the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Contrasting growth and mortality responses of different species lead to shifts in tropical montane tree community composition in a warmer climate
Bonaventure Ntirugulirwa
Etienne Zibera
Nkuba Epaphrodite
Aloysie Manishimwe
Donat Nsabimana
Johan Uddling
Abstract. The response of tropical trees and tree communities to climate change is crucial for the carbon storage and biodiversity of the terrestrial biosphere. Trees in tropical montane rainforests (TMFs) are considered particularly vulnerable to climate change, but this hypothesis remains poorly evaluated due to data scarcity. To reduce the knowledge gap on the response of TMFs trees to warming, we established a field experiment along a 1300–2400 m elevation gradient in Rwanda. Twenty tree species native to montane forests in East and Central Africa were planted in multispecies plots at three sites along the gradient. They have overlapping distributions but primarily occur in either transitional rainforest (1600–2000 m a.s.l) or mid elevation TMF (2000–3000 m a.s.l.), with both early- (ES) and late-successional (LS) species represented in each elevation origin group. Tree growth (diameter and height) and survival were monitored regularly over two years. We found that ES species, especially from lower elevations, grew faster at warmer sites while several of the LS species, especially from higher elevations, did not respond or grew slower. Moreover, a warmer climate increased tree mortality in LS species, but not much in ES species. ES species with transitional rainforest origin strongly increased in proportion of stand basal area at warmer sites, while TMF species declined, suggesting that lower-elevation ES species will have an advantage over higher-elevation species in a warming climate. The risk of higher-elevation and LS species to become outcompeted by lower-elevation and ES species in a warmer climate has important implications for biodiversity and carbon storage of Afromontane forests.
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Bonaventure Ntirugulirwa et al.
Status: final response (author comments only)
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RC1: 'Comment on bg-2023-42', Anonymous Referee #1, 17 Apr 2023
In this manuscript the authors describe the results of a large manipulative experiment where multiple tree species were planted at three different elevations in order to observe the effect of temperature change in their growth and mortality. In addition, these species represented different successional strategies (Early and Late) and different forests of origin (montane vs transitional). I find the experiment very impressive. The manuscript is very well-written and presented so I want to congratulate the authors. I do however, think that the story is quite complicated and the constant use of acronyms does not help to simplify it. The intro needs a few adjustments to make it shorter and more concise (see below) but it is on the Methods/Results and Discussion where it gets harder to follow and stay engaged. These are a few suggestions to make the manuscript more engaging:
- Try to reduce the number of acronyms or modify the current ones for something more explicit, for example: LVTF- Transitional forests, TMF – Montane forests; ES- Early-S, LS- Late-S
- Include a diagram that explains the experimental set up with the three elevations, species origins, data recorded (could be in Supplement)
- Include a summary figure with your main results. The figures (for example fig.2 and 3) have a lot of information and it is hard to focus on what is important and what does it mean.
Something that I do not understand is the role of the higher elevation site as the control site. That is mentioned in the methods but I do not see much discussion around this fact.
The title undersells the study. That could be the title for an observational study. I think it should reflect the enormous experimental work and novelty of it.
Some line comments:
Abstract
Very well written. The only doubt I have is at what stage where the plants transplanted (seedling, sapling…)
Introduction
45- negative effects on xxxx and where?
50 in the field? Need to be more specific, In mountains heat and drought do not always covary
59 – Large variability… This sentence is unclear needs to be reorganized.
64 – You may want to read Tovar et al 2022
65 – The lower elevation lmit of TMF varies widely between and within continents so I am not sure what this >1000 m a.s.l. refers to
70 – why species on the higher elevations are further away from their thermal optimum? You imply that they could tolerate a higher increase in temperature than lower elevation plants, but you need to justify that. For example Leon-Garcia and Lasso 2019. Although here they go all the way to the paramo ecosystem.
83-90 The intro is great until this paragraph. I think it is a bit repetitive, and there is one minor point that could be made more clearly and briefly. Maybe as a first line of the next paragraph a sentence linking the functional strategy with different chances to survive and then growth forms?
- I am more familiar with using ES and LS to refer to species but primary or old-growth and secondary forests to the forests. I think better to keep them separate.
- I don’t like the sentence: remains uncertain.. is it the forests? The species??. Also, these experimental indications – how are they different from what you are doing here? Are they greenhouse experiments, models? Better to indicate so you can highlight the novelty of your paper.
100 – defining ES species here is a bit late, should come at the beginning of the paragraph.
100 – suspectable= susceptible?
Overall this paragraph also needs some reorganizing and trimming
105-115. This seems like a good point but is it important enough to be a whole paragraph? I think the intro is really long and this is something that you should cut.
116 – I feel now we are back on track. This brings the story back to line 81 – I would probably talk about the growth strategies after this paragraph or even after the next one where you explain your objective. The reader needs to know early on the intro what you are doing on the manuscript, and it is not clear until here.
125 – identical plant material- not sure what it means
129 – at what stage are the trees transplanted? Saplings?
135 – are lower elevation species planted on even lower elevations? I get that from the hypothesis. Maybe include the elevational range of the species in the objective?
Overall, I would reduce the background information on precipitation and drought because now I realize that you are irrigating to isolate the effect of temperature, so better to focus on that.
Methods
150 – increased/decreased by
159 – I don’t understand why that site is the control at 2400 because the species grow nearby at (potentially) 1600m which is as low as the mid-elevation site.
In Table 1 PNV of HE should be the full name to make it easier to link with the text (TMF)
224 – I do wonder about the effect of solar radiation. The HE site is thus the control but also the more shaded one.
It is overall a bit confusing the species origin vs the planting sites. And where is the control evaluated?
Results (see comments above)
Discussion
405 – This drought period, was it a problem with the irrigation system? Has it been mentioned before? can it have an effect on other variables?
467 – Transplant experiments are also a good approach for this and should not be ignored (e.g. Tito et al 2020) but may be limited to one or few species.
There is also a lot of work into temperature sensitivity measured as leaf temperature tolerance and safety margin. I would change a bit the phrase to be more specific to what you want to say.
477 – pattern was or patterns were
482 – Moreover, other studies in the photosynthesis of Rwandan forests showed…
531- the mortality driven thermophilization is a mechanism at the community level. This study calculated the Community thermal index using the species optimum temperatures and their relative basal areas, and then its change through time. A change in CTI is easier/more obvious by the mortality of large trees than by the addition of new ones given their effect of the relative basal area. So I would not necessarily think that it is a big contradiction to your results of mortality per se, they are different approaches.
537- Amazonian plots with tropical forest? Not sure why the explanation, is this for the Australian plots?
Fig S1,S2,S3- I think you should mention here that the irrigation/nutrient experiment only started in 2019-9 and it had no effect on your results - to remind that it is not a variable taken into account in the manuscript
Indira V. Leon-Garcia ,Eloisa Lasso. 2019. Plos One. High heat tolerance in plants from the Andean highlands: Implications for paramos in a warmer world
Tito, Vasconcelos, Feeley, 2020. Journal of Ecology. Multi-population seedling and soil transplants show possible responses of a common tropical montane tree species (Weinmannia bangii) to climate change
Tovar et al. 2022. Journal of Biogeography. Understanding climate change impacts on biome and plant distributions in the Andes: Challenges and opportunities
Citation: https://doi.org/10.5194/bg-2023-42-RC1 - AC1: 'Reply on RC1', Göran Wallin, 03 Aug 2023
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RC2: 'Comment on bg-2023-42', Anonymous Referee #2, 14 Jul 2023
General comments
This is a very interesting and timely paper on an important subject. The authors make use of the exciting Rwanda TREE experiment to assess the performance of a number of tree species from different climates of origin and different successional stages (early and late successional). These data are very novel, covering a large number of species and able to assess closely temperature effects with limited influence of drought. The findings are logical and supported by other literature, showing a higher sensitivity of late successional species and high elevation species to temperature, as measured with data on growth, mortality and contribution to basal area. There are some issues around the interpretation due to the experimental rather than observational nature of the study, but these are mostly sufficiently discussed and acknowledged by the authors.
Specific comments
The main issue I found with the paper is that the site is planted with early and late successional species, and therefore in a high light environment it is possible there would be poor performance of the late successional species. However, this is raised and discussed in the paper, and the crucial thing is that this would be the case at all the sites, and therefore by looking at differences across the sites at different temperatures, this becomes less important. While this is in the discussion, I think it might be useful to add something on this, justifying the approach, in the Introduction so the reader is not wondering about it throughout.
I also think that because these are experimentally planted sites, it is harder to consider that composition is really being assessed. I think the study brings great insight into how different species respond, and how this affects composition within the sites, and the knowledge gained can be generalised to natural systems, but I think there needs to be a little more conservative language, for example line 134 could be phrased “tree community composition within the experimental sites”.
There is a lot of detail in the analysis, perhaps looking at too many variables. I am not sure the relative growth rate results are needed in addition to the analysis of final diameter and height. Unless the authors think this adds necessary insight, I suggest to remove it, or include in supplementary material. Some technical clarifications on the analysis are needed (see below).
Technical comments (typos and small clarifications)
line 30. not much -> had little effect
line 64. at -> within
line 87. is -> are
line 129. What is the difference between Afromontane and African highland? Please define.
line 150. with -> by, check spelling on approximately
line 175-180. This section could be written more clearly/simply. Please state what water and nutrient treatments were applied in chronological order. It seems the plants were watered except for one dry season – has this one dry season had an impact on results?
line 177. No significant effect of the treatments on what?
line 201. Define wildling
section 2.5 This section could be written more concisely.
line 248. know -> known
line 259. 3+ superscript
line 290. The basal area approach is somewhat confusing, but I understand the intention to standardize for different initial number of stems of each species group, but I think it could be explained better. The text mentions the dead tree contribution to basal area – presumably here considering the loss of trees to basal area composition, rather than basal area of dead trees? Clarify that only living trees are used for basal area. The approach to considering mortality impact on basal area was not easy to understand.
line 301. Here and other places it looks like there is a typo with repeated D-RGR_D10-25.
line 301. It needs to be specified that the D and h information is used only from the final census, so looking at the total effect of the two year study period. For the growth rates, is this averaged across all the census intervals?
line 302. Here mentions both species and species group for the ANOVA models. Presumably this is then two separate analyses? The results seem to just report statistical results of species (e.g table 3, figure 2) and not species groups. If they are not included, perhaps remove mention on species groups here. If the are included, differences between groups could be added to Figure 2 with another set of letters.
line 307. Using plot level means. This is ok, but individual tree data could be used within mixed effects models, with a random effect for block. This would account for multiple trees within the same block.
Table 3. Details of the anova results here and in other tables could be moved to the Supplement.
Fig 3. The axis labels are difficult to read.
line 369/370. Are these numbers referring to D and h or ME and LE? Not clear.
line 378. species name spelling and formatting needs correction.
Fig4. X-axis could be relabelled with month/year.
Line 415. Why is this important, does it mean it is before the irrigation was stopped and therefore that the results are temperature rather than moisture driven?
line 481. support -> supports
line 505. There is interesting discussion here. Over time, with a more closed canopy the LS species could increase following successional process, is this relevant here?
line 586. divers -> diverse
line 593. The text could be more explicit about the implications of the results for biodiversity, carbon, species selection, e.g. which species types does this research suggest would be best for restoration projects.
Citation: https://doi.org/10.5194/bg-2023-42-RC2 - AC2: 'Reply on RC2', Göran Wallin, 03 Aug 2023
Bonaventure Ntirugulirwa et al.
Bonaventure Ntirugulirwa et al.
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