Tree-ring Oxygen Isotope Research Articles

Deciphering the transfer of hydroclimate signals to tree-ring δ18O using a proxy system model in East Asia's Meiyu region

The stable oxygen isotope ratios of whole tree-ring α-cellulose (δ18OWR) have been interpreted as an indicator of early summer hydroclimate in the Meiyu region of East Asia. However, the underlying physical mechanism often remains unclear. Here we provide a mechanistic understanding through intra-annual tree-ring oxygen isotope analysis and process-based δ18OWR modelling over the period 1979–2006. The selected tree species for analysis is Pinus taiwanensis, whose δ18OWR exhibit the strongest linear relationship with relative humidity (RH) in June. The results indicated that the June RH signal is predominantly contained in tree-ring earlywood rather than latewood. The strong response of δ18OWR to June RH is not due to the legacy effect. Using the proxy system model (PSM) of δ18OWR, we obtained a modeled δ18OWR time series that is significantly positively correlated with the measured δ18OWR time series. The modeled and measured δ18OWR series show similar relationships with monthly RH. Sensitivity experiments with PSM revealed that the June RH signal is originated from the oxygen isotopes of source water and leaf water. Rapid cellulose formation in June plays a role in enhancing the June RH signal. Our study demonstrates how δ18OWR record early summer hydroclimate signals from a process perspective, and that the PSM is effective in modelling the interannual δ18OWR variability in the Meiyu region.

Tree ring oxygen isotope (δ18O) variations from western Himalaya and it linkage with vapor pressure and runoff water in India

Tree ring oxygen isotope (δ18O) variations from western Himalaya and it linkage with vapor pressure and runoff water in India

Seasonal and Depth Dynamics of Soil Moisture Affect Trees on the Tibetan Plateau

The soil moisture (SM) influences tree growth with climate change. However, the spatial and temporal dynamics of tree water use strategies in climate-sensitive areas remain uncertain. Therefore, we collected the tree-ring oxygen isotope (δ18OTR) chronologies and divided the wet–dry gradients according to the precipitation on the Tibetan Plateau (TP). Further, the relationship between the δ18OTR and environmental factors was analyzed across different gradients. We found the following: (1) The SM during the growing season was the most important factor for δ18OTR. (2) The response of the δ18OTR to the SM had a lag in arid areas than in humid areas. (3) Trees absorbed the SM on the surface in humid areas (r = −0.49 to −0.41, p < 0.01), while trees absorbed the SM from deep in the soil in arid areas (r = −0.48 to −0.29, p < 0.01). The results demonstrated that trees were better able to cope with drought stress in arid regions because they used more stable deep soil water than in humid regions. Therefore, the findings will provide a scientific basis for water use of trees using the δ18OTR in complex environmental contexts. Trees with single water use strategies should be given more attention to keep ecosystems healthy.

Summer climate information recorded in tree-ring oxygen isotope chronologies from seven locations in the Republic of Korea

The Republic of Korea is characterized by its north-to-south stretch and high mountain ranges along the eastern coast, resulting in terrain with higher elevation in the east and lower in the west. These geographical features typically lead to regional climate differences, either based on latitude or from east to west. In the present study, for effectiveness, the entire Korean peninsula was divided into four regions based on the geographical features: The Northeast Coast (NEC), Central Inland (MI), Southeast Coast (SEC), and South Coast (SC). Two test sites were chosen from each region, except for the SC. The linear relationship between the altitude of sites and the mean oxygen isotope ratio (δ18O) revealed a negative correlation; the highest (1,447 m a.s.l.) and the lowest altitude (86 m a.s.l.) sites had a mean δ18O of 27.03‰ and 29.67‰, respectively. The sites selected from the same region exhibited stronger correlation coefficients (0.75–0.79) and Glk (Gleichläufigkeit) (74–83%) between the tree-ring oxygen isotope chronologies (δ18OTR chronologies) than those from different regions (0.60–0.69/70–79%). However, subtle variations in pattern were observed in the comparison period during a few selected intervals (approximately 10 years). All the regional δ18OTR chronologies exhibited positive correlations with either June or July temperatures over Korea, whereas negative correlations with regional summer precipitation and SPEI-3. Moreover, the chronologies showed notable negative correlations with the water condition of western Japan. The findings of this study can be used as a scientific reference for the study of variations of rainfall in East Asia using δ18OTR chronology.

A past and present perspective on the European summer vapor pressure deficit

Abstract. The response of evapotranspiration to anthropogenic warming is of critical importance for the water and carbon cycle. Contradictory conclusions about evapotranspiration changes are caused primarily by their brevity in time and sparsity in space, as well as the strong influence of internal variability. Here, we present the first gridded reconstruction of the summer (June, July, and August) vapor pressure deficit (VPD) for the past 4 centuries at the European level. This gridded reconstruction is based on 26 European tree ring oxygen isotope records and is obtained using a random forest approach. According to validation scores obtained with the Nash–Sutcliffe model efficiency, our reconstruction is robust over large parts of Europe since 1600, in particular for the westernmost and northernmost regions, where most tree ring records are located. Based on our reconstruction, we show that from the mid-1700s a trend towards higher summer VPD occurred in central Europe and the Mediterranean region that is related to a simultaneous increase in temperature and decrease in precipitation. This increasing summer VPD trend continues throughout the observational period and in recent times. Moreover, our summer VPD reconstruction helps to visualize the local and regional impacts of the current climate change, as well as to minimize statistical uncertainties of historical VPD variability. This paper provides also new insights into the relationship between summer VPD and large-scale atmospheric circulation, and we show that summer VPD has two preferred modes of variability, namely a NW–SE dipole-like mode and a N–S dipole-like mode. Furthermore, the interdisciplinary use of the data should be emphasized, as summer VPD is a crucial parameter for many climatological feedback processes in the Earth's surface system. The reconstructed summer VPD gridded data over the last 400 years are available at the following link: https://doi.org/10.5281/zenodo.5958836 (Balting et al., 2022).

Tree ring oxygen isotope in Asia

Tree-ring oxygen isotope (δ18OTR) is a powerful proxy for paleoclimatology, ecology and archaeology, δ18OTR studies in Asia over the past ∼50 years have made significant achievements in improving our understanding of Asian climatic and ecological systems. Here, we review these developments in δ18OTR research in Asia. Most study sites are located within East Asia, Southeast Asia, South Asia, and North Asia, and δ18OTR records from conifers, broadleaf trees, and shrubs have been produced. Progress has been made in the following areas: (1) Experimental techniques. The novel plate method for cellulose extraction has been developed, which reduces experimental time and greatly improves efficiency. (2) Development of δ18OTR chronologies. Ninety-six chronologies exceeding 200 years in length have been produced. A 4354-year δ18OTR chronology with an annual-resolution has been produced in Japan, and a 6700-year δ18OTR chronology with a 1–5 year resolution has been produced in the northeastern Tibetan Plateau, which is the longest in Asia. (3) Biological influences on δ18OTR. Variations of intra- and inter-tree and inter-species δ18OTR have been investigated, and the relationship between tree age and δ18OTR has been explored. (4) Climatic implications. Seasonal δ18OTR records primarily reflect the variability of precipitation δ18O, facilitating high-resolution paleoclimate reconstructions, while interannual δ18OTR records mainly reflect summer moisture variability. (5) Climate reconstructions. Quantitative estimates have been produced for past precipitation, relative humidity, PDSI (Palmer Drought Severity Index), runoff, and other parameters from δ18OTR records. Since 1850, δ18OTR chronologies in monsoon regions show upward trends indicative of drying, while these from westerlies regions exhibit downward shifts implying wetting. (6) Dating. Intra- and inter-annual δ18OTR records help determine the annual growth of trees without rings and facilitate the cross-dating of trees with rings. Thus, the dating of archaeological and buried wood is feasible via comparisons with δ18OTR master chronologies. Promising areas of development include climatic implications (e.g., Position-specific isotope analysis, seasonal δ18OTR records on longer timescales and larger spatial scales), climate reconstructions (syntheses of multiple sites and records, development of long chronologies, deep time paleoclimate reconstructions) and ecological applications (e.g., the response of trees to climatic extremes, and the combinations with multiple tree-ring multiple proxies such as width and density).

Demonstration of process-based reconstruction of annual temperatures from tree ring oxygen isotope

Forecasting the global warming of the post-industrial period requires knowledge of natural variations in climatic parameters, especially temperature in preceding times. Due to its stable time resolution and known physiochemical formation process, tree ring cellulose isotope datasets have immense potential to yield climatic variability information. The first standardized site-independent temperature reconstruction model from tree-ring cellulose oxygen isotope data is demonstrated here using data from a montane site in the western Himalayas. This model does not require any statistical calibration and can be directly compared with instrumental or modelled data. The resulting temperature amplitude is dependent on moisture availability and this input is needed to modulate the reconstruction. The present work tests the possibility of input of carbon isotope discrimination as a proxy of relative humidity. This input achieved amplitude control but additional frequency components were introduced to the reconstruction.

Irrawaddy River experienced more frequent hydrological drought events with global warming: Evidence from a 400-year tree ring oxygen isotope record

Global temperature fluctuations have significantly impacted regional hydrological cycles. The Irrawaddy River (IR) is the largest river in Myanmar and has an important influence on socioeconomic conditions within its watershed and beyond. However, potential changes to the IR’s hydrological extremes under a future warming climate remain poorly understood. We reconstructed a 400-yr May–July streamflow time series for the IR using tree-ring cellulose oxygen isotopes (δ18O) and compared the variations of the hydrological extremes before and after the onset of global warming. The streamflow reconstruction shows that the IR experienced 65 extreme flood events and 67 extreme drought events during 1617–2017, with 58 extreme drought events occurred after 1850. Extreme drought events in this region have gradually increased and the frequency of extreme droughts has increased significantly since the 1850s. The occurrence of drought events in the last few decades has exceeded the natural variability. Especially, the persistent drought conditions during 2000–2017 are unprecedented compared with the record of the last 400 yr. This increase in drought events is significantly linked to the weakening of the ISM and increasing temperatures, hence the IR may experience more extreme drought events in the future under global warming.

Anthropogenic warming has exacerbated droughts in southern Europe since the 1850s

Widespread and frequent droughts have affected most parts of Europe over recent years, but it remains unclear when this synchronous drying trend began and how it has been influenced by anthropogenic forcing. Here we reconstruct and explore the history of drought in southern Europe over the past 300 years using an annual record of tree-ring oxygen isotopes from a site in Bosnia and Herzegovina. The reconstruction suggests that the onset of the drying trend in southern Europe occurred around the 1850s, which is consistent with previous studies demonstrating the extensive and long-lasting drying across central and western European areas. The evidence from CMIP6 model and reanalysis data demonstrates that anthropogenic warming has enhanced the strength of land–atmosphere coupling and exacerbated the widespread drying trend since the 1850s.

TWO NEW MILLENNIUM-LONG TREE-RING OXYGEN ISOTOPE CHRONOLOGIES (2349–1009 BCE AND 1412–466 BCE) FROM JAPAN

ABSTRACTWe present two new millennium-long tree-ring oxygen isotope chronologies for central and northern Japan, based on 9693 annually resolved measurements of tree-ring oxygen isotopes from 39 unearthed samples consisting mainly of Japanese cedar (Cryptomeria japonica). These chronologies were developed through cross-dating of tree-ring widths and δ18O data from multiple samples covering the periods 2349–1009 BCE (1341 yr) and 1412–466 BCE (947 yr) for central and northern Japan, respectively. In combination with our published chronology for central Japan, the tree-ring δ18O dataset currently available covers the past 4354 yr (2349 BCE to 2005 CE), which represents the longest annually resolved tree-ring δ18O dataset for Asia. Furthermore, the high-resolution temporal record of 14C contents independently developed by Sakurai et al. (2020) was reproduced by our 14C measurements of earlywood and latewood in annual rings for the period 667–660 BCE.

Regional tree-ring oxygen isotope deduced summer monsoon drought variability for Kumaun-Gharwal Himalaya

The precipitation in the Kumaun-Gharwal (Uttarakhand) Himalaya (KGH) is predominantly regulated by the Indian summer monsoon (ISM) and has been declining over the last century. However, because of limited historical data, it is difficult to place this recent decreasing trend in the context of pre-anthropogenic influences and understand a fuller range of hydroclimate scenarios for the region. Thus, we developed a 508-year regional tree-ring stable oxygen isotope (δ18OTR) record for multiple coniferous taxa (Abies spectabilis, Cedrus deodara and Picea smithiana) for the KGH, which spanned 1508–2015 CE. The δ18OTR record explained 35.8% of instrumental June–August Palmer Drought Severity Index (JJAPDSI) variance modelled from a nearby meteorological station. The JJAPDSI reconstruction shows regional coherency with both instrumental and proxy-based independent hydroclimatic records providing confidence that it is a reliable proxy to assess the long term hydroclimatic variability of the region. We identified decreasing strength of JJAPDSI reconstruction in KGH during recent decades indicating intensified drought. This is in the context of pre-anthropogenic influences and understand a fuller range of hydroclimate scenarios for the region. The δ18OTR also has coherency with Pacific and Indian Ocean sea surface temperature variability illustrating the broader teleconnections that influence the regional hydroclimate.

Inter-annual and intra-annual tree-ring oxygen isotope signals in response to monsoon rainfall in northwestern Thailand

We present a new teak ( Tectona grandis L.f.) tree-ring stable oxygen isotope (δ18OTR) chronology from Mae Tuen (MT) site, Tak province, northwestern Thailand at inter-annual (167-year, A.D.1850–2016) and intra-annual (A.D.2006–2016) timescales. The inter-annual δ18OTR chronology showed a significant negative correlation ( r = −0.678, p < 0.01) with the May–October amount of regional rainfall obtained from CRU TS4.03. We established a linear regression model that explained 46.0% of the actual rainfall variance and reconstructed monsoon season rainfall back to A.D.1850. The seasonal variations in moisture conditions were reflected in inter-annual δ18OTR variability. The rainfall amount and δ18O in precipitation (δ18Op) mainly controlled intra-annual variations in teak δ18OTR. These results show that the variation in Thai teak δ18OTR is dominated by rainfall, highlighting the strength of δ18OTR as a monsoon rainfall proxy.

A ~700 years perspective on the 21st century drying in the eastern part of Europe based on δ18O in tree ring cellulose

Numerical simulations indicate that extreme climate events (e.g., droughts, floods, heat waves) will increase in a warming world, putting enormous pressure on society and political decision-makers. To provide a long-term perspective on the variability of these extreme events, here we use a ~700 years tree-ring oxygen isotope chronology from Eastern Europe, in combination with paleo-reanalysis data, to show that the summer drying over Eastern Europe observed over the last ~150 years is to the best of our knowledge unprecedented over the last 700 years. This drying is driven by a change in the pressure patterns over Europe, characterized by a shift from zonal to a wavier flow around 1850CE, leading to extreme summer droughts and aridification. To our knowledge, this is the first and longest reconstruction of drought variability, based on stable oxygen isotopes in the tree-ring cellulose, for Eastern Europe, helping to fill a gap in the spatial coverage of paleoclimate reconstructions.

The modulation of Pacific Decadal Oscillation on ENSO-East Asian summer monsoon relationship over the past half-millennium

Monsoon precipitation affects natural and social systems in East Asia, one of the most densely populated regions in the world. Monsoon precipitation variability is strongly influenced by El Niño-Southern Oscillation (ENSO) and may be related to the phase of the Pacific Decadal Oscillation (PDO). However, a collective understanding of the long-term PDO-ENSO-monsoon relationship remains limited because related studies are almost exclusively based on short instrumental records. Although paleoclimate proxies for PDO and ENSO are currently available, there is a lack of high-quality proxies for East Asian summer monsoon (EASM) precipitation. Moreover, the strengthening of the ENSO-EASM relationship since the 1970s has raised the question of anthropogenic impact. Reconstructing EASM precipitation is thus crucial to understanding its variability under natural and anthropogenic forcings. In this study, we addressed these challenges using tree ring oxygen isotopes of red cypress (Chamaecyparis formosensis Matsum), a long-lived endemic tree species in Taiwan. We developed an annual-resolved and well-validated EASM precipitation proxy from 1533 CE to 2011 which explained 49 % of the variance in instrumental precipitation. In comparison with multiple paleoclimate proxies, we revealed that PDO persistently modulated the ENSO-EASM relationship over the past half-millennium. The ENSO-EASM relationship was enhanced during the positive PDO phases and dynamically weakened during the negative PDO phases, notably in the early-17th, 18th, and early to mid-20th centuries. The strengthened relationship since the 1970s concurred with an unusually high PDO and ENSO and fell within its natural variability. Nevertheless, as the amplitude of the PDO is predicted to weaken under warming, the modulation effects may become less predictable.

Different moisture regimes during the last 150 years inferred from a tree-ring δ18O network over the transitional zone of the Asian summer monsoon

The East Asian summer monsoon (EASM) and Indian summer monsoon (ISM) are two interactive climate systems dominating the moisture variability of Monsoon Asia. However, ISM-EASM interactions and their effects on regional moisture dynamics remain unclear. This study investigated the spatiotemporal characteristics of monsoon-related hydroclimate variability by establishing a new tree-ring oxygen isotope ratio (δ18Otree) network of eight δ18Otree records covering the last 150 years in southern China, which differs from previous studies that used data from individual sites. The δ18Otree chronologies were found to be sensitive to regional hydroclimatic changes during the monsoon season. The δ18Otree network indicated an east–west clustering pattern in the ISM-EASM transitional zone, which demonstrates an asynchrony in δ18Otree variability in the west and east of this region. Regional variability of the west and east δ18Otree modes reflected the different moisture signals, as indicated by their significant correlations with modern observations and paleo-δ18O records in disparate upstream regions of moisture transport (the Indochina Peninsula and Southeast China). This finding was confirmed by the east–west distinct pattern of spatial isotopic depletion of monsoon rainfall that originates from coastal areas of tropical oceans (Bay of Bengal and South China Sea) to the study area, as well as the consistent present-day pattern of atmospheric vapor transport during the monsoon season. In addition, the west and east modes exhibited stronger associations with ISM variability, whereas the east–west δ18Otree gradient captured the EASM-related hydroclimatic signal, suggesting that summer moisture variability in the ISM-EASM transitional zone was regulated more by ISM than by EASM during the last 150 years. The relationships between δ18Otree and large-scale ocean–atmosphere interaction modes revealed that the El Niño-Southern Oscillation (ENSO) dominantly modulates δ18Otree variability across southern China.

Tree ring oxygen isotope dating of wood recovered from a canal in the ancient capital of Japan

Trees growing in the lowlands of Japan are not climatically stressed; consequently, ring-width patterns are weakly correlated between different trees. Therefore, dendrochronological studies in Japan based on ring widths rely largely on climatically stressed conifers growing in mountainous regions and/or at higher latitudes. In contrast, oxygen isotope ratios (δ18O) are well-matched between trees, irrespective of climatic stress, indicating that the huge amounts of wood excavated in Japan could potentially be dated. Here, we tested oxygen isotope dating using unearthed stumps (Abies sp., Quercus sp., and Zelkova serrata) from Japan, which historical documents and archaeological evidence suggest date from 708 to 715 CE. A new 2470-year master chronology based on Japanese cypress samples (mainly Chamaecyparis obtusa) was developed, and our dating method was evaluated using modern samples collected from several species (Tsuga, Picea, Taxus, Pinus, and Quercus) across Japan and Korea. Our chronology was then used to date stumps recovered from the bank of a canal, which was constructed when Nara became the new capital city of Japan (Heijō-kyō in Japanese). Even though the samples had only 30–60 rings and comprised multiple species, including broad-leaf trees, their tree ring δ18O patterns were successfully cross-matched. The resulting site chronology was robustly dated against the master chronology, indicating that the felling dates of the stumps ranged from winter 697/8 to 709/10 CE. Overall, the dates are consistent with previous findings, but our study provides exact calendar dating related to the construction of the new capital, and demonstrates that oxygen isotope dendrochronology works well in Japan.

Tree-Ring Oxygen Isotope Variations in Subalpine Firs from the Western Himalaya Capture Spring Season Temperature Signals

We analyzed the tree-rings δ18O of Abies spectabilis (fir) growing at the subalpine treeline ecotone in the Magguchatti valley. The valley is located in the Indian summer monsoon (ISM) dominated region of western Himalaya and also receives snow precipitation derived by westerly disturbances (WDs) during the winter months. The 60 year developed (1960–2019 CE) tree-ring δ18O chronology revealed a strong positive correlation with the temperature of late winter and spring months (February to April). Strong negative correlations are also apparent for snowcover, soilmoisture, and relative humidity for the same spring season. Our findings partly contrast the significant correlation results of tree-ring δ18O with summer precipitation and drought indices recorded from other summer monsoon-dominated regions in the Himalayas. The spatial correlation analyses with sea surface temperatures (SSTs) and climate parameters showed subdued signals of tropical Pacific at the site, but with a shift to more moisture influx from the Arabian Sea during the last two decades. Moreover, a significant negative correlation with North Atlantic Oscillation further justifies the strongly captured spring temperature and snowcover signals and the weak effect of summer precipitation in fir trees. A temperature rising trend during the latter half of the 20th century and the elevation effect are taken as important factors controlling the moisture source at the treeline ecotone zones.

Reconstruction of alpine snowfall in southern Kazakhstan based on oxygen isotopes in tree rings

Reconstruction of alpine snowfall in southern Kazakhstan based on oxygen isotopes in tree rings

Tree-ring oxygen isotopes record a decrease in Amazon dry season rainfall over the past 40 years

Extant climate observations suggest the dry season over large parts of the Amazon Basin has become longer and drier over recent decades. However, such possible intensification of the Amazon dry season and its underlying causes are still a matter of debate. Here we used oxygen isotope ratios in tree rings (δ18OTR) from six floodplain trees from the western Amazon to assess changes in past climate. Our analysis shows that δ18OTR of these trees is negatively related to inter-annual variability of precipitation during the dry season over large parts of the Amazon Basin, consistent with a Rayleigh rainout model. Furthermore δ18OTR increases by approximately 2‰ over the last four decades (~ 1970–2014) providing evidence of an Amazon drying trend independent from satellite and in situ rainfall observations. Using a Rayleigh rainout framework, we estimate basin-wide dry season rainfall to have decreased by up to 30%. The δ18OTR record further suggests such drying trend may not be unprecedented over the past 80 years. Analysis of δ18OTR with sea surface temperatures indicates a strong role of a warming Tropical North Atlantic Ocean in driving this long-term increase in δ18OTR and decrease in dry season rainfall.

Improved El Niño Southern Oscillation signals extracted by principal component analysis of tree-ring oxygen isotope records from the East Asian monsoon region of China

There is a great need for the reconstruction of independent El Niño Southern Oscillation (ENSO) from different archives and locations to provide a robust understanding of its driving and feedback mechanisms on long time scales. Though tree-ring cellulose δ18O (δ18OTR) records from the East Asian monsoon (EAM) region of China have the potential to reconstruct ENSO on an annual time scale, δ18OTR ENSO signals may be masked by local climate effects since any individual δ18OTR record is influenced by both large-scale atmospheric circulation and local climate. Few attempts have been made to reduce the uncertainties of the reconstructed ENSO index by δ18OTR records from the EAM region of China by ensemble data analysis. In this study, twelve published δ18OTR records spanning over 100 years (1902–2003 CE) have been collected and processed using principal component analysis (PCA). The first principal component (PC1) extracted from the δ18OTR records shows a positive/negative relationship with the annual Niño 3.4 index/southern oscillation index (SOI), with correlation coefficients higher than those between single δ18OTR record and the Niño 3.4 index/SOI, suggesting the ENSO signal is improved by PCA. A 31-year running correlation on PC1- Niño 3.4 index/SOI reveals an increasingly stronger correlation between δ18OTR and ENSO since the last century, inferring that, in the EAM region, the δ18OTR-ENSO correlation may become much stronger under continuous global warming in the future. Spectrum and spatial correlation analysis further support the idea that PC1 responds to ENSO. The significant relationships between observational precipitation δ18O and Niño 3.4 SST anomaly and PC1 from 1973–2003 CE suggest that precipitation δ18O is a key link to connect δ18OTR with ENSO. This study shows the advantage of ensemble data analysis on δ18OTR records in EAM region of China, which could better extract ENSO signals and reduce the uncertainties of reconstructed ENSO variations by individual δ18OTR records from the EAM region of China. It provides an alternative independent annual indicator for ENSO reconstruction on long time scales when more δ18OTR records from this region are extended in the near future.