Search

Climate warming, rising atmospheric CO2 concentration (C a), and nitrogen (N) availability are exerting profound impacts on global forest ecosystems, particularly in high-altitude mountains. This study investigated the tree-growth dynamics of timberline Larix chinensis in the Taibai Mountain, central China, and explored its ecophysiological responses to environmental stresses by combining tree growth and stable isotopes. The results showed that the growth rate of L. chinensis has significantly increased since the 1960s, and that tree growth in this timberline was particularly sensitive to temperature in spring. Moreover, the continuously rising intrinsic water-use efficiency (iWUE), linked to higher C a and warmer environment, promoted the growth of L. chinensis. Before the 1960s, tree-ring δ15N gradually increased, then shifted to an insignificant decline with the acceleration of tree growth, and broke the preexisting carbon–nitrogen balance. Meanwhile, climate warming and increased iWUE have replaced N as the principal drivers of tree growth since the 1960s. It is believed that L. chinensis may gradually suffer a decline in N availability as it continues to grow rapidly. The insightful understanding of the biochemical mechanisms of plant responses to growth-related environmental conditions will improve our ability to predict the evolution of high-elevation mountain ecosystems in the future. [ABSTRACT FROM AUTHOR]

Resilience is a fundamental concept for vegetation health. The increasing drought frequency and severity may pose severe threat to vegetation resilience. However, it is still not clear how vegetation resilience is evolving in response to climate change in pivotal biographical zones. Here, we examined the resilience changes in terms of leaf area index (LAI, an indicator of canopy structure) and gross primary productivity (GPP, an indicator of carbon uptake) in responding to the Standardized Precipitation‐Evapotranspiration Index (SPEI) and vapor pressure deficit (VPD) over China's Loess Plateau and Qinling Mountains. Linking remote sensing variables and tree ring width allows the upscaling of plot‐based vegetation growth information. We further explored potential explanatory factors associated with the heterogeneous spatial distributions of resilience changes. Results revealed that the resilience of GPP weakened more than LAI in response to drought, suggesting that compared to LAI, productivity requires more time to recover to the pre‐drought levels. Regionally, the change of vegetation resilience on the Loess Plateau and in high‐altitude areas was highly susceptible to SPEI and VPD, respectively. The observed spatial heterogeneity in resilience changes was mainly attributed by climate zone, water deficit, and their interactions. Our findings provide direct and empirical evidence that the vegetation in the Loess Plateau and Qinling Mountains is gradually losing resilience. The results indicate that sustained ecosystem water deficit and atmospheric dryness will continue to threaten vegetation survival and terrestrial ecosystem service. Plain Language Summary: Vegetation resilience in pivotal biographical regions is altered by increasingly severe drought pressures. However, to what extent resilience changes and how the regional responses vary to different drought indicators have not yet been well explored. Here we quantify the resilience changes regarding vegetation physiological function and canopy structure after the significant changes in drought conditions over the Loess Plateau and Qinling Mountains of China. Results show that the vegetation physiological function has been damaged more seriously than the canopy structure, suggesting vegetation is gradually losing resilience. In relatively high‐altitude regions, the change of vegetation resilience is more sensitive to high vapor pressure deficit than the Standardized Precipitation‐Evapotranspiration Index, but in opposite direction on the Loess Plateau. Spatial heterogeneity analysis shows that these changes are mainly affected by climate zone, water‐related factors (precipitation, relative humidity, soil water), and their interactive effects. Characterizing of resilience changes as reference conditions for ecosystem management and restoration has significant implications in the context of global warming. Key Points: The resilience of vegetation productivity weakened more than that of canopy structure in response to droughtThe impacts of atmospheric dryness on vegetation resilience changes are much larger than those of Standardized Precipitation‐Evapotranspiration Index (SPEI) in high‐altitude regionsCombined effects of climate zone (CZ) and water deficits explain the spatial heterogeneity of vegetation resilience changes [ABSTRACT FROM AUTHOR]

Understanding the water-use strategy of human-planted species used in response to climate change is essential to optimize afforestation programs in dry regions. Since 2000, trees on the central Loess Plateau have experienced a shift from strengthening drought to weakening drought. In this study, we combined tree-ring δ13C and δ18O records from Pinus tabuliformis (syn. tabulaeformis) Carr. (a tree) and Sophora viciifolia Hance (a shrub) on the central Loess Plateau to investigate species-specific responses to rising atmospheric CO2 (Ca) and drought. We found summer relative humidity controlled the fractionation of tree-ring δ18O, but the magnitude of the climate influence on δ13C differed between the species. The intrinsic water-use efficiency (iWUE) trends of both species suggested a strongly active response to maintain constant intercellular CO2 concentrations as Ca rose. The tree-ring δ13C and δ18O of both species using first-difference data were significantly and positively correlated, with stronger relationships for the shrub. This indicated the dominant regulation of iWUE by stomatal conductance in both species, but with greater stomatal control for the shrub. Moreover, the higher mean iWUE value of S. viciifolia indicated a more conservative water-use strategy than P. tabuliformis. Based on our commonality analysis, the main driver of the increased iWUE was the joint effect of Ca and vapor-pressure deficit (25.51%) for the tree, while it was the joint effect of Ca and the self-calibrated Palmer drought severity index (39.13%) for the shrub. These results suggest S. viciifolia will be more drought-tolerant than P. tabuliformis and as Ca continually rises, we should focus more on the effects of soil drought than atmospheric drought on the water-use strategy of S. viciifolia. [ABSTRACT FROM AUTHOR]

Rising atmospheric carbon dioxide (CO2) may enhance tree growth and mitigate drought impacts through CO2 fertilization. However, multiple studies globally have found that rising CO2 has not translated into greater tree growth despite increases in intrinsic water-use efficiency (iWUE). The underlying mechanism discriminating between these two general responses to CO2 fertilization remains unclear. We used two species with contrasting stomatal regulation, the relatively anisohydric Qilian juniper (Sabina przewalskii) and the relatively isohydric Qinghai spruce (Picea crassifolia), to investigate the long-term tree growth and iWUE responses to climate change and elevated CO2 using tree ring widths and the associated cellulose stable carbon isotope ratios (δ13C). We observed a contrasting growth trend of juniper and spruce with juniper growth increasing while the spruce growth declined. The iWUE of both species increased significantly and with similar amplitude throughout the trees' lifespan, though the relatively anisohydric juniper had higher iWUE than the relatively isohydric spruce throughout the period. Additionally, with rising CO2, the anisohydric juniper became less sensitive to drought, while the relatively isohydric spruce became more sensitive to drought. We hypothesized that rising CO2 benefits relatively anisohydric species more than relatively isohydric species due to greater opportunity to acquire carbon through photosynthesis despite warming and droughts. Our findings suggest the CO2 fertilization effect depends on the isohydric degree, which could be considered in future terrestrial ecosystem models. [ABSTRACT FROM AUTHOR]

Drought occurrence and duration in central Asia are of important socioeconomic, ecological, and geophysical significance and have received increasing research attention in recent years. Understanding long-term drought trends and their driving forces require reliable records of past drought variability with broad spatial representativeness. Here, we compiled four tree-ring δ18O records from eastern central Asia (ECA) and composited them into a drought-sensitive proxy to explore regional ECA moisture variations over the past 301 years (1710-2010 CE). A robust regional standardized precipitation-evapotranspiration index (SPEI) reconstruction was established based on the tree-ring cellulose δ18O fractionation mechanism and statistically significant proxy-climate relationships. We identified prominent droughts in 1710-1770, 1810-1830, and the beginning of the twenty-first century, and a regime shift to a persistently wet period from the 1880s to 2000. Our reconstruction reveals the impact of drought and pluvial patterns on the decline of Zhungar Empire, and on historical agricultural and socio-economical activities, including increased migration into ECA during the 1770-1800 pluvial. Our findings also suggest that wet conditions in the twentieth century in ECA were related to a strengthening of the westerly circulation and thus shed light on large-scale atmospheric circulation dynamics in central Asia. [ABSTRACT FROM AUTHOR]

Tree-ring δ13C and δ18O of dominant Dahurian larch and Mongolia pine in the permafrost region of the northern Great Higgnan Mountains, China were used to elucidate species-specific ecophysiological responses to warming temperatures and increasing CO2 over the past century. Larch and pine stable carbon discrimination (Δ13C) 13C and δ18O in tree rings both showed synchronous changes during the investigated period (1901–2010), but with species-specific isotopic responses to atmospheric enriched CO2 and warming. Tree-ring Δ13C and δ18O were controlled by both maximum temperature and moisture conditions (precipitation, relative humidity and vapor pressure deficit), but with different growth periods (Δ13C in June–July and δ18O in July–August, respectively). In addition, stable isotopes of larch showed relatively greater sensitivity to moisture deficits than pine. Climatic conditions from 1920 to 1960 strongly and coherently regulated tree-ring Δ13C and δ18O through stomatal conductance. However, climatic-sensitivities of tree-ring Δ13C and δ18O recently diverged, implying substantial adjustments of stomatal conductance, photosynthetic rate and altered water sources over recent decades, which reveal the varied impacts of each factor on tree-ring Δ13C and δ18O over time. Based on expected changes in leaf gas-exchange, we isolated the impacts of atmospheric CO2 and climate change on intrinsic water-use efficiency (iWUE) over the past century. Higher intracellular CO2 in pine than larch from 1960 onwards suggests this species may be more resilient to severe droughts in the future. Our data also illustrated no weakening of the iWUE response to increasing CO2 in trees from this permafrost region. The overall pattern of CO2 enrichment and climate impacts on iWUE of pine and larch were similar, but warming increased iWUE of larch to a greater extent than that of pine over recent two decades. Taken together, our findings highlight the importance of considering how leaf gas-exchange responses to atmospheric CO2 concentration influence species-specific responses to climate and the alteration of the hydrological environment in forests growing in regions historically dominated by permafrost that will be changing rapidly in response to future warming and increased CO2. [ABSTRACT FROM AUTHOR]

Stable isotopes in tree rings are increasingly used as proxies for climatic and ecophysiological changes. However, uncertainties remain about the strength and consistency of their response to environmental variation at different temporal (i.e. seasonal to inter-decadal) scales., We developed 5 yr of intra-seasonal and 62 yr of early- and late-wood δ13C and δ18O series of Smith fir ( Abies georgei var. smithii) on the southeastern Tibetan Plateau, and used a process-based forward model to examine the relative importance of environmental and physiological controls on the isotopic data., In this temperate high-altitude region, the response, both δ18O and δ13C, is primarily to variations in relative humidity, but by different processes. In δ18O, the response is via source water δ18O but also arises from leaf water 18O enrichment. In δ13C, the response is via changes in stomatal conductance but is modified by carry-over effects from prior periods., We conclude that tree-ring δ18O may be a more robust climate proxy than δ13C, and δ13C may be more suited to studies of site-related physiological responses to the local environment. [ABSTRACT FROM AUTHOR]

• Vaganov–Shashkin lite was integrated with stochastic antecedent modeling. • Tree-ring width data of 109 conifers in temperate China were analyzed. • Tree-ring width series were clustered into warm-dry, cold-dry, and cold-wet. • Current and antecedent climatic drivers of tree-ring width series were quantified. • Our model improves the simulated results of tree-ring growth in diverse forests. Process modeling of tree growth responses to climate can improve our understanding of the functioning of tree growth under climate change, but these models do not consider so-called memory effects. Further, memory effects are unstudied in temperate forests of China, which is showing high vulnerability to climate warming. Here, we integrate a process-based Vaganov–Shashkin–Lite (VS-Lite) model of tree ring formation with the stochastic antecedent modeling (SAM) approach, to quantify the current and antecedent climatic drivers of the tree-ring width chronologies of 109 conifers in temperate China. Based on climate-response patterns, tree-ring width series were clustered into "warm-dry," "cold-dry," and "cold-wet." The VS-Lite results indicated growth in the "warm-dry" and "cold-dry" sites were primarily constrained by current soil moisture, while growth in the "cold-wet" sites was mainly constrained by cotemporaneous temperature. Compared to VS-Lite alone, we found significant improvements in model fit for the SAM framework in all forest types. Notably, antecedent drivers explained 14%, 11%, and 21% of the growth variations in "warm-dry," "cold-dry," and "cold-wet" sites, respectively, challenging the assertion that long memory is only important in dry forests. Commonality analysis revealed that growth in "cold-dry" and "cold-wet" sites was determined by antecedent climate while growth in the "cold-dry" and "warm-dry" clusters were determined by the endogenous (autoregressive) memory. In "warm-dry" and "cold-wet" sites, growth was significantly affected by antecedent temperature in opposite directions, whereas in the "cold-dry" sites it was affected by antecedent soil moisture conditions and soil moisture-temperature interactions. Our combined process-empirical model demonstrates that memory is prevalent across temperate forests in China and also improves representation of tree growth in diverse forests under novel climate conditions of the future. [Display omitted] [ABSTRACT FROM AUTHOR]

Incursion of the Indian Monsoon flow onto the Qinghai–Tibetan Plateau is an important regional moisture-delivery mechanism, but the causes of intra-seasonal and longer-term variations are poorly understood. We observed a clear intra-seasonal cycle in the oxygen isotope composition ( δ 18 O) of Smith fir ( Abies forrestii var. georgei) growing at two different altitudes (high-elevation site: 29°39′N, 94°42′E, 4200 m a.s.l.; low-elevation site: 29°39′N, 94°43′E, 3800 m a.s.l.) on a SE-facing slope in the Sygera Mountains, southeastern Qinghai–Tibetan Plateau. Mechanistic modeling suggests that δ 18 O of precipitation, which in turn depends on relative humidity and precipitation amount, is important in controlling the intra-seasonal tree-ring isotopic cycle, and we speculate that the same controls also operate on interannual and longer time scales. If so, multi-century δ 18 O records with intra-seasonal resolution may enable us to infer long-term variations in the incursion of the Indian Monsoon onto the Qinghai–Tibetan Plateau. [ABSTRACT FROM AUTHOR]

ABSTRACT We developed an annual tree-ring δ18O chronology from Larix sibirica in the eastern Tianshan Mountains of northwestern China. Climatic response revealed that tree-ring δ18O was significantly positively correlated with the mean and maximum July-August temperatures, whereas the July precipitation and relative humidity were significantly negatively correlated with tree-ring δ18O. The self-calibrating Palmer drought severity index (sc_PDSI) in July-August was significantly correlated with the tree-ring δ18O, which reflects the comprehensive effects of the three parameters on tree-ring δ18O fractionation. We established a robust reconstruction of July-August sc_ PDSI that accounted for 38.4% of the total variance of sc_PDSI from 1959 to 2008. The sc_PDSI reconstruction yields new insights on past drought that were not previously realized in other PDSI reconstructions (e.g. Monsoon Asia Drought Atlas) from the region. The reconstruction revealed several wet and dry periods but no trend towards a wetter climate in the eastern Tianshan Mountains during the last two decades. It also detected synergistic effects of the North Atlantic Oscillation and the El Niño-Southern Oscillation on regional moisture conditions as a result of teleconnections between tropical oceans and mid-latitude circulation patterns. [ABSTRACT FROM AUTHOR]

The rising atmospheric CO2 concentration (Ca) has increased tree growth and intrinsic water-use efficiency (iWUE). However, the magnitude of this effect on long-term iWUE and whether this increase could stimulate the growth of riparian forests in extremely arid regions remain poorly understood. We investigated the relationship between growth [ring width; basal area increment (BAI)] and iWUE in a riparian Populus euphratica Oliv. forest to test whether growth was enhanced by increasing CO2 and whether this compensated for environmental stresses in the lower reaches of the inland Heihe River, northwestern China. We accomplished this using dendrochronological methods and carbon (δ13C) and oxygen (δ18O) isotopic analysis. We found an increase in BAI before 1958, followed by a decrease from 1958 to 1977 and an increase to a peak around 2000. Tree-ring carbon discrimination (Δ) and δ18O indicated significant negative overall trends from 1920 to 2012. However, the relationship shifted in strength and direction around 1977 from significantly negative to a weak connection. The seasonal minimum temperature in April to July showed strong influence on Δ, and δ18O was controlled by relative humidity (negatively correlated) and temperature (positively correlated) in June and July. The patterns of internal to atmospheric CO2 (Ci/Ca) suggest a specific adaptation of tree physiology to increasing CO2. Intrinsic water-use efficiency increased significantly (by 36.4%) during the study period. The increased iWUE explained 19.8 and 39.1% of the observed yearly and high-frequency (first-order difference) variations in BAI, respectively, after 1977. Our results suggest significant CO2 stimulation of riparian tree growth, which compensated for the negative influences of reductions in river streamflow and a drying climate during the study period. [ABSTRACT FROM AUTHOR]

To reveal the influence of current warming on tree growth and δ18O in a sensitive high-latitude region that is undergoing rapid climate change, we examined tree width and the earlywood and latewood δ18O at two sites with a 400-m elevation difference in the Sygera Mountains of the southeastern Tibetan Plateau. The study period was from 1950 to 2011. The mean tree-ring index at the low site was higher than that at the high site during the study period. The climatic responses of earlywood and latewood δ18O at both sites were similar. Earlywood δ18O was mainly influenced by the June to August temperature and total cloud cover, whereas latewood δ18O was mainly controlled by relative humidity from July to August. Spatial correlations with CRU TS 3.1 regional data suggest that our δ18O chronologies can represent climatic changes over large regions. The high offset between earlywood δ18O at the two sites (2.3‰ higher at the low site) was mainly influenced by the high temperature lapse rate as a function of altitude during the earlywood growing season. Furthermore, meltwater with lower δ18O values might have affected earlywood δ18O at the high site, and thereby increased the earlywood δ18O offset between two sites. The low latewood δ18O offset between the two sites (0.4‰ higher at the low site) was not significant, but appears to have been primarily influenced by the low precipitation δ18O lapse rate as a function of altitude during the latewood growing season. Earlywood δ18O of Smith fir suitable for reconstructing past temperatures and latewood δ18O suitable for reconstructing past relative humidity on the southeastern Tibetan Plateau were identified. [ABSTRACT FROM AUTHOR]

We measured the annual variation in the stable isotopes of oxygen (δO) and hydrogen (δD) in tree rings of Abies georgei on the Batang-Litang Plateau of western China. Although correlations between tree-ring δO and δD are relatively weak in semi-arid regions, we found a strong correlation between the δO and δD time series from 1755 to 2009 under the wetter environment. Tree-ring δO and δD time series are both significantly and negatively correlated with moisture conditions from June to August, including relative humidity and total precipitation, respectively, from 1960 to 2009. Considering the difference in low-frequency domain between the two isotopes, the relative humidity histories from June to August, reconstructed separately from the tree-ring δO and δD data with instrumental climate data, reveal a persistent drying trend since 1850s, especially since the early 1970s. There is an obvious offset of reconstructed relative humidity from tree-ring δO and δD in the period 1755-1820, despite the strong similarity in their 21-year moving averages. The decreased relative humidity since the 1850s may be associated with the thermal contrast between the sea surface temperature of the Indian Ocean and the Qinghai-Tibetan Plateau, which determines the strength of moisture transfer via the Indian summer monsoon. [ABSTRACT FROM AUTHOR]

High-resolution tree-ring δ18O chronologies covering the last millennium, although scarce, are essential in understanding patterns of climatic changes in the northeastern region of the Qinghai-Tibetan Plateau. For this study, a tree-ring δ18O chronology with a temporal resolution of 3-years was developed from the long-lived Qilian juniper ( Sabina przewalskii Kom.), extending back in time to AD 991. This long δ18O chronology was significantly correlated with the yearly δ18O in tree rings during the common period from 1800 to 2006, and was an effective proxy for relative humidity during the growing season. A low-frequency moisture pattern signified the occurrence of a slight drought during the Medieval Climate Anomaly, a marked occurrence of a wet period during the Little Ice Age, and a trend in increasing moisture levels, although lower than average, alongside the Twentieth Century warming trend. Comparisons to other hydroclimatic reconstructions indicate that this tree-ring δ18O chronology serves as a reliable paleo-humidity proxy for the Qaidam Basin as well as documenting details of past humidity levels in the region. [ABSTRACT FROM AUTHOR]

The hydroclimate of southwestern China has experienced pronounced fluctuations over the past few decades. In this study, the total cloud cover variability of southwestern China during May–October for the past 276 years was reconstructed based on the composite tree-ring δ18O chronology. The record spans the common interval from 1734 to 2009 CE. Moisture-related factors, such as relative humidity, the precipitation, and cloud cover over the region, were the climatic factors controlling tree-ring δ18O. The reconstructed cloud cover exhibited a decreasing trend since 1900, which was linked to the weakening of the Indian summer monsoon. However, the relationship between cloud cover and the monsoon system was temporally unstable, which may be due to El Niño-southern oscillation and the Indian Ocean dipole. Furthermore, the sea surface temperature (SST) over the Indian and East Pacific oceans influenced the delivery of moisture over the study area; however, the influence of SST on the different oceanic regions varied between different periods. During the low-cloud-cover period, the link between SST and regional cloud cover weakened, and local circulation became the driving factor. Additionally, local circulation may disrupt the influence of monsoons on the hydroclimate in southwestern China, which was supported by the backward trajectory results. [Display omitted] • Reconstructed cloud cover variability based on tree-ring δ18O chronology in SW China. • The temporal relationship between the ISM and regional cloud cover varied. • The unstable relationship may be due to the interactions of ENSO and IOD. • The SST in the Indian and Pacific oceans interfere with the influence of the ISM. [ABSTRACT FROM AUTHOR]

Abstract: To understand how tree growth responds to rising atmospheric CO2 concentration ([CO2]) and climate change in arid and semi-arid regions, it is important to determine how trees in natural forests adjust physiologically to the changing atmospheric environment. Here, we developed an annual-resolution tree-ring stable carbon isotope (δ13C) chronology from the dominant natural spruce (Picea crassifolia) trees in the Xinglong Mountains, in the eastern part of northwestern China, from 1800 to 2009. The climate response recorded in the tree-ring δ13C residuals indicated that insufficient moisture during the previous autumn and the current growing season was the dominant climatic factor that controlled tree-ring carbon isotope discrimination in the semi-arid Xinglong Mountains. By combining the tree-ring δ13C, the atmospheric [CO2] (C a), and ring width data, and by calculating the variations in leaf intercellular [CO2] (C i), their ratio (C i/C a), intrinsic water-use efficiency (iWUE), and basal area increment (BAI), we found that the C i/C a remained mostly constant during the last century whereas C i and iWUE increased significantly. The increase in iWUE totaled about 40% by the end of the study period. However, after 1998, the trees switched from an active to a passive response to elevated atmospheric [CO2], leading to relatively stable iWUE. Since 1800, tree radial growth did not always parallel the increase in iWUE; the tree growth rate declined during drought periods, and especially during the severe droughts that occurred from 1923 to 1934 and recent decade. This response is a concern for future forest management and conservation under the current climate change scenario. [Copyright &y& Elsevier]

Climate warming-induced changes in the hydrological habitat for the alpine treeline can significantly impact tree growth and community dynamics. However, composite indicators that capture typical alpine species' responses to temperature and precipitation are lacking. This study investigates the use of the δ 2H values of lignin methoxy groups (δ 2H LM) from the dominant deciduous conifer tree (Larch chinensis) and evergreen broadleaf shrub (Rhododendron purdomii) in the upper treeline of the Qinling Mountains from 1960 to 2018 as potential hydroclimatic indicators. The δ 2H LM values of the R. purdomii shrubs were enriched by 23.3‰ in 2H compared with those of the L. chinensis trees. However, the multispecies δ 2H LM composite significantly reflected the temperature and precipitation during the growing season (May to September). Rapid warming and drying after the 1990s increased the interspecies consistency of δ 2H LM variability, accompanied by a decreasing enrichment trend. The weighted self-calibrated Palmer Drought Severity Index showed a similar variation tendency. Similar isotopic patterns were detected for the δ 2H values of precipitation (δ 2H Pre) and the composite δ 2H LM values. The latest empirical relationship mode can effectively verify the reconstructed δ 2H Pre values using the δ 2H LM composite from the alpine treeline. These results indicate that the cross-species composite tree-ring δ 2H LM can accurately indicate the alpine hydroclimatic conditions during the growing season, including the warming and drying periods. • Growing season temperature and precipitation correlate with tree-ring δ 2H LM in typical treeline species. • Reconstruction of precipitation δ 2H based on δ 2H LM across species significantly agrees with the empirical model. • Composite tree-ring δ 2H LM accurately indicates alpine hydroclimatic conditions. [ABSTRACT FROM AUTHOR]

In arid and semiarid regions, riparian forests are crucial for maintaining ecological biodiversity and sustainability, and supporting social and economic development. For the typical arid and semiarid ecosystem, streamflow variability is thought to be the dominant factor influencing the vulnerability and evolution of the riparian forests, which often leads to the neglect of other potentially important factors such as nutrient availability and transport. Here, we measured annual stable nitrogen isotopes (δ15N) and nitrogen concentrations (N%) in the tree rings of Populus euphratica Oliv. (Euphrates poplar) over a 90 year period (1920–2012), collected from the lower researches of the inland Heihe River, northwestern China. Coupling with our previous dual-isotope (δ13C and δ18O) chronologies and estimated intrinsic water-use efficiency (iWUE), we examined the linkages between tree-ring δ15N and δ18O, iWUE, streamflow, and then explored the contributions of each to tree growth during the study period. Our results show that after 1975, a statistically significant correlation between tree-ring δ15N and river streamflow appears, indicating the river as a potential carrier of nitrogen from the upper and middle reaches to the lower research trees. In addition, the linkage between tree-ring δ15N and iWUE suggests substantial influence of carbon and nitrogen together on photosynthesis and transpiration of trees, although this connection become decoupled since AD 1986. The commonality analysis revealed that the nitrogen impacts indicated by tree-ring δ15N on tree growth cannot be ignored when evaluating riparian forest development. The fertilization effects caused by rising CO 2 concentration complicate the nitrogen constraints on tree growth during the later part of the past century. Our results have potentially broad implications for identifying the limited factors for dryland forest ecosystems that are susceptible to natural water resource variations and human activities. • Tree-ring δ15N increased with a rate of 0.4‰ per decade but nitrogen concentration remained stable during 1930–2008. • Tree-ring δ15N and streamflow both showed rising trends, but they only significantly correlated over the period of 1975–1994. • The carbon and nitrogen cycling of riparian trees was associated but decoupled after 1986. • The δ15N and iWUE regulated Euphrates poplar tree growth together, but they have different dominant periods. [ABSTRACT FROM AUTHOR]

Examining the growth of trees in response to environmental factors is essential for evaluating the stability of forest ecosystems. In this study, using tree ring data obtained from 18 sites and climate diagnostic methods. we investigated relationships between the radial growth of Picea crassifolia in the mid-western Qilian Mountains and local climate/sea surface temperatures (SSTs) since 1851 C.E. The results revealed that the radial growth of P. crassifolia showed significant upward trends during three time periods (1884–1906, 1929–1946, and 1964–1983) and significant downward trends in a further three periods (1907–1928, 1947–1963, and 1984–1995). Variations in the growth of P. crassifolia showed a significant negative correlation with temperature in June and a positive correlation with precipitation from July in the previous year to June in the current year as a response to climate change. We also found that large-scale anomalies could influence the radial growth of P. crassifolia , which was reflected in results showing that extremely high radial growth is related to El Niño patterns in the central-eastern equatorial Pacific, whereas extremely low growth is related to anomalously warmer SSTs in the southern Indian Ocean. Notably, we found that the extremely low growth of P. crassifolia in response to SST was more stable than that of extremely high growth. Furthermore, on the basis of qualitative methodology, we established that years characterized by extremely high/low growth were largely/little influenced by the time window and threshold values that were selected when determining the positive/negative pointer years. Our results confirm the validity of using the relationships between extremely low growth and SSTs to predict forest dynamics. [ABSTRACT FROM AUTHOR]

The Qinling Mountains (Qinling Mts.) are characterized as a distinct transition of bioclimatic zones along the north-south geographic boundary in China. Although ongoing global warming may affect the growth of most tree species across elevation gradients, the effect of modern warming on the stable hydrogen isotopes of tree-ring wood is still unknown. In this study, we developed early- and late-wood δ 2H chronologies from 1900 to 2018 CE of lignin methoxy groups (δ 2H LM) at 700-m intervals from 1700 to 3100 m a.s.l. along the elevation gradient in Mount Taibai (Mt. Taibai), the peak of the Qinling Mts. The δ 2H LM values of the earlywood were relatively enriched in 2H compared to the latewood, and an elevation-dependent depleting trend along the altitudinal gradient was observed. Comparing the δ 2H LM values with modelled δ 2H values of precipitation (δ 2H Pre) of the sample sites, similar isotopic patterns can be obtained. The reconstructions of δ 2H Pre values confirm the strong coherence between the isotopic composition of the source water and the methoxy groups in the growing season. The consistency of δ 2H LM chronologies between the early- and late-wood at the higher sites was stronger than that at the lower sites. The additional lagged effect and climate response revealed significant seasonal and altitudinal differences. At the timberline site, δ 2H LM values of earlywood were mainly related to the January and February temperatures, whereas δ 2H LM values of latewood correlated most strongly with temperature from July to September, and both values were controlled by the amount of total precipitation. In addition, the combined new δ 2H LM chronologies, based on the lag-1 autocorrelation and the weighted average of the current year earlywood and previous latewood, could be used to merge the corresponding temperature signals from the middle- and low-elevation sites. Altitudinal correlations of elevation offset between the annual δ 2H LM chronologies and interpolations of precipitation and temperature suggested that the δ 2H LM values at higher elevations better represent regional climatic changes. Therefore, topographic effects such as elevation differences should be considered in large-scale applications of δ 2H LM values in future studies. • Altitude- and seasonal-differences in δ 2H values of both tree-ring wood and precipitation were revealed in Qinling Mountains, China. • Elevational effects of temperature and precipitation resulted in the variations of δ 2H LM of lignin methoxy. • Timberline δ 2H LM recorded the significantly lagged effects and strong hydroclimatic signals. • Altitude specificity of δ 2H LM should be considered in the large-scale investigations. [ABSTRACT FROM AUTHOR]

Stable hydrogen isotope ratios (δD) in tree rings are an attractive but still rarely explored terrestrial archive of past climatic information. Because the preparation of the cellulose nitrate for δD measurements requires more wood and a longer preparation time than preparation techniques for other isotopes in cellulose (δ 18 O or δ 13 C), it is challenging to obtain high-resolution records, especially for slow-growing trees at high elevations and in boreal regions. Here, we tested whether annually pooled samples of Qinghai spruce ( Picea crassifolia Kom.) trees from northwestern China provided results similar to those derived as the mean of individual measurements of the same trees and whether the resulting chronologies recorded useful climate information. Inter-tree variability of δD was higher than that of measured ring width for the same trees. We found higher and significant coherence between pooled and mean isotope chronologies than that among the individual series. It showed a logarithmic relationship between ring mass and δD; however, accounting for the influence of ring mass on δD values only slightly improved the strength of climatic signals in the pooled records. Tree-ring δD was significantly positively correlated with the mean, maximum, and minimum temperatures during the previous winter and with maximum temperature during the current August, and significantly negatively correlated with precipitation in the previous November to January and the current July. The winter climate signal seems to dominate tree-ring δD through the influence of large-scale atmospheric circulation patterns, i.e. the Arctic Oscillation. These results will facilitate reconstruction of winter atmospheric circulation patterns over northwestern China based on a regional tree-ring δD networks. [ABSTRACT FROM AUTHOR]

Abstract: We examined the temporal relationships between stable isotopes of carbon (δ13C) and oxygen (δ18O) from annual tree-ring cellulose of Abies georgei on the moist Batang–Litang plateau, western China. Climatic response analysis reveals that tree-ring δ13C contains a strong moisture signal, and that δ13C was especially influenced by relative humidity and precipitation in May through August during the period 1960–2005. In addition, tree-ring δ13C and temperature in May to August are positively correlated because of the high-elevation tree growth sites. Drought is also recorded in tree-ring δ13C, reflecting the integrated influences of temperature and moisture on stomatal regulation and photosynthesis. Generally, correlations between δ13C and δ18O remained positive, but not significant, in most of the investigated period, and this correlation switched to highly significant from 1969 onwards. Changes in the δ13C–δ18O relationship reflect changes in water stress on carbon isotopic discrimination, suggesting an increase in stomatal control of photosynthesis in recent decades, which is consistent with the overall drying trend in the region. Spatial correlation maps confirm that during the past century the response of tree-ring δ13C to temperature and precipitation differs before and after 1969. Using previously reported δ18O results and new δ13C results on the same samples, we determined that the climatic signal recorded in tree-ring δ18O is more stable than that of δ13C. Analysis of temporal changes in the δ13C-δ18O correlations indicates that dominant processes and the controlling factors on tree-ring δ13C potentially shift over the long term under temporal fluctuations of dry–wet spells, especially under recent global warming, and stomatal control of photosynthesis may adjust to region-wide changes in climate in this region where water stress on forest was formerly minor. [Copyright &y& Elsevier]

• We evaluated larch tree-ring δ2H LM and δ13C LM in individual and pooled, bulk and degreased wood from permafrost forest. • The total lignin content, but not G-lignin and methoxy isotopes, shifted at the boundary between sapwood and heartwood. • The May climate strongly affected the heartwood lignin content and tree growth. • δ2H LM series showed great potential as growing season temperature and VPD proxies. The hydrogen and carbon stable isotope ratios (δ2H LM and δ13C LM values) of tree-ring lignin methoxy groups have recently been recognized as valuable palaeoclimate indicators. However, the environmental factors but also sample preparation processes that might cause variations of δ2H LM and δ13C LM values have been not fully explored. Furthermore, the temporal dynamics of wood lignin content on both isotopes hasn't been investigated. To investigate the effects of total lignin content, removal of lipids and differences between individual and pooled tree-ring series on isotopic values, we analyzed ring samples of dominant Larix gmelinii trees from a typical permafrost region of northeastern China. We also examined relationships between δ2H LM and δ13C LM values with annual-resolution lignin content and climatic parameters over the common period of overlap 1901 to 2013. We found that the inter-tree variability between the individual and pooled isotopic chronologies was consistent, and the removal of lipids did not change the results of stable isotope measurements. The total lignin content shifted between the heartwood and sapwood, whereas the content of guaiacyl monomers (G-lignin), the only donor of methoxy groups, remained constant. Correlations analysis between lignin proxies and climate variables indicated that the heartwood total lignin content was positively correlated with the intrinsic water-use efficiency, temperature and vapor-pressure deficit (VPD), but negatively with the early-growing-season standardized precipitation evaporation index. Whilst δ2H LM values were positively correlated with the growing season temperature (May to August) and VPD, particularly in August, δ13C LM series showed insignificant correlation. Therefore, we suggest that the total lignin content and δ2H LM values of tree rings (both individual and pooled series) are suitable for tracking environment dynamics, as they can reveal the climate responses of tree growth. [ABSTRACT FROM AUTHOR]