The study by Lin et al. presents three new oxygen-18 tree ring cellulose chronologies (d18Otrc) from the Eastern European Plain. The authors present an investigation of the climate signal in the d18Otrc, identifying the regional differences in the hydroclimate response of d18Otrc, including a comparison to the climate signal in traditional tree ring width (TRW) records. The main conclusion is that d18Otrc from the Eastern European Plain is a superior hydroclimate proxy compared to TRW records, and that long-term records for this region can provide important climate constraints for hydroclimate reconstructions and provide context for current and future climate change.

The manuscript is well-written with clearly defined scientific objectives, and the evaluation of the d18Otrc is well designed with extensive testing with reference data ranging from meteorological station data (Temp., precip.), gridded climate data sets (Temp., Rh, …), and output from an isotope enabled climate model (ECHAM5-wiso). References to existing literature is present where appropriate, and references are generally up-to-date, including background information in the introduction and theoretical considerations about the climate signal in d18Otrc and the role of tree physiology.

In conclusion, I consider the study by Lin et al. to be mature and close to be ready for publication after minor revisions taking into account the comments below.

Specific comments.

The authors compare to the climate signal in TRW, but there hardly any mention about maximum late wood density (MXD), which is commonly considered to have a stronger link to climate. Please explain why or add a discussion related to this.

Similarly, quantitative wood anatomy (QWA) analysis shows stronger climate signal than TRW and MXD (e.g., Björklund et al., 2023). Could QWA be a way to extract more climate information for your Eastern European Plain sites?

The three different sites show different seasonality in the relations to climate parameters. Can you provide explanations for this? For example, spring ground water recharge from snow melt at the northern site which could account for the lagged winter signal recorded in d18Otrc, or is regional differences in auto-correlation of climate parameters/weather patterns enough to result in spurious significant correlations (for example due to large scale winter variability (NAO/EA/SCA patterns, Comas-Bru & Hernández, 2018))?

References

Björklund, J., Seftigen, K., Stoffel, M. et al. Fennoscandian tree-ring anatomy shows a warmer modern than medieval climate. Nature 620, 97–103 (2023). https://doi.org/10.1038/s41586-023-06176-4

Comas-Bru, L. and Hernández, A.: Reconciling North Atlantic climate modes: revised monthly indices for the East Atlantic and the Scandinavian patterns beyond the 20th century, Earth Syst. Sci. Data, 10, 2329–2344, https://doi.org/10.5194/essd-10-2329-2018, 2018.

This study by Lin et al. presents three tree-ring cellulose oxygen-isotope chronologies developed along a latitudinal gradient and evaluates their potential for reconstructing hydroclimatic variability across the East European Plain, a region where this proxy remains comparatively understudied.

The main finding from this study is that δ18O_TRC retains stronger sensitivity to atmospheric moisture conditions than TRW at the wetter, temperature-limited northern and central sites, whereas TRW more directly reflects soil-moisture limitation at the drier southern site.

Overall, the study is well-organised and shows a clear main objective, consistently developed throughout the manuscript. The methods used for sample preparation and the general analytical framework are appropriate. The manuscript is generally well grounded in the relevant literature and the discussion includes topics which are of great interest within this research field. 

I found the manuscript scientifically interesting and generally well prepared. My comments mainly concern methodological transparency, statistical interpretation and the clarity of some passages. The following comments should help improve the methodological transparency, clarity and interpretation of the results.

1. Lines 43–54: The paragraph provides a useful broad overview of the latitudinal transition from temperature- to moisture-limited TRW. However, it would benefit from a more explicit summary of the principal seasonal climate controls reported in previous studies, for example, which temperature and moisture variables dominate, during which months or seasons and in which parts of the region. This would provide a clearer basis for evaluating the additional information supplied by δ18O_TRC.
2. Line 72: Including some comments about the cited spatial heterogeneity would give important context on how the dominant controls and seasonal response windows of δ18O_TRC are expected to vary with moisture source, transport pathway and local hydroclimatic regime. Now it is only mentioned that the dominant controls "can shift with changes in moisture sources…", but it is not specified how those shifts actually are.
3. Lines 101-106: The description of summer temperature is potentially confusing because “temperatures decreased from the south to the central region” may be read as implying a temporal change rather than a spatial gradient. Please rephrase the sentence to state the site values directly or describe an increase in summer temperature from north to south.
4. Lines 131–132: Please explain and justify the criteria used to select cores for δ18O analysis. How many candidate trees and cores were available at each site, how many met the eligibility criteria and how were the four analysed trees selected from that eligible set? In particular, please clarify why cores containing missing or extremely narrow rings were excluded and whether these criteria were defined a priori and applied consistently across sites. Extremely narrow rings may record biologically and climatically meaningful growth-suppression events; excluding them could therefore introduce selection bias unless exclusion was analytically necessary, for example because they provided insufficient cellulose for reliable isotope measurement. The methodological justification should be supported with relevant literature. Please also explain why replication was increased only at the northern site and discuss the resulting imbalance among sites.
5. Lines 187–190 and 213–219: The central and southern isotope chronologies are based on only four trees and have EPS values of 0.84. Although these values indicate appreciable common signal strength, the statement that the chronologies “effectively capture a common environmental signal” may be too categorical, particularly because 0.85 is frequently used as a conventional benchmark and because EPS does not by itself eliminate the uncertainty associated with low replication. Please provide the precise calculation procedure and clarify whether EPS and Rbar vary through time. If available, time-varying sample depth, Rbar and EPS should be shown. The conclusions concerning regional representativeness and reconstruction potential should also acknowledge this limitation.
6. Lines 411–417 and 440–451, particularly lines 445-446: The conclusion that the northern chronology is sufficiently strong and stable to support a “reliable climate reconstruction” appears stronger than the analyses presented. The study demonstrates a statistically significant climate–proxy association and explores its temporal stability, but it does not perform a reconstruction or report calibration–verification statistics. I suggest consistently referring to “reconstruction potential” unless predictive skill is evaluated using an appropriate calibration and validation framework.