Microsites and Climate Zones: Seedling Regeneration in the Alpine Treeline Ecotone Worldwide
Round 1
Reviewer 1 Report
The manuscript summarizes studies on the ATE, detailing how relationships between microsite type and climate are currently influencing and will influence tree regeneration.This is an important topic for review, and the authors describe how these results could influence research and management/restoration in light of climate change.
I can tell the authors improved upon the introduction to the issues (clearly describing why microsites may vary by climatic region) and results (adding information to the tables, making the climate/95% confidence figure clearer and more informative). However, there are key issues that must be resolved to fully support the manuscript's aims and a few more issues that, if improved, will greatly increase the usefulness of this review.
ABSTRACT:
The abstract was well-written--except for a cosmetic issue: "our results" was repeated many times towards the end.
INTRODUCTION:
In lines 62 - 66, the authors make the claim that "the type of microsite most associated with seedling regeneration may vary by local climate." This is claim is the central motivation of the manuscript, yet there are no citations to back up this claim in this paragraph. And because I can imagine any number of situations in which this claim would not be true (for instance, perhaps all microsite types affect tree regeneration in all climates--albeit to varying degrees), citations supporting the claim/motivation are necessary.
METHODS:
In lines 97 - 99, the selection criteria are not clearly justified. For instance, the authors say they chose only one study per genus/climate/microsite type combination, but how did they choose this study? It is not clear whether the authors simply drew a study from each combination at random, if they chose the "most representative" study, etc. This opens the door for biased choices/results, and it leaves the reader in the dark concerning variation within genus/climate/microsite type combinations. Perhaps there was a large range of responses of Picea in Montana to microsite type or perhaps there wasn't, but as-written, I have no idea.
RESULTS:
Overall, I felt the results were much improved. A few minor suggestions and one hopeful suggestion:
In figure 2--does the "shade" microsite type refer to the "object" type?
In table 2--I had difficulty understanding columns 2 and 3. The wording was packed a bit tightly.
One gap I feel these results have is reporting the scale (in time and space) of the studies under review. For example, it seems highly relevant what the range of time-spans and spatial extents were for each microsite/climate combination. This could potentially be added to the supplementary table and greatly increase the interpretability and usefulness of the review.
DISCUSSION:
The discussion was very thorough. My only comment is that the sentence at line 229 is quite long and confusing to read.
Author Response
Please see the PDF.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
forests-601733:
In this study, the Authors present the results of a review and synthesis of studies focusing on tree regeneration at high elevation microsites. I enjoyed reading this paper and appreciate the Authors' efforts to submit a manuscript for peer review that is in good shape. My comments are minimal.
General comments:
General: I recommend reducing the length of many of your sentences, some of which are as many as 4 lines long. Examples are lines 44, 56 and 178.
General: Some changes in font size or type. Is there a reason for this?
Intro:
44: Change While to Although.
64, 65: Recommend removing these sentences
62: Recommend "features/structures on the scale of cm to m that have the potential..."
62: Recommend combining this paragraph with subsequent paragraph
85: I'm not sure if a discussion is an appropriate objective. Perhaps "We were especially interested in evaluating evidence supporting the role of microsites in upward..."
Discussion:
187: Is it possible that the tendency for speci_c microsites in a climate zone is because researchers focus on what is qualitatively understood or observed to be the dominant local microsite
supporting regeneration? That is, sub-dominant microsites may have some effect but are less-studied?
249: Fix reference.
334: Ample for what? See throughout.
Figures and Tables:
Figure 1: Recommend smaller points on the map.
Figure 2: Perhaps change "warm/wet" to "warm & wet" throughout manuscript. Fewer X and Y axis tick labels.
Author Response
Please see the PDF.
Author Response File: Author Response.pdf
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
Introduction
The authors clearly communicated the importance of microsites in Alpine Treeline Ecotone (ATE) tree recruitment/regeneration, and the citations were adequate.
However, the objectives, particularly 2 and 3, were unclear. For instance, it seems likely that many or all microsite types (convex, concave, object, wood substrate) could exist in any ATE on any continent (although whether seedlings occupy these microsites certainly may vary by location), but the objective 2 appears to say that a given microsite will only exist within a certain climate zone. This sort of confusion is prevalent throughout the manuscript and compelled several readthroughs before the intended meaning (which microsite types in which climatic zones most often lead to successful tree recruitment—I think?) could be determined.
Methods
While I appreciated the thoroughness of the literature review, the presentation of the meta-analysis methods was vague, and how they aligned with/answered the objectives was not clear. The most difficult (and most crucial point!) to understand was how microsites were divided into climatic zones (objective 2) and how “upward forest advance potential by microsite type” (objective 3) was assessed. For example, in lines 122 – 125, the authors begin to convey how they evaluated the literature to answer these questions, but again, they appear to say that microsites were evaluated simply by their presence (see comment for objective 2 in Introduction). Similarly, it would seem quite relevant to fulfill objective 3 with questions such as “how many seedlings per microsite type, per climate type did each study find?” or “what was the likelihood of positive tree recruitment by microsite by climate type?” These would have produced interesting quantitative answers and provided support for conclusions regarding objective 3—as it is, the authors seem to rely only on the existence of a microsite in a climatic zone as evidence for upward forest potential…
The placement of the literature review result tables in the Methods was confusing: it was unclear whether the authors created the delineations (especially for Table 2) or if the authors derived them from literature. Also, these tables appeared to be important results in and of themselves that could have easily been placed and outlined in the Results section.
Results
The Results provide little or no support for and do not align with objectives 2 – 3. Again, it seems the authors are saying a given microsite type only exists in a certain climatic zone. After several rereads, I believe the authors are saying certain microsite types (e.g., object) are positively associated with tree recruitment in certain climatic zones (e.g., for object microsites, warm/dry climates are best for tree recruitment), but this is not at all clear from an initial read of the manuscript.
Also, using the term “threshold” here and in the Methods does not appear warranted: all the authors show is separation of microsite/climatic types by arbitrarily-drawn lines. Further, because the “warm/wet” zone had n = 0, can that climatic zone be considered to exist at all in regards to microsites?
Discussion
The Discussion is well-written and thoughtful regarding the literature and the importance of microsites. But at the same time, the Discussion is where the authors seek to answer objective 3—but there is little justification for this due to lack of support in the Methods and Results.
Reviewer 2 Report
In upper mountain forests and the Alpine Treeline Ecotone (ATE) microsites are indisputable of significant importance for successful tree recruitment. This in turn is a prerequisite for the regeneration of forest stands as well as the expansion of forests into higher altitudes under climate change. Consequently, a global study of the importance of microsites for tree recruitment in the ATE is highly relevant to better understand the factors governing potential forest expansion in a warmer and drier future climate.
The authors have conducted a literature search regarding this topic and incorporated 52 studies into the data analysis. Since microsites are defined as “all local site features/structures serving to ameliorate local seedling regeneration stresses” (line 63-64) only those studies reporting a positive effect of microsites on recruitment were considered. Beforehand microsites were classified into four broad categories: wood, convex, concave and objects. These categories were further differentiated by location and tree genus present. The main question addressed was, whether there is an association between annual temperature and precipitation and certain types of microsites. Following, the authors aimed at assessing the upward forest advance potential by microsites.
Generally, I rate the paper worth to be published in Forestry. It is concisely written, addresses an issue interesting to the readership and draws on a substantial database. Nevertheless, in my view, there are several shortcomings needing amelioration.
I regard the restriction to studies which have reported positive effects of microsites as a major shortcoming of the study. This will most probably lead to an over-estimation of the importance of microsites. Rather, it would be more important to get to know the probability microsites favour upward forest expansion and additionally to estimate their absolute contribution to the overall expansion rate. To this end, however, also studies which did not find a positive or even a negative effect of microsites on tree recruitment in the ATE have to be considered. As a prerequisite, the definition of microsites needs to be revised. If microsites would be defined as local features/structures which have the potential to alter the environment for seedling recruitment significantly positive as well as negative and missing effects are enclosed. For instance, concave microsites may have negative effects in cold/wet climate because snow melt is delayed and infection by snow mold may be increased.
Furthermore the study submitted lacks transparency in respect of the deduction of the results. Although a reference list is given in the supplement there is no further information available about the contents of the studies. I miss a table presenting details of the studies to be able to comprehend how the results were achieved (information per study incorporated might be: continent, temperature, precipitation, type of microsite, tree genus, ….).
To improve clarity the methods chapter should be complemented with an even more detailed description of the parameters extracted from the literature. As far as I have understood, a typical meta-study using standard deviation and mean values as basis to deduce effect size has not been conducted. Instead, did you use binary data (microsite type present: yes or no)? Were the results of the statistical analysis of the studies taken into consideration (kind of inference tests applied, significance level, …)? Additionally, it remains unclear which type of ANOVA was applied, and whether normality and homogeneity of variances were observed as prerequisites.
Also, the results chapter offers only rather broad information about the outcome of the statistical analysis. Results are mainly presented about the effect of temperature and precipitation on the occurrence of microsites. There are no results available regarding the effect of location (continent) and/or tree genus. Especially the latter would be of high importance as it can be assumed that microsite effects are strongly correlated with tree species or genus, respectively. Finally, the method of exactly separating the four climatic types remains unclear. It seems that they have been separated visually between the confidence intervals.
The following remarks should also be considered:
line 100: I do not understand what you mean by “… associated with microsite association …”
line 109-110: Since animal grazing is a dominant factor controlling treelines in many regions worldwide it needs consideration at least in the discussion.
line 114: In how many cases was it necessary to add climate data from other sources? Are data pooled with different observation periods? In my eyes it is advantageous to use a comparable source for climatic data based on a standard observation period, e. g. WorldClim Database.
line 130: These are genus not species
line 139-140 in combination with Figure 3: The abszissa and ordinate to not span the whole climatic range.
lines 172-191: It is not always clear on which basis these detailed causal relationships are deduced. Have typical relationships been systematically extracted from all 52 publications analysed? I would like to recommend to add references to all statements made. For instance, the following statements in lines 173-175 are not underpinned by concrete citations: “… for sites in cold/wet locations, there is limited carbon uptake (carbon assimilation) of seedlings due to cold temperatures and time under snow, and these stresses are ameliorated by convex sites. Further, cold/wet locations may be infested by snow mold.”
lines 255 ff: Mainly, the conclusions chapter has the character of a summary. The last practical conclusions in the discussion chapter and the conclusions for the validity of models could be implemented here. In my view the summarizing sentences are obsolete.
