the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Sep 2026
Coupling Tree-Ring Blue Intensity and Carbon Isotope (δ13C) Reveals Shifting Climate Change Drivers on Tree Growth in Kurdistan Forests, Northern Iraq
Abstract. This study investigates the relationships between climate changes and growth in semi-arid forests located in the Kurdistan Region of northern Iraq by utilizing tree-ring Blue Intensity (BI) parameters alongside stable carbon isotopes (δ¹³C). The sensitivity of detrended tree-ring width (TRW), earlywood blue intensity (EWBI), latewood blue intensity (LWBI), and delta blue intensity ΔBI to temperature, precipitation, and physiological responses are measured on three species of tree Quercus aegilops, Juglans regia and Fraxinus. Tree-ring Blue Intensity chronologies for 100 years, are used to look at climate-growth interactions over time for all three species. Results show a dominant temperature control on wood formation. EWBI is correlates with temperature (r ≈ 0.37, p < 0.001) and with precipitation in Quercus aegilops, whereas ΔBI is a positive temperature proxy (r ≈ 0.37, p < 0.001), this shows that when years are warmer, the density contrast of earlywood and latewood is increased. In Juglans regia, the strongest temperature sensitivity in TRW is found in LWBI (r ≈ -0.45, p < 0.001). The positive correlation of ΔBI with δ¹³C (r ≈ 0.30, p < 0.01) indicates a positive structural and physiological response to drought stress. The TRW sensitivities of Fraxinus are weaker (r = 0.128 in Erbil, r = 0.122 in Duhok), and the temperature BI(EW-LW) relationships are negative and significant (r ≈ -0.340, p < 0.01) (r ≈ -0.280, p < 0.01). Wavelet coherence analysis indicates a shift in the growth trend from precipitation-driven growth prior to ~1980 to the temperature-related growth thereafter, with increasing non-stationarity and episodic drought signatures. The integrated BI-δ¹³C approach captures anatomical and physiological adjustment, revealing that recent warming led to a decrease in radial growth, less earlywood development and changed wood density structure across species. The results show the potential of BI as an effective indicator of detecting climate-driven ecosystem shifts and temperature variability.
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CC1: 'Comment on egusphere-2026-4943', Stefan Klesse, 12 Oct 2026
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CC2: 'Reply on CC1', khalat mohammed, 31 Oct 2026
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We sincerely appreciate the input, and the points raised have our full attention.
The comment regarding multiple sampling replication warrants special consideration. However, the conclusion that an n = 1 sampling design for each species necessarily renders the results uninterpretable does not distinguish the individual-tree proxy analyses, inference and objective from the population-level (or EPS) dendroclimatic representation and objectives. Our sampling design does not, in any inference says the objectives and targets embedded in the EPS representation of a species-level response to climatic signatures embedded and preserved on an individual level but collectively, represent the species response or the tree inter-variability and are used to establish whether the observed relationships are representative of the respective species or wider forest populations.
Nevertheless, we contend that the absence of species-level sample replication should constrain the scope of interpretation, rather than disqualify the scientific investigation of annually resolved individual-tree records. We rightly share with others (see further) the way of work and objective, that the individual-tree growth records are not merely a source of residual variance around a population signal; those individual records carry strong, meaningful and interpretable, tree-specific responses to climate that a mean chronology may obscure or smooth out. In this context, Carrer (2011, p.5) clearly argued that “the individualistic approach is able to unveil the significant differences between the responses of the two species; differences that would have been concealed analyzing just the mean values.” Moreover, and with much more discussion on the subject, Trouillier et al. (2018, Sections 1.3 and 4: Visualizing Individual Tree Differences in Tree-Ring Studies. Forests, 9(4), 216.) similarly argued positively (among other discussion points) that the “individual-based assessments can offer additional insights into tree growth and ecosystem dynamics”.
More importantly, the scientific and methodological soundness of individual-tree proxy investigations is supported not merely by earlier arguments, but by direct precedents in published research, specifically, Weigl et al. (2007, 2008), Gray and Thompson (1977), and Libby and Pandolfi (1974), providing a substantive basis for recognizing individual-tree climatic records as scientifically meaningful and interpretable within their explicitly defined research objectives.
Weigl et al. (2007: Dendrochronologia, 24(2–3), 117–122), investigated the axial variability of latewood width and stable carbon and oxygen isotope ratios (δ¹³C and δ¹⁸O) in a single sessile oak (Quercus petraea). Their investigation compared measurements taken at different stem heights and found that the influence of sampling height was relatively weak for both latewood width and stable-isotope ratios. Importantly, relationships between samples collected at different stem heights were strong when corresponding calendar years were compared, whereas relationships were weaker when corresponding cambial ages were compared. Their findings highlighted the importance of calendar-year correspondence and demonstrated that multiple tree-ring parameters from an individual tree can be investigated to understand the variability and consistency of the recorded signals. This provides a methodological precedent for examining different parameters within an individual-tree record, while recognizing that such an approach does not establish population-level representativeness.
Weigl et al. (2008: Science of the Total Environment, 393(1), 154–161.) provided a further and particularly relevant example by investigating radial growth and stable carbon and oxygen isotope ratios (δ¹³C and δ¹⁸O) in a single sessile oak (Quercus petraea). They separated the tree rings into earlywood, latewood, complete annual rings, and transfer-ring components, with the latter comprising latewood from one year and earlywood from the following year. Their analyses included mean sensitivity, simple correlation, partial correlation, and autocorrelation. The authors reported that the different tree-ring parameters exhibited distinct statistical behaviors: δ¹⁸O and ring-width measurements generally showed lower autocorrelation, whereas δ¹³C displayed stronger autocorrelation in several ring components. They also found that relationships within the same proxy groups were generally stronger than relationships between growth and isotope parameters. Importantly, although their investigation focused on one tree, the results were compared with average site data, and the authors demonstrated the potential value of individual-tree measurements for understanding the environmental information preserved in different tree-ring parameters. This study is particularly relevant to our approach because it illustrates the value of analyzing complementary proxy parameters within the same individual tree and evaluating their statistical properties separately.
Gray and Thompson (1977: Nature, 270, 708–709) measured oxygen-isotope ratios (¹⁸O/¹⁶O) in cellulose, lignin, and whole wood from the annual rings of a single white spruce (Picea glauca) that grew in the Edmonton area of Canada between 1882 and 1969. By comparing the isotope measurements with meteorological records, they found a strong relationship between cellulose oxygen-isotope ratios and mean annual temperature, a weaker but still significant relationship for whole wood, and no significant relationship for lignin. Their findings demonstrated that the climatic information obtained from an individual-tree record depends on the wood component and proxy being examined. This work therefore supports a careful, proxy-specific evaluation of individual-tree records and illustrates that different measurements from the same tree may preserve different levels of climatic information.
An earlier contribution is provided by Libby and Pandolfi (1974: Proceedings of the National Academy of Sciences, 71(6), 2482-2486), who investigated the temperature dependence of stable carbon, oxygen, and hydrogen isotope ratios in tree rings. They analyzed wood samples representing approximately three-year intervals from a German oak covering 1712-1954 and compared the isotope measurements with historical meteorological records from England, Basel, and Geneva to evaluate empirical temperature relationships. They also examined isotope measurements from a second oak covering 1530-1800, identifying climatic variability associated with the Little Ice Age and subsequent warmer intervals. Although their sampling resolution and analytical approach differ from modern annually resolved dendroclimatic investigations, their work represents an early demonstration of the use of individual-tree isotope records to investigate climatic information preserved in wood. Collectively, these studies provide methodological precedents for individual-tree proxy investigations, while not replacing the need for biological replication when the objective is to establish population-level representativeness.
In our study, blue intensity (BI) was measured in three individual trees representing oak (Quercus aegilops), walnut (Juglans regia), and Fraxinus. The walnut BI time series covers 1925-2024, the oak BI time series covers 1928-2023, and the Fraxinus BI time series covers 1940-2024. For each tree, earlywood blue intensity (EWBI), latewood blue intensity (LWBI), and delta blue intensity (ΔBI) were analyzed across all available annual rings, generating separate annually resolved time series for each parameter. Annual δ¹³C measurements were obtained from the same walnut individual used for BI analysis, covering the period 1928-2024.
Instrumental temperature and precipitation records covering 1901-2024 were also used to provide the climatic context for examining the proxy records over their respective time periods. This design enables us to examine the temporal behavior of the measured proxy records and their potential associations with instrumental climate data. In the walnut record, the availability of both BI and δ¹³C measurements also permits an examination of the relationship between complementary proxies obtained from the same individual. The availability of EWBI, LWBI, and ΔBI for all three trees further allows us to investigate different aspects of the annual blue-intensity signal within each individual record. These multiple parameters provide complementary analytical perspectives on the same annual growth sequences rather than independent biological replication.
Accordingly, we interpret our results as individual-tree proxy evidence rather than as replicated species chronologies or definitive population-level or regional climate reconstructions. Additional sampling across multiple trees and, ideally, sites would be necessary to quantify between-tree variability, assess reproducibility, and determine the broader applicability of the observed relationships. Our argument is not that the current sampling design is a representative of species-level chronology; rather, it is that the scientific interpretability of an individual-tree record and its representativeness of a wider population are distinct methodological questions. We therefore respectfully maintain that the limited sample size constrains the scope of inference but does not, by itself, establish that the measured individual-tree proxy relationships are uninterpretable.
Finally, methodological rigor and biological replication address different questions. Rigorous dating, measurement, and proxy-specific analysis are fundamental to establishing the reliability and interpretability of each individual-tree record and the environmental information it may preserve. Biological replication, in turn, is necessary to quantify between-tree variability and determine whether the observed relationships can be generalized to a wider population. Within this scope, this work establishes a baseline record for the Kurdistan region, where no comparable data previously existed.
We appreciate the comments and the scientific discussion they have initiated.
For the authors,
Khalat M.
31/10/2026
References
Libby, L.M. and Pandolfi, L.J. (1974). Temperature dependence of isotope ratios in tree rings. Proceedings of the National Academy of Sciences, 71(6), 2482–2486. https://doi.org/10.1073/pnas.71.6.2482
Gray, J. and Thompson, P. (1977). Climatic information from ¹⁸O/¹⁶O analysis of cellulose, lignin and whole wood from tree rings. Nature, 270, 708–709. https://doi.org/10.1038/270708a0
Weigl, M., Grabner, M., Helle, G., Schleser, G.H. and Wimmer, R. (2007). Variability of latewood-widths and stable isotope ratios in a sessile oak tree (Quercus petraea (Matt.) Liebl.). Dendrochronologia, 24(2–3), 117–122. https://doi.org/10.1016/j.dendro.2006.10.003
Weigl, M., Grabner, M., Helle, G., Schleser, G.H. and Wimmer, R. (2008). Characteristics of radial growth and stable isotopes in a single oak tree to be used in climate studies. Science of the Total Environment, 393(1), 154–161. https://doi.org/10.1016/j.scitotenv.2007.12.016
Carrer, M. (2011). Individualistic and time-varying tree-ring growth to climate sensitivity. PLoS One, 6(7), p.e22813. https://doi.org/10.1371/journal.pone.0022813
Trouillier, M., Van der Maaten-Theunissen, M., Harvey, J.E., Würth, D., Schnittler, M. and Wilmking, M. (2018). Visualizing individual tree differences in tree-ring studies. Forests, 9(4), p.216. https://doi.org/10.3390/f9040216
Citation: https://doi.org/10.5194/egusphere-2026-4943-CC2
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CC2: 'Reply on CC1', khalat mohammed, 31 Oct 2026
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RC1: 'Comment on egusphere-2026-4943', Anonymous Referee #1, 26 Oct 2026
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Regarding the manuscript ‘Coupling Tree-Ring Blue Intensity and Carbon Isotope Reveals Shifting Climate Change Drivers on Tree Growth in Kurdistan Forests, Northern Iraq’, I have one important concern that, in my view, justify the rejection of the paper, despite its potential scientific interest.
My main concern is methodological. As I understand it, only 3 samples/individual trees (one per species) are used to perform statistical analyses and draw conclusions, which, to my knowledge, does not meet quality standards used in dendroclimatology. This prevents me from evaluating everything that follows.
Secondly, I think the manuscript would benefit from another proofreading by the authors to correct typos throughout the text, as well as restructuring, particularly in the abstract and introduction sections. In particular, in my view, it is a bit difficult to follow the line of reasoning in the introduction that leads to the study’s objective. This second point, in itself, does not justify the rejection of the manuscript, since it can be easily improved. I am more concerned about the first point I raised here. I could provide more details regarding the formal aspects of the text, if the methodological concerns are addressed.
Citation: https://doi.org/10.5194/egusphere-2026-4943-RC1 -
AC1: 'Reply on RC1', khalat mohammed, 31 Oct 2026
reply
We appreciate the effort and time invested by Anonymous Referee #1 in reviewing and commenting on our manuscript. We would like to address the referee's first comment regarding the limited number of sampled trees. This response shares many elements in our response to community commenter.
Biological replication is important for quantifying between-tree variability and determining whether observed climate-proxy relationships are representative of a species or site. However, we contend that the absence of such replication does not, by itself, establish that all measurements and relationships obtained from individual-tree records are scientifically uninterpretable. A distinction is necessary between investigating the temporal behavior and potential climatic associations of individual-tree proxy records and establishing a representative species-level or regional climate reconstruction. Our sampling design constrains the latter, but it does not eliminate the scientific value of investigating the environmental information preserved in the sampled individual-tree records.
Specialized literature has previously investigated climatic information preserved in individual-tree growth and stable-isotope records. These studies provide methodological precedents for examining individual-tree proxy behavior, although they do not replace the biological replication required to establish population-level representativeness. Gray and Thompson (1977) measured oxygen-isotope ratios (¹⁸O/¹⁶O) in cellulose, lignin, and whole wood from the annual rings of a single white spruce (Picea glauca) from the Edmonton area of Canada, covering the period 1882-1969. By comparing the isotope measurements with meteorological observations, they demonstrated that the relationship with mean annual temperature differed among the analyzed wood components. Cellulose exhibited a strong temperature relationship, whole wood showed a weaker but significant relationship, and lignin showed no significant relationship. Their study illustrates that the climatic information obtained from an individual-tree record depends on the proxy material and the environmental relationship being investigated. It also demonstrates the importance of evaluating the climatic behavior of different proxy components separately.
Weigl et al. (2007) investigated latewood width and stable carbon and oxygen isotope ratios (δ¹³C and δ¹⁸O) in a single sessile oak (Quercus petraea). Their study focused on the axial variability of these parameters at different stem heights. They found that sampling height had a relatively weak influence on latewood width and stable-isotope ratios, whereas relationships between samples from different stem heights were stronger when identical calendar years were compared than when identical cambial ages were considered. These findings highlight the importance of calendar-year correspondence and demonstrate the value of examining the consistency of multiple tree-ring parameters within an individual tree.
A further relevant study is Weigl et al. (2008), who analyzed tree-ring width and stable carbon and oxygen isotope ratios (δ¹³C and δ¹⁸O) in a single sessile oak (Quercus petraea). They separated the tree rings into earlywood, latewood, complete annual rings, and transfer-ring components, the latter consisting of latewood from one year and earlywood from the following year. Their statistical assessment included mean sensitivity, simple correlation, partial correlation, and autocorrelation analyses. They reported that δ¹⁸O and ring-width measurements generally exhibited lower autocorrelation, whereas δ¹³C showed stronger autocorrelation in several tree-ring components. They also found that relationships within the same proxy groups were generally stronger than relationships between growth and isotope parameters. Although their investigation focused on a single tree and did not provide a replicated population-level climate reconstruction, the study demonstrated the potential of examining different tree-ring parameters within one individual and highlighted the importance of evaluating their individual statistical characteristics. This is particularly relevant to our multi-proxy approach, in which different BI parameters and carbon-isotope measurements are used to investigate complementary environmental information preserved in the sampled tree rings.
Libby and Pandolfi (1974) provide an earlier example of investigating climatic information through tree-ring stable isotopes. They measured stable carbon, oxygen, and hydrogen isotope ratios in wood samples representing approximately three-year intervals from a German oak covering 1712-1954. These measurements were compared with historical weather records from England, Basel, and Geneva to evaluate empirical temperature relationships. They also examined isotope measurements from a second oak covering 1530-1800, which provided information on climatic variability during the Little Ice Age and subsequent warmer intervals. Although their sampling resolution and analytical approach differ from modern annually resolved dendroclimatic investigations, this study demonstrates the historical use of individual-tree isotope records to investigate climatic information preserved in wood. We consider these publications methodological precedents for individual-tree proxy investigation, rather than evidence that limited biological replication is sufficient for representative population-level reconstruction. In addition, the work by Vogel et al., (2001: Radiocarbon adjustments to the dendrochronology of a yellowwood tree) stands as an important study based on one species and one tree stump. Similar published literature is available that covers the 80-90ies of the last century. But here is only to show relevance and precedents and discussion not a literature survey.
In the present study, blue intensity (BI) measurements were obtained from three individual trees representing oak (Quercus aegilops), walnut (Juglans regia), and Fraxinus. The walnut BI time series covers 1925-2024, the oak BI time series covers 1928-2023, and the Fraxinus BI time series covers 1940-2024. For each tree, earlywood blue intensity (EWBI), latewood blue intensity (LWBI), and delta blue intensity (ΔBI) were analyzed across all available annual rings, generating separate annually resolved time series for each parameter. Annual δ¹³C measurements were obtained from the same walnut individual used for BI analysis, covering the period 1928-2024. Instrumental temperature and precipitation records covering 1901-2024 were also used to provide the climatic context for examining the proxy records over their respective time periods.
This design enables examination of the temporal behavior of the measured proxy records and their potential associations with instrumental climate data. In the walnut record, the availability of both BI and δ¹³C measurements also permits an examination of the relationship between complementary proxies measured from the same individual. Furthermore, the separate EWBI, LWBI, and ΔBI records allow us to investigate different aspects of the annual blue-intensity signal within each sampled tree. These measurements provide complementary analytical information about the individual records, while we recognize that they do not increase the number of independent biological samples.
Accordingly, our findings should be interpreted as individual-tree proxy evidence rather than as replicated species chronologies or definitive population-level or regional climate reconstructions. Additional biological replication would be necessary to quantify between-tree variability, assess reproducibility, and establish the wider applicability of the observed relationships. Nevertheless, the scope of the present investigation is to examine the environmental information preserved in the sampled individual-tree records and assess their potential associations with instrumental climate observations. Within this scope, this work establishes a baseline record for the Kurdistan region, where no comparable data previously existed. We therefore maintain that the individual-tree records are scientifically interpretable within this defined scope, while recognizing that their representativeness at the species and population levels requires further investigation through additional sampling. The limited number of sampled trees constrains generalization, but does not, by itself, invalidate the examination of the temporal behavior and potential climatic associations of the measured records.
Regarding the referee's additional concerns about proofreading, typographical errors, and the structure of the abstract and introduction, we acknowledge the importance of improving the manuscript's clarity and organization. These aspects will be addressed through careful proofreading, correction of typographical and grammatical errors, and restructuring of the abstract and introduction where necessary to improve the presentation of the research objectives, methodology, and findings.
We hope that these responses clarify the methodological basis, analytical scope, and limitations of our study. We remain available to address any further questions or points requiring clarification.
For the authors,
Khalat M.
31/10/2026
References
Libby, L.M. and Pandolfi, L.J. (1974). Temperature dependence of isotope ratios in tree rings. Proceedings of the National Academy of Sciences, 71(6), 2482–2486. https://doi.org/10.1073/pnas.71.6.2482
Gray, J. and Thompson, P. (1977). Climatic information from ¹⁸O/¹⁶O analysis of cellulose, lignin and whole wood from tree rings. Nature, 270, 708–709. https://doi.org/10.1038/270708a0
Weigl, M., Grabner, M., Helle, G., Schleser, G.H. and Wimmer, R. (2007). Variability of latewood-widths and stable isotope ratios in a sessile oak tree (Quercus petraea (Matt.) Liebl.). Dendrochronologia, 24(2–3), 117–122. https://doi.org/10.1016/j.dendro.2006.10.003
Weigl, M., Grabner, M., Helle, G., Schleser, G.H. and Wimmer, R. (2008). Characteristics of radial growth and stable isotopes in a single oak tree to be used in climate studies. Science of the Total Environment, 393(1), 154–161. https://doi.org/10.1016/j.scitotenv.2007.12.016
Vogel, J.C., Fuls, A. and Visser, E. (2001). Radiocarbon adjustments to the dendrochronology of a yellowwood tree. South African Journal of Science, 97(3), pp.164-166. https://hdl.handle.net/10520/EJC97292
Citation: https://doi.org/10.5194/egusphere-2026-4943-AC1
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AC1: 'Reply on RC1', khalat mohammed, 31 Oct 2026
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An n=1 for each species does not meet any data quality standards typically applied in dendrochronology. The results are thus not interpretable.