Phenological Differentiation in Sugar Maple Populations and Responses of Bud Break to an Experimental Warming

Round 1

Reviewer 1 Report

The paper describes a simple investigation of adaptation of sugar maple’ ecotypes (populations?) to local climate. The data presented clearly and professionally analyzed applying methods of descriptive statistics. This exhibited high-level mastering of the authors. However, the topic lacks any theoretical novelty, wider and generalizing predictions, and thus scientific importance. The tendencies and impact of changing climate on common plant’ phenology are widely analyzed Moreover, the aim is not circumstantial and includes only data description without wider interpretation, particularly practical suggestions.

Author Response

Thanks for your comments. Our topic is of practical importance, although there is no theoretical novelty.

Sugar maple is a key species in the hardwood forests of the northern parts of the Central and Eastern United States and Eastern Canada. This species is of great social and economic value due to its bright fall foliage and xylem sap, which is the source for maple syrup production. Thus, the effect of warming on the bud break in sugar maple is not only of the broad interest in climate change ecology but also related to industry and agriculture.

To highlight the practical and scientific importance of this study, we added the related part in line 70-72 as: “Moreover, given the great social and economic value of sugar maple, this work is of great importance not only in forest ecology under climate change but also in the related industry and agriculture.”

In addition, our results found similar responses to warming among provenances, which is novel for the species. This finding suggested that similar advancements of bud break could be expected under a scenario of homogeneous warming across the Canadian range of sugar maple. Moreover, the parameters of phenological models set for specific populations could be applied reliably to other populations of the same species, which is very helpful to widen the range of model predictions.

To highlight the novelty, we added the related part in line 179 as: “This finding is novel for sugar maple…”.

Reviewer 2 Report

This is an interesting paper that first examines geographic variation in phenology among sugar maple populations and then experimentally examines phenological responses of sugar maple seedlings to warming. The aim of the study is good, the work will be of broad interest in the field of climate change ecology, and the paper is mostly well written. I make the following comments to assist the authors in revising their manuscript.

• The Abstract would benefit from some tightening up of the writing. For example, the following sentence is long and once the reader reaches the second half it is difficult to follow “The timings differed among provenances, with the earlier events being observed in seedlings originating from the colder sites according to a negative relationship with mean annual temperature of the provenance origin of 4.8 days delay per °C.” Perhaps have two separate sentences and rework the second one for clarity.
• The Abstract sentence “the question whether and how the phenological responses to warming” should read as “questions”.
• It would be helpful to define what ‘bud break’ is, it may not be immediately clear what this means, and to describe what the bud break stages are (i.e. more than just providing the terms in the table).
• The Abstract mentions two experiments and then presents results in terms of “the experiment”. This needs fixing and I think in general there needs to be more clarity around the experimental design (see below).
• The following sentence in the Introduction (first paragraph) is unclear “the response of spring phenology to increasing temperatures in seven dominant European tree species has significantly decreased”. How does phenology decrease?
• This sentence in the last paragraph of the Introduction doesn’t make sense “sugar maple has shown that bud break occurred” and needs fixing.
• You need to have a better explanation for your expectation that “sugar maple populations also exhibit different phenological responses to warming conditions”. Just because there are geographic differences in phenology, doesn’t necessarily mean that you would expect these different populations to show different rates of change under warming conditions. Explain why you would expect this link between to the two.
• There needs to be more detail provided about the experimental design and analyses: (i) In the second paragraph of section 2.1, what were the growth conditions of the seedlings between collection and October during which they completed the first growing season? (ii) Provide detail of the growth chambers, e.g. the models/dimensions etc. Did they have glass windows to observe the seedlings or did the doors need to be opened to see the seedlings? If the latter, could this have affected the temperature treatments? (iii) It would be much clearer to state that the same experiment was performed at two different times (January and April) and then to include time as a factor in the analyses. At the moment, it is often unclear in the paper as to whether the results being discussed refer to one or both of the times of the experiment. Importantly, given the emphasis on timing in this research, we need to know if time is a factor in generating different patterns. More simply, do phenological patterns differ depending on the time of year of the experiment? (iv) Why was a bootstrap procedure with 1,000 replications used in the ANOVA? I don’t see the need for this?
• What do ‘I’ and ‘II’ refer to in the legend for Table 3?
• Degrees of freedom are needed in Table 2.
• In the Fig. 2 legend, state which colour circles represent the control and the treatment (that would be easier to follow than the temperature range currently used in the bottom right-hand figure).
• I’m really curious about all points in each experimental group falling along straight lines in Fig. 2. Does this mean that there is an r^2 value of 1 for all models?

 

Author Response

This is an interesting paper that first examines geographic variation in phenology among sugar maple populations and then experimentally examines phenological responses of sugar maple seedlings to warming. The aim of the study is good, the work will be of broad interest in the field of climate change ecology, and the paper is mostly well written. I make the following comments to assist the authors in revising their manuscript.

R: Thanks for your positive comments. We have taken into account all suggestions and comments in our revision.

 

The Abstract would benefit from some tightening up of the writing. For example, the following sentence is long and once the reader reaches the second half it is difficult to follow “The timings differed among provenances, with the earlier events being observed in seedlings originating from the colder sites according to a negative relationship with mean annual temperature of the provenance origin of 4.8 days delay per °C.” Perhaps have two separate sentences and rework the second one for clarity.

R: We re-wrote the abstract and the sentence was rephrased in line 21-22 as: “The earlier events were observed in seedlings originating from the colder sites. Bud break was delayed by 4.8 days per additional degree Celsius in the mean annual temperature at the origin site.”

 

The Abstract sentence “the question whether and how the phenological responses to warming” should read as “questions”.

R: We corrected “question” to “questions” in line 16.

 

It would be helpful to define what ‘bud break’ is, it may not be immediately clear what this means, and to describe what the bud break stages are (i.e. more than just providing the terms in the table).

R: ‘Bud break’ is a well-known process in plants and has frequently appeared in the recent literature for example Dhont et al (2010), Rossi (2015), and Zhang et al (2019). We have described each stage of bud break in line 109-113.

Reference:

Dhont, C., Sylvestre, P., Gros-Louis, M., and Isabel, N. 2010. Field guide for identifying apical bud break and bud formation stages in white spruce. Natural Resources Canada, Quebec, Ottawa, ON, Canada. pp. 17.

Rossi, S. 2015. Local adaptations and climate change: converging sensitivity of bud break in black spruce provenances. Int. J. Biometeorol. 59(7): 827-835. doi:10.1007/s00484-014-0900-y.

Zhang, S., Isabel, N., Huang, J.G., Ren, H., and Rossi, S. 2019. Responses of bud-break phenology to daily-asymmetric warming: daytime warming intensifies the advancement of bud break. Int J Biometeorol. doi:10.1007/s00484-019-01776-0.

 

The Abstract mentions two experiments and then presents results in terms of “the experiment”. This needs fixing and I think in general there needs to be more clarity around the experimental design (see below).

R: We corrected “the experiment” to “the experiments” in line 20. We also added the details of the experimental design (see below).

 

The following sentence in the Introduction (first paragraph) is unclear “the response of spring phenology to increasing temperatures in seven dominant European tree species has significantly decreased”. How does phenology decrease?

R: Here, the response of spring phenology to increasing temperatures, instead of spring phenology, has significantly decreased. To make it clear, we rephrased the sentence in line 37-38 as: “For example, the relationship between spring phenology and spring temperature in dominant European species has significantly decreased….”

 

This sentence in the last paragraph of the Introduction doesn’t make sense “sugar maple has shown that bud break occurred” and needs fixing.

R: We rephrased this sentence in line 63- 64 as: “In a common garden experiment, an earlier bud break of sugar maple was observed in northern populations,...”.

 

You need to have a better explanation for your expectation that “sugar maple populations also exhibit different phenological responses to warming conditions”. Just because there are geographic differences in phenology, doesn’t necessarily mean that you would expect these different populations to show different rates of change under warming conditions. Explain why you would expect this link between to the two.

R: The common garden experiment showed the different timings of bud break among populations, which demonstrated different sensitivities to temperature among sugar maple populations. Thus, the effects of warming on bud break can be expected to be divergent among different populations. To make it clear, we added the sentence in line 64-65 as : “… which demonstrated different sensitivities to temperature among sugar maple populations”

 

There needs to be more detail provided about the experimental design and analyses: (i) In the second paragraph of section 2.1, what were the growth conditions of the seedlings between collection and October during which they completed the first growing season? (ii) Provide detail of the growth chambers, e.g. the models/dimensions etc. Did they have glass windows to observe the seedlings or did the doors need to be opened to see the seedlings? If the latter, could this have affected the temperature treatments? (iii) It would be much clearer to state that the same experiment was performed at two different times (January and April) and then to include time as a factor in the analyses. At the moment, it is often unclear in the paper as to whether the results being discussed refer to one or both of the times of the experiment. Importantly, given the emphasis on timing in this research, we need to know if time is a factor in generating different patterns. More simply, do phenological patterns differ depending on the time of year of the experiment? (iv) Why was a bootstrap procedure with 1,000 replications used in the ANOVA? I don’t see the need for this?

R: (i)We added the details about seed sowing and seedlings production in line 88-92 as: “ In May 2018, the seeds were stratified, and sown in plastic containers with 15 cavities of 320 cm³ volume in the forest nursery of Berthier (QC, Canada). The resulting seedlings were grown in tunnels until the end of June. In July, after two weeks under a shading net, the seedlings were grown in an open space under full light. In November, the seedlings were removed from the containers, and stored in the dark, at a temperature of -2 °C and relative humidity of 85%.”

(ii) The experiments were performed in two growth chambers (CMP 3023, about 240 cm wide, 89 cm deep, and 198 cm tall, Conviron, Winnipeg, MB, Canada). Because the model or dimension of the growth chamber cannot affect the experiment, we do not show this information in the manuscript.

When observing the seedlings, we opened the door and moved the containers to outsides. The observations take less than 5 minutes per container (including several seedlings). After the observation, we moved back the containers to the growth chamber. Given that the process of the opening door is short, its impacts on temperature treatments are clearly marginal.

(iii)We re-wrote the related part in line 94-96 as: “ In 2019, we conducted two experiments starting on January 27^th and April 8^th (experiment 1 and 2, respectively) to assess the effect of chilling accumulation on budbreak. Seedlings in experiment 2, with a longer winter by 70 more days, accumulated more chilling compared to experiment 1.”

(iv) Here, we use a bootstrap procedure with 1,000 replications to increase the reliability of the analysis. Bootstrap produces results that are independent from the sample size, thus improving the sensitivity and quality of our statistical tests. To make it clear, we added the related part and rephrased the sentences in line 119-120 as: “ Due to the sample size, a bootstrap procedure with 1,000 replications was applied to make the results independent from the sample size and improve the reliability of the statistics.”

 

What do ‘I’ and ‘II’ refer to in the legend for Table 3?

R: ‘I’ and ‘II’ refer to the timings of bud break at day/night temperatures of 14/10 °C and 18/14 °C, respectively. To make it clear, we replaced them with “Colder” and “Warmer”.

 

Degrees of freedom are needed in Table 2.

R: In Table 2, the degree of freedom is 24 for each phase.

 

In the Fig. 2 legend, state which colour circles represent the control and the treatment (that would be easier to follow than the temperature range currently used in the bottom right-hand figure).

R: We added the related part to the caption in line 148 as: “White and grey dots refer to day/night temperatures of 14/10 and 18/14 °C, respectively.”

 

I’m really curious about all points in each experimental group falling along straight lines in Fig. 2. Does this mean that there is an r^2 value of 1 for all models?

R: Here, we used the predicted values of the linear mixed model. Thus, all points fall along the straight lines.

Reviewer 3 Report

I enjoyed reviewing this manuscript about variation in budburst response to warming among provenances of sugar maple. The experiments appear solid, but more information is needed about the experimental design as I mention below. The finding of variation among provenances in budburst confirms previous research on the topic, and the finding of a similar response to warming among provenances is novel for the species. My comments and questions are below.

1. L 44: The term “it” is not clear here. Please use a clearer subject about the decline of 35%.
2. L45-6: The phrase “Except for spatial differences in warming speed..” is not clear in the context of the rest of the sentence.
3. L51: I recommend replacing “Besides” with “In addition”
4. L56: More context is needed about the results of Chuine et al. For example, was the work on tree species?
5. L58: Replace “represents” with “is”
6. L62: The structure of this sentence is awkward. Consider revising as “…bud break occurred earlier in northern than southern populations of sugar maple.”
7. L65: Revise as “The aim of the study was to compare timing of budburst and leaf development among sugar maple populations ….”
8. L71: Revise as “The stands containing the mother trees were located…”
9. L82-4: More information is needed about:
   1. The experimental design or arrangement of seed sources in the seed-sowing stage.
   2. The location and environment for early growth. For example, did growth occur in a greenhouse, outside nursery, or somewhere else?
10. L88-9: Need more information about the experimental design for the growth chamber experiments. How were the seedlings arranged in the growth chambers? Randomly? In blocks or groups by seed source?
11. L 105-7: I do not fully understand the analysis approach. Why did you use a bootstrap in a one-way anova? Why not just use a normal anova since you appeared to have replication of each seed source? Also, what was the experimental unit used in the analysis? Was the unit an individual seedling? Or, was the unit a group of seedlings from a seed source? These questions relate to the lack of information about the experimental design.
12. L125: Change “earlier” to “earliest”
13. L127-8: Do you mean the same for all budbreak stages?
14. L132-4: The Figure 2 caption needs to better describe the codes used for the two temperature treatments.
15. L155: I do not understand “advancement of 17.48 to 27.07 days in the first phase.” Does this represent the range in advancement over seed sources, or over bud burst phases?
16. L157-8: Revise as: “No provenance x thermal treatment interaction was observed…”
17. L160: Replace “bud” with “bud burst”
18. L170-1: I do not fully understand the description of Kriebel’s results. Do you mean leaf-out occurs earlier in northern than southern provenances in a common garden with the same thermal regime, or in nature when provenances grow at their native sites? This is an important distinction for both Kriebel’s results and your results.
19. L177-9: I cannot tell whether this statement refers to an earlier study or your study. Be clearer.
20. L178-9: The distinction here between chilling requirement and bud burst forcing requirement is helpful. I encourage you to use these terms more when describing your study. For example, your study is focused on understanding differences among provenances in the forcing requirement.
21. L182-3: The term “optimal growth” is not clear here. Is this related to or the same as bud burst? I recommend focusing the topic here on bud burst, not optimal growth.
22. L188: Insert “reported” before “different”
23. L196: Delete “in any”
24. L210: Revise as “The provenance x thermal treatment interaction…”

 

Author Response

I enjoyed reviewing this manuscript about variation in budburst response to warming among provenances of sugar maple. The experiments appear solid, but more information is needed about the experimental design as I mention below. The finding of variation among provenances in budburst confirms previous research on the topic, and the finding of a similar response to warming among provenances is novel for the species. My comments and questions are below.

R: Thanks for your positive comments. When revising, we have taken into account all suggestions and comments.

 

1. L 44: The term “it” is not clear here. Please use a clearer subject about the decline of 35%.

R: Here, “it” means “phenological shift”. We corrected “it” to “the phenological shift” in line 45.

 

2. L45-6: The phrase “Except for spatial differences in warming speed..” is not clear in the context of the rest of the sentence.

R: We rephrased the sentence in line 47-48 as: “ The underlying reasons were attributed to a spatial divergence between warming and the phenological response to temperature.”

 

3. L51: I recommend replacing “Besides” with “In addition”

R: We replaced “Besides” with “In addition” in line 53.

 

4. L56: More context is needed about the results of Chuine et al. For example, was the work on tree species?

R: Yes, the result of Chuine et al. showed the clinal variations in the phenological response to temperature between tree population. We re-wrote the sentence in line 58-59 as: “Chuine et al. [28] observed the variations in phenological response to temperature between populations of six tree species, but their significance depends on species.”

 

5. L58: Replace “represents” with “is”

R: We replaced “represents” with “is” in line 60.

 

6. L62: The structure of this sentence is awkward. Consider revising as “…bud break occurred earlier in northern than southern populations of sugar maple.”

R: We rephrased this sentence in line 63-64 as: “In a common garden experiment, an earlier bud break of sugar maple was observed in northern populations...”.

 

7. L65: Revise as “The aim of the study was to compare timing of budburst and leaf development among sugar maple populations ….”

R: We did the revision in line 69 as: “The aim of the study was to compare the timing of bud break and leaf development among sugar maple populations….”

 

8. L71: Revise as “The stands containing the mother trees were located…”

R: We did the revision in line 77 as: “The stands containing the mother trees were located…”

 

9. L82-4: More information is needed about:
10. The experimental design or arrangement of seed sources in the seed-sowing stage.

2.The location and environment for early growth. For example, did growth occur in a greenhouse, outside nursery, or somewhere else?

R: We added the related parts about seed sowing and seedlings production in line 88-92 as: “ In May 2018, the seeds were stratified, and sown in plastic containers with 15 cavities of 320 cm³ volume in the forest nursery of Berthier (QC, Canada). The resulting seedlings were grown in tunnels until the end of June. In July, after two weeks under a shading net, the seedlings were grown in an open space under full light. In November, the seedlings were removed from the containers, and stored in the dark, at a temperature of -2 °C and relative humidity of 85%.”

 

10. L88-9: Need more information about the experimental design for the growth chamber experiments. How were the seedlings arranged in the growth chambers? Randomly? In blocks or groups by seed source?

R: We added the details about the arrangement of seedlings in the growth chambers in line 99-102 as: “Each growth chamber involved five containers with 15 cavities. Each container was filled by seedlings from 5 seed sources, with 3 seedlings per seed source, for a total of 150 seedlings monitored per experiment.”

 

11. L 105-7: I do not fully understand the analysis approach. Why did you use a bootstrap in a one-way anova? Why not just use a normal anova since you appeared to have replication of each seed source? Also, what was the experimental unit used in the analysis? Was the unit an individual seedling? Or, was the unit a group of seedlings from a seed source? These questions relate to the lack of information about the experimental design.

R: Here, we use a bootstrap procedure with 1,000 replications to increase the reliability of the analysis. With bootstrap, we produced results independently from the sample size and thus improve the sensitivity and quality of our statistical tests. To make it clear, we added the related part and rephrased the sentences in line 119-120 as: “Due to the sample size, a bootstrap procedure with 1,000 replications was applied to make the results independent from the sample size and improve the reliability of the statistics.”

 

12. L125: Change “earlier” to “earliest”

R: We corrected “earlier” to “the earliest” in line 139.

 

13. L127-8: Do you mean the same for all budbreak stages?

R: Yes, we re-wrote the sentence to make it clear in line 141-143 as: “The earliest and latest provenances were the same for all stages of bud break, except for green tip stage in experiment 2 and the stages of bud break and full leaf expansion in experiment 1.”

 

14. L132-4: The Figure 2 caption needs to better describe the codes used for the two temperature treatments.

R: We added the related part to the caption in line 148 as: “ White and grey dots refer to day/night temperatures of 14/10 and 18/14 °C, respectively.”

 

 

15. L155: I do not understand “advancement of 17.48 to 27.07 days in the first phase.” Does this represent the range in advancement over seed sources, or over bud burst phases?

R: Here, the advancement refers to advancement over bud burst phases. To make it clear, we re-wrote the sentence in line 168-170 as: “ In experiment 1, seedlings submitted to warmer temperature exhibited an advancement of 17.48 to 27.07 days depending on the bud break phases.”

 

16. L157-8: Revise as: “No provenance x thermal treatment interaction was observed…”

R: We did the revision in line 171 as: “No provenance x thermal treatment interaction was observed…”.

 

17. L160: Replace “bud” with “bud burst”

R: We replaced “bud phenology” with “the timings of bud beak” in line 175.

 

18. L170-1: I do not fully understand the description of Kriebel’s results. Do you mean leaf-out occurs earlier in northern than southern provenances in a common garden with the same thermal regime, or in nature when provenances grow at their native sites? This is an important distinction for both Kriebel’s results and your results.

R: Kriebel’s results are based on a common garden experiment. To make it clear, the sentence was rephrased in line 185 as: “ …, which was in accordance with the common garden experiment of Kriebel [31], showing that….”.

 

19. L177-9: I cannot tell whether this statement refers to an earlier study or your study. Be clearer.

R: This statement refers to a previous study. To make it clear, we rephrased the sentence in line 192 as: “ As shown by a previous garden experiment of sugar maple,…”

 

20. L178-9: The distinction here between chilling requirement and bud burst forcing requirement is helpful. I encourage you to use these terms more when describing your study. For example, your study is focused on understanding differences among provenances in the forcing requirement.

R: Yes, we added the related part in line 94-96 to highlight the chilling requirement as: “In 2019, we conducted two experiments starting on January 27^th and April 8^th (experiment 1 and 2, respectively) to assess the effect of chilling accumulation on bud break. Seedlings in experiment 2, with a longer winter by 70 more days, accumulated more chilling compared to experiment 1.”. Besides, we also re-wrote the sentence in line 103 to highlight the forcing as: “Two thermal treatments were set in each experiment, specifically day/night temperatures of 14/10 °C and 18/14 °C, to assess the effect of forcing on budbreak.”

 

21. L182-3: The term “optimal growth” is not clear here. Is this related to or the same as bud burst? I recommend focusing the topic here on bud burst, not optimal growth.

R: We replaced “optimal growth” to “optimal timing of bud break” in line 197-198.

 

22. L188: Insert “reported” before “different”

R: We inserted “reported” before “different” in line 203.

23. L196: Delete “in any”

R: We deleted “if any” in line 212.

 

24. L210: Revise as “The provenance x thermal treatment interaction…”

R: We did the revision in line 225-226 as: “The provenance x thermal treatment interaction…”.

Round 2

Reviewer 2 Report

The authors have done a great job addressing my review comments and they have provided the necessary detail and clarity. The paper is looking really sharp.