence “....led to overstory dominant-specific impacts of particular native tree species.” Please rephrase.