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The effects of climate warming are widely recognized; however, some studies have suggested that the rate of warming has slowed since the end of the twentieth century. However, the relationships between climate and the radial growth of trees during different periods of climate change have yet to be investigated fully. In the present study, tree-ring core samples were collected from Picea crassifolia and Juniperus przewalskii in the Qilian Mountains of northwest China to study the relationships between climate and radial growth. The core samples were divided into two groups: one with significant tree-growth reduction and another with insignificant reduction. Trees showing with significant growth reduction were more sensitive to climate change than trees showing insignificant growth reduction during the period of climate warming. P. crassifolia exhibited a stronger response to precipitation during May–June of the current year and from August of the previous year to June of the current year, both before and after climate warming. P. crassifolia exhibited a positive coherence with precipitation from August–September of the previous year and a negative coherence with the temperature of the current June during climate warming. After climate warming, J. przewalskii responded positively to precipitation in the current May and from the previous August to the current May. J. przewalskii also responded positively to temperature during January–February of the current year after warming. P. crassifolia responded to precipitation more strongly than J. przewalskii, whereas J. przewalskii showed a closer relationship to temperature than P. crassifolia. [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]

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]

• 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]

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]

• 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]

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]