Fire and Herbivory Interactively Suppress the Survival and Growth of Trees in an African Semiarid Savanna

Round 1

Reviewer 1 Report

The manuscript is well written, concise, and understandable and from the standpoint of English, it is ok. The manuscript presents some findings in order to understand better the interactions between fire and different herbivore groups. From my point of view, the manuscript is interesting and related to the profile of the journal.

Some specific comments and suggestions to improve manuscript readability:

Line 81: Please explain what you mean with “herbivore treatment” and “fire treatment” (line 220).

Line 84-86: Why you hypothesized that?

Line 155: I found the range of the humidity quite wide (23-80). Is there any chance to affect the results, in respect to fire intensity/severity?

Line 159: Why the plots (boxes) in the figure 1 have different sizes?

Line 173: typographical error “trees” instead of “treas”

Line 239-240: must be explain better in the discussion chapter

Line 270-271: must be explain

Line 321-323: explain please also in line 325 why are generally cooler?

The Chapter 2 “Material and Methods” is divided very well and is well-understood. I will suggest add more discussion. Fires are known to stimulate high quality herbaceous regrowth. Make clear how in reburned plots the regrowth will be greater. Also, refer how dense are the plots, especially after the fire. It is known that herbivores reduce fuel loads and consequently the frequency and severity of the fire, maybe this is important to discuss it.

 

Author Response

Reviewer 1

The manuscript is well written, concise, and understandable and from the standpoint of English, it is ok. The manuscript presents some findings in order to understand better the interactions between fire and different herbivore groups. From my point of view, the manuscript is interesting and related to the profile of the journal.

• Thank you

Some specific comments and suggestions to improve manuscript readability:

Line 81: Please explain what you mean with “herbivore treatment” and “fire treatment” (line 220).

• ” Herbivore treatment” refers to the particular herbivore combination that is excluded from a given plot. There are six herbivore treatments (O, C, W, WC, MW, and MWC). On the other hand, “fire treatment” refers to whether a plot was burned or unburned. We have clarified this in line 79 as well as in line 252-254

Line 84-86: Why you hypothesized that?

• we have rephrased this hypothesis as follows “Tree subjected to repeated burns and herbivory would generally have lower growth rate than trees in plots burned once and protected from herbivores because both fire and herbivory additively suppress tree growth”

Line 85-87Line 155: I found the range of the humidity quite wide (23-80). Is there any chance to affect the results, in respect to fire intensity/severity?

• We agree with the reviewer that differences in humidity could have affected the intensity and severity of the burns. As pointed out in the methods section (line 157-160), we prioritized burning plots with higher fuel loads earlier in the day when humidity was high. Consequently, our estimates for intensity and severity in these plots is actually conservative as the third reviewer notes. We have included a statement in the discussion (Line 324-331) to highlight this.

Line 159: Why the plots (boxes) in the figure 1 have different sizes?

• All the burned plots and the respective control plots (unburned) were 30 x 30m. Figure 1 is generated from a low precision GPS (+7m) hence the plots may look distorted

Line 173: typographical error “trees” instead of “treas”

• corrected: line 176

Line 239-240: must be explain better in the discussion chapter

• We believe that this is well explained in the discussion (Line 334-339 “For all cattle plots (C, WC, and MWC), we found lower tree mortality rates in repeat burns than single burns. We attribute this to reduction of post-fire biomass accumulation; where cattle maintain lower herbaceous vegetation cover in previously burned areas thus reducing severity of subsequent fires. We have previously demonstrated in this system that cattle may affect fire spread by creating bare patches that do not carry a fire [30].”

 

Line 270-271: must be explain

• We attribute the lower severity in previously burned subplots to positive feedback from cattle grazing. All previously burned subplots in herbivore plots accessible to cattle experienced lower tree mortality after the second set of burns. We have clearly explained this in the third paragraph of the discussion (Line 339-347)

Line 321-323: explain please also in line 325 why are generally cooler?

• We have clarified that herbivores that are attracted previously burned area may retard fuel load accumulation, thus reducing the severity of subsequent fires (Line 332-347)

The Chapter 2 “Material and Methods” is divided very well and is well-understood. I will suggest add more discussion. Fires are known to stimulate high quality herbaceous regrowth. Make clear how in reburned plots the regrowth will be greater. Also, refer how dense are the plots, especially after the fire. It is known that herbivores reduce fuel loads and consequently the frequency and severity of the fire, maybe this is important to discuss it.

• We have clarified this (Line 344-345)

 

Author Response File: Author Response.docx

Reviewer 2 Report

Dear authors,

This is a very interesting and well-written study that shed’s light on the complex interactions among fire, herbivory presence/absence and tree growth and survival rates. The manuscript is not very long, and you avoid adding to much information, a fact that is positive since it can be read without tiring the readers.

I propose some minor changes below that can clarify some issues or improve your manuscript.

Page 3, l.101: How ants protect trees from herbivores – it is not clear. Please provide more information.

Page 3, l.119: Are ungulates different from herbivores or the same? If the same, please use consistent terms everywhere e.g. if you choose herbivores use herbivores everywhere.

Page 3, l.120: wild ungulates and but exclude cattle – delete “and”

Page 4, l.143: areas instead of area

Page 5, l.194-198: Please explain the difference between open and bounded scale. Why you had to do this transformation since both scales are 0-1. Explain it for all the readers that are not familiar with this transformation.

Page 5, l.200: For every package use before its name the R, e.g., generalized linear mixed models (GLMMs) in the R package glmmTMB

Page 5, l.200: the interactive effect of herbivore. Same for the line 202, use “of”

Table 1: what does the x2 values denote? Maybe use the units in the first column (response variables) to clarify, e.g. cm.

Figure 2: what are the units of mortality rate?

Page 7, l.245: use a space between a number and units everywhere in the manuscript, e.g., 100 cm and not 100cm

Figure 4: still not clear what the a, b and c stand for. Please explain better in the figure caption

Page 8, l.272: Top-kill is caused by the effect of herbivores? Or from something else. Maybe some photos of top-killed trees or trees with evident effects of browsing/grazing will help readers that are not from Kenya to familiarize with what you describe.

Page 8, l.276: replace “in burns than unburned” with “in burned compared to unburned areas”

Page 8, l.279: I recommend a table that will summarize all the results from all the combinations described, denoting where and in what combinations we have low, moderate or high mortality, canopy volume and tree height etc. Without the table, readers are struggling to catch the take home message. A table could help a lot to simplify the understanding of your findings.

Page 8, l.284: replace intermediate with moderate

Page 9, l.309: maybe also because by removing the biomass, the trees that remain are more vigorous since they face less stress for resources and have better access to nutrients and water (antagonistic vegetation is removed by browsing)

 

Author Response

Reviewer 2

This is a very interesting and well-written study that shed’s light on the complex interactions among fire, herbivory presence/absence and tree growth and survival rates. The manuscript is not very long, and you avoid adding to much information, a fact that is positive since it can be read without tiring the readers.

• Thank you

I propose some minor changes below that can clarify some issues or improve your manuscript.

Page 3, l.101: How ants protect trees from herbivores – it is not clear. Please provide more information.

• Done: by swarming and biting herbivores – Line 102-104

Page 3, l.119: Are ungulates different from herbivores or the same? If the same, please use consistent terms everywhere e.g. if you choose herbivores use herbivores everywhere.

• For clarity, we have replaced all references to “ungulates” with ”herbivores”

Page 3, l.120: wild ungulates and but exclude cattle – delete “and

• Done: Line 122

Page 4, l.143: areas instead of area

• Done: Line 145

Page 5, l.194-198: Please explain the difference between open and bounded scale. Why you had to do this transformation since both scales are 0-1. Explain it for all the readers that are not familiar with this transformation.

• Done: Line 198-203

Page 5, l.200: For every package use before its name the R, e.g., generalized linear mixed models (GLMMs) in the R package glmmTMB

• Done

Page 5, l.200: the interactive effect of herbivore. Same for the line 202, use “of”

• inserted “of” between “effect” and “herbivore” in both cases

Table 1: what does the x2 values denote? Maybe use the units in the first column (response variables) to clarify, e.g. cm

• χ² denotes Type II Wald Chi-squareχ² We have clarified this in (now) Table 2

Figure 2: what are the units of mortality rate?

• Under data analysis section, we have explained how mortality rate was calculated: Line 194-197

Page 7, l.245: use a space between a number and units everywhere in the manuscript, e.g., 100 cm and not 100cm

• Done: Line 263

Figure 4: still not clear what the a, b and c stand for. Please explain better in the figure caption

• Done: Sampling periods where average tree height differs significantly from the subsequent years are annotated with different letters.  

Page 8, l.272: Top-kill is caused by the effect of herbivores? Or from something else. Maybe some photos of top-killed trees or trees with evident effects of browsing/grazing will help readers that are not from Kenya to familiarize with what you describe.

• we have provided a contextual definition in Line 306-307: “complete death of the aerial biomass, regardless of whether the plant recovers by resprouting”

Page 8, l.276: replace “in burns than unburned” with “in burned compared to unburned areas”

• Done: Line 311

Page 8, l.279: I recommend a table that will summarize all the results from all the combinations described, denoting where and in what combinations we have low, moderate or high mortality, canopy volume and tree height etc. Without the table, readers are struggling to catch the take home message. A table could help a lot to simplify the understanding of your findings.

• The information requested by the reviewer is captured in Table 2

Page 8, l.284: replace intermediate with moderate

• Done: Line 321

Page 9, l.309: maybe also because by removing the biomass, the trees that remain are more vigorous since they face less stress for resources and have better access to nutrients and water (antagonistic vegetation is removed by browsing)

• We agree with the reviewer that competition from grasses may be a major bottle-neck to recruitment of tree seedlings. Indeed, work by LaMalfa al. 2021 and Porensky & Veblen 2012 has demonstrated that grass competition may limit growth of Acacia drepanolobium seedlings in this study system. However, in this section, our focus is on trees that are beyond the height (age) category where grass competition is the primary limiting factor

 

Reviewer 3 Report

General:

This paper takes an in-depth look at fire x herbivory interactions, including consideration of different herbivore types, and the experimental design and analysis methods appear robust. The quality of presentation and writing is excellent.

I could identify some problems, however, mostly revealed in the Discussion. One concern is that no explanation is offered for why saplings grew faster on burned than unburned subplots. Another concern is that temperature data are missing from the results, even though they are cited in the Discussion. I also don’t understand how temperature was measured using the ceramic tiles. Knowing the temperature data trends as a function of the treatments and the other variables measured is necessary to understand the mortality results, and subsequent veg response. As such, I can’t fully evaluate the merits of the study.

 

Specific Comments:

L84-86. Hypothesis 3 doesn’t include a “because” explanation like Hypotheses 1 and 2.

L143. I think “area” should be made plural.

L144. I think “source” should be made plural.

L156-158. Safety is an understandable and justifiable reason to burn the higher fuel load plots earlier in the day. However, fuel loads do affect fire behavior, so it seems likely that had the plots with higher fuel loads burnt later in the day, greater fire intensity and more woody topkill would have resulted. As such, burning the potentially more lethal fires sooner would mitigate fuel loading as a source of variation between the burn plots. This may have served as a useful control on the experiment, whether or not it was intended. It at least merits mentioning and further discussion.

L167. I think “sapling” should be made plural.

L170, 172. Hyphens have white space after but not before.

L171. Delete the comma after “2018”.

L176. Forward slash has white space after but not before.

L200. There’s a superfluous white space before the citation “[36]”.

L204. Insert “the” after “in”.

Fig. 4. Were tree heights not tracked after the 2018 burns?

L270. I don’t see how “subplots” is relevant here, only the burning. Change “previously burned subplots” to “previous burning”?

L274. This would be the right place to offer your explanation(s) for why this result refuted your hypothesis.

L280-284. Where in the Results were temperatures presented? You can’t expect the reader to find and read references 19 and 33 to learn this information essential to understanding this result.

L315. I understand why fires should be more intense in the O plots (no herbivory), but why the C plots?

 

Author Response

Reviewer 3

This paper takes an in-depth look at fire x herbivory interactions, including consideration of different herbivore types, and the experimental design and analysis methods appear robust. The quality of presentation and writing is excellent.

• Thank you

I could identify some problems, however, mostly revealed in the Discussion. One concern is that no explanation is offered for why saplings grew faster on burned than unburned subplots.

• Our study did not compare sapling growth between unburned subplots, but rather compared growth in plots burned once and those burned twice. In absence of megaherbivores, we found greater sapling growth in plots that were burned for the second time than those burned only once. We have explained possible mechanism for such differences under the discussion: Line 357-376

Another concern is that temperature data are missing from the results, even though they are cited in the Discussion. I also don’t understand how temperature was measured using the ceramic tiles. Knowing the temperature data trends as a function of the treatments and the other variables measured is necessary to understand the mortality results, and subsequent veg response. As such, I can’t fully evaluate the merits of the study.

• We have now included temperature data for the three sets of burns and we believe this has enriched the manuscript: Table 1. We have also clarified how fire temperature were measured in Line 149-151; “We monitored fire temperatures using ceramic tiles painted with an array of six different temperature sensitive Tempilaq (LA-CO industries, Elk Grove Village, Illinois, USA) paints, designed to melt at 79, 107, 149, 204, 260, and 316^oC”

Specific Comments:

L84-86. Hypothesis 3 doesn’t include a “because” explanation like Hypotheses 1 and 2.

• we have revised this as follows: “Trees subjected to repeated burns and herbivory would generally have lower growth rate than trees in plots burned once and protected from herbivores because both fire and herbivory additively suppress tree growth”: Line 85-88

L143. I think “area” should be made plural.

• Done: Line 145

L144. I think “source” should be made plural.

• Done: Line 146

L156-158. Safety is an understandable and justifiable reason to burn the higher fuel load plots earlier in the day. However, fuel loads do affect fire behavior, so it seems likely that had the plots with higher fuel loads burnt later in the day, greater fire intensity and more woody topkill would have resulted. As such, burning the potentially more lethal fires sooner would mitigate fuel loading as a source of variation between the burn plots. This may have served as a useful control on the experiment, whether or not it was intended. It at least merits mentioning and further discussion.

• Good point. We have mentioned this in the methods section (Line 160 -161) as well as in the discussion (335 - 339)

L167. I think “sapling” should be made plural.

• Done: line 170

L170, 172. Hyphens have white space after but not before.

• Done: line 173

L171. Delete the comma after “2018”.

• Done: Line 174

L176. Forward slash has white space after but not before.

• Corrected: Line 179

L200. There’s a superfluous white space before the citation “[36]”.

• Deleted: Line 205

L204. Insert “the” after “in”

• Done: Line 210

Fig. 4. Were tree heights not tracked after the 2018 burns?

• We did not track tree heights after 2018; hence that data is not available

L270. I don’t see how “subplots” is relevant here, only the burning. Change “previously burned subplots” to “previous burning”?

• Done: Line 305

L274. This would be the right place to offer your explanation(s) for why this result refuted your hypothesis.

• We have provided explanation of the above concern in line 359 - 362. “We attribute the higher resprout vigor in repeat burns to the fact that trees may have suffered less tissue damage in repeat burns than in single burns, therefore retaining more above ground tissue after fire”

L280-284. Where in the Results were temperatures presented?

• Information requested by the reviewer has been added in Line 222 - 225 and temperature data provided in table 1.

L315. I understand why fires should be more intense in the O plots (no herbivory), but why the C plots?

• We refrained from grazing cattle in their designated KLEE plots for the 5 months prior to the burns to help ensure sufficient fuel loads. Hence, C Plots which were only accessible to cattle had much similar fuel loads to O plots. We have clarified this in the manuscript: Line 129-130