the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 May 2025
Radial growth decline in a tropical Andean treeline in Bolivia
Abstract. Relative to research efforts in higher latitiudes, the impact of climate shifts in the tropical treeline remains understudied. Little is known about the tree growth dynamics and climate response at this treeline over the past few centuries, and at present under a rapidly changing environment. Here we provide information on recent changes in tree-ring patterns of Polylepis pepei BB.Simpson, a tropical tree species that grows in a monospecific forest at the elevational treeline in the Andes-Amazon ecotone of Bolivia and identify factors that limit its radial growth. We first developed a ring width (RW) chronology spanning 1867–2018 C.E. using dendrochronological methods and independently verified annual periodicity with radiocarbon dating. The RW chronology indicates a significant (p < 0.01) radial growth decline in P. pepei since 1997, a trend that mirrors a decrease reported in other Polylepis species from the drier central Andes of South America. P. pepei tree-ring width (RW) was mostly limited by mean, minimum, and maximum temperature and precipitation during austral summer (November–January). Over the instrumental period (1981–2019) prior-year temperatures negatively affected current-year tree growth (p < 0.05), while prior-year wet conditions were associated with higher growth (p < 0.05). Gridded temperature records (1901–2019) showed a significant increase in minimum temperatures and a decline in the diurnal temperature range since 1967, which may reduce orographic convection and water availability at higher elevations where our forest is located. In situ daily measurements from dataloggers in the forest recorded higher temperatures and lower relative humidity values when data was available. Our results suggest less moisture availability associated with warming conditions was related to the observed tree-growth decline. If temperature continues to rise at current rates, one of the highest-elevation tree species on the globe, P. pepei, could face severe consequences. This work provides insights into the past and historical trends of a tropical Andean treeline, which shows a recent decline also observed in other high-elevation forests (4657–4800 m.a.s.l.) of tropical South America (>17° S).
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-2032', Anonymous Referee #1, 17 Jul 2025
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AC2: 'Reply on RC1', Rose Oelkers, 11 Sep 2025
On behalf of the co-authors, we thank-you for your time and the suggestions to improve this manuscript. We are open to implementing these changes during the review process and will generate a table to summarize the chronology statistics. We invite you to view the document we’ve included in our response to Referee #2 that highlights potential revisions based on the feedback.
Citation: https://doi.org/10.5194/egusphere-2025-2032-AC2
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AC2: 'Reply on RC1', Rose Oelkers, 11 Sep 2025
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RC2: 'Comment on egusphere-2025-2032', Anonymous Referee #2, 15 Aug 2025
Reviewer Comments
I have carefully read and reviewed the manuscript entitled “Radial growth decline in a tropical Andean treeline in Bolivia”. The study highlights the importance of dendrochronological research at a tropical Andes treeline dominated by Polylepis pepei, particularly the role of minimum temperature and its changes over the past ~20 years.
While the topic is relevant and the results could be valuable, I have several issues that, in my opinion, mean the manuscript requires major revision before it can be considered for publication.
My main concerns are:
The manuscript is not well structured, particularly in the Introduction, Methodology, and Results sections. Some methodological details are presented in the Results and vice versa.
The Introduction lacks coherence and focus. For example, the first paragraph discusses Northern Hemisphere treelines, which are not the focus of this study. I suggest beginning with tropical treelines in the Andes, then the focal species, and finally the dendrochronological potential of P. pepei.
The quality of English throughout the manuscript is poor, making comprehension difficult.
Methodology: it is unclear why two different climatic datasets were used for the climate–growth response, how many samples were collected, their age distribution, and whether growth differences were observed between the south- and west-facing slopes, or if the two campaigns contributed to the growth decline.
Terminology: please be careful with word choice, e.g., “shift in tree-ring width” (line 39), “common radial growth” (line 42).
Authorship: the 'Author Contributions' section does not list all authors. Please revise this section and check the grammar throughout.Below are detailed comments by section.
Abstract
The first sentence mentions tree growth dynamics over “a few centuries and present,” yet the study focuses on the past 40 years. Please revise it.
Avoid unnecessary acronyms, e.g., delete “RW” for ring width.
Lines 24–25: I suggest moving the phrase “a trend that mirrors a decrease reported in other Polylepis” to the end of the sentence “…tropical Andean treeline, shows a recent decline…”.
I recommend removing the data logger results from the Abstract, as these are weak given the data quality.Introduction
Please consider restructuring the Introduction. My suggestion is to start with tropical treelines in the Andes, then the main species, especially P. pepei, the disturbances (anthropogenic and climatic), and finally the previous dendrochronological studies in similar areas.
Lines 37–49: Remove the discussion of Northern Hemisphere treelines; it is misleading, and references are outdated.
Line 39: Clarify what is meant by “shifts in tree-ring width have been linked to upward recruitment.”
Line 53: Specify why tropical treelines, more than other ecosystems, are of great concern, and explain the link with anthropogenic influences.
Line 65: Clarify the use of the term “timberline.”
Lines 69–72: Add the time period of the correlation analysis and discuss how your results compare to Morales et al. (2004) in the Discussion.
Line 78: Clarify what “respectively” refers to.
Line 83 and elsewhere: Standardise the expression for meters above sea level (“m asl” or “m a.s.l.”).
Lines 84–85: Add the link to Figure 1 in the text.
Lines 93–94: Avoid acronyms that are not used later (e.g., TNA, TSA, MNP).
Lines 96–98: Rewrite for clarity; the current sentence is vague and overly complex.
Lines 108–112: Consider whether these sentences are relevant.Materials and Methods
Lines 129–131: Rewrite for clarity and avoid repeating “dendrochronological” twice.
Lines 134–140: Clarify if two different climate datasets were used for temperature and precipitation, whether they were compared, and if the climate–growth correlation was conducted for the same period.
Lines 183–188: Move this ecological description of P. pepei to the species ecology paragraph.
Figure 2: Add a photo of the whole plant. If possible, compare phenological and growth differences between the open-canopy south-facing slope and the closed-canopy west-facing slope.
Line 214: Check the reported correlation values. Consider omitting the section on chronology correction, as the issue appears not to be ring detection but miscounting an incomplete ring, if I understood correctly.
Line 218: State whether the raw time series shows a negative trend.
Line 219: Provide the number of samples from 2012 and 2019, distinguishing between living and dead trees. A table with tree status, age, interseries correlation, number of cores, and diameter would be useful.
Lines 219–220: Use precise terminology, did the authors “compare” or “correlate” with climate data?
Line 223: Indicate which program was used to detrend the series.
Line 233: Specify the climate variables (temperature and precipitation) used and the exact period covered.
Line 279/Table 1: Move this to the Results section. Clarify the years classified as ENSO events, why 13 years in each column, if there should be 24 years total?Results
Lines 283–286: Ensure that methods are not presented here, and results are not in the Methods section.
Line 288: Provide more details on the dendrochronological suitability of P. pepei.
Lines 302–306: Correct the figure caption; the current “B” panel does not match the description. Reduce the font size of the legend in panel C. Clarify whether the pointer-year analysis covers a specific period. Discuss whether reduced sample depth (2012 vs. 2019) could explain the negative trend in the raw chronology and report the age distribution of samples.
Figure 4: Make month labels more legible. Consider correlating the standardised chronology with the normalised diurnal temperature range (which shows a negative trend). Also, test correlations for November–February minimum temperature. Include seasonal correlations here and keep spatial correlations in a separate figure.
Include a map of the species’ spatial distribution to help interpret climate response patterns.Discussion
Lines 405–407: Be specific about whether growth differences exist between the south- and west-facing slopes. State the number of samples from each slope and how these differences may influence results.Citation: https://doi.org/10.5194/egusphere-2025-2032-RC2 - AC1: 'Reply on RC2', Rose Oelkers, 11 Sep 2025
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The manuscript Radial growth decline in a tropical Andean treeline in Bolivia to be considered for publication in Egusphere. The manuscript is well written and the information is clear. In my opinion, the research is interesting for international readership. However, I think that the manuscript needs to be revised before being accepted. Minor modifications as follows:
Line 116-121: This paragraph move to after of line 81.
Line 149: Add a table with the information of sites and samples.
Line 295: “Notable growth” change to Extreme growth.
Line 309-310: Delete sentence. The next sentences repeat the same.