Long-term decrease in Asian monsoon rainfall and abrupt climate change events over the past 6,700 years

Early-Middle Holocene high lake levels of Rinqen Shubtso on the southern Tibetan Plateau and the formation mechanisms

The lake-level highstands on the southern Tibetan Plateau (TP) during the Early-Middle Holocene have traditionally been attributed to increased monsoonal precipitation. However, there has been limited discussion and evaluation regarding how the elevated shoreline indicates the formation of mega-paleolakes and the effects of glacial meltwater on rising lake levels. In this study, we conducted an investigation into the well-preserved paleoshorelines of Rinqen Shubtso, a closed-basin lake system located on the southern TP. By utilizing 14C dating and analyzing shoreline elevations, the Holocene lake-level fluctuation history of Rinqen Shubtso was reconstructed. Through examining strontium (87Sr/86Sr) and oxygen isotopes (δ18O), as well as Rb/Sr ratios in tufa samples from the shoreline, we evaluated the relative contribution of glacial meltwater and East Asian Monsoon precipitation to the lake-level expansion throughout this period. Our findings indicate that prior to 8.5 cal ka BP, the lake level reached its highest elevation before experiencing a rapid drop by approximately 44 m within a short timeframe. Subsequently, maintaining a stable highstand between 8.5 and 5.8 cal ka BP before gradually declining to its present elevation thereafter. We argue that the glacial meltwater induced by rising temperature due to solar insolation likely played a significant role in contributing to these large amplitude high lake levels prior to 8.5 cal ka BP, whereas the maximum East Asian Monsoon precipitation was responsible for sustaining high water levels during 8.5-5.8 cal ka BP when the mean latitudinal position of the summer Intertropical Convergence Zone shifted northward until reached its northernmost point at 8.5 cal ka BP. Following 5.8 cal ka BP, with the weakening of summer monsoon precipitation observed, gradually decreased lake level occurred accordingly. Our results provide valuable insights into understanding past changes in lake level, which are of great importance to predicting future lake variations on the TP.

Mid-to-late Holocene climate variability in coastal East Asia and its impact on ancient Korean societies

The sustainability of human societies is contingent upon our ability to accurately predict the effects of future climate change on the global environment and humanity. Wise responses to forthcoming environmental alterations require extensive knowledge from historical precedents. However, in coastal East Asia, a region with a long history of agriculture, it is challenging to obtain paleoenvironmental proxy data without anthropogenic disturbances that can be used to assess the impact of late Holocene climate change on local communities. This study introduces a high-resolution multi-proxy sedimentary record from an isolated crater in Jeju Island, Korea, to elucidate the mechanisms underlying mid-to-late Holocene climate change and its impacts on ancient societies. Our findings suggest that hydroclimate changes were predominantly governed by sea surface temperature fluctuations in the western tropical Pacific, with low-frequency variability in solar activity and a decrease in summer insolation identified as primary drivers of temperature change. Moreover, ancient societies on the Korean peninsula were significantly affected by recurring cooling events, including the 2.8 ka event, 2.3 ka event, Late Antique Little Ice Age, maunder minimum, and others.

Assessing the influence of local environment, regional climate and tree species on radial growth in the Hexi area of arid northwest China

Radial growth is influenced by the local environment, regional climate, and tree species. Assessing the influence of these variables on radial growth can help to reveal the relationships between tree growth and the environment. Here, we used standard dendrochronological approach to explore the response of radial growth to climate factors. We reported ring-width (TRW) residual chronologies from five sites along a longitudinal gradient in the Hexi area, arid northwestern China, based on a total of 249 Qinghai spruce (Picea crassifolia) ring-width records. We found that Qinghai spruce in the west of the Hexi area is more sensitive to climate change than in the east, and that drought condition in the previous growing season and the early growing season (March to June) limits spruce growth. Comparison between the regional standard chronologies of Qinghai spruce and Qilian juniper (Juniperus przewalskii) in the Hexi area during 1813-2001 showed that both chronologies were more consistent in the high-frequency domain than in the low-frequency domain. The findings emphasize the impacts of local environment, regional climate and tree species on radial growth, suggesting that accounting for these variables could improve large-scale and multi-species dendrochronological studies.

Responses of Tree Growth and Intrinsic Water Use Efficiency to Environmental Factors in Central and Northern China in the Context of Global Warming

The Loess Plateau is a fragile ecological zone that is sensitive to climate change. The response, adaptation, and feedback of tree growth in forest ecosystems to global warming and CO2 enrichment are urgent scientific issues. Intrinsic water use efficiency (iWUE) is an important indicator for understanding forest ecosystem adaptability to climate change and CO2 enrichment. In this study, tree-ring width, tree-ring stable carbon isotope ratio (δ13C), and iWUE of P. tabulaeformis Carr. were established. Climate response analysis showed that temperature was the main limiting factor affecting radial tree growth and that relative humidity significantly affected the stable carbon isotope fractionation of tree rings. During 1645–2011, the iWUE increased by 27.1%. The responses of iWUE to climate factors and atmospheric CO2 concentrations (Ca) showed that the long-term variation in iWUE was affected by Ca, which could explain 69% of iWUE variation, and temperature was the main factor causing iWUE interannual variation. The ecosystem of P. tabulaeformis showed a positive response to rising Ca, as its carbon sequestration capacity increased. In response to global warming and CO2 enrichment, rising Ca promoted increases in iWUE but ultimately failed to offset the negative impact of warming on tree growth in the study area.

Appeal: the protection of ancient tree species around the world, taking qilian juniper (Juniperus przewalskii) as an example

Juniperus przewalskii (the Qilian juniper) is a dominant species in the northeast region of the Qinghai-Tibet Plateau. As such, it is of great significance for maintaining the balance and long-term stability of the ecosystem in this biodiversity hotspot. In this paper, we review the literature related to J. przewalskii published in the China National Knowledge Infrastructure and Web of Science. Here, we summarize the main research achievements with regard to this species from ten aspects: tree morphology and phenology, population structure and life history, radial growth and climate response, tree-ring chronology-based history reconstruction, soil physical and chemical properties, chemical composition and activity, physiological ecology, biogeography, breeding and propagation techniques, and occurrence and control of diseases and pests. Considering the ecological importance and research value of J. przewalskii, as well as the shrinking population size, we discuss future research prospects and directions for the conservation of J. przewalskii from four aspects: global climate change, human disturbance, tree regeneration, and pest control. This work provides an important reference for conservation studies of alpine tree species in other biodiversity hotspots around the world.

Modern aridity in the Altai-Sayan mountain range derived from multiple millennial proxies

Temperature and precipitation changes are crucial for larch trees growing at high-elevation sites covered by permafrost in the Altai-Sayan mountain range (ASMR). To contextualize the amplitude of recent climate fluctuations, we have to look into the past by analyzing millennial paleoclimatic archives recording both temperature and precipitation. We developed annually resolved 1500-year tree-ring cellulose chronologies (δ¹³C_cell, δ¹⁸O_cell), and used these new records to reconstruct the variability in local summer precipitation and air temperature. We combined our new local reconstructions with existing paleoclimatic archives available for the Altai. The data show a strong decreasing trend by ca. 49% in regional summer precipitation, along with a regional summer temperature increase towards the twenty-first century, relative to the preceding 1500 years. Modern dry conditions (1966–2016 CE) in the ASMR are the result of simultaneous summer warming and decreased precipitation. Our new reconstructions also demonstrate that climate change in the ASMR is much stronger compared to the global average.

Spatiotemporal Distribution and Geographical Impact Factors of Barley and Wheat during the Late Neolithic and Bronze Age (4000–2300 cal. a BP) in the Gansu–Qinghai Region, Northwest China

During the Late Neolithic and Bronze Age, the spread of Triticeae crops gradually transformed local millet agriculture in the Gansu–Qinghai region. However, few studies focused on the distribution characteristics and geographical factors influencing Triticeae agriculture. Here, geographical data from 65 sites with barley (Hordeum vulgare) and wheat (Triticum aestivum) remains in Gansu–Qinghai region were studied by spatial analysis in ArcGIS. The results showed that spatiotemporal distribution of barley and wheat exhibited three stages: firstly appeared in the central Hexi Corridor at ~4000 cal. a BP, then spread throughout the Hexi Corridor and Qinghai Lake area during 3600–3200 cal. a BP, and continued to move southward to the Hehuang Valley and Weihe River Basin during 3200–2300 cal. a BP. The studied sites were mainly distributed along rivers and certain altitude with suitable climatic conditions. The average distance to the nearest river was ~8 km, with wheat sites ~300 m closer than barley’s. The average elevation was 3500–1500 m, with wheat sites ~200 m lower than barley’s. The variations in spatiotemporal distribution were attributed to greater environmental tolerance of barley. These findings provide important environmental insights into the spatiotemporal distribution and transmission routes of barley and wheat in northwest China.

Tree-Ring Stable Carbon Isotope as a Proxy for Hydroclimate Variations in Semi-Arid Regions of North-Central China

Carbon and oxygen isotope ratios (δ13C and δ18O) were measured in annual tree-ring cellulose samples dated from 1756 to 2015 CE. These samples were extracted from Chinese pine (Pinus tabulaeformis Carr.) trees located in a semi-arid region of north-central China. We found that tree-ring δ13C and δ18O values both recorded similar climatic signals (e.g., temperature and moisture changes), but found that tree-ring δ13C exhibited a stronger relationship with mean temperature, precipitation, average relative humidity, self-calibrating Palmer drought severity index (scPDSI), and standard precipitation evaporation index (SPEI) than δ18O during the period 1951–2015 CE. The strongest correlation observed was between tree-ring δ13C and scPDSI (previous June to current May), which explains ~43% of the variance. The resulting 130-year reconstruction reveals severe drought events in the 1920s and a sustained drying trend since the 1980s. This hydroclimate record based on tree-ring δ13C data also reveals similar dry and wet events to other proxy data (i.e., tree-ring width and historical documentation) that have allowed reconstructions to be made across the northern fringe of the Asian summer monsoon region. Our results suggest that both large-scale modes of climate variability (e.g., El Niño-Southern Oscillation, Pacific Decadal Oscillation, and North Atlantic Oscillation) and external forcing (e.g., solar variability) may have modulated moisture variability in this region. Our results imply that the relationship between tree-ring δ18O and local climate is less well-characterized when compared to δ13C and may be affected more strongly by the influences of these different atmospheric circulation patterns. In this semi-arid region, tree-ring δ13C appears to represent a better tool with which to investigate historical moisture changes (scPDSI).

Origin of Intra-annual Density Fluctuations in a Semi-arid Area of Northwestern China

Intra-annual density fluctuation (IADF) is a structural modification of the tree ring in response to fluctuations in the weather. The expected changes in monsoon flow would lead to heterogeneous moisture conditions during the growing season and increase the occurrence of IADF in trees of the arid ecosystems of continental Asia. To reveal the timings and physiological mechanisms behind IADF formation, we monitored cambial activity and wood formation in Chinese pine (Pinus tabuliformis) during 2017-2019 at three sites in semi-arid China. We compared the dynamics of xylem formation under a drought event, testing the hypothesis that drought affects the process of cell enlargement and thus induces the production of IADF. Wood microcores collected weekly from April to October were used for anatomical analyses to estimate the timings of cambial activity, and the phases of enlargement, wall thickening, and lignification of the xylem. The first cells started enlargement from late April to early May. The last latewood cells completed differentiation in mid-September. Trees produced IADF in 2018. During that year, a drought in June limited cell production in the cambium, only 36% of the xylem cells being formed in IADF trees, compared to 68% in normal tree rings. IADF cells enlarged under drought in early July and started wall thickening during the rainfall events of late July. The drought restricted cell enlargement and affected wall thickening, resulting in narrow cells with wide walls. Cambium and cell enlargement recovered from the abundant rainfall, producing a new layer with large earlywood tracheids. IADF is a specific adaptation of trees to cope with water deficit events occurring during xylem formation. Our findings confirmed the hypothesis that the June-July drought induces latewood-like IADFs by limiting the process of cell enlargement in the xylem. Our finding suggests a higher occurrence of IADF in trees of arid and semi-arid climates of continental Asia if the changes to monsoon flows result in more frequent drought events during the earlywood formation in June.