the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Feb 2025
Treeline species Betula ermanii are more adaptable to alpine environments than non-treeline species Picea jezoensis: evidence from leaf functional traits
Abstract. Understanding functional trait differences between treeline and non-treeline species is key to exploring their adaptive strategies under environmental stress and predicting subalpine forest dynamics. On Changbai Mountain, Betula ermanii dominates over 90 % of the treeline zone, while Picea jezoensis accounts for over 70 % of the lower elevation zone. It remains unclear whether P. jezoensis, a treeline genus elsewhere, would eventually shift upward and replace B. ermanii. We thus investigated leaf functional traits, their intraspecific variation, and inter-trait relationships for both species along the elevational gradient. B. ermanii exhibited higher LDMC, N, P, and gs, but lower WUE and δ18O at higher elevations, with the greatest intraspecific variability in photosynthetic and hydraulic traits, and tighter linkages among traits. In contrast, P. jezoensis exhibited an increase in δ13C and a decrease in SLA with elevation, accompanied by the greatest intraspecific variability in photosynthetic traits and weaker correlations among traits. Overall, B. ermanii employs a resource-acquisition strategy enabling it to occupy resources and space, while P. jezoensis adopts a resource-conserving strategy by emphasizing shade and drought-tolerance, resource conservation, and long-term adaptation at lower elevation, limiting its ability of upward range expansion. These findings enhance our understanding of their adaptive strategies and responses to elevational change, informing predictions of subalpine forest dynamics.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-369', Anonymous Referee #1, 31 Mar 2025
The manuscript has an important scientific significance by giving new information about functional traits and their inter- and intraspecific variation of two tree species in an elevation gradient in Northeast China: a deciduous treeline species Betula ermanii and a conifer tree species Picea jezoensis ( non-treeline species), and a discussion on the species adaptive strategies and ability of upward range expansion. However, there are some important issues to be fixed, regarding the analyses, explanations of the obtained results and result interpretation. After these have been fixed, the manuscript is of good quality to be published in EGUsphere, according to my opinion. Below are my detailed comments, which are also marked as comments to the pdf file attached:
L23-24: What is expected from these in relation to treeline environment?
L33-34: This was against expectations, right?
L36-37: Why these traits are expected to relate adaptation to lower elevation? I would expect an opposite effect, as high elevation environments are sometimes also dry, and resource conservation strategy may be beneficial there due to harsh conditions.
L61: Do you refer here inter- or intraspecific variation in traits? Or both?
L71-73: Isn't resource conservation strategy something that would be beneficial at the treeline due to harsh environmental, not the resource acquisition strategy?
L83-85: How about the evolutionary aspect here? Physiological traits, such as photosynthesis, is an old trait in evolutionary perspective, and is therefore expected to be rather fixed, i.e. not very plastic, due to the conservative inheritance of the trait caused by its complex genetic basis.
L121-123: Why we are expecting reduced leaf trait connectivity when the construction costs need to be minimized? Could you give a short clarification for this.
L147-148: How much of leaf trait differences are explained by the fact that the other sp is conifer and other is deciduous? A short discussion on this would be good to have.
L216-218_ What were used here as fixed and random factors?
L221: Can you explain here what Q3 and Q1 are?
L251: This is an unclear statement.
L263-264: Adaptation to higher elevation (colder or drier conditions)?
L266-267: The datapoints in each elevation are not independent, as they are from the same tree. Therefore, a simple linear regression is not appropriate here. Mean values per tree should be used, but this is also a problem as there is only 6 datapoints then. Thus, some other method than simple lm should be used. Maybe using a tree as a random factor (?)
L269-270: Can you give a little bit more explanation here, e.g. was the other analysis done combining all the data together (both species)?
L280-283: I am wondering how much of this variation is due to measurement challenges, as these traits, especially photosynthetic traits, are very sensible, and therefore their measurement usually need several repetitions? A discussion about this would be good to add.
L312: Surprising that there is no relationship between A and SLA, and only a weak relationship between A and gs in P. jezwoensis and no relationship at all in B. ermanii ? There is a discussion about this in the discussion section., but it should be extended and made more clear.
L313: This is confusing as the same color combination is used to separate the species.
L317: Does this figure give significantly more information than fig 3 and 4? If so, can you explain a bit more what does each point (a, b and c) mean?
L324-325: What does the radical water use strategy mean and how was that observed? This is discussed later on (in 4.2) a bit more, but if it is mentioned here it needs some explanation.
L328-329: But there was no connection between these traits, which is surprising (fig 4). This is discussed a bit more later on in 4.2, but same as above: if it is mentioned here, it needs some explanation.
L331-335: I think this is a bit controversial, as harsh treeline environments are not resource-rich. Can you explain better this idea?
L337-339: What kind of mechanisms is this based on? Can you explain better, so that the next sentence would be more justified.
L351-352: But on the other hand, the traits showed more intraspecific variation, indicating that there is potential.
L355-356: It may limit a rapid expansion, but on the other hand, it can better ensure survival in harsh conditions. This should be also discussed.
L362-363: Can you explain what is the reasoning behind this statement?
L364-365: Is this due to a stronger adaptation to a certain type of environment (cold /dry)?
L365: Is there something missing from this sentence?
L367-371: I think that this part is somehow aiming to give an explanation for my questions /comments (regarding e.g. the lines 324-325 and 328-329), but it could be a bit more clear, expanded and combined with the statements in the corresponding lines.
L374-376: I think that this statement is not so clear, especially what comes to photosynthesis, which is an old plant trait and thus expended to be quite fixed.
What do you mean by anisotropic traits?
L377-378: Maybe some of them, but photosynthesis is an old trait and therefore expected to be more fixed.
L382-384: This makes sense, indicating that this is thus more as a species-specific difference than environmental adaptation.
L397-399: This sentence seems incomplete, or something is missing.
L417-418: What do you mean by "the central traits"?
L421-427: Could you explain these interesting mechanisms a bit more. For example, why an increase in photosynthetic rate was not observed in B. ermanii when stomatal opening increased? How the independent regulation of water and carbon is expected to work?
L433-435: This was not tested with an experimental setting. So, it is difficult to claim that the differences were driven only by the environment and not by species-specific differences. And the difference in WUE seemed to be species-specific difference anyway. This aspect should not be forgotten from the discussion.
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RC2: 'Comment on egusphere-2025-369', Anonymous Referee #2, 23 Apr 2025
This study compares the leaf functional traits of Betula ermanii, a treeline species and Picea jezoensis, a non-treeline species, along an elevational gradient on Changbai Mountain to understand their adaptive strategies. B. ermanii exhibits traits favoring resource acquisition and adaptability at higher elevations, while P. jezoensis shows a resource-conserving strategy more suited to lower elevations.
I have a fundamental concern regarding the conceptual approach of comparing only two species in relation to the treeline. While the study aims to draw broader conclusions about vegetation stratification, analyzing just two species—each with distinct ecological strategies—limits the robustness of the findings. This approach risks confounding species-specific differences with general patterns, making it difficult to extrapolate broader ecological insights. Additionally, the manuscript often lacks sufficient detail and clarity, and contains multiple errors that hinder comprehension. It would benefit greatly from thorough proofreading and substantial rewriting to improve both readability and logical flow. I also have reservations about aspects of the data analysis and figure construction, which are at times unclear or insufficiently justified. These elements should be revised to enhance clarity and relevance.
Below are my specific comments:
Major issues:
- I have a conceptual concern with the study’s approach. The manuscript aims to explain vegetation stratification through functional traits, yet it compares two species that naturally occupy different elevations and likely have inherently different physiological thresholds. The comparison is further complicated by the contrasting functional types—Betula ermanii being a deciduous broadleaf and Picea jezoensis a conifer—each with fundamentally different life strategies. Moreover, the claim that P. jezoensis inhabits a resource-rich and less stressful environment is not well supported by empirical data. Since this species is at its own altitudinal limit, the level of stress it experiences is likely comparable—in relative terms—to that faced by B. ermanii at the treeline. Elevational limits represent ecological thresholds for both species, and interpreting lower elevation as inherently less stressful may overlook species-specific constraints. For instance, at comparable altitudes, the two species show markedly different trait values, and their trait responses to elevation differ substantially. These factors cast doubt on the validity of directly comparing their functional traits, and ultimately complicate the interpretation of adaptive strategies or elevation-driven processes. In my view, a more informative approach might have been to compare two co-occurring treeline species—or two non-treeline species—to identify shared adaptive strategies within each group.
- While I’m not a specialist in functional traits, I wonder if the analysis could be made more accessible by reducing redundancy among highly correlated traits—especially among leaf traits, which often show strong interdependence. Simplifying the dataset by selecting a representative trait from each highly correlated set could help clarify the figures and make them easier to interpret. Similarly, for trait group analyses (e.g., hydraulic, foliar), it might be useful to present either a single representative trait per group or an average value per group with associated uncertainty. This could streamline the presentation and highlight broader patterns more effectively.
- In the abstract you say that P. jezoensis is a treeline genus elsewhere. Does that mean that elsewhere it's on the treeline but not on your site? If so, it calls into question your study comparing a treeline and a non-treeline species.
- The manuscript requires careful proofreading, as it contains numerous typos, errors, and repeated phrases (e.g., lines 87, 97, 121, 160, 225, 237). The introduction lacks clarity and does not effectively communicate the main objectives or ecological significance of the study. Strengthening the justification for the research questions and explicitly outlining their broader relevance would greatly enhance this section. Figure legends are overly succinct and should be expanded to allow readers to interpret the figures without referring back to the main text. In the Methods section, essential parameters related to the trait network analysis—such as modularity and closeness—are not defined. While some of these are included in the supplementary materials, they should be clearly explained and contextualized within the main text. Additionally, the ecological relevance of these network metrics should be supported with appropriate references and further discussed in the Discussion section to better integrate them into ecological theory. The Results section would benefit from more explicit descriptions of key findings, including numerical values drawn from the figures to improve interpretability. The Discussion, meanwhile, needs editing for clarity and coherence, and should include a section addressing the limitations and potential weaknesses of the study’s approach. Finally, the authors should place greater emphasis on the broader ecological or practical implications of their findings to strengthen the study's impact.
- Several times throughout the manuscript—particularly in the Discussion—you suggest that conifers, such as Picea jezoensis, are less adapted for upward range shifts. However, I find this conclusion somewhat overreaching, given that it is based solely on a limited set of adult physiological traits. Key processes related to establishment, such as seed production, dispersal mechanisms, germination success, and seedling survival, are not addressed in the study. Without considering these life history stages, it is difficult to draw robust conclusions about a species' capacity to expand its range in response to climate change. I would recommend framing these interpretations more cautiously or supporting them with broader evidence.
- Your results highlight two distinct adaptive strategies—resource acquisition and resource conservation—across the two species studied. However, given the complexity and number of traits analyzed, it can be challenging to synthesize the findings as a whole. I suggest including a summary figure or diagram in the Discussion section that visually distills the main results for each species and clearly contrasts their respective strategies. This would greatly enhance the reader’s ability to grasp the overarching patterns and takeaways of the study.
Minor issues:
- it's best to avoid using acronyms in the abstract
- L84: ref ?
- L87: repetition of “functional traits”
- L96-99: lots of repetitions, improve the sentances
- L107-108: You say there are relatively few studies, but you don't cite any. Are you sure that no study has looked at the dynamics and physiological mechanisms of non-treeline species in response to climate change?
- L113: what do you mean by “integration”?
- L134: ‘a mean temperature of -7.3 to 4.9 °C in the growing season and annual precipitation of 800 to 1800 mm.’ Why do you give two figures each time? are they averages? quantiles? please specify.
- L140: You say B. ermanii is a treeline species, do you have a ref to support this?
- L142-145: Over what period were these figures calculated and what are the values for your sites?
- In methods (2.1.), could you specify if the area is grazed or mown. If yes is it still the case? And if the forest is managed? These informations are important to understand the structure and the responses to climate change.
- I think we're missing a study site map showing the sites sampled by species and their elevation. It is also in this figure that you could put the temperature values of the environments of each species (S1). You could add pictures of the two species.
- Figure 1: ‘environment’ corresponds to elevation ? the term needs to be changed. What's more, we don't know what altitude range it corresponds to, e.g. 1700-1800?
- I wonder if you run a PCA with the two species together, would that give two clusters corresponding to each species? Or would you still find altitudinal clusters?
- Table 2: What are F and p? You need to explain. What does ‘Elevation*species’ mean? It needs to be explained much better.
- Figure 2: equations and pvalues are displayed only for significant regressions ? specify.
- Figure 3: please add dotted ablines for certain % (25, 50, 75) to facilitate reading (panels a and b). For panel c, it's an average QCD per trait group? we don't understand what you've done here, please explain.
- Figure 4: I don't see the advantage of panels a and b over PCA. Justify it to me or put it in sup mat.
- Parameters of figures 4 and 5 need to be explained and ecological significances detailed and justified in methods.
- S1 : “DBH” meaning ? same, “High” what is high? Is it soil/air temperature? Specify.
- L368-369: ref ?
- Could you detail in methods the ecology of the two species: pioneer species? Place in the ecological succession, mode of dispersal.
- L420-432: not just a difference between conifer and broadleaved species?
- L434-435: “contrasting growth environments” in terms of what? Explain.
Citation: https://doi.org/10.5194/egusphere-2025-369-RC2
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