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The response of the growing season to the ongoing global warming has gained considerable attention. In particular, how and to which extent the growing season will change during this century is essential information for the Tibetan Plateau, where the observed warming trend has exceeded the global mean. In this study, the 1960-2014 mean length of the tree-ring growing season (LOS) on the Tibetan Plateau was derived from results of the Vaganov-Shashkin oscilloscope tree growth model, based on 20 composite study sites and more than 3000 trees. Bootstrap and partial correlations were used to evaluate the most significant climate factors determining the LOS in the study region. Based on this relationship, we predicted the future variability of the LOS under three emission scenarios (Representative Concentration Pathways (RCP) 2.6, 6.0, and 8.5, representing different concentrations of greenhouse gasses) derived from 17 Earth system models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5). The averaged LOS on the Tibetan Plateau is 103 days during the period 1960-2014, and April-September minimum temperature is the strongest factor controlling the LOS. We detected a general increase in the LOS over the twenty-first century under all the three selected scenarios. By the middle of this century, LOS will extend by about 3 to 4 weeks under the RCPs 2.6 and 6.0, and by more than 1 month (37 days) under the RCP 8.5, relative to the baseline period 1960-2014. From the middle to the end of the twenty-first century, LOS will further extend by about 3 to 4 weeks under the RCPs 6.0 and 8.5, respectively. Under the RCP 2.6 scenario, however, the extension reaches a plateau at around 2050 and about 2 weeks LOS extension. In total, we found an average rate of 2.1, 3.6, and 5.0 days decade−1 for the LOS extension from 2015 to 2100 under the RCPs 2.6, 6.0, and 8.5, respectively. However, such estimated LOS extensions may be offset by other ecological factors that were not included into the growth model. The estimated lengthening of the growing season could substantially affect carbon sequestration and forest productivity on the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

The variability of tree stem phenology plays a critical role in determining the productivity of forest ecosystems. Therefore, we aim to identify the relationships between the timings of cambium phenology, and forest growth in terms of tree-ring width over a long-term scale. A meta-analysis was performed that combined the timings of xylem formation, which were calculated by a tree-ring formation model of the VS (Vaganov-Shashkin)-oscilloscope during the period 1960-2014, and a tree-ring width series at 20 composite sites on the Tibetan Plateau. Both the start and length of the growing season significantly affected the formation of wood at 70% of the 20 composite sites within the study region. A wider tree ring probably resulted from an earlier start and a longer duration of the growing season. The influence of ending dates on tree-ring width was less evident, and more site-dependent. Weak relationships were identified between the start and end of the growing season at 85% of the composite sites. Compared to the monitoring results, which could only detect the relationships between cambium phenology and xylem cell production from a limited number of trees and years, our long-term relationships deepened such connections, and therefore should be used to improve mechanism models for the accurate evaluating and predicting of wood production and carbon sequestration in forest ecosystems under current and future climate change. [ABSTRACT FROM AUTHOR]

In recent decades, the Tibetan plateau (TP) experienced a distinctive temperature increase, with fundamental consequences for the hydrological system. As meteorological time-series extending back more than 60 years are scarce, there is a strong need for proxy data providing insight into the regional hydroclimatic history as well as the long-term variability. Within this study, a recently developed annually resolved 1500 year-long tree-ring stable oxygen isotope (δ 18 O) time from the southeastern TP is presented. Climate–proxy relationships reveal a strong impact of relative humidity (rH) during the summer months (May to September) on tree-ring δ 18 O, explaining around 45% of its variance. The derived reconstruction of relative humidity reveals a recent trend towards drier conditions on the TP. However, the present low rH level is not unprecedented in regard to the last 1500 years. In comparison to recent climate conditions, the Medieval Warm Period (MWP) was characterized by more stable conditions with higher humidity values. Hydroclimatic conditions during the Little Ice Age (LIA) suggest a contrasting two-phased period, with a clear shift from drier conditions prevailing between 1400 and 1650 AD to more humid conditions since the second half of the 17th century. Comparisons with other local proxies from lake levels and Pollen data from the southern part of the Tibetan plateau indicate a common regional climate forcing during the MWP and the LIA which can be related to changes in summer monsoon activity. However, the strength of the distinct dryness trend during the 20th century seems unique. [ABSTRACT FROM AUTHOR]

We present a newly developed, annually resolved tree-ring cellulose δO chronology for the southeastern Tibetan Plateau (TP) from Sikkim larch ( Larix griffithii), spanning between 1684 and 2012. Comparisons with local and regional climate data reveal strong positive correlations with monthly sunshine hours, temperature and daily temperature amplitude as well as strong negative correlations with relative humidity, vapor pressure, rain days per month and cloud cover of August. Relationships with local and regional tree-ring δO chronologies are stable and highly significant. Over the 20th century, we find no long-term climatic trends. This is consistent with other tree-ring δO chronologies of other tree species south of the Himalayas, but contrasts with results from isotope studies north of the Himalayas. This suggests stable macroclimatic flow patterns throughout the last centuries for the southern tree stands. In terms of large-scale climate dynamics, we find evidence of a significant 30-year wave influencing our tree-ring oxygen chronology, most probably induced by the Indian Ocean Dipole and influencing tree-ring oxygen isotope chronologies along the southeastern Himalaya and the southeastern rim of the TP. This pattern is spatially and temporarily consistent among the chronologies and has apparently strengthened during the last century. During periods of strong positive dipole mode activity, the dipole mode index shows positive correlations with the δO of tree-rings on the southeastern TP. [ABSTRACT FROM AUTHOR]

Knowledge of Asian monsoon variability remains limited because of sparse instrumental data available only for short time series. Here, an updated tree-ring width record covering the period ad 1037–2009 was developed for the south-central Tibetan Plateau (TP). Correlation analysis revealed a significant relationship (r = 0.71) between the tree-ring index and annual (previous July to current June) precipitation series for the instrumental period 1963–2008, which accounts for 50.41% of the rainfall variability. Based on a linear regression model, the longest available regional precipitation history was reconstructed. Spatial correlation between tree ring width and annual precipitation data from previous July to current June indicates that the reconstruction is representative of precipitation changes on the south-central TP. Regional wet conditions occurred during ad 1095–1161, 1376–1403, 1414–1446, 1518–1537, 1549–1572, 1702–1757, 1848–1878 and 1891–1913, while dry periods were identified during ad1189–1242, 1256–1314, 1329–1357, 1470–1491, 1573–1623, 1636–1686, 1761–1821, 1823–1847, 1879–1890 and 1931–1985. The negative correlation between our reconstructed precipitation and India monsoon rainfall series indicates the seesaw pattern over northern and southern monsoon Asia. It is suggested that solar radiation-induced sea surface temperature (SST) anomalies over the tropical Pacific influence regional rainfall patterns. The degree of this influence has been stable at the multidecadal scale during the past 1000 years. [ABSTRACT FROM AUTHOR]

Abstract: High-resolution proxy data are still scarce in the southeastern Tibetan Plateau, which limit our understanding of climatic variability before instrumental records. In this study, we developed 595-year tree ring-width chronology from Larix speciosa near the timberlines of the Gaoligong Mountains, southeastern Tibetan Plateau. Ring-width site chronologies and a well-replicated regional chronology (RC) showed significant positive correlations with warm season temperatures from May to August. Using RC as predictor, we reconstructed mean May–August temperature for the study region that extends back to A.D. 1585. Cold conditions prevailed during the periods 1600s, 1620–30, 1640–50s, 1700s, 1730–40s, 1760s, 1810–20s, 1850s, 1900–10s and 1955–70. Warm episodes occurred during 1610s, 1660–1680s, 1710–20s, 1750s, 1780–90s, 1820–40, 1920–50 and 1980–present. Spatial correlations with gridded land surface temperatures revealed that our reconstruction represents regional temperature signal for the southern Tibetan Plateau. Comparison with other tree ring-based temperature reconstructions from surrounding areas implies a high degree of confidence for our reconstruction. [ABSTRACT FROM AUTHOR]

The article presents a study on the temperature history recorded by tree rings in the Tibetan Plateau (TP), China during 1612-1998. The study develops two ring-width index chronologies such as traditionally standardized chronology (TSC) and regional curve standardization (RCS) for detrending. The results show that low-frequency and high-frequency temperature variations are captured by RCS and TSC reconstructions, and the response patterns of RSC chronology to climate data are similar to the TSC.

tree ring-width record spanning from AD 1377-1998 was developed from Tibetan juniper (Cupressus gigantea) growing at sites north of the deep gorge of the Yarlung Tsangbo River of southeast Tibet. A linear regression model between ring width and mean January-June temperature accounts for 35% of January-June temperature variance for the period 1961-1998. Based on this model, we reconstructed January-June temperature variation history for the study region during the past 622 years. Warm conditions occurred during AD 1385-1418, 1443-1466, 1482-1501, 1523-1548, 1570s, 1705-1759, 1770-1790, 1851-1888, 1910s, and 1938-1965, and periods of relatively cold years are identified forAD 1419-1442, 1470s, 1502-1522, 1550-1569, 1610-1640, 1680-1700, 1760s, 1791-1850, 1900s and 1965-1995. Spatial correlation between tree rings and observed temperatures indicates that the reconstruction is representative of temperature change for southeast Tibet. Regional cold conditions during AD 1625, 1685, 1760, 1800-1850, 1890-1930 and 1965-1995, and warm conditions during 1710, 1730-1750, 1850-1890 and 1930-1960 can be identified in the eastern Tibetan Plateau. Our reconstruction shows good agreement with modelled regional temperatures derived from an ECHO-G model simulation. Common cold periods around 1470, 1685 and 1845 corresponding to the Spörer Minimum, the Maunder Minimum and the Dalton Minimum periods of low solar activity and increased volcanic activity were identified, indicating that solar and volcanic activity played an important role in temperature change of this region. The discrepancy between modelled and reconstructed temperatures was noted around 1965, reflecting that temperature decrease in the late 20th century (1965-1995) is anomalous. [ABSTRACT FROM AUTHOR]

Abstract: The climatological signal of δ 18O variations preserved in ice cores recovered from Puruogangri ice field in the central Tibetan Plateau (TP) was calibrated with regional meteorological data for the past 50 years. For the period AD 1860–2000, 5-yearly averaged ice core δ 18O and a summer temperature reconstruction derived from pollen data from the same ice core were compared. The statistical results provide compelling evidence that Puruogangri ice core δ 18O variations represent summer temperature changes for the central TP, and hence regional temperature history during the past 600 years was revealed. A comparison of Puruogangri ice core δ 18О with several other temperature reconstructions shows that broad-scale climate anomalies since the Little Ice Age occurred synchronously across the eastern and southern TP, and the Himalayas. Common cold periods were identified in the 15th century, 1625–1645 AD, 1660–1700 AD, 1725–1775 AD, 1795–1830 AD, 1850–1870 AD, 1890–1920 AD, 1940–1950 AD, and 1975–1985 AD. The period 1725–1775 AD was one of the most prolonged cool periods during the past 400 years and corresponded to maximum Little Ice Age glacier advance of monsoonal temperate glaciers of the TP. [Copyright &y& Elsevier]

Abstract: In this preliminary study, we compare two tree-ring width chronologies from the Tibetan Plateau and Scandinavia. At their altitudinal limit of distribution Juniperus tibetica (Tibet) and Pinus sylvestris (Scandinavia) are sensitive to climate variability, and both chronologies cover the last 1000 years. We used regional curve standardization (RCS) of the tree-ring data, to reveal growth variability on annual to centennial timescales. Both records showed similar evolution in tree-growth variability, on decadal and longer time scales, between c. 1100 and 1550. However, the tree-ring data indicated a gradually varying climate in Tibet compared to the more frequent oscillations seen in the Scandinavian record. The impact of the Medieval Warm Period is not clearly evident at either site. Tree growth was reduced during the Little Ice Age (LIA) both in Tibet and Scandinavia, but the timing and magnitude of the LIA differ between the two regions. In Tibet, unfavourable climate was inferred mainly between c. 1400 and 1550 and in the mid-17th century, while in Scandinavia harsh growth conditions prevailed more or less from the late 16th century to the early 20th century. Around 1550 the two chronologies diverge, and it is suggested that this was caused by a strengthening of the Asian monsoon. Warming in the 20th century is more pronounced in Scandinavia. In Tibet, tree growth is declining, which may be a response to a strengthening of the Asian summer monsoon during the last four centuries. [Copyright &y& Elsevier]

Identifying what determines the high elevation limits of tree growth is crucial for predicting how treelines may shift in response to climate change. Treeline formation is either explained by a low-temperature restriction of meristematic activity (sink limitation) or by the photosynthetic constraints (source limitation) on the trees at the treeline. Our study of tree-ring stable isotopes in two Tibetan elevational transects showed that treeline trees had higher iWUE than trees at lower elevations. The combination of tree-ring δ13C and δ18O data further showed that photosynthesis was higher for trees at the treeline than at lower elevations. These results suggest that carbon acquisition may not be the main determinant of the upper limit of trees; other processes, such as immature tissue growth, may be the main cause of treeline formation. The tree-ring isotope analysis (δ13C and δ18O) suggests that Tibetan treelines have the potential to benefit from ongoing climate warming, due to their ability to cope with co-occurring drought stress through enhanced water use efficiency. [ABSTRACT FROM AUTHOR]

For the mechanistic understanding of hydroclimate variability, stable oxygen isotopic ratios (δ 18 O) of tree-ring α-cellulose can play a key role in paleoclimatic research. On the north-east Tibetan Plateau (TP), a region particularly sensitive to climate change, there exists a distinct lack of δ 18 O research in tree-ring archives. The few currently existing tree-ring δ 18 O chronologies were obtained by pooling the wood material from the same year from different trees prior to preparation and analysis. Although this method is time and cost efficient, it might impede the analysis of changes within the internal variability of oxygen isotope signals and a deeper understanding of the resulting climate reconstruction. We selected five Qilian juniper ( Sabina przewalskii Kom .) trees covering the period 1951–2011 on the north-east TP to investigate the climate signals contained in stable oxygen isotopic composition records obtained using single tree analysis. Although the inter-series correlation between the five individual δ 18 O series indicates a highly significant relationship (mean r = 0.59, n = 59, p = 0.01), the five individual δ 18 O series show significantly differently mean values. We found significant negative correlations between drought/hydroclimatically triggered climate parameters (PDSI, precipitation) and each of the individual δ 18 O series. Spatial correlation maps indicate negative correlation of tree-ring δ 18 O and Indian Ocean sea surface temperatures, and inverse correlation patters of moisture conditions on the northern and southern TP. Our results provide a reference for future research on stable oxygen isotope/climate signals in the species S. przewalskii Kom .. [ABSTRACT FROM AUTHOR]

Abstract: Qilian juniper (Juniperus przewalskii Kom.) is a widely distributed tree species growing on south-facing slopes in the northeastern Tibetan Plateau in arid northwestern China. We established a tree-ring width network based on two new chronologies and four previously published chronologies. Correlation and response function analyses demonstrate that precipitation positively influences radial growth. Despite of minor differences in local climate–growth relations, precipitation for the annual window between previous July and current June shows consistent positive correlations with ring width at all study sites. Similar to the so called ‘pointer year’ approach, ‘anomalous’ growth years were defined to extract extreme climate events for the period AD 1450–2006. We defined a dryness–wetness grade series with five grades of climate events inferred from anomalous year analysis. During the last 50years, the frequency of wet events increased and that of drought events decreased noticeably, implying that the probability of occurrence of dry years in the northeastern Tibetan Plateau will further decrease in the future if regional warming continues. Combining our proxy records with a historical dryness–wetness record from eastern China, we mapped dryness–wetness patterns over large parts of China. By analyzing the atmospheric pressure patterns at the 850hPa level over China for selected extreme event years, we found that the confluence of cold and hot air is a precondition for a flood event in the northeastern Tibetan Plateau. Thus, a counter-clockwise atmospheric circulation centered in south of Lake Baikal only occurs in flood event years. [Copyright &y& Elsevier]

Abstract Oxygen isotope ratios (δ 18 O) are the most commonly used parameters recorded in paleoclimate archives since they link different natural archives via the water cycle. Tree-ring δ 18 O (δ 18 O TR) has been widely used for hydroclimate reconstructions in the Himalaya. However, few of them record temperature signals, which are dominant in Himalaya ice-core δ 18 O. We hypothesize that the "precipitation amount effect" due to the South Asian Summer Monsoon (SASM) may overprint temperature signals in δ 18 O TR series. The purpose of this study is to investigate whether temperature signals could be found in the δ 18 O TR in locations where the influence of SASM is weak. We developed a 105-yr δ 18 O TR chronology from the northern slope of the western Himalaya which greatly blocks the SASM. Our δ 18 O TR clearly shows stronger correlations with temperature (dominant winter and weak summer) than summer precipitation signals. It also agrees well with summer soil moisture δ 18 O simulated by the global isotope model LMDZ4 (r = 0.72, 1979–2010). In LMDZ4, low winter temperature was found to increase winter snowfall and subsequent snow melt, and thus to increase the contribution of winter snowfall to soil moisture in summer at the expense of summer precipitation. Since winter snowfall is more depleted than summer precipitation, this leads to lower summer soil moisture δ 18 O. The temperature signals found in our δ 18 O TR series are consistent with those found in the Dasuopu ice-core δ 18 O. This implies that δ 18 O TR series from the southwest Tibetan Plateau (TP), with a weak monsoon, hold great potential to capture temperature signals. Climate interpretations of δ 18 O proxies in the Himalaya largely depend on the influence of seasonal water from the dominant atmosphere circulation systems of the westerlies or monsoon. The δ 18 O proxies from the monsoon-affected region have a higher potential for the reconstruction of boreal summer hydroclimate, whereas δ 18 O proxies from westerly-affected sites have a higher potential for temperature reconstructions. Highlights • A tree-ring δ 18 O record was developed from the northern slope of the Himalayas. • The δ 18 O TR recorded dominant winter and weak summer temperature signals. • No summer monsoon precipitation signals were found. • Winter temperature signals were mainly transferred into δ 18 O TR through snowmelt. [ABSTRACT FROM AUTHOR]

Little is known about whether increasing iWUE (intrinsic water use efficiency) can stimulate tree growth in the temperature-limited natural timberlines. Here, we presented the basal area increment (BAI) and iWUE chronologies of Smith fir ( Abies georgei var. smithii ) from 1900 to 2006 at a high-elevation (ca. 4400 m a.s.l.) timberline in the humid Sygera Mountains, southeastern Tibetan Plateau (TP). The commonality analysis model was applied to investigate the relationships among BAI, temperatures, atmospheric CO 2 concentration ( C a ) and iWUE during 1961–2006, taking into account of both pure and joint effects. As illustrated by the commonality analysis model, the pure effect of C a (39.15%) had more stronger influence on iWUE than that of the Tmean (annul mean temperature, 0.12%), but the joint effect between C a and Tmean (49.79%) on iWUE was stronger than any pure effect for the raw data with an increasing trend. For the first-difference data with year-to-year variations, the pure effect of C a (7.72%) on iWUE was stronger than that of Tmean (0.59%) and the joint effect between them (0.59%). All above imply the C a is the dominant factor for iWUE both for the 46-year trend and interannual variations. In addition, as showed by the commonality analysis model, the pure effect of iWUE (17.57%) played a much more important role on BAI than that of temperatures (smt, mean temperature during June, July, August of current year, 5.92%; amt, mean temperature during September, October, November of previous year, 3.04%), while joint effects of iWUE and temperatures contributed more (27.96%; 13.90%; 16.47%) to the BAI than their pure effects for the raw data with an increasing linear trend. For the first-difference data with interannual variations, the pure effect of smt (12.45%) had much more effect on BAI than that of iWUE (5.49%), at the same time the joint iWUE and temperatures contributed less (3.56%; 1.9%; 1.31%) to the BAI than their pure effects. These results suggest that an increasing iWUE could enhance 46-year increasing tree growth trend at humid and high-elevation timberlines, supporting the CO 2 fertilization hypothesis, while temperatures dominate the interannual variations of tree growth. [ABSTRACT FROM AUTHOR]

• A new method that isolates the RH signal from tree-ring δ18O is proposed. • Reconstructed millennium RH revealed cycles of ∼ 35, ∼85, 130–160 and ∼ 200 years. • Local RH may decrease in the future with continuous global warming. As one of the driest regions in the Asia, the ecological conditions in Northwest China are very fragile. Although relative humidity (RH) plays a vital role in the environment, the lack of long-term RH records hinders the development of reliable scenarios for future extreme environmental conditions in this region. We developed a new method for quantitative regional RH reconstruction; in this method, the signals of precipitation oxygen isotope ratios (δ18O) and temperature in the tree-ring δ18O series are removed to extract the RH signal from the tree-ring δ18O records to the greatest extent. We then combined published millennium tree-ring δ18O, ice core δ18O, and paleo-temperature data to reconstruct a new millennium-long RH record around Delingha, Qinghai, China, on the edge of the northeastern Tibetan Plateau (TP). The reconstructed RH was closely related to both temperature and rainfall, and revealed cycles of approximately 35, 85, 130–160 and 200 years, which are associated with solar insolation variations, Pacific decadal oscillation, and the Atlantic multidecadal variability. With continuous global warming, RH may decrease as the contribution of the temperature increase becomes larger than that of the precipitation over the northeastern TP. This study provides a basis for a new method of RH reconstruction based on different paleoclimate archives. [ABSTRACT FROM AUTHOR]

Advances and retreats of glaciers on the Tibetan Plateau (TP) are widely regarded as indicators for climate changes sensitive to macroclimatic forcing mechanisms like the Asian summer monsoon. However, it often remains unclear why some glaciers retreat or advance earlier than others, particularly regarding the timing of the so-called ‘Little Ice Age (LIA) maximum’. Within this study, we present newly acquired tree-ring data from four glacier forefields in the Nyainqêntanglha Range on the southeastern TP and link them to previous studies. Two of the glaciers formed a double-tongue moraine set during the LIA, with trees providing minimum ages for two different major glacier extents. In combination with evidence from a third glacier, three warm phases within the generally cold LIA can be deduced during the first half of the 16th century, and the mid-17th and 18th centuries. This implies the occurrence of three preceding major cold spells. A minor cool phase between 1790 and 1850 marks the end of the LIA on the southeast TP. Minimum ages for beginning retreat from the last major LIA glacial advance differ by only 40 years for all sampled monsoonal temperate valley glaciers in the Nyainqêntanglha Range. This implies a common forcing, whose impact on glacier dynamics is only slightly altered by topographical factors or local climatic conditions. Regression analysis suggests that the delay of the glacier retreat from the maximum extent is mainly related to glacier size, with minor influence of ablation area aspect. All moraine ages highlight intermediate LIA warm phases on the southeast TP corresponding to warm phases on the northern Hemisphere and thus prove monsoonal temperate glaciers to be highly sensitive archives for northern hemispheric climatic conditions. [ABSTRACT FROM AUTHOR]