Annotation of the results obtained in 2023
The calculated module of hydrogen isotope composition in tree-ring cellulose is included in the global process model. By modeling oxygen and hydrogen isotope ratios, it is necessary to change the parameters in the freeze-thaw model block because permafrost degradation and consequent thawing of permafrost soils in the Mackenzie River Delta appears to be faster compared to the northeastern part of the Siberian regions. Likely, the degradation of permafrost and consequent thawing of permafrost soils for the Delta Mackenzie River is faster compared to the northeastern part of Siberia, where the model has been successfully tested. The multi-parametric approach proposed in this project, combining triple stable isotopes (d13C-d18O-dD) allowed us to identify ecophysiological mechanisms of tree response to changes in eco-hydrological environmental conditions (precipitation, permafrost thawed water, relative air humidity, water vapor pressure deficit), and to identify and evaluate strategies and mechanisms of adaptation of different conifer species (Larix cajandri Mayr, Larix gmelinii Rupr., Picea glauca (Moench) Voss.) to environmental changes (Churakova (Sidorova) et al. 2023, Science of the Total Environment, https://doi.org/10.1016/j.scitotenv.2023.161644). Our results reveal a remarkably consistent relationship between GMST and δMW throughout the CE, despite relatively constant ice–ocean boundary conditions. Between 1850 and 2000, global δprecip increased by at least 0.56‰, indicative of a warmer, more humid troposphere. Global δMW and regional δMW appear to adjust to changing temperature and atmospheric circulation patterns, respectively, within decades—similar to the timescale of the forcing itself. Expansion of δMW observational networks will be critical for detecting and attributing shifts in rainfall, drought and circulation as the planet continues to warm (Konecky et al. 2023, Nature Geoscience).

 

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Annotation of the results obtained in 2021
The 1498 -year reconstructions from stable carbon and oxygen isotope data in tree-ring cellulose revealed increased droughts during the 20th and 21st centuries that are unprecedented compared to the last millennium, when average monthly precipitation in July, from 60.4 mm (for the period 516 to 1799 AD), decreased to 44.3 mm (1800 to 2009) (Churakova (Sidorova) et al. 2021). Moreover, the significant decrease in precipitation (> 80%) relative to the mean and the increase in summer temperature (daily maximum > 30 °C) corresponds to extremely dry phases in July (> 3σ) in 1717, 1732, 1957, 1979, 1980, 2008, 2009, as captured by our July precipitation reconstruction. The Arctic Oscillation (AO) influences winter temperature and precipitation patterns in Eurasia, North America, and in Siberia. Positive phases of the Arctic Oscillation (AO+) in May bring warm and often dry weather conditions to the Siberia. The results of our study showed that AO+ shows a higher amplitude of oscillations also during the Medieval Warm Period compared to the current one. However, the frequency of positive oscillations shows a prolonged trend during the modern period. Whereas the negative phases of the AO- led to cold conditions caused by stratospheric volcanic eruptions in 536, 541-542, 1257, 1812, 1814-1815, 1822, 1835, 1883 CE, which are clearly recorded in the reconstruction. During the modern twentieth to twenty-first centuries, the AO fluctuations do not exceed the amplitude of the second half of the first millennium and the Medieval Warming (9-10 CE). The chronologies agree significantly mainly during the Little Ice Age (LIA, 1350-1800 CE), a positive anomaly during the Late Antique Little Ice Age (LALIA, 516-600 CE), the Early Medieval Climatic Anomaly (MCA, 700-1000 AD) are meaningfully recorded in the seasonally reconstructed AO index. A comparison of the regional AO reconstruction with the North Atlantic Oscillation (NAO), showed a significant correlation with AO in May (p < 0.0001) for the 1049-1969 AD period (Churakova (Sidorova) et al. 2021). According to stable oxygen isotope data in tree-ring cellulose (δ18O) (Larix cajanderi Mayr., Larix gmelinii Rupr. Rupr., Picea glauca (Moench) Voss) from subarctic regions in Siberia (Russian Federation): northeastern Yakutia, YAK, 69◦ N, 149◦ E; eastern Taimyr, TAY, 70◦ N, 103◦ E; and Mackenzie Delta, CAN, 68◦ N, 133◦ W, (Canada), we examined the response of conifers to current climate change (1900-2000 AD). We compared local and gridded air temperature, precipitation, and vapor pressure deficit (VPD) data, and analyzed trends and their changes over the last century for all three study areas (Zharkov et al. 2021). The analysis of climatic trends of the local weather stations Chokurdakh, Khatanga (Russian Federation) and Inuvik (Canada), and gridded data (68-69° N - 133-134° W) showed that dramatic changes in climate started since the 1980s, with a significant increase in temperature trends and decrease in precipitation from 2000 to 2020. The current warming and development of dry conditions are more pronounced in the western hemisphere (Canadian subarctic) compared to the Siberian subarctic. Abrupt changes in precipitation, temperature increases, and VPD vapor pressure deficits mainly occur in winter, spring, and fall in subarctic regions. We found a mixed signal in δ18O-YAK of July temperature (r = 0.49; p = 0.001), precipitation (r = -0.37; p = 0.02), and VPD (r = 0. 31; p = 0.02). Whereas the δ18O-CAN data showed a longer and more pronounced signal with spring air temperature (March-May, r = 0.37, p = 0.001) and summer July temperature (r = 0.32, p < 0.05), and spring VPD (r = 0.54, p = 0.001). Significant correlations were also found between δ18O-TAY and air temperature in July (r = 0.23, p = 0.04), and VPD in March (r = -0.26, p = 0.03). The obtained ecohydrological relationships (Zharkov et al. 2021) indicate the importance of temperature and humidity to varying degrees, which can be explained by site- and species-specific differences. Newly updated chronologies of stable isotopes, which include updated data from recent years (2000-2021), are urgently needed to get a complete picture of current changes compared to the past, and are planned to be obtained in this project. Accurate reconstruction of past climate data using local and gridded data for remote subarctic sites will improve the quality of hydroclimatic reconstructions and climate prediction models.

 

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Annotation of the results obtained in 2022
Significant air temperature and precipitation changes have occurred since the 2000s in vulnerable Siberian subarctic regions and urged updates of available chronologies towards the third millennium. It is important to better understand recent climatic changes compared to the past decades, centuries and even millennia. In this study, we present the first annually resolved triple tree-ring isotope dataset (δ13C, δ18O and δ2H) for the eastern part of the Taimyr Peninsula (TAY) and northeastern Yakutia (YAK) from 1900 to 2021. We found that the novel and largely unexplored δ2H of larch tree-ring cellulose was linked significantly with δ18O for the YAK site, which was affected by averaged April–June air temperatures and evaporation. Simulated by the Land Surface Processes and Exchanges (LPX-Bern 1.0) model, the water fraction per year for soil depths at 0–20 and 20–30 cm was significantly linked with the new eco-hydrological tree-ring δ2H data (Figure 1). Our results suggest increasing evapotranspiration and response of trees’ water relation to rising thaw water uptake from lower (20–30 cm) soil depth. A positive effect of July air temperature on tree-ring δ18O and a negative impact of July precipitation were found, indicating dry conditions (Figure 2). The δ13C in larch tree-ring cellulose for both sites showed negative correlations with July precipitation and relative humidity, confirming dry environmental conditions towards the third millennium. Churakova (Sidorova) OV, Fonti MV, Barinov VV, Zharkov MS, Taynik MS, Trushkina TV, Kirdyanov AV, Arzac A, Saurer M (2022) Towards the third millennium changes in Siberian triple tree-ring stable isotopes. Forests 13(6), 934, https://doi.org/10.3390/f13060934, Zenodo doi: 10.5281/zenodo.6635877 Funding: This research was funded by the Russian Science Foundation (RSF) grant number 21-17-00006 titled “Hydrogen isotopes and deuterium excess (d-excess) in conifer tree-ring cellulose as indicator of extreme eco-hydrological changes in boreal forests (ECO-HYDROTREE)”. The boreal forests are widely expanded from subarctic forest to tundra, and from taiga to forest-steppe zone (from 50 °N to 70 °N). We reviewed available stable isotope chronologies in tree-ring cellulose (δ13C, δ18O and δ2H) from 16 sites located in the Russian Federation; 4 research sites from Fennoscandia (Finland, Sweden and Norway); 5 sites from Canada, and 1 site from Alaska (USA) to evaluate impact of climatic changes from seasonal to annual scale across boreal forest ecosystems (Figure 3). Results of our review of carbon isotope data showed that drought conditions (mainly high vapour pressure deficit) are prevalent for western and central regions of Eurasia, Alaska and Canada, while northeastern and eastern sites of Eurasian subarctic are showing water shortage developments resulting from decreasing precipitation. Oxygen isotope chronologies show increasing trends towards the end of the twentieth century mainly for all chronologies, except for the Siberian northern and southern sites. The application of the multiple stable isotope proxies (δ13C, δ18O, δ2H) is beneficial to study responses of boreal forests to climate change in temperature-limited environments. However, a deeper knowledge of hydrogen isotope fractionation processes at the tree-ring cellulose level is needed for a sound interpretation and application of δ2H for climate reconstructions, especially for the boreal forest zone where forest ecosystems are more sensitive to climatic and environmental changes. Churakova (Sidorova) OV, Porter T, Kirdyanov AV, Myglan V, Fonti M, Vaganov EA (2022) Stable Isotopes in Tree Rings of Boreal Forests. Chapter 20 In Book Stable Isotopes in Tree Rings: Inferring Physiological, Climatic and Environmental Responses, Eds. RTW Siegwolf, JR Brooks, J Roden, M Saurer Springer International Publishing, ISBN 978-3-030-92697-7, https://doi.org/10.1007/978-3-030-92698-4_20 This work was supported by the Russian Science Foundation (RSF) (project 21-17-00006) granted to O.C. (S.) and (project 19-14-00028) granted to V.M.

 

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