the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Mar 2025
Tree Growth and Water-Use Efficiency at the Himalayan Fir Treeline and lower altitudes: Roles of Climate Warming and CO2 Fertilization
Abstract. Alpine forests are increasingly exposed to rising temperatures and intensified drought, potentially pushing species beyond their tolerance limits. However, the extent to which rising atmospheric CO₂ (Cₐ) can mitigate these stressors by enhancing tree intrinsic water-use efficiency (iWUE) remains unclear. We investigated the growth and physiological responses of Himalayan fir (Abies spectabilis) using basal area increment (BAI) and δ¹³C data to track ecophysiological processes over recent decades along an elevational gradient in warming and drying sites on the Tibetan Plateau. Significant growth increases were observed at all elevations in wet regions, while negative growth trends were noted at lower elevations in dry regions. Climate–growth correlation analysis revealed that growth is primarily constrained by growing season temperatures and spring moisture availability. Tree iWUE increased over time at all elevations, with a stronger increase in wet regions. Tree growth at lower elevations in dry stands was negatively related to iWUE, whereas BAI in wet regions was positively associated with iWUE. Leaf intercellular CO₂ (Cᵢ) increased proportionally to Cₐ after 1965. Structural equation modeling indicated that temperature was a key driver of BAI and iWUE at all elevations in wet regions, while temperature had negative effects on BAI at lower elevations in dry regions. These results suggest that elevated Cₐ and temperature can stimulate tree growth in high-elevation forests in wet regions, but the positive effects do not compensate for the negative impacts of reduced water availability at lower elevations in dry regions. Warming-induced drought stress may thus emerge as a more significant driver of growth compared to increasing Cₐ levels in comparable alpine forests. Our findings provide critical insights for refining assumptions about CO₂ fertilization and climate change effects in ecophysiological models.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
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Supplement
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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Supplement
(998 KB) - BibTeX
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- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2025-952', Anonymous Referee #1, 14 Apr 2025
- AC1: 'Reply on RC1', Lixin Lyu, 30 May 2025
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RC2: 'Comment on egusphere-2025-952', Anonymous Referee #2, 16 Apr 2025
General comments
This manuscript examines the long-term physiological and growth responses of Abies spectabilis across elevational gradients in wet and dry regions of the Tibetan Plateau, using tree-ring width, basal area increment (BAI), and δ¹³C-derived intrinsic water-use efficiency (iWUE). The authors aim to disentangle the relative roles of climate warming and atmospheric CO₂ in modulating tree growth. The study addresses timely questions in forest ecophysiology and uses standard dendrochronological and isotope techniques. However, the manuscript suffers from several significant conceptual, methodological, and shortcomings. The analysis relies heavily on correlative patterns/scenarios with limited mechanistic interpretation. Basic sample metadata is omitted and critical information on detrending and sample replication is lacking.
Moreover, while the Tibetan Plateau is an understudied region, the manuscript largely reproduces well-established findings regarding iWUE trends, growth–climate relationships, and physiological strategies under CO₂ enrichment. The analysis does not introduce new mechanisms, theory, or methods. Thus, its novelty lies primarily in applying standard approaches to a geographically distinct context — which is of regional interest, but not a substantial conceptual advancement for the field.
Major concerns
1- The authors fail to report key information about the sampled trees, such as sample sizes per site, tree ages (range and mean), diameter at breast height (DBH), or inter-individual variability. Given the long temporal scope, this omission undermines the interpretation of growth trends. Tree age can confound long-term growth patterns and sensitivity to climate or CO₂. This basic metadata must be included in a table or appendix.
2- While the authors use ARSTAN with negative exponential or linear detrending, they do not explain how this affects long-term trends — the central focus of the paper. Detrending can remove real long-term growth signals (e.g., due to CO₂ fertilization). Were raw BAI series analyzed? Were alternative detrending methods tested (e.g., Regional Curve Standardization)? This needs clarification and discussion.
3- The use of theoretical Ci/Ca trajectories (Scenarios 1–3) is oversimplified. These scenarios assume fixed relationships that rarely hold across environmental gradients or time. The authors treat alignment with Scenario 1 ("Ci = constant") as evidence of physiological strategy but offer no underlying reasoning in terms of stomatal control, leaf traits, or drought response. This needs much deeper physiological interpretation.
4- The authors repeatedly attribute growth changes to rising Ca or warming based on correlations, without ruling out confounding variables (e.g., age, stand density, soil conditions). The SEM framework is promising but underdeveloped, and the model structure, diagnostics, and assumptions are not detailed.
5- Many of the main findings — that iWUE increased, growth responded positively to warming in wet regions, and drought limits growth at lower elevations — are well established in the literature. The manuscript would benefit from a clearer articulation of what is truly new.
6- The manuscript references nutrient availability as a potentially limiting factor (e.g., lines 54–56, 238), citing literature that suggests it can constrain the CO₂ fertilization effect. However, no nutrient data (e.g., soil N or P) are presented or analyzed. This creates a mismatch between the framing and execution of the study. Either include relevant data or remove unsupported speculation.
7- While I believe that using AI to assist with scientific writing is acceptable, the manuscript shows clear signs of overreliance, including inconsistent use of abbreviations, redundancy of abbreviations, and occasional lapses in technical accuracy. The dataset presented is valuable, but the manuscript requires substantial revision to improve clarity, coherence, and scientific precision before it can be considered for publication.
Minor comments
1- Use species name in italic.
2- More information on the ice core data are needed
3- Figures: Captions are too brief (especially in Figs. 2, 3, 4, 5) (also, there are no legend for coloring in figures).
4- Figure 4, it is hard to believe the results about iWUE since a lot of p-values shoz no significance of trends.
5- Any more information on the SPEI data? How were the data collected? Where they gridded data? Were the data extracted at specific locations?
Citation: https://doi.org/10.5194/egusphere-2025-952-RC2 - AC2: 'Reply on RC2', Lixin Lyu, 30 May 2025
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2025-952', Anonymous Referee #1, 14 Apr 2025
This study investigates the physiological and growth responses of Himalayan fir to climate warming and CO2 fertilization across elevational gradients in wet and dry regions of the Tibetan Plateau. The research addresses a critical knowledge gap regarding the interplay between CO2 fertilization, temperature, and drought stress in high-elevation forests. While the manuscript presents valuable insights, several issues need clarification to strengthen the conclusions.
Major Concerns:
1 Interpretation Issue of Ci/Ca Scenarios: The comparison of observed Ci/Ca trends to theoretical scenarios lacks quantitative validation. The statement that Ci/Ca "largely followed Scenario 1" is qualitative and not statistically tested.Please quantify deviations from theoretical scenarios using goodness-of-fit metrics (e.g., RMSE, AIC) and report significance tests.
- Temporalcoverage issue: The results section mentions post-1965 trends for most of the analyses, but we lack a temporal changes of temperatures to justify its relevance to modern climate change. Consider add a figure to show the climate warming in the study region.
Specific Concerns:
L14: The species name should be in italic. The same for L64.
L86: Liang et al., (2016) did not analysed this tree species, please remove it from here.
L120: The text briefly mentions that the data on atmospheric carbon dioxide concentration is derived from ice core data, but does not elaborate on the specific source. Please provide the source and the reference literature.
L217: The sections 4.2 and 4.3 should be merged and summarised by a more physiologically meaningful title.
Citation: https://doi.org/10.5194/egusphere-2025-952-RC1 - AC1: 'Reply on RC1', Lixin Lyu, 30 May 2025
-
RC2: 'Comment on egusphere-2025-952', Anonymous Referee #2, 16 Apr 2025
General comments
This manuscript examines the long-term physiological and growth responses of Abies spectabilis across elevational gradients in wet and dry regions of the Tibetan Plateau, using tree-ring width, basal area increment (BAI), and δ¹³C-derived intrinsic water-use efficiency (iWUE). The authors aim to disentangle the relative roles of climate warming and atmospheric CO₂ in modulating tree growth. The study addresses timely questions in forest ecophysiology and uses standard dendrochronological and isotope techniques. However, the manuscript suffers from several significant conceptual, methodological, and shortcomings. The analysis relies heavily on correlative patterns/scenarios with limited mechanistic interpretation. Basic sample metadata is omitted and critical information on detrending and sample replication is lacking.
Moreover, while the Tibetan Plateau is an understudied region, the manuscript largely reproduces well-established findings regarding iWUE trends, growth–climate relationships, and physiological strategies under CO₂ enrichment. The analysis does not introduce new mechanisms, theory, or methods. Thus, its novelty lies primarily in applying standard approaches to a geographically distinct context — which is of regional interest, but not a substantial conceptual advancement for the field.
Major concerns
1- The authors fail to report key information about the sampled trees, such as sample sizes per site, tree ages (range and mean), diameter at breast height (DBH), or inter-individual variability. Given the long temporal scope, this omission undermines the interpretation of growth trends. Tree age can confound long-term growth patterns and sensitivity to climate or CO₂. This basic metadata must be included in a table or appendix.
2- While the authors use ARSTAN with negative exponential or linear detrending, they do not explain how this affects long-term trends — the central focus of the paper. Detrending can remove real long-term growth signals (e.g., due to CO₂ fertilization). Were raw BAI series analyzed? Were alternative detrending methods tested (e.g., Regional Curve Standardization)? This needs clarification and discussion.
3- The use of theoretical Ci/Ca trajectories (Scenarios 1–3) is oversimplified. These scenarios assume fixed relationships that rarely hold across environmental gradients or time. The authors treat alignment with Scenario 1 ("Ci = constant") as evidence of physiological strategy but offer no underlying reasoning in terms of stomatal control, leaf traits, or drought response. This needs much deeper physiological interpretation.
4- The authors repeatedly attribute growth changes to rising Ca or warming based on correlations, without ruling out confounding variables (e.g., age, stand density, soil conditions). The SEM framework is promising but underdeveloped, and the model structure, diagnostics, and assumptions are not detailed.
5- Many of the main findings — that iWUE increased, growth responded positively to warming in wet regions, and drought limits growth at lower elevations — are well established in the literature. The manuscript would benefit from a clearer articulation of what is truly new.
6- The manuscript references nutrient availability as a potentially limiting factor (e.g., lines 54–56, 238), citing literature that suggests it can constrain the CO₂ fertilization effect. However, no nutrient data (e.g., soil N or P) are presented or analyzed. This creates a mismatch between the framing and execution of the study. Either include relevant data or remove unsupported speculation.
7- While I believe that using AI to assist with scientific writing is acceptable, the manuscript shows clear signs of overreliance, including inconsistent use of abbreviations, redundancy of abbreviations, and occasional lapses in technical accuracy. The dataset presented is valuable, but the manuscript requires substantial revision to improve clarity, coherence, and scientific precision before it can be considered for publication.
Minor comments
1- Use species name in italic.
2- More information on the ice core data are needed
3- Figures: Captions are too brief (especially in Figs. 2, 3, 4, 5) (also, there are no legend for coloring in figures).
4- Figure 4, it is hard to believe the results about iWUE since a lot of p-values shoz no significance of trends.
5- Any more information on the SPEI data? How were the data collected? Where they gridded data? Were the data extracted at specific locations?
Citation: https://doi.org/10.5194/egusphere-2025-952-RC2 - AC2: 'Reply on RC2', Lixin Lyu, 30 May 2025
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(4255 KB) - Metadata XML
-
Supplement
(998 KB) - BibTeX
- EndNote
- Final revised paper
This study investigates the physiological and growth responses of Himalayan fir to climate warming and CO2 fertilization across elevational gradients in wet and dry regions of the Tibetan Plateau. The research addresses a critical knowledge gap regarding the interplay between CO2 fertilization, temperature, and drought stress in high-elevation forests. While the manuscript presents valuable insights, several issues need clarification to strengthen the conclusions.
Major Concerns:
1 Interpretation Issue of Ci/Ca Scenarios: The comparison of observed Ci/Ca trends to theoretical scenarios lacks quantitative validation. The statement that Ci/Ca "largely followed Scenario 1" is qualitative and not statistically tested.Please quantify deviations from theoretical scenarios using goodness-of-fit metrics (e.g., RMSE, AIC) and report significance tests.
Specific Concerns:
L14: The species name should be in italic. The same for L64.
L86: Liang et al., (2016) did not analysed this tree species, please remove it from here.
L120: The text briefly mentions that the data on atmospheric carbon dioxide concentration is derived from ice core data, but does not elaborate on the specific source. Please provide the source and the reference literature.
L217: The sections 4.2 and 4.3 should be merged and summarised by a more physiologically meaningful title.