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• 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]

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]

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]

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]

• 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]