the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
A shift in circadian stem increment patterns in a Pyrenean alpine treeline precedes spring growth after snow melting
Abstract. Changing snow regimes and warmer growing seasons are some climate factors influencing productivity and growth of high-elevation forests and alpine treelines. In low-latitude mountain regions with seasonal snow and drought regimes such as the Pyrenees, these climate factors could negatively impact forest productivity. To address this issue, we assessed the relationships between climate, snow, and inter- and intra-annual radial growth and stem increment data in an alpine Pinus uncinata treeline ecotone located in the central Spanish Pyrenees. First, we developed tree-ring width chronologies of the study site to quantify climate-growth relationships. Second, radial growth, tree water deficit, and shrinking/swelling cycles were quantified and identified at monthly to daily scales using fine-resolution dendrometer data. These variables were extracted for three climatically different years, including one of the hottest summers on record in Spain (2022), and were related to soil water content, soil and air temperature, and the dates of snow duration across the treeline ecotone. Warmer February and May temperatures enhanced tree radial growth, probably because of an earlier snow melt-out and start of the growing season and higher growth rates in spring, respectively. The characteristic circadian cycle of stem increment, defined by night swelling and day shrinking, was detected in summer and autumn. However, this pattern inverted during the snow season from November through April, suggesting a transition phase characterized by wet soils and swollen stems preceding the spring onset of growth. Air temperature, soil temperature and moisture, and the presence of snow are strong indicators of how much and for how long mountain trees can grow. Shifts in daily stem increment patterns reveal changes in early growth phenology linked to snow melting.
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RC1: 'Comment on egusphere-2024-3385', Edurne Martinez Del Castillo, 29 Nov 2024
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The manuscript egusphere-2024-3385, entitled “A shift in circadian stem increment patterns in a Pyrenean alpine treeline precedes spring growth after snow melting” tackles a timely and important ecological issue, namely the effects of climate change on high-elevation ecosystems. The authors use dendrometer data, tree-ring chronologies, and climate data to examine the influence of air and soil temperatures, snowpack duration, and soil water content on tree growth at inter- and intra-annual scales. The results highlight that warmer February and May temperatures promote earlier snowmelt and longer growing seasons, affecting the growth patterns. The figures are well designed and informative, complementing the text and effectively communicating the results. The analyses are succinct to only three years of data, but they are climatically distinct years, which adds variability to the findings. The study introduces a novel perspective on circadian stem increment cycles, giving insights into the inversion of these patterns during snow seasons and their potential phenological implications. Overall, the manuscript would be a valuable contribution to Biogeosciences, but several aspects need to be improved, see my detailed comments below.
Introduction
The introduction is comprehensive, but it could better synthesize the relevance of circadian cycles in tree physiology and tree line dynamics Additional studies could strengthen the literature review and help linking the ecological context of the article. Some examples:
- Lázaro-Gimeno, C. Ferrari, N. Delhomme, M. Johansson, J. Sjölander, R. Kumar Singh, M. Mutwil, M. E Eriksson (2024) The circadian clock participates in seasonal growth in Norway spruce (Picea abies), Tree Physiology, Volume 44, Issue 11. https://doi.org/10.1093/treephys/tpae139
- Lüttge, U., Hertel, B. Diurnal and annual rhythms in trees. Trees 23, 683–700 (2009). https://doi.org/10.1007/s00468-009-0324-1
L88. The hypothesis should be formulated at the end of the last paragraph to be quickly identified.
Materials and Methods
The methodological approach is robust but could be more transparent in addressing potential biases or limitations, such as the reliance on snow presence inferred from soil temperature oscillations (L170-176), which might benefit from validation using direct snowpack observations or satellite images. If this method is commonly used to calculate the period of snow presence, other studies should be cited. It is not clear to me whether snow season definitions based on temperature oscillations are an innovative approach, but they could be better validated with alternative snow metrics.
Some methodological details are sparse. For instance, while dendrometer calibration is mentioned, specific steps to address potential biases in measurements (e.g., sensitivity to environmental factors) could be elaborated. The relatively small sample size (only nine trees for dendrometer data) limits the generalizability of the conclusions.
Statistical approaches (e.g. Pearson correlations and moving averages) are valid but might benefit from additional justification regarding their selection.
Results
The findings related to soil water content (SWC) are briefly discussed. Since SWC is a critical factor influencing tree growth, a more detailed exploration of its role during critical phenological transitions could enhance the discussion.
Table 2. It is not clear what the letters after the average±SE mean. The caption says that they indicate significance, but it is not clear what the difference between a, b, or c is.
Table 3. Check the precipitation values for year 2021, those values cannot be mm.
Table 3. The total precipitation of the growing season in 2023 cannot be 114mm if there was 118 mm during May. Check the values, please.
Figure 4. I understand that this analysis starts in mid-April as this is the beginning of the radial increase of the trees, however, the highest historical correlations with climatic variables (in fact, with temperature) were detected in February. In my opinion, these climate-growth relationships are not sufficiently explored and discussed in the article.
Figure 6. This is an excellent summary figure (could be a very illustrative graphical abstract) but might benefit from additional labeling or annotation for clarity. What is the orange square on the tree trunk of the “normal pattern” and “Transition” trees? Why is one filled and the other is not?
Discussion
This is the major weakness of the article, in my opinion. The discussion is rather superficial, and there is a limited exploration of the broader ecological and global implications of the findings. It effectively connects findings to prior research, but a deeper exploration of how these results might generalize to other alpine ecosystems would increase the manuscript's broader applicability. The authors could explore deeper into the ecological significance of phenological shifts, particularly their long-term impacts on carbon sequestration and forest dynamics under climate change.
While the study provides detailed and novel insights into circadian stem increment cycles and the influence of snow dynamics on tree phenology, it largely focuses on a specific alpine tree line in the Spanish Pyrenees. The findings are not sufficiently contextualized within a broader ecological framework, such as global alpine ecosystems or potential feedback mechanisms with climate change.
Additionally, the climate-growth relationships shown in the results are not sufficiently explored and are contextualized only using two research papers (Sanmiguel-Vallelado et al., 2019, 2021). For instance, in L283 the authors mention prior studies without citation.
A paragraph explaining the study's limitations is needed. For example, the small sample size of the dendrometer data (i.e., nine trees) undermines the generalizability of the results, especially considering the spatial heterogeneity often present in tree ecotones.
Conclusion
L300 – The last phrase should be an overall conclusion of your findings, not a justification for further studies.
Citation: https://doi.org/10.5194/egusphere-2024-3385-RC1
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