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The Himalayas experienced long-term climate changes and recent extreme weather events that affected plant growth and the physiology of tree species at high-elevation sites. This study presents the first statistically robust δ18OTR chronologies for two native pine species, Pinus roxburghii, and Pinus wallichiana, in the lower Nepalese Himalaya. The isotope chronologies exhibited 0.88‰ differences in overall mean isotope values attributed to varying elevations (460–2000 m asl). Comparative analysis of climate response using data sets from different sources and resolutions revealed the superiority of the APHRODITE (Asian Precipitation – Highly-Resolved Observational Data Integration Towards Evaluation) data set calibrated for the South Asian Summer Monsoon (SASM)-dominated region. Both species exhibited negative correlations with monsoon precipitation and positive correlations with temperature. However, during the peak monsoon season (July-August), daily resolved climate data disentangled statistically insignificant relationships, and revealed that δ18OTR is influenced by atmospheric moisture. Both congeneric species showed a decoupling between the chronologies after 1995. However, no significant change in air moisture origin and monsoon regime between the study sites was observed, indicating a consistent dominant moisture source during different monsoon seasons. Besides, we also observed the decreased inter-series correlation of both δ18OTR chronologies after 1995, with P. wallichiana experiencing a steeper decrease than P. roxburghii. The weakening correlations between and within the chronologies coincided with a regional drought during 1993–1995 in both sites, highlighting the strong regulation of local climate on the impact of regional extreme climate events. Our findings emphasise the importance of employing climate data with optimal spatial and temporal resolution for improved δ18OTR-climate relationships at the intra-annual scale while considering the influence of site-specific local environmental conditions. Assessing climate data sets with station data is vital for accurately interpreting climate change's impact on forest response and long-term climate reconstructions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Tree-ring δ18 O values are a sensitive proxy for regional physical climate, while their δ13 C values are a strong predictor of local ecohydrology. Utilizing available ice-core and tree-ring δ18 O records from the central Himalaya (CH), we found an increase in east–west climate heterogeneity since the 1960s. Further, δ13 C records from transitional western glaciated valleys provide a robust basis for reconstructing about 3 centuries of glacier mass balance (GMB) dynamics. We reconstructed annually resolved GMB since 1743 CE based on regionally dominant tree species of diverse plant functional types. Three major phases became apparent: positive GMB up to the mid-19th century, the middle phase (1870–1960) of slightly negative but stable GMB, and an exponential ice mass loss since the 1960s. Reasons for accelerated mass loss are largely attributed to anthropogenic climate change, including concurrent alterations in atmospheric circulations (weakening of the westerlies and the Arabian Sea branch of the Indian summer monsoon). Multi-decadal isotopic and climate coherency analyses specify an eastward declining influence of the westerlies in the monsoon-dominated CH region. Besides, our study provides a long-term context for recent GMB variability, which is essential for its reliable projection and attribution. [ABSTRACT FROM AUTHOR]

To better understand long-term drought variations in the central Himalaya, we developed new tree-ring width chronologies of Himalayan spruce ( Picea smithiana (Wall.) Boiss.) from three sites in the north-western Nepal. The local site chronologies showed high cross correlations and similar growth-climate responses to regional spring drought variability. We thus combined all site chronologies into a regional composite (RC) standard chronology that spans 516 years (1498–2013 CE). The RC chronology showed significant positive (negative) correlations with spring (March–May) precipitation (temperature) variability. Meanwhile, RC chronology showed the highest correlation with spring self-calibrating Palmer drought severity index (scPDSI, r = 0.652, p < 0.001), indicating that radial growth of P. smithiana is strongly limited by spring moisture availability. Using RC chronology, we reconstructed the spring drought variability for the period 1725–2013, which explained 42.5% variance of the actual scPDSI during the calibration period 1957–2012. Our reconstructed spring drought variability in the central Himalaya showed consistent wet-dry episodes with other regional drought and precipitation reconstructions from the Himalaya and nearby regions. Spectral peaks and spatial correlation analysis indicate that spring drought variability in the central Himalaya may be linked to large scale climatic drivers, mainly Atlantic Multidecadal Oscillation activities due to sea surface temperatures variation in the Atlantic Ocean. Our reconstruction revealed a continuous shift toward drier conditions in the central Himalaya since early 1980s that coincide with continental-scale warming and reduced spring precipitation in the central Himalaya. [ABSTRACT FROM AUTHOR]

• Tree growth in the Karnali river catchment was sensitive to moisture availability. • March–July streamflow was reconstructed for Karnali River Basin since 1628 CE. • An increasing frequency of extreme events was found during the recent decades. • Streamflow variability of the Karnali river is mainly linked with ENSO and PDO. The Karnali River is one of the major transboundary rivers of the Nepalese central Himalaya and a major tributary of the Ganges River. In order to investigate regional historical hydroclimate variability, we compiled eleven tree ring-width chronologies based on 354 exactly dated tree cores from five coniferous species for the Karnali River catchment. The chronologies were moisture-sensitive and the composite chronology was significantly correlated with spring-to-early summer (March–July) (R = 0.67, P < 0.001) discharge at the Chisapani gauging station. The robust calibration (R2 = 0.45, 1962–2010) enabled us to reconstruct March–July streamflow variability of Karnali River Basin (KRB) over the past four centuries. The streamflow reconstruction showed prolonged pluvial periods during 1650–1661, 1711–1718, 1723–1731, 1824–1831, 1877–1889, 1924–1931, and 1971–1981, while prolonged low-flow periods occurred during 1639–1649, 1689–1710, 1753–1769, 1777–1784, 1803–1823, 1867–1876, 1942–1959, and 1963–1970. Our reconstructed March–July streamflow series correlated positively with the seasonally averaged drought index and precipitation in the central-west Himalayan region and vicinities. The streamflow reconstruction revealed an increasing frequency of extreme low-flow and pluvial events in the central-west Himalayan region, especially during the past century. We found short (2.1–3.3, 6.2–6.3, and 8.5–8.7 years) to medium (12.2–12.5 years) periodicities in the reconstructed streamflow series, implying teleconnections with broad-scale climate modes like the El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). Our Karnali River streamflow reconstruction represents an update of regional hydroclimate variability, which may serve as a useful reference for water resource management and appropriate water-use policies in the context of current and future climate change. [ABSTRACT FROM AUTHOR]

There are knowledge gaps in the long-term changes in Indian summer monsoon rainfall (ISMR, June–September) behaviour, which affects the livelihood of large population in the Indian subcontinent. To better understand ISMR variability, we reconstructed June–July rainfall for the period 1743–2015 CE from Dingad valley, Uttarakhand, central Himalaya using δ18O variations in tree rings. For this, regional mean tree-ring isotope chronology, prepared by merging δ18O chronologies of two conifer (Abies pindrow (Royle ex D.Don) Royle and Picea smithiana (Wall.) Boiss) and one broadleaf deciduous species (Aesculus indica (Wall. ex Camb.) Hook), was used. The reconstructed rainfall series revealed significant correlations with the observational monsoon records from different regions of India. The most conspicuous feature in our reconstructed series is a decreasing rainfall trend since 1743 CE with the driest period in the late 20th and early 21st centuries, which is consistent with other monsoon rainfall proxies from a large part of the south Asian monsoon region (India, Nepal, Bhutan, Myanmar and Tibet Plateau). Consistency of reconstructed series with instrumental records and proxies from different regions revealed the capture of regional scale rainfall features in our data testifying its utility in understanding ISMR variability in long-term perspective. • First 273-years long (1743–-2015 CE) June–July rainfall for the Dingad valley, Uttarakhand, central Himalaya, India using a regional mean isotope tree-ring chronology prepared by averaging species-specific 18O of two conifer (Abies pindrow and Picea smithiana) and one broadleaf deciduous species (Aesculus indica). • The rainfall reconstruction revealed significant correlations with instrumental monsoon records from different regions of India validating the reliability of the dataset in understanding Indian Summer Monsoon Rainfall behaviour for larger region. • The most conspicuous feature in our reconstruction is a decreasing rainfall trend since 1743 CE with the driest period in the late 20th and early 21st centuries. • The reconstruction is consistent with proxy and instrumental records from different regions further underscore the utility in understanding the Indian summer monsoon variability in a long-term context. [ABSTRACT FROM AUTHOR]

The potential of shrub-ring oxygen isotopes (SRO) as a paleoclimatic proxy is not well demonstrated. In this study we explored the climate signals embedded in an SRO chronology from Saga, on the northern slope of the central Himalayas by correlation analysis with climate variables and comparison with tree-ring and/or ice-core δ18O records. The Saga SRO had significant and positive correlations with May–July temperatures in both local (r = 0.43) and moisture source areas (r = 0.49) during the period 1955–2015 (CE), while no significant correlations were found with hydroclimatic variables. It is also in good agreement with two tree-ring δ18O series from the southern slope of the central Himalayas, the source of moisture to the site. The temperature signals are further verified by an ice core δ18O record from the Tanggula Mountains on the central Tibetan Plateau. However, it shows little similarity with the nearest Himalayan ice core δ18O record from Dasuopu, demonstrating temperature signals of different seasons. This study suggests that SRO could be used as an acceptable climate proxy, like tree-ring and ice-core δ18O, to broaden the spatial extent of the paleoclimatic records network. However, we should be cautious when combining different proxies with climate signals of different seasons in paleoclimate studies. • A high-elevation shrub-ring δ18O (SRO) chronology was developed. • The SRO captures summer temperature of both local and moisture source areas. • The SRO is comparable with δ18O of tree rings and ice cores. • SRO is a promising high-resolution proxy for paleoclimatology. [ABSTRACT FROM AUTHOR]

Abstract Knowledge of the long-term frequency and intensity of drought events in an area is crucial since drought has adverse effects on natural ecosystems, food security, economy, society, and civilization. We developed a 405-year long (1611–2015C.E.) tree-ring chronology of Pinus wallichiana (Blue pine) from the Dolpo area of the trans–Himalayan region in Nepal to reconstruct drought variability in this remote region. Correlation analysis revealed significant positive relationships with February–August precipitation, but negative correlations with temperature. High positive correlations with the self-calibrated Palmer drought severity index (scPDSI) confirmed that moisture availability during the early growing season and full growing season is the primary limiting factor for Blue pine tree growth in the trans–Himalayan region. We used a linear regression model between our tree-ring record and regional climate data to reconstruct a 319-year long (1697–2015C.E.) February–August scPDSI series. This reconstruction accounts for 39.4% of the total variance in actual scPDSI over the calibration period (1957–2015C.E.). The reconstruction showed that the area was under slightly dry conditions for most of the reconstructed period, with below normal scPDSI values. An extreme drought was observed in the year 1707. Also, the years 1705, 1706, 1784, 1786, 1809, 1810, 1813, 1821, 1849, 1858, 1861, 1909, 1967, 2006, and 2009 experienced severe drought conditions. Consecutive years with moderate drought were 1702–1706, 1783–1789, 1796–1798, 1812–1814, 1816–1827, 1846–1848, 1856–1864, 1873–1877, 1879–1882, 1899–1901, 1908–1911, and 1967–1968. Three historic mega-drought events that occurred in Asia were also captured in our reconstruction: Strange Parallels drought (1756–1768), the East India drought (1790–1796), and the late Victorian Great Drought (1876–1878). Very few wet years (1776–1979, 1989, 1991, and 2003) were observed during the reconstruction period. Power spectrum analysis revealed drought variability at frequencies of 2.0–2.5, 3.0, 12.0, and 128 years, suggesting that drought in the region might be linked to broad-scale atmospheric-oceanic variabilities such as the El Niño-Southern Oscillation (ENSO) and the Atlantic Multidecadal Oscillation (AMO). The results not only improve our understanding on regional drought variability, but are also helpful to make decisions on adaptation measures for protecting the marginal communities of the trans–Himalayan region from the anticipated adverse impacts of future droughts. Highlights • Radial growth of Pinus wallichiana is mainly limited by moisture availability. • February-August scPDSI during 1697–2015 C.E. was reconstructed for trans-Himalaya region • The trans-Himalayan region experienced prolonged drought during 19th and 20th century. • Drought variability in the trans-Himalayan regions has teleconnections with ENSO and AMO. [ABSTRACT FROM AUTHOR]

The Kashmir region in the western Himalayas is located in a transition zone between areas dominated by the South Asian Summer Monsoon (SASM) and the North Atlantic Oscillation (NAO). Currently being primarily influenced by westerly disturbances (WDs), the area is important to decipher teleconnections between these two important circulation systems for the assessment of past climate variability. We evaluated climate-growth relationships of Abies pindrow (Royle ex D. Don) and reconstructed April to June (AMJ) self-calibrated Palmer drought severity index (scPDSI) for the south Kashmir region during the period 1643–2016 CE. Our reconstructed scPDSI revealed a long wet phase during 1650–1816 CE, indicating the impact of the Little Ice Age (LIA) over the region, followed by prominent drier post-LIA episodes. The mid 18th century (1730–1760 CE) was the wettest period in the past four centuries, whereas the period 1817 to 1865 CE marked the driest phase. These phases are consistent with other precipitation reconstructions from the WD-dominated western and Trans-Himalayan regions, but inconsistant with summer precipitation reconstructions from the SASM-dominated Himalayan regions. A significant positive correlation between our scPDSI reconstruction and the North Atlantic Oscillation (NAO) for the wet phase of the LIA suggests that the NAO remained dominant in modulating the winter and spring precipitation at the study region. During the 19th and 20th centuries, scPDSI was either weakly or negatively correlated with the NAO index, indicating the influence of other atmospheric circulation systems in driving the spring/summer precipitation in the study area. This study, augmented with other moisture records, contributes to analyze the temporal and spatial extent of moisture variability in a regional perspective. • Abies at westerly dominated Kashmir Himalaya are sensitive to spring moisture. • A 373-yrs of drought reconstruction revealed moist Little Ice Age and dry post-LIA. • Radiative forcings due to volcanic eruption affected the annual growth of Abies. • North Atlantic Oscillations influence winter/spring precipitation at study region. • Strong teleconnection with NINO 3.4 in last four decades due to ocean warming. [ABSTRACT FROM AUTHOR]