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The Qilian Mountain National Nature Reserve is an important ecological barrier in northwestern China; therefore, it is of great importance to accurately evaluate the carbon storage of its main arbor forest. In this study, the Tianlaochi Basin of Qilian Mountain Nature Reserve was used as the research area to measure the carbon storage of the main arbor forests and determine the relationship between neglect of understory vegetation and errors in estimated carbon storages. The results showed that: (1) The total carbon storages of Picea crassifolia and Sabina przewalskii in Qilian Mountain Nature Reserve were 6.05 × 107 and 9.48 × 106 Mg and the carbon density were 426.07 Mg/hm2 and 338.72 Mg/hm2, respectively. (2) The understory carbon storage of Picea crassifolia and Sabina przewalskii accounted for 7.03% and 5.5% of the total carbon storage, respectively (6.81% combined). When the understory vegetation was neglected, the carbon storage in Qilian Mountain Nature Reserve was estimated to be less than 9.72 × 105 Mg. (3) There are large areas of Picea and Sabina forests worldwide; therefore, ignoring understory vegetation in these systems when estimating the carbon stock will lead to underestimations. Our research can provide a reference when estimating the carbon storage of similar tree species.

The temperature in northwestern China has increased significantly since the 1990s. However, the responses of mountainous forests to warming have not been extensively examined. We collected tree rings of two dominant coniferous species of Qinghai spruce (Picea crassifolia) and Chinese pine (Pinus tabulaeformis) in the eastern part of the Qilian Mountains, and analyzed the differences in the response dynamic of the radial growth of two species to climate change. The results showed that (1) the annual radial growth of Qinghai spruce was mainly restricted by the minimum temperature in July and October, and the growth of Chinese pine was mainly restricted by the mean temperature in September of the previous year, January, and July and the maximum temperature in March, May, and July. In particular, Qinghai spruce increased its sensitivity to total precipitation in the growing seasons in March, May, and July after the temperature abruptly increased. (2) In comparison to Qinghai spruce, Chinese pine showed a consistent response to the main climatic factors and was more severely affected by drought stress. Qinghai spruce had divergent responses to mean temperatures in March and May and minimum temperatures in April and June. (3) The growth of Qinghai spruce increased with a significant fluctuation at the end of the twentieth century, while the growth of Chinese pine first showed an increase and then a significant decreasing trend. At present, the increase in temperature has adversely affected the growth of Chinese pine in the eastern Qilian Mountains and promoted the growth of Qinghai spruce. However, a continuous temperature increase could negatively affect the growth of Qinghai spruce because of the increasing probability of drought stress. Therefore, we should pay more attention to the growth dynamics of Qinghai spruce, especially with the different water supply and demand, and to the effects of drought on Chinese pine in forest ecosystems in arid and semiarid areas.

Ecological stoichiometry is an effective tool for analyzing and explaining changes and connections between plants and the environment. However, the changes in ecological stoichiometry throughout the leaf-root-litter-soil continuum during different seasons in plantation ecosystems are not fully understood. In this study, the C, N and P contents of leaves, roots, litter and soil were measured in four plantations growing Larix principis-rupprechtii, Picea crassifolia, Betula platyphylla and Populus cathayana in spring, summer and autumn. The stoichiometric characteristics and seasonal dynamics along the continuum were analyzed to reveal nutrient limitations on the plantations and coupling relationships among the components. Our analysis revealed that the C and N contents and C:P and N:P ratios of leaves, the P content of litter, and the C:N ratio of roots were the highest among all components. The seasonal dynamics showed that the N and P contents of leaves and roots and the P content of soil and litter were the highest in spring on all plantations, while the C:N, C:P and N:P of leaves, the C:N and C:P of roots and soil, and the C:P and N:P of litter were the lowest in spring. In all plantations, the N contents of BP and PR in each component were higher, while the P content and C:P and N:P ratios in roots of LR in summer were lower. The N element between litter-soil-root-leaf had a strong coupling relationship, and the P element between leaf-litter-root had a strong coupling relationship. Our results demonstrated that plants and the environment in each season on the Loess Plateau were mainly coupled and regulated by the N element in litter-soil-root-leaf and by the P element in leaf-litter-root. The plants absorbed N mainly from the soil and P mainly from litter. Plantations were limited by N in spring and summer and by P in autumn; this information may help guide plantation restoration and management.

In situ observations indicate that different levels of micro-standing could induce divergent temperature responses in trees at similar elevations but different slope aspects, which is supported by physiological process model simulation of radial incremental growth. The unstable response of tree growth to climate change is a well-documented, yet poorly understood phenomenon. It has been reported that Picea crassifolia exhibits a systematically variable response to climate forcing at a single high-mountain study area located at the upper treeline in the Qilian Mountains, China. This study used in situ meteorological observations to force a simplified but nonlinear mechanistic model of tree-ring width to identify how variable conditions translate into differences in radial growth within this site. The simulations suggest no significant difference in soil moisture between sites, but disparate diurnal micro-site temperature maxima, apparently amplified by a uniform warming, drive the divergent growth trends between subsites. For tree-ring-based reconstructions of surface temperature, such divergent growth trends challenge the validity of the assumption that a linear growth response to environmental conditions within and across study sites. Therefore, we recommend the development of precise, high-resolution data sets for calibrating, validating and reconstructing climatic change, given the potential for emergent non-stable tree growth–climate relationships.

For trees in the east from 1980 to 2001 and after 2001 and trees in the west from 1980 to 2001, younger trees responded to precipitation and older trees responded to temperature more strongly. Global warming has now persisted for more than 100 years, but an unusual pause in this rising temperature began in the 2000s, at least for the climate of China. This shift presents an unusual opportunity to investigate possible differences in the radial growth of trees during a climate warming period (1980–2001) and the warming hiatus (after 2001) for different age groups under different moisture conditions. We sampled a total of 304 tree-ring cores of Picea crassifolia in the eastern and western Qilian Mountains, which were divided into four age groups for study. The results indicate that trees of the young group and half-mature group in the wetter eastern region showed insignificant growth trend, while all trees in the drier west showed significant growth reduction related to climate warming. Radial growth of trees in the east showed weak correlation with precipitation, and that in the west positively correlated with precipitation from the previous August–September and current May–June from 1980 to 2001. In contrast, trees in the east positively correlated with precipitation from the previous August–September and current July, and those in the west positively correlated with precipitation from the current April–May during the temperature pause after 2001. Trees in the east showed negative correlation with temperature in the winter dormancy period, and those in the west negatively correlated with temperature from the previous July, November and the current June from 1980 to 2001. Trees in both the east and west showed weak correlations with temperature after 2001. Older trees (> 150 years old) showed a weaker response to precipitation than the younger trees (

Climate change has been documented in the Qilian Mountains of China, and the response of tree stem growth to environmental factors may be changed, affecting regional estimates of seasonal growth patterns and carbon sequestration. The objective of this study was to provide initial evidence for responses of intra-annual tree stem radial changes of sapling Qinghai spruce (Picea crassifolia) to climate warming in the Qilian Mountains. We monitored stem radial changes of Qinghai spruce in an open-top chamber (OTC) and out of chamber (OC) over three growing seasons (2014–2016) using high-resolution point dendrometers. Statistical analysis indicated that the differences in annual average air temperature between OTC and OC were about 1 °C. Our results showed that the onset of the main growth period in OTC was about one week earlier than in OC, and soil temperature played an important role in controlling the onset of growth in sapling Qinghai spruce. However, the timing of radial growth cessation exhibited large variability in different growth conditions. These results indicated that the end of growth may not be related to environmental factors. Daily average air temperature, vapor pressure deficit, relative air humidity, and precipitation were significantly correlated with daily stem radial increment (SRI) and tree water deficit (TWD). Further, soil water content and TWD were also correlated. SRI in OTC was larger than that in OC, while TWD exhibited no significant differences between the two growth conditions. However, cumulative seasonal growth may not be influenced by air temperature alone, but by interacting environmental factors.

Vegetation reconstruction from fossil pollen records depends on an adequate understanding of the relationship between modern pollen and vegetation. In this study, we present the modern pollen dispersal and representation of the “Forest Island” and its surrounding areas within the Chinese desert-loess transitional zone. Forty-four pollen carpet traps, which imitate the natural pollen sediment process, were collected from the Ningxia Daluoshan Mountain and its surrounding areas along a southeast–northwest transect included five vegetation types. The pollen spectra, spatial distribution patterns of the main pollen types and discriminant analysis of pollen data show that: Pinus Diploxylon-type has high pollen production and good dispersal ability and that the over-represented, with values exceeding the 20% (> 100 grains cm− 2 year− 1) within vegetation dominated by Pinus tabuliformis, below 7% ( 100 grains) within the spruce forest, decreased abruptly and even disappeared completely outside the forest. Another interesting finding of this study is that Poaceae have higher values and higher pollen production. The modern pollen assemblage of the desert steppe, mountain steppe and Picea crassifolia forest, but especially the desert steppe and mountain steppe, can be separated from each other as follows: (1) desert steppe: Chenopodiaceae–Poaceae pollen assemblage; (2) mountain steppe: Poaceae–Artemisia–Aster type pollen assemblage; and (3) Picea crassifolia forest: Picea–Artemisia–Poaceae pollen assemblage. However, overlap occurs between deciduous broadleaved shrubs and coniferous and broadleaved mixed forests. The very low concentration of deciduous broadleaved shrubs may due to an increased contribution of pine pollen. Our study may facilitate the confident use of fossil pollen data to reconstruct vegetation in the desert-loess transitional zone in China and may be able to be used to correctly interpret fossil pollen records from similar areas.

Climate change has dramatically altered the frequency and intensity of droughts, thereby altering tree growth. Understanding whether tree growth in semiarid areas in response to long-term drought and the post-drought recovery rate of tree growth vary along moisture gradients is crucial for predicting future forest change. Here, we assessed the spatial variation in both the growth resistance of Qinghai spruce (Picea crassifolia Kom.) to long-term drought and its post-drought recovery using a stand-total sampling strategy along a moisture gradient that covered three sites (with an annual precipitation of 330.4, 394.2, and 515.9 mm for the western, middle, and eastern sites, respectively) with six plots. Resistance and recovery were evaluated by analyzing the tree growth trends during a long-term drought period (1980–2001) and the subsequent post-drought period (2001–2013), respectively. Our results indicate that the trees with the highest temporal stability were those at the wetter eastern site; specifically, during the long-term drought period, the trees at the wetter eastern site showed the highest resistance (−0.015) and the lowest recovery (0.002). The trees in moderately arid conditions were much more sensitive to climate change than those at the relatively arid western site, showing the lowest resistance (−0.050) and highest recovery (0.020). Climate change had the strongest impact on tree growth at the moderately arid site, contributing 60.6% to the tree growth decreasing trend during the long-term drought period and 65.4% to the tree recovery during the post-drought period, respectively. Climate change had a lower impact on tree growth at the wet and dry sites, contributing less than 50% to the tree growth trends at these sites. The results indicate that a trade-off relationship exists between resistance and recovery at the different sites; that is, the highly resistant trees at the wetter eastern site tend to have lower recovery, whereas the weakly resistant trees at the moderately arid site tend to have higher recovery. These results have implications for predicting tree growth in response to future climate change.

Hydroclimatic conditions and related water resources change in the Tibetan Plateau is one of the main concerns for future sustainable development in China. This study presents a 254-year precipitation reconstruction from August of the previous year to June of the current year for the northeastern Tibetan Plateau based on tree-ring width data of tree-ring cores of Picea crassifolia from three sampling sites. The precipitation reconstruction explained 51.4% of the variance in instrumental precipitation during the calibration period 1958–2013. Dry periods with precipitation below the 254-year average value occurred during 1848–1865, 1873–1887, 1898–1923, and 1989–2003, and wet periods (precipitation above the mean) occurred during 1769–1785, 1798–1833, 1924–1938, 1957–1968, and 2004–2013. Spatial correlation analyses with the precipitation gridded dataset showed that our reconstruction contains some strong regional-scale precipitation signals for the upper Yellow River Basin. Our precipitation reconstruction also agreed in general with other dendroclimatic precipitation reconstructions from surrounding regions. In addition, reconstructed precipitation changes were consistent with the streamflow variation of the Yellow River.

Global climate change is reshaping the geographical distribution of forests, especially in mountain regions. The alpine forest ecotone is expected to be affected first, due to the climatic limitations on tree growth, and is thus considered as an indicator of the forest succession to climate change. However, current studies involving tree radial growth and their responses to climate change at alpine forest ecotone remain scarce, posing challenges for predictions of forest dynamics and management strategies under different climate scenarios. Here, we studied 94 Chinese Pine (Pinus tabulaeformis; Pinaceae) and 86 Qinghai Spruce (Picea crassifolia; Pinaceae) trees along the alpine forest ecotone [2100–2400 m above sea level (a.s.l.)] to assess tree growth differences and corresponding responses to climate change in the Helan Mountains, northwestern China. The results revealed that growth rates of Chinese Pine at lowest altitudes and Qinghai Spruce at highest altitudes are strongly affected by water shortage and intraspecific competition, respectively, leading to the tree growth in these regions are inferior to those of other trees in the ecotone (the tree age at maximum width is about 20 years later than in the ecotone), especially for seedling and sapling stages. The responses of tree radial growth to dryness were broadly similar in each sample site, but wetness was more positive in trees at low altitudes than at high altitudes. Climate warming has inhibited the radial growth of Qinghai Spruce at lower distribution limits since 2005, while for Chinese Pine in the same area was not significantly affected by rising temperature, and even the upper limits of the Chinese Pine might migrate to higher altitudes as the calorific limit becomes higher. If climate warming continues, the above process may erode the Qinghai Spruce habitat, resulting in the transitional zone between Chinese Pine and Qinghai Spruce also moving to higher elevations.

Background Large-scale afforestation can significantly change the ground cover and soil physicochemical properties, especially the soil fertility maintenance and water conservation functions of artificial forests, which are very important in semi-arid mountain ecosystems. However, how different tree species affect soil nutrients and soil physicochemical properties after afforestation, and which is the best plantation species for improving soil fertility and water conservation functions remain largely unknown. Methods This study investigated the soil nutrient contents of three different plantations (Larix principis-rupprechtii, Picea crassifolia, Pinus tabuliformis), soils and plant-soil feedbacks, as well as the interactions between soil physicochemical properties. Results The results revealed that the leaves and litter layers strongly influenced soil nutrient availability through biogeochemical processes: P. tabuliformis had higher organic carbon, ratio of organic carbon to total nitrogen (C:N) and organic carbon to total phosphorus (C:P) in the leaves and litter layers than L. principis-rupprechtii or P. crassifolia, suggesting that higher C:N and C:P hindered litter decomposition. As a result, the L. principis-rupprechtii and P. crassifolia plantation forests significantly improved soil nutrients and clay components, compared with the P. tabuliformis plantation forest. Furthermore, the L. principis-rupprechtii and P. crassifolia plantation forests significantly improved the soil capacity, soil total porosity, and capillary porosity, decreased soil bulk density, and enhanced water storage capacity, compared with the P. tabuliformis plantation forest. The results of this study showed that, the strong link between plants and soil was tightly coupled to C:N and C:P, and there was a close correlation between soil particle size distribution and soil physicochemical properties. Conclusions Therefore, our results recommend planting the L. principis-rupprechtii and P. crassifolia as the preferred tree species to enhance the soil fertility and water conservation functions, especially in semi-arid regions mountain forest ecosystems.

Mountain ecosystems are significantly affected by climate change. However, due to slow vegetation growth in mountain ecosystems, climate-induced vegetation shifts are difficult to detect with low-definition remote sensing images. We used high-definition remote sensing data to identify responses to climate change in a typical Picea crassifolia Kom. forest in the Qilian Mountains, China, from 1968 to 2017. We found that: (1) Picea crassifolia Kom. forests were distributed in small patches or strips on shaded and partly shaded slopes at altitudes of 2700&ndash, 3250 m, (2) the number, area, and concentration of forest patches have been increasing from 1968 to 2017 in relatively flat and partly sunny areas, but the rate of area increase and ascend of the tree line slowed after 2008, and (3) the establishment of plantation forests may be one of the reasons for the changes. The scale of detected change in Picea crassifolia Kom.forest was about or slightly below 30 m, indicating that monitoring with high-resolution remote sensing data will improve detectability and accuracy.

In the past few decades, industrialization has caused a large number of pollutants to be released into the atmosphere. Forest ecosystems play an important function in regulating the biogeochemistry and the circulation of metal ions pollutants. Forest ecosystems affect the absorption of pollutants and dissolution of nutrients from the atmosphere and vegetation canopy, thereby influencing the content and composition of forest floor leachate and soil solution. This study examined changes in acid anions (NO3−, SO42−, Cl−) and metal cations (K+, Ca2+, Na2+, Mg2+, Fe3+, Pb2+, Cu2+, Cd2+) in rainfall, throughfall, stemflow, and forest floor leachate for five different forests (Larix principis-rupprechtii, Picea wilsonii, Picea crassifolia, Betula platyphylla and Rhododendron communities). The results showed that the enrichment capacity of acid anions and metal cations in the vegetation canopy of the coniferous forests (L. principis-rupprechtii, P. wilsonii, P. crassifolia) was stronger than that of the broad-leaved forests (B. platyphylla and Rhododendron communities). The content of acid anions and metal cations in stemflow of coniferous forests were 3.7–5.6 times and 0–9.3 times higher than those of broad-leaved forests, respectively. Corresponding values in throughfall were 1–1.4 times and 0.3–2.4 times, respectively. The contents of NO3−, Cl−, K+, Mg2+, Fe3+, Pb2+, Cu2+, and Cd2+ in leachate filtered from the soil layers that are deepening gradually showed consistent decreasing trend for all the forest stands. In addition, NO3−, Cl−, K+, Mg2+, Fe3+, and Pb2+ were also concentrated in the topsoil, except for Cu2+ and Cd2+. Nevertheless, SO42− and Na+ were concentrated in the subsoil, whereas Ca2+ was concentrated in the upper soil layers. Soil organic carbon (SOC) and total nitrogen (TN) contents in coniferous forest stands were 20–37% and 34–63% higher than those in broad-leaved forest stands, respectively. This results also shown that the contents of OC and TN has a strong correlation with the content of partial metal cations in soil and litter, indicating that coniferous forest stands had stronger ion scavenging and adsorption capacity in soil layer and litter layer than broad-leaved forest stands. Therefore, L. principis-rupprechtii, P. wilsonii, P. crassifolia had higher air pollutant adsorption and soil pollution remediation capacities than the other two forests. Thus, we recommend planting coniferous tree species (L. principis-rupprechtii, P. wilsonii and P. crassifolia) for eco-rehabilitation and water purification to improve the ecological service function of forest ecosystems.

It is important for integrated forest-water management to develop a better understanding of the variation of tree transpiration among different canopy layers in the forests and its response to soil moisture and weather conditions. The results will provide insights into water consumption by trees occupying different social positions of the forests. In the present study, an experiment was conducted in the Qilian Mountains, northwest China, and 13 trees, i.e., 4&ndash, 5 trees from each one of dominant (the relative tree height (HR) &gt, 1.65), subdominant (1.25 &lt, HR &le, 1.65) and intermediate-suppressed (HR &le, 1.25) layers) were chosen as sample trees in a pure Qinghai spruce (Picea crassifolia Kom.) forest stand. The sap flux density of sample trees, soil moisture of main root zone (0 to 60 cm) and meteorological conditions in open field were observed simultaneously from July to October of 2015 and 2016. The results showed that (1) The mean daily stand transpiration for the study period in 2015 and 2016 (July&ndash, October), was 0.408 and 0.313 mm&middot, day&minus, 1, and the cumulative stand transpiration was 54.84 and 40.97 mm, accounting for 24.14% (227.2 mm) and 16.39% (249.9 mm) of the total precipitation over the same periods, respectively. (2) The transpiration varied greatly among canopy layers, and the transpiration of the dominant and codominant layers was the main contributors to the stand transpiration, contributing 86.05% and 81.28% of the stand transpiration, respectively, in 2015 and 2016. (3) The stand transpiration was strongly affected by potential evapotranspiration (PET) and volumetric soil moisture (VSM). However, the transpiration of trees from the dominant and codominant layers was more sensitive to PET changes and that from the intermediate-suppressed layer was more susceptible to soil drought. This implied that in dry period, such as in drought events, the dominant and codominant trees would transpire more water, while the intermediate-suppressed trees almost stopped transpiration. These remind us that the canopy structure was the essential factor affecting single-tree and forest transpiration in the dryland areas.

Mining causes serious destruction of the surface morphology and soil structure of lands, and vegetation restoration on post-mining lands provides an effective way for soil and water conservation. To determine the influence of mining and vegetation restoration on soil properties in the eastern margin of the Qinghai-Tibet Plateau, four land sites, including two vegetation restoration sites (restorated by Elymus nutans and Picea crassifolia, respectively), one non-vegetated mining site and one native grassland site, were selected. Fifty-two topsoil (0&ndash, 10) samples were collected from these four sites, and then soil properties, trace metals and soil enzyme activities were analyzed. The results showed that there was an increase in soil pH (&gt, 8.0) after mining, while vegetation restoration decreased the soil pH compared with native grassland, the soil organic matter and total nitrogen in the site restored with E. nutans increased by 48.8% and 25.17%, respectively, compared with the site restored with P. crassifolia. The soil enzyme activities decreased after mining, and there were no significant increases in urease, phosphatase, &beta, glucosidase and &beta, 1,4-N-acetylglucosaminidase activities after five years of restoration. In addition, the contents of soil trace metals (cadmium, chromium, mercury, lead and zinc) after mining were lower than the Chinese threshold (GB 15618/2018), but the content of arsenic in non-vegetated soil and P. crassifolia-restored soil exceeded the threshold by 22.61 times and 22.86 times, respectively. Therefore, As-contaminated land areas should be accurately determined and treated in a timely way to prevent arsenic from spreading, and plant species with tolerance to alkaline soil should be selected for vegetation restoration on post-mining lands.

Canopy and stem phenology of Qinghai spruce, central Qilian Mountains, respond to different environmental factors depending on season and elevation. To understand vegetation species response to climate change, much research has been devoted to changes in forest phenology. Results of such studies are not only of scientific interest; they are potentially of great use in forest management. This study focuses on variations in canopy and stem phenology as affected by climate and elevation. We collected data on canopy phenology (as recorded in the Normalized Differential Vegetation Index) and stem phenology [using the Vaganov–Shashkin (V–S) model] in Qinghai spruce (Picea crassifolia) growing at two sites in the central Qilian Mountains, Northeast Tibetan Plateau. One site was at a higher elevation, near the local alpine tree-line, and the other was near the local lower tree-line. At both sites, a significant correlation was found between canopy and stem spring phenology. This would seem to be mainly due to spring temperatures. No such correlation was found between canopy and stem autumn phenology. The study suggests that the main factors affecting stem growth after the beginning of growing season would be temperature and soil moisture, and that these have different effects depending on elevation. At the lower elevation, soil moisture seems to be the main factor limiting growth. At the higher elevation, temperature was the determining factor. Climate change will have different effects depending on elevation.

It is important to develop a better understanding of the climatic and soil factors controlling the stem diameter growth of Qinghai spruce (Picea crassifolia Kom.) forest. The results will provide basic information for the scientific prediction of trends in the future development of forests. To explain the seasonal pattern of stem diameter growth of Qinghai spruce and its response to environmental factors in the Qilian Mountains, northwest China, the stem diameter changes of 10 sample trees with different sizes and soil and meteorological conditions were observed from May to October of 2015 and 2016. Our results showed that the growth initiation of the stem diameter of Qinghai spruce was on approximately 25 May 2015 and 20 June 2016, and stem diameter growth commenced when the average air and soil temperatures were more than 10 &deg, C and 3 &deg, C, respectively. The cessation of growth occurred on approximately 21 August 2015 and 14 September 2016, and it was probably controlled by soil moisture. Stem diameter growth began earlier, ended later, and exhibited a larger growth rate as tree size increased. For the period May&ndash, October, the cumulative stem diameter growth of individual trees was 400 and 380 &mu, m in 2015 and 2016, respectively. The cumulative stem diameter growth had a clear seasonal pattern, which could be divided into three growth stages, i.e., the beginning (from day of year (DOY) 120 to the timing of growth initiation with the daily growth rate of less than 2 &mu, m&middot, day&minus, 1), rapid growth (from the timing of growth initiation to the timing of growth cessation with the daily growth rate of more than 2 &mu, 1), and ending stages (from the timing of growth cessation to DOY 300 with the daily growth rate of less than 2 &mu, 1). The correlation of daily stem growth and environmental factors varied with growth stages, however, temperature, vapor pressure deficit (VPD), and soil moisture were the key factors controlling daily stem diameter growth. Overall, these results indicated that the seasonal variation in stem growth was regulated by soil and climatic triggers. Consequently, changes in climate seasonality may have considerable effects on the seasonal patterns of both stem growth and tree growth.

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.

The dominant climate-controlling system of the southwestern Gobi Desert and its surrounding areas in China at interannual to centennial time scale has not been determined. It is necessary to improve our understanding of the humidity variations in the past and forcing mechanisms in the southwestern Gobi Desert to enhance the accuracy of climate model predictions and cope with future desert advance/retreat crises resulting from climate change. We measured the annual stable oxygen isotope ratio (δ18O) in the cellulose of four Picea crassifolia trees growing on the fringe of the southwestern Gobi Desert from 1806 to 2011. Our tree-ring cellulose δ18O values (34.49‰) were considerably higher than those reported in other regions of Asia, suggesting that the southwestern Gobi Desert has been extremely arid over the past two centuries. The chronology of tree-ring cellulose δ18O is significantly (p

The effect of warming-induced drought stress on tree growth has been frequently discussed, and a decline in the growth of forest and increased mortality has been reported for many areas of the world. A hiatus in the global warming trend began around the year 2000, but how tree growth has responded to this warming slowdown remains unclear. Here we present a study of the recent impacts of climate change on Qinghai spruce (Picea crassifolia) growth using 38 tree-ring records collected from the Qilian Mountains in northwestern China. Results indicate that there was a clear increase in Qinghai spruce growth during the recent warming hiatus. Similarly, regional vegetation growth also improved as indicated by NDVI variations, which is closely associated with regional climate changes. The mean air temperature increased dramatically during the period 1980–2001, whereas precipitation decreased slightly. This combination of warmer temperatures and reduced precipitation created drought conditions that limited the growth of Qinghai spruce during this period. Since 2001, the warming trend has slowed, whereas precipitation has clearly increased, creating moist conditions as reflected by the higher drought index (SPEI). A decline in drought pressure, coupled with an increasingly long growing season, might be the main reason for the recent improvement in the growth of Qinghai spruce in the Qilian Mountains. Our results suggest that the growth of Qinghai spruce is very sensitive to climate change. A better understanding of future climate changes and their likely impact on tree growth should be of significant interest to forest managers.

Close-to-nature forest management has been proposed as an effective method for improving the quality of plantation forests. Knowledge of spatial distribution patterns, structure, and succession trajectories in natural forests can provide guidelines for the establishment of close-to-nature forest plantations. Such knowledge is lacking in natural spruce (Picea crassifolia) forests in the Qilian Mountains of China, impeding the establishment of production forests. We conducted a case study in the Qilian Mountains to analyze the relationships between the naturally-formed forest patches and terrain factors, spatial heterogeneity of stand characteristics, and stand structure following harvesting disturbance. Our results suggested that spruce plantations will be effective on the N, NE, and NW slopes, at elevations between 2700 and 3300 m, and on slopes ranging from 15° to 45°. Further, planted forest patches should occupy 64% of the slope area on semi-shady slopes (NE, NW). Spatial patterns in the studied forest exhibited a strong scale-effect, and an area of 0.25 ha could be used as the most efficient plot scale for the management of spruce plantations. Partial logging is an effective method for the conversion of spruce planted forests into nearnatural forests, and the intensity of partial logging can be determined from the negative exponential function relationship between stand density and DBH. Our results provided critical information for the development of spruce plantations and conversion of existing plantations.

Here we report a new find of abundant woody debris and cones in stratum of two sections located to the east of the Qinghai Lake basin in China. Analysis of the anatomical structure of the wood and cones confirmed that they are Picea crassifolia Kom. The results of accelerator mass spectrometry 14C dating indicate that the buried Qinghai spruce grew during 9.7–4.2 ka, and the ages of the large trunks or branches are mainly concentrated within the interval 7.5–6.5 ka. This finding gives direct evidence at the species level about the presence of coniferous forest in the early–middle Holocene in Qinghai lake basin. In addition, the buried cones suggest that the early-middle Holocene environment was suitable for the propagation of Picea crassifolia Kom. The variations in the occurrence of Qinghai spruce forest in the Holocene probably reflect changes in humidity/moisture. The humid early-middle Holocene was suitable for the growth and reproduction of Qinghai spruce forest, while a shift toward an increasingly arid climate during the late Holocene resulted in the disappearance of Picea crassifolia Kom. from the Qinghai Lake basin, although human activities may also have contributed to the environmental change.

The frequency of stem-growth cessation in Picea crassifolia has increased significantly as the frequency of extreme climate events has increased. Extreme climatic events are increasingly recognized as important drivers of tree growth, forest dynamics, and range contractions. Understanding tree growth responses to extreme events is important for forest conservation and management, especially under climate change. Here, we studied the patterns of growth cessation of Picea crassifolia Kom., an endemic species to Central Asia, across its distributional range in the Helan Mountains of Northwestern China, to test the hypotheses that (1) tree growth and growth cessations are limited by moisture availability, and (2) that this relationship is constant over the diverse set of conditions covered in our elevational gradient. While tree growth across our gradient was significantly limited by low precipitation in June, we found growth cessation events at the lower distributional margin to increase in severity and frequency in recent decades. We found that the combination of low precipitation early in the monsoon season (June) and high mid-summer (July) vapor pressure deficit likely caused the increased frequency of growth cessation events. Because these populations are already experiencing a high frequency of growth cessation events, the mortality rate of the lower distributional margin trees could further increase if the current trends of decreasing moisture and increasing heat stress continue into the future. Our results strongly suggest that growth cessation events are more than the dendrochronological curiosity they are usually considered to be, and can be important indicators of increased tree growth stress and be potentially useful in identifying tipping points prior to forest change.

Study of the radial growth response to climate factors at different tree ages is essential for predicting forest dynamics and formulating correct management policies. In this study, we analysed the growth responsiveness of Picea crassifolia to climate conditions, and evaluated its relationship to tree age at the individual tree scale, in the transitional zone between regions affected by the East Asian monsoon and winds blowing from the west (westerlies) in the Qilian Mountains. On three study sites, 150 cores were taken from 75 trees. Pearson correlation coefficients were calculated between the standardized tree-ring series of each core (and chronology) and climatic factors which were utilized to research the climate-growth relationships. Quadratic polynomial fitting was used to test the relationships between tree age, chronological parameters, and the radial growth response to climate. Radial growth was positively influenced by total precipitation for the previous July-September and the current May-July. Additionally, radial growth was negatively correlated with the total monthly precipitation for the current September, and the mean monthly temperature for the previous July. Trees younger than 80 years old showed a stronger response to accumulative precipitation in the previous July-September, those older than 160 years old showed a stronger response to precipitation at the end of current growing season, while those of 140–160 years old showed a weaker response to precipitation in the current May-July.

Ongoing climate changes posed significant threats to forest growth, structure and ecological functions. However, in our field survey in the east Qilian Mountains, we found that growth condition of Qinghai spruce under climate changes was better in anthropogenic management forest than disturbance-free stands. Accordingly, we suggested a hypothesis that the environmental change induced suppression in Qinghai spruce growth could be mitigated by anthropogenic management. Such a hypothesis was tested by comparing Qinghai spruce growth and population dynamic patterns between anthropogenic-managed forests and disturbance-free ones. We found that Qinghai spruce radial growth persistently declined since the late 1950s in the disturbance-free forests, while the radial growth of anthropogenic-managed trees almost kept steady-going trends since the 1970s in which anthropogenic-management appeared. Besides, some anomalous growth-declining years occurred in disturbance-free forests did not appear in anthropogenic-managed forests in the same year after anthropogenic-management emerged. Moreover, missing ring and tree mortality took place in disturbance-free stands and became increasingly frequent after the mid-1980s. The frequencies and proportions of missing rings and tree die-off were much lower in anthropogenic-managed forests than disturbance-free ones. Additionally, the occurrence of Qinghai spruce radial growth decline and increased mortality in recent time in our disturbance-free forests could be well explained by global-change-type drought in conjunction with elevated temperature and warming-related pest outbreaks. These results would be of fundamental significance for the further understanding of the changing forests, which could provide new clues for forest management and conservation in the future.

Soil moisture controls the functioning of semi-arid ecosystems. The response of soil moisture to forest stand thinning determines planting density and sustainability of forest development. However, consequences of stand thinning are poorly known in semi-arid ecosystems of the Qilian Mountains of China. We investigated long-term effects of three thinning intensities in Picea crassifolia plantations on soil hydrological responses and soil moisture dynamics at 10, 20, 40, 60, and 80 cm depths, and compared them to those of a natural Picea crassifolia forest. Results revealed that soil hydrological response may be temporarily modified by thinning according to changes in canopy structure, precipitation properties, and antecedent soil moisture conditions. Soil moisture in natural forest rapidly infiltrated into deep soil, which greatly improved the efficiency of precipitation use. Thinning significantly increased the capacity for soil infiltration, and moderate thinning intensity may be conducive to deep soil-water recharge. Soil moisture content changed drastically after thinning, with a significant decrease near-surface (10 cm), and a significant increase in sub-surface (60 and 80 cm) soil. High planting density was the main cause of severe soil moisture deficits in the long-term, but it could be mitigated by 20–40% thinning (∼3139 trees ha−1). Changes in precipitation patterns that include larger but less frequent rainfall events during the growing season will benefit the growth of vegetation planted at high densities in this semi-arid region.

Exploring the response of intra-annual tree growth to environmental factors is critical for the understanding of the effects of climate changes on tree seasonal growth patterns. We monitored stem radial changes of Qinghai spruce ( Picea crassifolia ) over two growing seasons (2014 and 2015) in the Qilian Mountains of China using high-resolution point dendrometers. Our results showed that the onset of the main growth period occurred around 11 May. Soil temperature was a key environmental factor influencing the onset of tree growth. Stem radial growth commenced when soil temperature at 40 cm depth was >0 °C. The timing of radial growth cessation exhibited greater variability than onset, ranging from late July to early August, and was probably unrelated to climatic conditions. We found close relationships between maximum daily shrinkage (MDS) and air temperature, precipitation, soil water content, and soil temperature. Maximum and mean air temperature, relative air humidity, vapor pressure deficit, and precipitation had significant effects on daily stem radial increment (SRI). Precipitation was one of the most important factors influencing stem radial growth. Moreover, the number of precipitation days was more significant in promoting cumulative seasonal growth of Qinghai spruce than total precipitation. With climate warming, the onset of radial growth may occur earlier than before, and seasonal growth patterns of trees may change.

The northern Tibetan Plateau has been subject to recent warming far above the global average. With few instrumental climate records available for this region before the 1950s, paleoclimatic reconstructions must be used to understand annual-to-centennial-scale climate variations and local climate response to large-scale forcing mechanisms. We developed a maximum latewood density chronology of Qinghai spruce (Picea crassifolia) from the southern slope of the Qilian Mountains, northern Tibetan Plateau. Based on the chronology, we reconstructed August–September temperature for 1780–2008. The temperature reconstruction model accounts for 39.7% of instrumental temperature variance from 1957 to 2008, successfully capturing the most recent warming. Superposed epoch analysis indicated a volcanic forcing for temperature, resulting in pulses of cooler conditions that can persist for 2–4 years. Tree-ring data indicated that warm-dry and cold-wet climate combinations mainly occurred in northern Tibetan Plateau...

Studying the spatial distribution pattern of soil organic carbon and its influencing factors is essential for understanding the carbon cycle in terrestrial ecosystems. Soil samples from four active layers of typical vegetation types (Populus, subalpine shrubs, Picea crassifolia Kom, and alpine meadow) in the upper reaches of Shiyang River basin in the Qilian Mountains were collected to determine the soil organic carbon content and physicochemical properties. The results show the following: (1) There are significant differences in the vertical distribution of Soil organic carbon in the watershed, and the Soil organic carbon content decreases significantly with increasing soil depth. (2) Mainly affected by biomass, the organic carbon content of different vegetation types in different soil layers is as follows: Alpine meadow > Picea crassifolia Kom > Populus > Subalpine shrub, and the soil organic carbon content increases with increasing altitude. Under different vegetation types, the Soil organic content is the highest in the 0–30 cm soil profile, and the maximum value often appears in the 0–10 cm layer, then gradually decreases downward. (3) When soil organic carbon is determined in different vegetation types in the study area, the change of hydrothermal factors has little effect on soil organic carbon content in the short term.

Tree growth strongly responds to climate change, especially in semiarid mountainous areas. In recent decades, China has experienced dramatic climate warming, however, after 2000 the warming trend substantially slowed (indicative of a warming hiatus) in the semiarid areas of China. The responses of tree growth in respect to elevation during this warming hiatus are poorly understood. Here, we present the responses of Qinghai spruce (Picea crassifolia Kom.) growth to warming using a stand-total sampling strategy along an elevational gradient spanning seven plots in the Qilian Mountains. The results indicate that tree growth experienced a decreasing trend from 1980 to 2000 at all elevations, and the decreasing trend slowed with increasing elevation (i.e., a downward trend from &minus, 10.73 mm2 year&minus, 1 of the basal area increment (BAI) at 2800 m to &minus, 3.48 mm2 year&minus, 1 of BAI at 3300 m), with an overall standard deviation (STD) of 2.48 mm2 year&minus, 1. However, this trend reversed to an increasing trend after 2000, and the increasing trends at the low (2550&ndash, 2900 m, 0.27&ndash, 5.07 mm2 year&minus, 1 of BAI, p &gt, 0.23) and middle (3000&ndash, 3180 m, 2.08&ndash, 2.46 mm2 year&minus, 0.2) elevations were much weaker than at high elevations (3300 m, 23.56 mm2 year&minus, 1 of BAI, p &lt, 0.01). From 2000&ndash, 2013, the difference in tree growth with elevation was much greater than in other sub-periods, with an overall STD of 7.69 mm2 year&minus, 1. The stronger drought conditions caused by dramatic climate warming dominated the decreased tree growth during 1980&ndash, 2000, and the water deficit in the 2550&ndash, 3180 m range was stronger than at 3300 m, which explained the serious negative trend in tree growth at low and middle elevations. After 2000, the warming hiatus was accompanied by increases in precipitation, which formed a wetting&ndash, warming climate. Although moisture availability was still a dominant limiting factor of tree growth, the relieved drought pressure might be the main reason for the recent recovery in the tree growth at middle and low elevations. Moreover, the increasing temperature significantly promoted tree growth at 3300 m, with a correlation coefficient between the temperature and BAI of 0.77 (p &lt, 0.01). Our results implied that climate change drove different growth patterns at different elevations, which sheds light into forest management under the estimated future climate warming: those trees in low and middle elevations should be paid more attention with respect to maintaining tree growth, while high elevations could be a more suitable habitat for this species.

It is important to explore the responses of radial tree growth in different regions to understand growth patterns and to enhance forest management and protection with climate change. We constructed tree ring width chronologies of Picea crassifolia from different regions of the Qilian Mountains of northwest China. We used Pearson correlation and moving correlation to analyze the main climate factors limiting radial growth of trees and the temporal stability of the growth–climate relationship, while spatial correlation is the result of further testing the first two terms in space. The conclusions were as follows: (1) Radial growth had different trends, showing an increasing followed by a decreasing trend in the central region, a continuously increasing trend in the eastern region, and a gradually decreasing trend in the isolated mountain. (2) Radial tree growth in the central region and isolated mountains was constrained by drought stress, and tree growth in the central region was significantly negatively correlated with growing season temperature. Isolated mountains showed a significant negative correlation with mean minimum of growing season and a significant positive correlation with total precipitation. (3) Temporal dynamic responses of radial growth in the central region to the temperatures and SPEI (the standardized precipitation evapotranspiration index) in the growing season were unstable, the isolated mountains to total precipitation was unstable, and that to SPEI was stable. The results of this study suggest that scientific management and maintenance plans of the forest ecosystem should be developed according to the response and growth patterns of the Qinghai spruce to climate change in different regions of the Qilian Mountains.

The evergreen coniferous creeping shrub, Sabina vulgaris, is widely distributed in China, from high and cold mountain sites to low-elevation arid desert areas. This paper performed dendroclimatological analysis of the samples of S. vulgaris from three sampling sites (2700–2900 m a.s.l.) at sites in the northern border of the middle Qilian Mountains, Northwest China. We found that the radial growth of S. vulgaris was mainly limited by rainfall during the growing season, especially in May and June, but was also limited by low temperatures in the late growing season. It presents the regional differention of the climate response of the shrub radial growth from the other distributed areas, such as the sandland and mountains. We constructed a 274-year drought record by calibrating growth-ring data and relating this data to the regional monthly Palmer Drought Severity Index (PDSI56), which represents regional moisture conditions. The PDSI56 reconstruction accounted for 35.5 % of the variation of the actual May to June PDSI56 during the period for which meteorological data was available (1950–2014). The most severe drought occurred during the 1920s, followed by the 1950s, 1740s, and the period from the 1870s to the 1880s; the wettest years were during the 1980s, followed by the 2010s, 1770s, and 1750. The changes between drought, normal, and wet periods were consistent with the reconstructed results from previous conifer ring series for Sabina przewalskii and Picea crassifolia in the study area. Our results demonstrate the value of S. vulgaris in dendrochronology.

Understanding the effects of environmental variables on seedling-sapling distributions can help gain insight into changes in population recruitment in the context of global warming. Six transects were used to survey the distribution of seedling-saplings and the variability in environmental variables along altitudinal gradients. In addition, laboratory experiments measuring the effects of moss on Picea crassifolia seedling emergence were conducted to identify seedling development characteristics of Picea crassifolia. The results show that the seedling-sapling distribution of Picea crassifolia is determined by altitudinal gradients over the large scale and is influenced by moss at the small scale. We found the largest number of seedling-saplings occurring at middle altitudes (3050-3250 m a.s.l.). The crucial abiotic factors in the seedling-sapling distribution for this species are precipitation at lower altitudes and temperature at higher altitudes. Moss mats can promote the recruitment of Picea crassifolia when the moss depth is less than 5 cm and moss cover is less than 50%. Conversely, with increasing moss depth and cover, the effects are shifted towards a negative relationship. Moss can promote moist conditions, implying that recruitment may be facilitated by moss in lower altitude areas where drought stress may occur. However, both air temperature in moss layers and soil temperature below moss are reduced, suggesting that the presence of moss may result in a shortened growing season for Picea crassifolia. More research into the seedling-sapling distribution of Picea crassifolia at both the large and small scales is necessary for artificial forestation and forest management, especially for those alpine plant species that are sensitive to climate change. (C) 2016 Elsevier B.V. All rights reserved.

The influence of climate in the previous year on radial growth was stronger than that in the current year and climatic influence became weaker as the altitude increased. The climate conditions during the previous year often influence the radial growth of trees in the current year, which is known as the lingering effect of climate. To explore this lingering effect in depth, we compare the influence of climate in the previous and current year on radial growth and find variations in climatic effects on growth along the altitudinal gradient. We also relate radial growth of Picea crassifolia to climate variables from 24 different timescales (from 1 month to the previous 24 months) at different altitudes in the middle of the Qilian Mountains. In the study area, accumulated monthly precipitation before November in the previous year benefited radial growth in the current year at all altitudes. At lower altitudes, July could be seen as a demarcation point that precipitation from the current May–July significantly affected radial growth in the current year, but precipitation from the current July–October significantly affected radial growth in next year. At the same time, radial growth there responded stronger to precipitation from the previous July–October than that from the current May–July. Radial growth at higher altitudes responded weaker to climatic factors than that at lower altitudes and rarely correlated with precipitation from the current year.

The current work adopted the Biome-BioGeochemical Cycle model to simulate the net primary productivity (NPP) of a subalpine forest (Picea crassifolia forest) under four representative concentration pathway (RCP) scenarios in the Qilian Mountains of northwest China. This study also investigated the responses of forest’s NPP to different combinations of climatic changes and CO2 concentration increase. Results showed that (1) under the RCP scenarios, greater increases in temperature, precipitation, and CO2 concentration caused larger increments in forest NPP; (2) the effect of CO2 concentration (increased NPP from 19.9% to 21.7%) was more significant than that of climate change (increased NPP from 7.5% to 17.1%); (3) the simultaneous increments in climatic change and atmospheric CO2 concentration led to a remarkable increase in P. crassifolia forest NPP (ranging from 33.1% to 41.3%), with the combination of the two exerting strong interactive effects on forest NPP; and (4) the response of the forest’...

Climate affects Picea crassifolia growth and climate change will lead to changes in the climate–growth relationship (i.e., the “divergence” phenomenon). However, standardization methods can also change the understanding of such a relationship. We tested the stability of this relationship by considering several variables: 1) two periods (1952–1980 and 1981–2009), 2) three elevations (2700, 3000, and 3300 m), and 3) chronologies detrended using cubic splines with two different flexibilities. With increasing elevation, the climatic factor limiting the radial growth of Picea crassifolia shifted from precipitation to temperature. At the elevation of 2700 m, the relationship between radial growth and mean temperature of the previous December changed so that the more flexible spline had a greater precipitation signal. At the elevation of 3000 m, positive correlation of radial growth with mean temperature and precipitation in September of the previous year became more significant. At the elevation of 3300 m, positive correlation between radial growth and precipitation of the current summer and the previous spring and autumn was no longer significant, whereas the positive correlation between radial growth and temperature of the current spring and summer strengthened. The detrending with the most flexible spline enhanced the precipitation signal at 2700 m, while that with the least flexible spline enhanced the temperature signal at 3300 m. All results indicated that the divergence phenomenon was affected by the climatic signals in the chronologies and that it was most dependent on the detrending method. This suggests it is necessary to select a suitable spline bootstrap for studies of growth divergence phenomena.

The Lenglongling Mountains (LLM) located in northeastern part of the Tibet Plateau, belong to a marginal area of the East Asian summer monsoon (EASM) and are sensitive to monsoon dynamics. Two tree-ring width chronologies developed from six sites of Picea crassifolia in the LLM were employed to study the regional drought variability. Correlation and temporal correlation analyses showed that relationships between the two chronologies and self-calibrated Palmer Drought Severity Index (sc_PDSI) were significant and stable across time, demonstrating the strength of sc_PDSI in modeling drought conditions in this region. Based on the relationships, the mean sc_PDSI was reconstructed for the period from 1786 to 2013. Dry conditions prevailed during 1817-1819, 1829-1831, 1928-1931 and 1999-2001. Relatively wet periods were identified for 1792-1795 and 1954-1956. Spatial correlations with other fourteen precipitation/drought reconstructed series in previous studies revealed that in arid regions of Northwest China, long-term variability of moisture conditions was synchronous before the 1950s at a decadal scale (1791-1954). In northwestern margin of the EASM, most of all selected reconstructions had better consistency in low-frequency variation, especially during dry periods, indicating similar regional moisture variations and analogous modes of climate forcing on tree growth in the region.

Knowledge of nutrient traits, especially carbon (C), nitrogen (N), and phosphorus (P), is essential for understanding the biogeochemical cycle and ecosystem functioning. However, spatial patterns in soil and plant tissues stoichiometry in the arid and semiarid forest ecosystem are not yet fully understood. Here, a regional-scale study was conducted on the Qilian Mountains to illustrate the macroecological patterns of soil C:N:P stoichiometry and the plant tissue N and P contents in Qinghai spruce (Picea crassifolia) forest and their driving forces. Soil C, N, and P contents and C:N:P ratios at all four soil depth layers (0–5, 5–10, 10–20, and 20–40 cm) showed depths, latitudinal, and longitudinal trends (p

Precipitation is one of the most important climate factors controlling tree growth, yet it is not fully understood how changes in precipitation affect the relationship between growth and temperature. On the northeastern edge of the Tibetan Plateau, nine tree-ring chronologies of Picea crassifolia were developed along a precipitation gradient from semi-arid (mean annual precipitation, 255 mm) to semi-humid (710 mm). We analyze the growth-climate relationships along this precipitation gradient and assess whether these associations are regulated by local precipitation. From 1960 to 2014, temperature increased significantly while precipitation remained stable at the nine sampling sites. The radial growth of P. crassifolia decreased at the semi-arid sites but increased at the semi-humid sites. Growth-temperature relationships gradually changed from negative to positive along the precipitation gradient (from dry to wet sites), particularly during summer. The moist P. crassifolia sites are also characterized by positive correlations with the Palmer Drought Severity Index. The temporal growth-temperature relationships varied significantly among the different spruce sites over the last five decades. Although temperature remains the main factor controlling the growth of P. crassifolia, local precipitation variability is becoming increasingly important. Our findings indicate that considering species distribution areas supports the analyses of the impact of climate change on tree growth.

Northwestern China has experienced dramatic climate change characterized by rapid warming since the 1980s with the warming trend substantially slowing after 2000. Qinghai spruce (Picea crassifolia Kom.), a key tree species in northwest China, has been predicted to be strongly coupled with climate change. However, how the trends in biomass growth change at different canopy positions under climate change and whether climate–growth responses vary with canopy position remain unclear. A total of 222 trees were sampled by a stand-total sampling strategy in the central Qilian Mountains. Trees were assigned to four canopy positions according to height and distance from neighbors: dominant, codominant, intermediate, and suppressed. Our results indicate that trees in dominant and codominant canopy positions dominate the decreasing trend in stand-level biomass from 1980 to 2000 and the increasing trend from 2000–2013, contributing 81.3 % and 86 %, respectively, whereas trees in the intermediate and suppressed canopy positions contributed less. This result was attributed to a more sensitive response of biomass growth in trees in dominant and codominant canopy positions to climate change. From 1980 to 2000, the stronger decreasing trend in biomass growth at dominant and codominant canopy positions is mostly accounted for by increasing temperature. A more pronounced water deficit might have restricted biomass growth more than that at the intermediate and suppressed canopy positions. However, from 2000 to 2013, drought stress was relieved and summer standardized precipitation evapotranspiration index became a leading factor, which promoted the recovery in biomass at dominant and codominant canopy positions. In a word, compared with intermediate and suppressed canopy trees, those in dominant and codominant positions are less resistant to drought, but dominant and codominant canopy position's biomass can recover more when drought stress is relieved. A more robust understanding of canopy-level growth response and resilience to climate change is crucial to fully understand forest growth dynamics under fluctuating climate conditions.

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 Nino 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.

Water deficiency is the primary limiting factor for tree growth in arid and semi-arid areas. Droughts associated with rising temperatures have increased in severity and frequency globally over the past few decades, making the trees in the drought-prone sites first be affected by water shortages. However, our understanding of tree growth status in these areas, and of their response to drought, is currently insufficient; especially in the context of global warming. Here, we studied 94 Chinese pine (Pinus tabulaeformis) and 86 spruce (Picea crassifolia) trees from different altitudes [2,100–2400 m above sea level (a.s.l.)] distributed at the desert margins of Northwestern China to explore tree growth and drought response from multiple perspectives using dendroecological approaches. Significant growth decline, across all tree species and altitudes, was detected in response to an interdecadal trend towards a drier climate. Moreover, the extent of tree growth decline, the proportion of affected trees, and the degree of moisture dependence have all tended to increase in each sample site, most likely due to enhanced drought severity and duration in recent decades. The more sensitive and susceptible trees were found at lower elevations (drier sites) and may signify a higher vulnerability to heating-induced drought stress. Tree resistance to drought showed strong negative correlation with drought severity across all sample sites. However, the connection between post-drought tree resilience and drought intensity is weak, perhaps because the samples were all collected from living trees, while those that had died were not sampled. The priority for future work should be to combine surviving and dead trees simultaneously, thus achieving a more representative view of tree resilience to drought; this will improve our knowledge of forest dynamics and even ecosystem succession in these vulnerable and sensitive environments.

Understanding the mechanisms for how tree growth responds to drought is important for forest conservation and management in semi-arid regions, particularly under ongoing climate change. However, most previous studies have been relied on linear statistical analysis, unsupported by insights into tree physiological processes. In this study, we applied process-based Vaganov–Shashkin (VS) model to investigate the stem radial growth response and resilience of Qinghai spruce (Picea crassifolia) and Chinese pine (Pinus tabuliformis) to intense drought in semi-arid region of China from 1960 to 2013. Correlation analysis indicated that soil moisture availability in the early growing season (May to July) significantly limit stem growth for both tree species, but Chinese pine was affected more strongly by drought legacies. The model results validated our statistical modeling results, and quantified the effects of temperature and soil moisture on the intra-seasonal variation of the relative stem radial growth rate. The influence of soil moisture on the relative stem radial growth rate during the early growing season was significantly higher for positive pointer years (years with greater than mean growth) than for negative pointer years (years with less than mean growth) for both tree species. Interestingly, the two species displayed similar patterns of growth resilience in response to drought. Based on our successful application of the VS model, our results provide important insights into the two main tree species in the semi-arid Helan Mountains, and offer a new perspective on tree growth processes and their varying responses and resilience to drought.

We reconstructed August–May precipitation from AD 1765 to 2010 for the eastern part of the Hexi Corridor, northwest China, using tree rings of Picea crassifolia. The precipitation reconstruction explains 44.1% of the actual precipitation variance during the common period of 1951–2010. The precipitation reconstruction is representative of precipitation conditions over a large area of the Hexi Corridor. Multi-taper spectral analysis reveals the existence of significant variability with periods of 9.3, 6.7, 3.1, and 2.6 years. Comparison between the precipitation reconstruction of the eastern part of the Hexi Corridor and other nearby precipitation/drought reconstructions shows high coherency in the timing of dry/wet episodes on annual to decadal scale. The divergences existing between the reconstructions may reflect the influence of different geographic features in Gansu and differences in seasonality of the various precipitation/drought reconstructions.

Semi-arid forests at the limit of their existence close to the Gobi Desert in Inner Asia might be vulnerable to warming-induced drought stress. Yet, not much is known about the impact of global-change-type droughts on these forests. Here, we show that warming-related tree mortality is recently taking place in high-elevation semi-arid Qinghai spruce (Picea crassifolia Kom.) forests of the north-eastern margin of the Tibetan Plateau (Qilian Mountains). Tree-ring samples were collected from 24 Qinghai spruce forest plots (20 m × 20 m) at three elevations (2600, 2700, 2800 m) along eight elevation transects on north-facing slopes. Three lines of evidence suggest that these forests are increasingly at risk of increased tree mortality as a consequence of global warming, (i) a strong precipitation and air humidity dependence of radial growth, (ii) increasing frequency of missing tree rings, and (iii) a rising tree mortality rate in recent decades. The recent drought episode on the north-eastern Tibetan Plateau may represent a precursor of future global-change-type drought events in large parts of Inner Asia. Warming-related tree mortality of the semi-arid forests may be interpreted as early-warning signs for the densely populated artificial oases surrounding the Gobi Desert, which largely depend on river run-off from the mountain forests on the edge of the Tibetan Plateau.

The tree species, elevation and climate change jointly influenced the tree-ring growth, directing us to carefully consider the comprehensive effects of these variables when performing large-scale and multi-species-based dendroclimatic studies. The rate of tree growth varies by species and is influenced by elevation and climate gradients. Assessing the influence of these variables on tree-ring growth can improve projections for tree growth under future climate conditions. Here, we evaluate the relationships between tree species, elevation, climate and tree-ring growth in the Qilian Mountains of northwest China by using the tree-ring records of Qinghai spruce (Picea crassifolia) and Qilian juniper (Juniperus przewalskii) trees. The rotated principal component analysis was applied to examine the individual tree growth and the associated influencing factors. The tree-ring chronologies were used to compare tree growth patterns and the climate–growth relationships between the two species and elevations. The results indicated that trees from the Qinghai spruce sites showed similar growth patterns and climate–growth relationships, even though the elevation differed. The Qilian juniper chronologies from the high and low elevation sites revealed different growth patterns during recent decades that were closely related to the discrepant climate–growth relationships with the temperatures and the recent warming trend. The differences in tree growth patterns between both species and elevation could be attributed to plant physiological traits. The results of this study demonstrate the impact of tree species, elevation and climate change on tree-ring growth, suggesting that accounting for the influences of these variables could improve large-scale and multi-species-based dendroclimatic studies.

The vast alpine grasslands of the Tibetan Plateau are lined by a belt of oroboreal coniferous forests in the transition zone to the Central Asian semi-deserts and deserts which may play important roles in the hydrology and biogeochemistry of the region. Many of these forests exist under semi-arid climates, are exposed to rapid climate warming and suffer from intensive human impact. We investigated the carbon stocks in biomass and soil in the Picea crassifolia (Qinghai spruce) forests of the Qilian Mountains, north-eastern Tibetan Plateau. In 18 forest plots at 2600–2800 m elevation, we found ecosystem carbon stocks of 348 Mg C ha−1 with carbon densities of 43 Mg C ha−1 in the live and dead aboveground biomass, 12 Mg C ha−1 in roots, 3 Mg C ha−1 in litter and 305 Mg C ha−1 in the soil (SOC; 0–100 cm). The belowground carbon pools exceed averages reported for northern boreal forests and even more clearly for zonal forest vegetation in the temperate zone. The high SOC density in the P. crassifolia forests are probably the result of slow decomposition rates due to low soil temperatures in combination with low soil moisture. The widespread degradation of mountain spruce forests to shrubland reduces the ecosystem carbon stock by >85 Mg C ha−1, and is partly caused by a 15%-reduction in SOC. We conclude that the remaining mountain forests of spruce and other conifers at the northern fringe of the Tibetan Plateau play an important role in the regional carbon budget and need urgent conservation.

Global climate change is likely to change precipitation patterns with consequences for tree water use and growth in semi-arid areas. However, little is known about the effects of variability in precipitation on growth- and water-related physiological processes of native trees in dry areas of northwestern China. In this study, sap flow and stem radial variability in four Qinghai spruce trees (Picea crassifolia) were monitored in the Qilian Mountains, China. Tree water deficit (ΔW) and basal area increment (BAI) were calculated using stem radial variation; water-use efficiency (WUE) was then estimated as the ratio of BAI and sap flow (Jt). The results showed that sap flow density (Js) increased logarithmically with increasing ΔW when ΔW < 50 μm, and then gradually stabilized. Multiple factor generalized additive models (GAM) showed that Js was closely related to all measured environmental variables except for daily mean temperature and relative air humidity. ΔW was related to the minimum daily temperature and soil water content. WUE exhibited higher values in early July. Low WUE was observed under conditions of prolonged dry weather, but it quickly increased during rainy days. WUE decreased after precipitation events due to high transpiration. We concluded that, in these semi-arid areas, precipitation is the most important controlling factor in tree growth and transpiration.