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10 pags.- 4 Figs. Under Licence CC BY 4.0, Global warming delays the autumn date of foliar senescence (DFS) in recent decades, with positive implications for growing season length and therefore global carbon storage. However, warming-associated drought, leading to water limitation, may conversely stimulate earlier DFS. Using ground observations since 1940s and 34 years of satellite greenness data (1982‒2015) over the Northern Hemisphere (>30° N), we show the increased impact of drought on DFS. Earlier DFS is linked to decreased precipitation under warming and weaker drought resistance associated with various plant functional traits. For example, isohydric plants with strict regulation of water status may drop leaves fast during droughts. We derive an improved set of phenology models based on this influence and project earlier DFS by the end of the century, particularly at high latitudes (>50° N). Our results limit uncertainties in the later end of plant growth with warming, aiding estimation of carbon uptake of terrestrial ecosystems., This work was funded by the Strategic Priority Research Programme of the Chinese Academy of Sciences (XDA19040103), the National Natural Science Foundation of China (42125101) and the CAS Interdisciplinary Innovation Team (JCTD-2020-05). J.P. and P.C. were funded by European Research Council Synergy grant ERC-SyG-2013-610028 IMBALANCE-P. J.P. was also financially supported by the Fundación Ramon Areces grant ELEMENTAL-CLIMATE, the Spanish Government grant 2013-610028 and the Catalan Government grant SGR 2017-1005.

Climatic warming alters the onset, duration and cessation of the vegetative season. While previous studies have shown a tight link between thermal conditions and leaf phenology, less is known about the impacts of phenological changes on tree growth. Here, we assessed the relationships between the start of the thermal growing season and tree growth across the extratropical Northern Hemisphere using 3,451 tree-ring chronologies and daily climatic data for 1948-2014. An earlier start of the thermal growing season promoted growth in regions with high ratios of precipitation to temperature but limited growth in cold-dry regions. Path analyses indicated that an earlier start of the thermal growing season enhanced growth primarily by alleviating thermal limitations on wood formation in boreal forests and by lengthening the period of growth in temperate and Mediterranean forests. Semi-arid and dry subalpine forests, however, did not benefit from an earlier onset of growth and a longer growing season, presumably due to associated water loss and/or more frequent early spring frosts. These emergent patterns of how climatic impacts on wood phenology affect tree growth at regional to hemispheric scales hint at how future phenological changes may affect the carbon sequestration capacity of extratropical forest ecosystems.

Uncertainty still exists on the directions and intensity of changes in leaf herbivory under scenarios of global warming. We, therefore, conducted an investigation on insect herbivory along an elevational gradient to explore how leaf herbivory may respond to future climate warming using a space-for-time substitution approach. We hypothesize that the leaf herbivory for alpine woody species should decline with elevation. We also guess the leaf herbivory may not differ between different leaf-age groups, for the old leaves are less attractive to insect due to their lower nutrients. To approve these assertions, we measured different aspects of leaf herbivory, i.e., the intensity (leaf area consumed per damaged leaf), frequency (percentage of leaves damaged), and rate (percentage of consumed leaf area over the total number of leaves), across different leaf-age groups for Rhododendron aganniphum var. schizopeplum along an elevational gradient (4280–4640 m) in the Sergymla Mountains, southeast Tibet. Related leaf traits of leaf mass per area (LMA) and nitrogen (Nmass), as well as total phenolics for 1-year-old leaves, were also investigated. In contrast with our expectation, the rate of herbivory did not vary with elevation, while the frequency and intensity reflected increasing and declining patterns, respectively. LMA and total phenolics tended to increase with elevation, while Nmass significantly declined. Further analysis indicated that Nmass and total phenolics mainly explained the variation of frequency of herbivory, while Nmass reflected a significant effect on the variation of intensity. No differences in herbivory were found between the leaf-age groups. Our results suggest that the lower intensity of leaf herbivory at higher elevations is mainly associated with the declined nutritional levels, while the higher frequency might be related to the higher costly anti-herbivore defenses like phenolics and the lower nutritional levels. Although the older leaves are exposed to herbivore attacks for a longer time, they possessed the same herbivory levels as current-year leaves partly due to their lower nitrogen concentrations. Both supporting the nutrient limitation hypothesis, i.e., plants with lower leaf nutrient levels possess less leaf herbivory. In all, the herbivory on the alpine Rhododendron is small in magnitude, but given the higher herbivory (for intensity at least) under persistent warming conditions and its potential impacts on mediating plant–insect interactions, insect herbivory should be included in predictions of climate change impacts on alpine ecosystems.

Despite growing interest in predicting plant phenological shifts, advanced spring phenology by global climate change remains debated. Evidence documenting either small or large advancement of spring phenology to rising temperature over the spatio-temporal scales implies a potential existence of a thermal threshold in the responses of forests to global warming. We collected a unique data set of xylem cell-wall-thickening onset dates in 20 coniferous species covering a broad mean annual temperature (MAT) gradient (−3.05 to 22.9°C) across the Northern Hemisphere (latitudes 23°–66° N). Along the MAT gradient, we identified a threshold temperature (using segmented regression) of 4.9 ± 1.1°C, above which the response of xylem phenology to rising temperatures significantly decline. This threshold separates the Northern Hemisphere conifers into cold and warm thermal niches, with MAT and spring forcing being the primary drivers for the onset dates (estimated by linear and Bayesian mixed-effect models), respectively. The identified thermal threshold should be integrated into the Earth-System-Models for a better understanding of spring phenology in response to global warming and an improved prediction of global climate-carbon feedbacks.

To provide a long-term context for understanding the “Karakoram Anomaly” in recent glacier studies, we originally aimed to develop an annually resolved multi-century winter–spring precipitation record using tree-ring earlywood cellulose δ18O (δ18OEW) of Pinus wallachina from the Karakoram, northern Pakistan. Out of expectation, winter (January–May) temperature rather than precipitation is found to be the dominant climate signal (r = 0.63, p

AIM: To better understand how climate change drives altitudinal treeline dynamics at large spatial scales. LOCATION: Northern Hemisphere. TIME PERIOD: 1901–2018. MAJOR TAXA STUDIED: Tree species that constitute alpine treelines. METHODS: We conducted a meta‐analysis of annual treeline shift rates at 143 sites from 38 published studies. For each site, we calculated current change rates in annual or seasonal temperatures, precipitation and drought (standardized precipitation evapotranspiration index). We then evaluated relationships between treeline shift rates, and site characteristics and climate using standard and partial correlation analyses, as well as generalized linear models. These analyses were conducted at the hemispheric scale and for two geographic subsets of subarctic (north of 60° N) and temperate regions (south of 60° N). RESULTS: Treelines ascended at 88.8%, remained stable at 10.5% and descended at 0.7% of the sites. The mean hemispheric shift rate (0.354 m/year) was half of what would be expected from climate warming alone. Treeline shifts were significantly faster in subarctic than temperate regions. The precipitation effect was more important than temperature for predicting treeline shift rate. In the subarctic regions, autumn precipitation mostly determined treeline shift rates. In the temperate region, warmer temperatures and higher autumn precipitation accelerated shift rates, whereas wetter springs reduced them. Autumn precipitation and winter mean minimum temperature best explained the treeline shift rates across the Northern Hemisphere. MAIN CONCLUSIONS: A combination of thermal and hydrological factors drives treeline shift rates across the Northern Hemisphere, with precipitation assuming an important modifying role of the general temperature‐driven treeline ascent. Regional treeline shift rates, therefore, co‐depend on drying and wetting trends, which should be considered in future estimates of global change impacts on alpine ecosystems.

Shrubs growing at higher latitudes and altitudes are considered to be highly sensitive to climate warming. In particular, alpine shrubs are potential climate proxies for understanding responses of ...

Climate change is altering phenology; however, the magnitude of this change varies among taxa. Compared with phenological mismatch between plants and herbivores, synchronization due to climate has been less explored, despite its potential implications for trophic interactions. The earlier budburst induced by defoliation is a phenological strategy for plants against herbivores. Here, we tested whether warming can counteract defoliation-induced mismatch by increasing herbivore-plant phenological synchrony. We compared the larval phenology of spruce budworm and budburst in balsam fir, black spruce, and white spruce saplings subjected to defoliation in a controlled environment at temperatures of 12, 17, and 22°C. Budburst in defoliated saplings occurred 6-24 days earlier than in the controls, thus mismatching needle development from larval feeding. This mismatch decreased to only 3-7 days, however, when temperatures warmed by 5 and 10°C, leading to a resynchronization of the host with spruce budworm larvae. The increasing synchrony under warming counteracts the defoliation-induced mismatch, disrupting trophic interactions and energy flow between forest ecosystem and insect populations. Our results suggest that the predicted warming may improve food quality and provide better growth conditions for larval development, thus promoting longer or more intense insect outbreaks in the future.

Aging is a universal property of multicellular organisms. Although some tree species can live for centuries or millennia, the molecular and metabolic mechanisms underlying their longevity are unclear. To address this, we investigated age-related changes in the vascular cambium from 15- to 667-y-old Ginkgo biloba trees. The ring width decreased sharply during the first 100 to 200 y, with only a slight change after 200 y of age, accompanied by decreasing numbers of cambial cell layers. In contrast, average basal area increment (BAI) continuously increased with aging, showing that the lateral meristem can retain indeterminacy in old trees. The indole-3-acetic acid (IAA) concentration in cambial cells decreased with age, whereas the content of abscisic acid (ABA) increased significantly. In addition, cell division-, cell expansion-, and differentiation-related genes exhibited significantly lower expression in old trees, especially miR166 and HD-ZIP III interaction networks involved in cambial activity. Disease resistance-associated genes retained high expression in old trees, along with genes associated with synthesis of preformed protective secondary metabolites. Comprehensive evaluation of the expression of genes related to autophagy, senescence, and age-related miRNAs, together with analysis of leaf photosynthetic efficiencies and seed germination rates, demonstrated that the old trees are still in a healthy, mature state, and senescence is not manifested at the whole-plant level. Taken together, our results reveal that long-lived trees have evolved compensatory mechanisms to maintain a balance between growth and aging processes. This involves continued cambial divisions, high expression of resistance-associated genes, and continued synthetic capacity of preformed protective secondary metabolites.

Shrub recruitment, a key component of vegetation dynamics beyond forests, is a highly sensitive indicator of climate and environmental change. Warming-induced tipping points in Arctic and alpine treeless ecosystems are, however, little understood. Here, we compare two long-term recruitment datasets of 2,770 shrubs from coastal East Greenland and from the Tibetan Plateau against atmospheric circulation patterns between 1871 and 2010 Common Era. Increasing rates of shrub recruitment since 1871 reached critical tipping points in the 1930s and 1960s on the Tibetan Plateau and in East Greenland, respectively. A recent decline in shrub recruitment in both datasets was likely related to warmer and drier climates, with a stronger May to July El Niño Southern Oscillation over the Tibetan Plateau and a stronger June to July Atlantic Multidecadal Oscillation over Greenland. Exceeding the thermal optimum of shrub recruitment, the recent warming trend may cause soil moisture deficit. Our findings suggest that changes in atmospheric circulation explain regional climate dynamics and associated response patterns in Arctic and alpine shrub communities, knowledge that should be considered to protect vulnerable high-elevation and high-latitude ecosystems from the cascading effects of anthropogenic warming.

Aim: Initiation of autumnal leaf senescence is critical for plant overwintering and ecosystem dynamics. Previous studies focused solely on the advanced stages of autumnal leaf senescence and claimed that climatic warming delays senescence, despite the fundamental differences among the stages of senescence. However, the timing of onset of leaf coloration (DLCO), the earliest visual sign of senescence, has been rarely studied. Our aim is to assess the response of DLCO to temperature across the Northern Hemisphere.Location: 30–75°N in the Northern Hemisphere.Time period: 2000–2018.Major taxa studied: Deciduous vegetation.Methods: We retrieved DLCO from high temporal-resolution satellite data, which was then validated by PhenoCam observations. We then investigated the temporal changes in DLCO and the relationship between DLCO and temperature by using DLCO from satellite data and ground phenological observations. We further estimated the timing of onset of autumnal decline in maximum canopy photosynthetic capacity, an indicator of leaf senescence earlier than DLCO, from FLUXNET CO2 fluxes data, and assessed its response to temperature.Results: DLCO was not significantly (P < 0.05) delayed between 2000 and 2018 in 94% of the area. DLCO was positively correlated with pre-DLCO mean daily minimum temperature (Tmin) in only 9% of the area, likely because of the overriding photoperiodic control, particularly for woody vegetation, whereas the end of leaf coloration (DLCE) was positively correlated with pre-DLCE mean Tmin over a larger area (34%).Main conclusions: The coloration progress rate is more sensitive than its start date to temperature, indicating an extension of the duration of leaf senescence under warming. This study indicates that the leaf coloration onset was not responsive to climate warming and provides observational evidence of photoperiod control of autumnal leaf senescence at biome and continental scales.

Understanding synergies and trade-offs of ecosystem services (ESs), finding influencing factors related to these ESs, and determining how these factors regulate spatiotemporal patterns are vital to the sustainability of ecosystem management. However, the spatialization of the correlation between the relationships among ESs and the influencing factors has remained a difficult analysis for the Qinghai-Tibet Plateau (QTP). Consequently, three ESs are quantified using different models, and synergies and trade-offs among the ESs are analyzed. The precipitation changes, minimum temperature changes, gross domestic product changes, population changes, normalized difference vegetation index (NDVI) changes, and the digital model (DEM) were selected as influencing factors. The influence of these influencing factors on the relationship among the ESs was then measured using the methods of logistic regression (LR) and the geographically weighted logistic regression (GWLR). The authors showed that (1) the spatially concordant high supplies of water yield (WY) and carbon storage (CS) were distributed in the southeastern forest areas and the eastern grassland areas, whereas high meat supply (MS) was primarily distributed in the eastern and the middle of the grassland areas. (2) The relationships between the WY and MS and between the CS and MS were primarily trade-offs, and their area ratios were 40% and 41%, respectively. However, the relationship between the WY and CS was primarily synergistic, and the area ratio was 54%. (3) Precipitation was more highly related to the relationships among the ESs than the other influencing factors among the six independent variables. Compared to the LR, the GWLR model provides an improved calculation to explain the correlation between the relationships among the ESs and these influencing factors. Overall, it was found that these influencing factors on the impacts of the relationship among the ESs were different, and they lacked spatial concordance. These results highlight the importance of managing nearly all areas by specifically controlling local factors.

Warming-induced drought stress leads to convergent and negative growth responses to temperature between sympatric tree species, implying an increasing interspecific competition for soil moisture. In mixed forests, sympatric tree species avoid competition by partitioning their niches according to available environment resources. We raise the hypothesis that climate warming leads to a convergence in growth responses to climate, thus increasing the competition among sympatric species in drought-prone forests. In this study, we selected a mixed forest located at ca. 3600 m a.s.l in the Baima Snow Mountains, an inner dry valley of the southeastern Tibetan Plateau. We measured width of the tree rings produced during 1910–2016 in 60 trees belonging to three sympatric species: Abies georgei, Picea likiangensis, and Betula delavayi. We analyzed the changes in radial growth and their responses to climate. We detected shifts in the responses to climate after the 1990s. The radial growth of all species was positively correlated with precipitation from 1964 to 1990, but negatively correlated with March–June temperature from 1991 to 2016. Compared to the period 1964–1990, convergent and negative growth responses to warmer temperatures in the period 1991–2016 probably reflect less available soil moisture for growing in this mixed forest. We conclude that climate warming will affect the niches of sympatric species in mixed forests subjected to seasonal drought, thus increasing competition and altering structure and composition of the stands in dry regions.

The resumption of stem growth varies across the ontogenetic development of trees. Compared with younger trees, older ones have thicker outer bark with a temperature-insulating effect that could potentially prevent the stem from warming in the spring. However, the question of whether xylogenesis in old trees is influenced by the thick bark still remains unresolved.We investigated the onset of xylogenesis across the ontogenetic development of Smith fir (Abies georgei var. smithii) trees in the Sygera Mountains, southeastern Tibetan Plateau. The outer bark of older trees was also removed. Xylogenesis was monitored in microcores we collected every 3 days during May and June in 2017.Xylogenesis began in late May in young (50 yr) and mature (50-100 yr) trees, 1 week earlier than in adult (100-150 yr) and old (150-200 yr) trees. Older (200 yr) trees had the latest onset of xylogenesis, 2 weeks after young trees. The resumption of xylogenesis was similar between the control and bark-removed trees.Growth resumption was delayed in older and bigger trees. Outer bark did not affect the onset of xylogenesis, which indicated that the delayed resumption of growth during the lifespan of trees could be more related to endogenous factors than to an insulating effect of the thick bark of older individuals.

Forest ecosystems provide environmental services that benefit human communities. Despite their importance, forests have experienced deforestation and land-use changes that are particularly severe in the tropics and subtropics. Increasing concern about forest sustainability management and conservation supports the need for studies on forest recovery after deforestation. Many studies have estimated that recovery of forest structure, species composition and richness takes more than 50 years to reach a state similar to that of adjacent undisturbed forests. To date, most of these studies were carried out in the tropics and at high latitudes. However, less is known about the recovery process of sub-alpine forest after deforestation. The natural sub-alpine forest on the southeastern Tibetan Plateau is an important ecosystem, which ranges from 3000 to 4400 m above the sea level (a.s.l.), and these forests were disturbed by human before 1998. In the forests near the town of Lulang, Smith fir ( Abies georgei var. smithii ) is the dominant tree species above 3500 m a.s. l. The climate is mainly influenced by the Indian monsoon and westerlies. In summer, the Indian monsoon brings abundant precipitation to our study area. The winter climate is controlled by the westerlies producing cold and dry conditions. According to the instrumental record from the Nyingchi meteorological station, mean annual temperature and total annual precipitation are 8.9°C and 672.7 mm, respectively. This area has experienced a significant warming trend during the past five decades. Our study focused on logged and burnt sites of Smith fir forests (3460–3865 m a.s. l.) near Lulang in Nyingchi. Three and five rectangular plots (30 m ´ 30 m) were established in the logged (azimuth angle: 326°–328°) and fire-affected (azimuth angle: 243°–260°) gaps, respectively. Global Positioning System (GPS) was used to record the locations of the 8 plots. For each plot, the upper and lower sides were perpendicular to the direction of slope aspect. The lower-left side of each plot was regarded as the origin for each sampling grid. To assess the regeneration in each plot, we tagged all of the trees and measured their position coordinates ( x and y ). The tree species were also recorded. The ages of the gaps were estimated by taking increment cores from nearby adult trees surrounding the gap and examining the synchronous and abrupt growth release events. These cores were then air dried, carefully sanded, and crossdated in the lab. The ring-width series were measured under the Lintab 6 system (Rinntech, Heidelberg, Germany). We used several methods to estimate the age of each tree. For the seedlings and saplings, we estimated their age by counting the number of bud scars along the main stem. For the adult trees, we used the increment borer to get a core (close to the pith) at the base of the trunk. The ages of these samples were obtained by counting the number of rings under the stereomicroscope. The date of the burnt gap formation was estimated as 1942 and the logging gap formed in 1991, both based on abrupt growth release of nearby adult trees surrounding the gaps after fire and logging. According to the surveyed data and tree age, we obtained the decadal recruitment series for each tree species. In the burnt gaps (southwestern facing slope), we found that the regenerated tree species mainly consist of spruce ( Picea likiangensis var . linzhiensis ), juniper ( Sabina squamata ) and oak ( Quercus aquifolides ). In the logged site (northwestern facing slope), the regenerated tree species is mainly Smith fir. Compared with the natural Smith fir forest, the tree density is lower in the burnt and logged sites. In summary, once the forest on the southeastern Tibetan Plateau is logged or burnt, it will take longer time to recover to undisturbed Smith fir forest.

The Tibetan Plateau, also known as the “third pole of the Earth”, houses a diverse array of alpine-ecosystem types and serves as a critical ecological security shield for China and even for many other regions of Asia. In recent decades, the rapid climate change in the Tibetan Plateau has led to profound changes in the structure and functioning of its ecosystem. Such changes in the ecosystem of the Tibetan Plateau can not only profoundly impact the environment of the high plateau itself but also extend significant influence over that of surrounding areas. With the continuous growth of data obtained via long-term in situ monitoring, manipulative experiments, satellite remote sensing, and model simulations, scientists have recently made significant advances in the research on the responses and feedback of the Tibetan Plateau’s alpine ecosystem to climate change. Aiming to identify knowledge gaps and to stimulate future research, we provide a comprehensive review of past efforts to understand how climate change has impacted the Tibetan Plateau’s alpine ecosystem, which in turn also provides feedback to the climate. In particular, we focus on the impacts of climate change on the structure and functioning of the ecosystem, including vegetation phenology, treeline position, species biodiversity, ecosystem productivity, and carbon sink, along with feedback involving vegetation changes to the regional climate and hydrology through local and teleconnected biophysical loops. A number of key findings emerge based on cumulative knowledge from old as wells as recent researches. (1) Climate warming during the past several decades has significantly advanced spring vegetation phenology in the Tibetan Plateau. (2) Further, warming has significantly shifted the treeline upward with varying amplitudes that may have been regulated by other factors such as precipitation and interspecific interactions. (3) The plant-community structure of the Tibetan Plateau’s alpine steppe ecosystem is sensitive to climate change, with climate warming considerably reducing its biodiversity and species abundance. However, for the alpine-meadow ecosystem, the impact of climate warming on the diversity and species abundance is still inconclusive. (4) Furthermore, warming has significantly increased vegetation productivity, which can consequently lead to an enhanced carbon sink. Such warming-induced carbon accumulation by vegetation is higher in alpine meadows than in alpine steppes. However, the effect of climate change on soil carbon stock remains highly uncertain mainly because of the high spatial heterogeneity of soil properties and lack of information regarding deep-layer soil processes. (5) Warming-induced vegetation greening of the Tibetan Plateau provides an overall cooling effect countering the local warming and modulates the local and far-reaching precipitation patterns through teleconnected feedback to the East Asian monsoon. In particular, the modeling results suggest that this greening trend of the Tibetan Plateau increases precipitation in South China but reduces precipitation in the region between the Yellow and Yangtze Rivers. Even with the recent significant progress in the study on the ecosystem-climate interaction in the Tibetan Plateau, many knowledge gaps remain. These gaps provide opportunities for future research, which needs to expand and optimize long-term ecological observation networks to improve the understanding of key ecological processes and deepen the comprehension of the response and acclimation mechanisms of the alpine ecosystem under the influence of climate warming. The knowledge enhancement thus obtained will provide important guidelines for improving ecosystem management and safeguarding the ecological security in the Tibetan Plateau.

Mass movements in China typically encompass landslides, rockfalls, and debris flows, and these processes affect almost 70% of all mountain areas of China. An improved understanding of geomorphic process dynamics and their drivers and process histories are important prerequisites for a proper assessment and a sustainable management of mountain hazards. Tree rings have on varied occasions proven to be a reliable tool for the acquisition of data on past mountain hazards. Here we present the first reconstruction of past landslide activity on a slope in the Qilian Mountains, northeast Tibetan Plateau, China. To this end, a total of 543 Qilian juniper (Sabina przewalskii Kom.) samples were selected from 239 trees growing on or next to a complex landslide body, with the aim to reconstruct past landslide histories at the site. The dataset of disturbed trees was complemented with 174 increment cores from 84 reference Qilian juniper trees growing on three adjacent, yet undisturbed sites. Based on the dating of the samples and the interpretation of growth disturbances (GD) in the tree-ring series of affected trees, we provide the first reconstruction of past landslides as retrieved from a dendrogeomorphic record covering more than a millennium (956–2011 CE). Results show that the first detectable GD occurred as early as in 1042 CE, followed by seven major landslide reccurrences in 1703, 1816, 1848, 1863, 1913, 1970, and 1982. Comparison between our landslide record and a dendroclimatic precipitation reconstruction points to the crucial role of major, longer-term wet precipitation anomalies on landslide occurrence.

Large tropical volcanic eruptions can cause short-term global cooling. However, little is known whether large tropical volcanic eruptions, like the one in Tambora/Indonesia in 1815, cause regional hydroclimatic anomalies. Using a tree-ring network of precisely dated Himalayan birch in the central Himalayas, we reconstructed variations in the regional pre-monsoon precipitation back to 1650 CE. A superposed epoch analysis indicates that the pre-monsoon regional droughts are associated with large tropical volcanic eruptions, appearing to have a strong influence on hydroclimatic conditions in the central Himalayas. In fact, the most severe drought since 1650 CE occurred after the Tambora eruption. These results suggest that dry conditions prior to monsoon in the central Himalayas were associated with explosive tropical volcanism. Prolonged La Nina events also correspond with persistent pre-monsoon droughts in the central Himalayas. Our results provide evidence that large tropical volcanic eruptions most likely induced severe droughts prior to monsoon in the central Himalayas.

Most Tibetan people make their living through pastoralism, with livestock grazing being widespread throughout the Tibetan Plateau. The Tibetan Plateau is also home to the northern hemisphere’s highest treelines but the effects of human activities, in particular pastoralism, on these treelines and their influence on the treeline responses to climate change remain unclear. To disentangle the effects of pastoral disturbances and climate on treeline dynamics, we surveyed three plots at grazing-disturbed Balfour spruce (Picea likiangensis var. balfouriana) treelines in the Yushu region (eastern Tibetan Plateau) and compared this data with published results of five undisturbed spruce treeline sites. Spatiotemporal changes in the position and structure of treelines during the past 400 years were reconstructed using dendrochronology. Tree recruitment was significantly linked to long-term summer and winter temperature. Under low local population densities and grazing pressure but consistent climate warming, disturbed treelines shifted upwards by 23.9–82.6 m during the 1611–1910 period. However, under higher local population densities, increased grazing pressure and climate warming over the last century, disturbed treelines showed no significant upward shift. In contrast, undisturbed treelines shifted upward by 25–68.5 m over the last century, thus implying that pastoralism was suppressing treeline expansion in disturbed sites. Together, these results underscore the importance of anthropogenic disturbances in influencing the structure and position of alpine treelines on the Tibetan Plateau. The role played by human disturbance should be carefully considered when evaluating the potential for treeline advance under future climate warming.

The potential of shrub-ring oxygen isotopes (SRO) as a paleoclimatic proxy is not well demonstrated. In this study we explored the climate signals embedded in an SRO chronology from Saga, on the northern slope of the central Himalayas by correlation analysis with climate variables and comparison with tree-ring and/or ice-core δ18O records. The Saga SRO had significant and positive correlations with May–July temperatures in both local (r = 0.43) and moisture source areas (r = 0.49) during the period 1955–2015 (CE), while no significant correlations were found with hydroclimatic variables. It is also in good agreement with two tree-ring δ18O series from the southern slope of the central Himalayas, the source of moisture to the site. The temperature signals are further verified by an ice core δ18O record from the Tanggula Mountains on the central Tibetan Plateau. However, it shows little similarity with the nearest Himalayan ice core δ18O record from Dasuopu, demonstrating temperature signals of different seasons. This study suggests that SRO could be used as an acceptable climate proxy, like tree-ring and ice-core δ18O, to broaden the spatial extent of the paleoclimatic records network. However, we should be cautious when combining different proxies with climate signals of different seasons in paleoclimate studies.

A number of studies have suggested that growing season length determines carbon sequestration of forest ecosystems. Given the possibility that drought-induced growth decline will be caused by a prolonged growing season under a warming climate, we investigated the effect of growth rate and duration of xylem production on annual wood biomass in drought-prone environments. We analyzed the intra-annual dynamics of wood formation in Qilian junipers (Juniperus przewalskii) from the semi-arid north-eastern Tibetan Plateau, China (2009–2014) and in ponderosa pine (Pinus ponderosa) from the hyperarid Mojave Desert in Nevada, USA (2015–2016). Most variability in the number of xylem cells (Ncell) was related to growth rate (rm) rather than duration of cell production (Dcell). At the Tibetan sites, 69.9% and 54.7% of variability in Ncell was attributable to rm for the lower and upper treeline, respectively. Within the Mojave Desert site, 53.9% of the variability in Ncell was related to rm. The growth rate in the Tibetan Plateau forest is affected by minimum temperature and precipitation. Thus, rm is a primary control on wood biomass in conifer species of semi-arid forests. Under warmer and drier conditions, a longer growing season will not benefit xylem formation of conifers, and in turn warming-induced drought could limit carbon sequestration by reducing the rate of cell production.

Knowledge of past climatic variability for the Tibetan Plateau (TP), especially the northwestern part of it, is still limited due to the lack of long-term meteorological records. Within this study, we developed a summer (June–July) minimum temperature reconstruction since 1681 based on annual tree-ring width of Schrenk’s spruce (Picea schrenkiana Fisch. et Mey.) from a humid timberline on the western Kunlun Mountains. Our reconstruction passed the statistical calibration and validation tests, and accounted for 37.69% of the variance for the instrumental record during the period 1960–1996. The reconstruction mainly exhibited annual to decadal temperature variability, with cold periods in 1694–1696, 1745–1749, 1823–1824, and 1915–1921, and warm periods in 1755–1760, 1795–1799, and 1894–1896. Spatial correlations and comparisons with the Asia summer temperature reconstructions further verified our temperature reconstruction and its capability to serve as a proxy for large-scale temperature variability on the northwestern TP. Our results clearly indicate the great potential to perform dendrochronological studies on the western Kunlun Mountains.

The treeline ecotone refers to the transition from the timberline to treeless alpine vegetation. As either the upper-elevation or northern-most limit of erect trees, treeline will be sensitive to environmental change. An increasing number studies have focused on influence of climatic change on treeline dynamics. Treeline shift rate is also considered to be a fingerprint of influence of global change on terrestrial ecosystems. However, less is known about how disturbance, as an important non-climatic factor, mediates the response of treelines to climate change. Such studies can provide useful information to guide implementation of key ecological protection and rehabilitation measures in these comparatively cold biomes. Herein, we review research progress relating to impacts of different disturbances on ecological processes on the treeline ecotone. Based on the Web of Science and CNKI database, only a modest 10.8% of 1002 treeline-related publications from 1950 to 2018 focused on disturbance. Disturbances include natural and human-caused categories according to their causes. Natural disturbances mainly include volcanic eruptions, glacier fluctuations, wildfire, snow avalanche, biological activities, extreme low temperature, and strong wind. Human-caused disturbances mainly include deforestation, grazing, human-caused fire, mining, farming, medicinal plant collecting and tourism. In most cases, due to differences in intensity, frequency and time duration, different disturbances could have different effects on the ecological processes of the treeline ecotone. Under some circumstances, the impacts of natural and human-caused disturbances on treelines may not differ significantly. For example, natural wildfires and human-caused fires, as well as wildlife activities and livestock grazing, may contribute similar forcing in driving treeline dynamics. In disturbed areas, based on soil habitat regimes, treeline changes would follow two main stages, i.e., primary succession or secondary succession processes driven by multiple environmental factors and climate-driven stand changes. Observational data for the common disturbance influences (e.g. fire and grazing) were rare in recent studies, failing to distinguish relative effects of disturbances and climate change on ecological processes in the treeline ecotone. Additionally, attention should be paid to both the negative and positive effects of disturbances at the treeline ecotone. In particular, more studies are necessary to better understand positive effects of disturbances driving treeline shifts and ecological strategies of trees at disturbed treelines. To adapt to frequent disturbances, treeline trees might counteract disturbance through bark and cone characteristics, by clumped distributions, or by clonal reproduction (e.g. sprout-regeneration). Ecological strategies of trees under different disturbance regimes provide a new perspective for explaining non-linear responses of treeline shift to climate change. Humans cannot control most natural disturbances (e.g. volcanic eruption, glacier fluctuations, wildfire and snow avalanche), whereas we can control human-caused disturbances (e.g. grazing, deforestation and tourism). In this context, human-caused disturbance is a key issue for forest management policies. Further work should consider incorporating disturbance factors into treeline modelling. In most forest regions, monitoring data for common disturbance factors are very scarce. In order to have a better understanding of treeline ecological processes, it is necessary to model changes of disturbed treelines in response to disturbance regimes. Based on multiple approaches, researchers should focus on key ecological processes and perform long-term tracking surveys at the disturbed treelines. It should be noted that the Tibetan Plateau not only hosts the Northern Hemisphere’s highest natural treelines but also diverse disturbed treelines. The Tibetan Plateau could be an ideal location to investigate different contributions of disturbance and climatic change in driving treeline dynamics. In particular, it would be interesting to investigate how fire or grazing shapes treeline structure, and how interactions between disturbances and climatic change drive treeline shift on the Tibetan Plateau.

Climatic change is exhibiting significant effects on the ecosystem of the Tibetan Plateau (TP), a climate-sensitive area. In particularly, winter frost, freezing events and snow avalanche frequently causing severe effects on ecosystem and social economy, however, few long-term winter temperature records or reconstructions hinder a better understanding on variations in winter temperature in the vast area of the TP. In this paper, we present a minimum winter (November–February) temperature reconstruction for the past 668 years based on a tree-ring network (12 new tree-ring chronologies) on the southeastern TP. The reconstruction exhibits decadal to inter-decadal temperature variability, with cold periods occurring in 1423–1508, 1592–1651, 1729–1768, 1798–1847, 1892–1927, and 1958–1981, and warm periods in 1340–1422, 1509–1570, 1652–1728, 1769–1797, 1848–1891, 1928–1957, and 1982–2007. As suggested by the comparisons with existing winter temperature series and spatial correlations with Climatic Research Unit gridded data, our reconstruction is reliable and indicative, and it can represent large-scale winter temperature variability on the southeastern TP. Furthermore, it shows an overall agreement with winter temperature from the northeastern TP on decadal to inter-decadal timescales. It also shows the possible effects of volcanic eruption and reducing solar activity on the winter temperature variability for the past six centuries on the southeastern TP.

The Tibetan Plateau and its surrounding mountains exhibit the largest concentration of glaciers at lower latitudes on earth, providing source water for the large river systems in Asia. However, glacier observations in this region are short and scarce and hence limit a better understanding of glacier behaviors under a warming climate and thereto coupled processes. The purpose of this research is to investigate the potential and the uncertainty for using pioneering trees on glacial deposits/glacier forefields as a proxy record for past glacier fluctuations in the Himalaya-Nyanqentanggula-Hengduan (H2N) area. It was found that Hippophae tibetana spent 4 years to colonize the glacier forefields at Hailuogou glacier and Pinus wallichiana germinated 11 years after glacier retreating at Gangapuna and Raikot glaciers. At deposit sites of glacier avalanche and glacial lake outburst, it took 8 years for Larix griffithii to germinate after the historical events. As shown by bootstrap resampling, the precision of the sampled maximum tree age could reach 2–14 years, which is related to the sample size to ensure the possibility of obtaining the eldest trees. However, if using the traditional sampling method at 100-cm height, errors of tree age estimation may reach up to ±20 years due to uncertainties of pith offset estimation and height-age calibration. If samples are collected with pith from ≤20 cm tree height, an error of

Oxygen isotope ratios ( δ 18 O) are the most commonly used parameters recorded in paleoclimate archives since they link different natural archives via the water cycle. Tree-ring δ 18 O ( δ 18 OTR) has been widely used for hydroclimate reconstructions in the Himalaya. However, few of them record temperature signals, which are dominant in Himalaya ice-core δ 18 O. We hypothesize that the “precipitation amount effect” due to the South Asian Summer Monsoon (SASM) may overprint temperature signals in δ 18 OTR series. The purpose of this study is to investigate whether temperature signals could be found in the δ 18 OTR in locations where the influence of SASM is weak. We developed a 105-yr δ 18 OTR chronology from the northern slope of the western Himalaya which greatly blocks the SASM. Our δ 18 OTR clearly shows stronger correlations with temperature (dominant winter and weak summer) than summer precipitation signals. It also agrees well with summer soil moisture δ 18 O simulated by the global isotope model LMDZ4 (r = 0.72, 1979–2010). In LMDZ4, low winter temperature was found to increase winter snowfall and subsequent snow melt, and thus to increase the contribution of winter snowfall to soil moisture in summer at the expense of summer precipitation. Since winter snowfall is more depleted than summer precipitation, this leads to lower summer soil moisture δ 18 O. The temperature signals found in our δ 18 OTR series are consistent with those found in the Dasuopu ice-core δ 18 O. This implies that δ 18 OTR series from the southwest Tibetan Plateau (TP), with a weak monsoon, hold great potential to capture temperature signals. Climate interpretations of δ 18 O proxies in the Himalaya largely depend on the influence of seasonal water from the dominant atmosphere circulation systems of the westerlies or monsoon. The δ 18 O proxies from the monsoon-affected region have a higher potential for the reconstruction of boreal summer hydroclimate, whereas δ 18 O proxies from westerly-affected sites have a higher potential for temperature reconstructions.