Contrasting Effects of Broadleaf Thinning Treatments on Spruce Growth in Central British Columbia, Canada
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsReview of the manuscript Griesbauer, H., Hawkins, C.: Contrasting effects of broadleaf thinning treatments on spruce growth in central British Columbia, Canada
In my opinion, the submitted manuscript (MS) is a pretty solid study and I consider it an interesting contribution to the question of the role of broadleafs admixture in conifer-dominated stands of western North America. The MS has a clear and concise structure, the text is reader-friendly, using the sound and standard methodologies. Results of the study confirmed conifer stands can benefit from a reasonable admixture of broadleafs, despite a certain level of yield reduction. I appreciate also a very practical recommendation of this reasonable amount (500 broadlief trees per hectare) given in the Discussion.
As I did not identify any substantial flaws to be corrected, my recommendation is to accept this MS, after correcting few minor points, for publication in journal Forests.
Specific comments:
L133: What form and health factors? Could you be more specific here?
L135: The 4cm dbh threshold was also applied in 2004? If yes, please add it in L126, if no, please explain why different thresholds were used in 2004 and 2024.
L137: delete one “multiple”
L468: typo “FMV”
L571: typo “Hakwins”
Finally, a very good and interesting work, best regards!
Author Response
Comment 1: L133: What form and health factors? Could you be more specific here?
Response 1: We thank the reviewer for pointing this out, as it could create confusion for the reader. Data collected during this study included a forest health assessment on each tree, where the presence of biotic factors such as insects, diseases and animal browse, as well as abiotic factors, such as frost damage, were recorded. Because factors present in the stand generally did not affect growth and form of trees (other than weevil), we suggest that it may be better to simply remove this line of text from the manuscript. This may help avoid potential confusion for the reader. The removed text is on Line 142, Page 4.
Comment 2: L135: The 4cm dbh threshold was also applied in 2004? If yes, please add it in L126, if no, please explain why different thresholds were used in 2004 and 2024.
Response 2: This is a helpful observation. The 4 cm DBH threshold was not applied in 2004 but was applied in 2023. This is because of the different vertical structure and seral dynamics of the stand during the two measurements. During the 2004 measurements, the stand was early seral, simpler in structure, and simpler in species composition, relative to the 2023 measurements. At this time, trees that were at least 1.3 m tall were tagged with an identifier on a branch. In 2023, the stand’s vertical structure and species composition had become more complex (i.e., the broadleaves and spruce were significantly taller than late-seral species such as hemlock and cedar that had regenerated in the understory of the stand), and the trees were substantially larger. This required a different measurement protocol and tree tagging system. We limited the tagging and measurements of new trees to those having a minimum 4.0 cm DBH for the following reasons:
1) Trees smaller than this size are shade-tolerant species (mostly hemlock and cedar) that regenerated naturally sometime after the stand was planted with spruce, and now reside in the suppressed canopy layer, well below the heights of the broadleaves and spruce. As such, trees smaller than 4.0 cm DBH are unlikely to be affecting the growth of the focal species for this study;
2) The ongoing maintenance of this study requires that trees are tagged with metal tags affixed to aluminum nails at 1.3 m on the bole of tree, which requires the tree to have a minimum size of 4.0 cm DBH to accommodate the nail at that height; and
3) Ingress of new trees will be an ongoing process in this stand. In order to record ingress during subsequent remeasurements, we will continue to tag and record trees as they reach the 4.0 cm threshold.
In order to avoid any potential confusion related to these details, and because these differences in measurement thresholds do not have a meaningful impact on our research data or conclusions, we would prefer to leave the text as-is and not make any changes. If the Editor feels that further clarification is necessary, we can elaborate.
Comment 3:L137: delete one “multiple”
Response 3: Thank you for catching this. We fixed this on Line 146, Page 4.
Comment 4:L468: typo “FMV”
Response 4: Thank you for catching this. We fixed this on Line 480, Page 20.
Comment 5:L571: typo “Hakwins”
Response 5: Thank you for catching this. We fixed this on Line 583, Page 23.
Reviewer 2 Report
Comments and Suggestions for AuthorsDear Editor,
Thank you for inviting me for reviewing the manuscript entitled Contrasting effects of broadleaf thinning treatments on spruce growth in central British Columbia, Canada. It is an interesting study examining the impact of broadleaf thinning treatment on spruce growth. I have generally found the article well written. However, some suggestions have been made to improve the introduction section, some recent theories on tree competition are required to be reviewed and presented. And the conclusion section has to be rewritten in the light of presented results. The following points may help.
- It would be interesting if the authors could present tree species of choice (economic point of view / biodiversity richness). Hence it will be clear about the context, generally mixed forests (conifer + broadleaved) have been considered positive in terms of long-term ecosystem functions. Please also compare and contrast economic value of birch and spruce.
- Please add the study area map in the manuscript, I would also suggest the close-up pictures of tree species as inset in the map.
- A brief introduction of all used models and their accuracy are required while explaining the methods.
- Figure 6: I was wondering if the older stand is more susceptible to weevil attack, would you please explain the case in the discussion section clearly.
- Figure 8, the X-axis for upper planes is not clear. Is it the same with the X-axis at the bottom?
- Conclusion: this section has to be re-written. Please remove 550-553 (this was already presented in the introduction section). Write the main conclusion of the study on the basis of results. Highlighting ………spruce height and diameter growth declined with broadleaf competition (competition-related reduction in stand-level spruce).
…The fastest spruce growth occurred in treatments where all broadleaf trees were removed
……complete broadleaf removal also resulted in higher rates of weevil attack on spruce, which also caused height and diameter growth reductions.
Comments for author File: 
Comments.pdf
Author Response
Comment 1: It would be interesting if the authors could present tree species of choice (economic point of view / biodiversity richness). Hence it will be clear about the context, generally mixed forests (conifer + broadleaved) have been considered positive in terms of long-term ecosystem functions. Please also compare and contrast economic value of birch and spruce.
Response 1: We agree with this suggestion and feel that it will help the introduction. We added two sentences as per the reviewer’s suggestion on Lines 53-58, Page 2 that compare the economic value of aspen/birch and spruce, and also discusses the potential ecosystem benefits from broadleaf trees and broadleaf-conifer mixtures [“In many regions of western North America, aspen and birch are considered to have limited economic value relative to conifers, however, these trees can provide valuable wood products and this potential remains relatively undeveloped [22,25,26]. Further, broadleaf trees, and stands with a mix of broadleaves and conifers, can pro-vide a range of ecosystem services that are increasingly desired by society, including non-timber forest products [27], enhanced biodiversity [28], and wildfire mitigation [29].”]
Comment 2: Please add the study area map in the manuscript, I would also suggest the close-up pictures of tree species as inset in the map.
Response 2: We agree and note one of the other reviewers made a similar suggestion. We have included this map as Figure 1. Regarding the suggestion to include photos of tree species in the map: we feel this is an interesting idea but our preference is not to do this for two main reasons: (i) most readers should be familiar with aspen, birch and spruce, as their genera are widely distributed and commonly managed; and (ii) readers unfamiliar with these tree species can easily obtain photos online. If the Editor feels that it’s important to include photos, perhaps we could include them in the Supplementary materials.
Comment 3: A brief introduction of all used models and their accuracy are required while explaining the methods.
Response 3: We agree with this suggestion and have placed introductory sentences describing mixed effects models on Lines 156-159, Page 4, and logistic regression models on Lines 210-212, Page 6.
The linear mixed effects model approach is now described as follows: “We analyzed growth data using a linear mixed effects (LME) modeling approach. LME models extend the classic linear regression model by accounting for the variance com-ponent associated with a nested data structure (in this case, tree data nested within plots, clustered within a treatment unit).”, and the logistic regression model is described as follows: “A logistic regression model is similar to linear regression, except that it estimates probability (in this case, the probability of tree mortality) as a function of independent variables.”
Comment 4: Figure 6: I was wondering if the older stand is more susceptible to weevil attack, would you please explain the case in the discussion section clearly.
Response 4: This is a helpful observation, and the reviewer is correct in that weevil susceptibility varies with stand age. We note that we discuss this in the introduction (please see Lines 63-69), and provide supporting literature. Given that we’ve discussed this in the introduction, and because stand age was not a controlling factor in this study, we would prefer not to add text regarding this in the discussion. However, we can do so if the Editor feels this is important.
Comment 5: Figure 8, the X-axis for upper planes is not clear. Is it the same with the X-axis at the bottom?
Response 5: The reviewer is correct, the same grid lines and units. We feel that this should come across the reader, however, we can change the figure if the Editor feels this is necessary.
Comment 6: Conclusion: this section has to be re-written. Please remove 550-553 (this was already presented in the introduction section). Write the main conclusion of the study on the basis of results. Highlighting ………spruce height and diameter growth declined with broadleaf competition (competition-related reduction in stand-level spruce).…The fastest spruce growth occurred in treatments where all broadleaf trees were removed……complete broadleaf removal also resulted in higher rates of weevil attack on spruce, which also caused height and diameter growth reductions.
Response 6: We agree with this suggestion, and have altered the conclusion on Lines 568-579, Page 21, to focus on the main results, as follows: ” Forest managers seeking to manipulate stand structure to retain broadleaf trees or maximize conifer yields should consider tree size and competitive interactions that will continue in the stand after treatment. Broadleaf competition had a strong effect on spruce growth across a range of residual broadleaf densities after thinning treat-ment, and these effects are projected to persist in the stand for its rotation. While spruce growth was maximized in treatments that removed all deciduous competition, the complete removal of deciduous also resulted in an increase in weevil attack, which reduced spruce growth and caused stem defects. Therefore, the retention of relatively small amounts of broadleaf trees may reduce weevil attack and have an acceptable impact to conifer yields. Our results highlight the need for forest managers to con-sider the balance of competitive and facilitative effects of different tree species in managed stands in highly productive temperate rainforest ecosystems. “
Reviewer 3 Report
Comments and Suggestions for Authors1. The location of the experimental area and sample site should have a diagram, and the experimental design should be listed in a table, it is more clear.
2. The paper results suggest that maintaining a density of approximately 500 broadleaf trees per hectare may achieve a stand condition that balances spruce growth reductions from competitive interactions with broadleaf trees while providing some protections from white pine weevil attack. Such a statement is incomplete, for it should not only be the density of about 500 broad-leaved trees per hectare, but also the question of the reasonable density of stands, that is, what should be the reasonable density of stands and what should be the proportion of broad-leaved trees among them. It's more accurate. In addition, the spatial distribution of trees should also be considered, for mixed forests, the spatial distribution of different tree species has an important impact on growth and health, not just a density problem.
Author Response
Comments 1: The location of the experimental area and sample site should have a diagram, and the experimental design should be listed in a table, it is more clear.
Response 1: We agree and feel this will clarify the study design. We have developed a new Figure (Figure 1) that shows the study design, with treatment units identified on a map. In addition, we have identified the treatment level (i.e., the target residual broadleaf density after thinning) for each treatment unit. We also show the control plot locations. The figure is inserted on page 4, and the figure caption is found on lines 132-134. Please note that all subsequent figure numbers have also been updated due to the addition of this figure.
Comments 2: The paper results suggest that maintaining a density of approximately 500 broadleaf trees per hectare may achieve a stand condition that balances spruce growth reductions from competitive interactions with broadleaf trees while providing some protections from white pine weevil attack. Such a statement is incomplete, for it should not only be the density of about 500 broad-leaved trees per hectare, but also the question of the reasonable density of stands, that is, what should be the reasonable density of stands and what should be the proportion of broad-leaved trees among them. It's more accurate. In addition, the spatial distribution of trees should also be considered, for mixed forests, the spatial distribution of different tree species has an important impact on growth and health, not just a density problem.
Response 2: We agree with the reviewer that we should clarify that our results are specific to optimally stocked stands (i.e., total density must be considered), and also that FVM treatments need to consider the spatial distribution of trees. We have added text to clarify this on Line 427 of page 18 and Lines 434-435 on page 19.
With respect to the need to clarify the importance of considering reasonable total stand density when discussing the broadleaf threshold, we added text on Lines 440-441 on page 19 ["Most importantly, our study suggests that in stands managed primarily for conifer yields in this region, a potential broadleaf density threshold of 500 SPH may provide a balance between competitive and facilitative effects of overtopping broadleaf trees on conifer growth. " ]. Here, we added “in stands managed primarily for conifer yields in this region”, to infer that the stands are being managed for optimal conifer and total stand density to achieve objectives.
To account for the reviewer’s comment regarding the importance of considering spatial distribution of trees, we added text on line 427 of page 18 [“However, the effects of FVM treatments vary across a suite of factors including tree species, the spatial distribution of trees within the stand, tree size at the time of treatment, site productivity and characteristics of the FVM treatment itself. "]
