Abies spectabilis shows stable growth relations to temperature,but changing response to moisture conditions along an elevation gradient in the central Himalaya

A better understanding of growth-climate responses of high-elevation tree species across their distribution range is essential to devise an appropriate forest management and conservation strategies against adverse impacts of climate change. The present study evaluates how radial growth of Himalayan fir (Abies spectabilis D. Don) and its relation to climate varies with elevation in the Manaslu Mountain range in the central Himalaya. We developed tree-ring width chronologies of Himalayan fir from three elevational belts at the species’upper distribution limit (3750−3900 m), in the middle range (3500−3600 m), and at the lower distribution limit (3200−3300 m), and analyzed their associations with climatic factors. Tree growth of Himalayan fir varied synchronously across elevational belts, with recent growth increases observed at all elevations. Across the elevation gradient, radial growth correlated positively (negatively) with temperature (precipitation and standardized precipitation-evapotranspiration index, SPEI-03) during the summer (July to September) season. However, the importance of summer (July to September) temperatures on radial growth decreased with elevation, whereas correlations with winter (previous November to current January) temperatures increased. Correlations with spring precipitation and SPEI-03 changed from positive to negative from low to high elevations. Moving correlation analysis revealed a persistent response of tree growth to May and August temperatures. However, growth response to spring moisture availability has strongly increased in recent decades, indicating that intensified spring drought may reduce growth rates of Himalayan fir at lower elevations. Under sufficient moisture conditions, increasing summer temperature might be beneficial for fir trees growing at all elevations, while trees growing at the upper treeline will take additional benefit from winter warming.     

The growth-ring variations of alpine shrub Rhododendron przewalskii reflect regional climate signals in the alpine environment of Miyaluo Town in Western Sichuan Province,China

Rhododendron przewalskii is an important dwarf shrub species in alpine environments of western Sichuan that offers a unique opportunity to expand current dendrochronological networks into extreme environments beyond the survival limit of trees. Our objectives in this study are to evaluate the dendroclimatological potentials of R. przewalskii and determine the major limiting climate factor for the species’ growth rings. We sampled 25 cross-sections of R. przewalskii at an elevation of 4050 m, about 150 m above treeline, on Zhegu Mountain of Miyaluo in western Sichuan of southeastern Tibetan Plateau. R. przewalskii has well-defined growth rings such that most stem sections could be cross-dated. The resulting 61-year long standard chronology (A.D. 1949–2009) was derived from 38 series from 19 cross-sections. Response analysis revealed that radial growth of the Zhegu Mountain R. przewalskii is significantly and negatively correlated to late winter temperature (January–February), mainly driven by maximum temperatures. This correlation indicates that colder daytime temperatures during late-winter lead to improved growth the following growing season. Minimum winter temperatures do not appear important for radial growth of this population. R. przewalskii ring widths are also strongly correlated to late-winter maximum temperatures over the southeastern Tibetan Plateau region. April maximum and July minimum temperatures are positively and significantly correlated to radial growth suggesting that warm April days and warm July nights promote current-season radial growth. In contrast, radial growth is only negatively correlated to April precipitation, indicating that wet soil conditions inhibit total radial increment. The main differences of R. przewalskii compared to tree species living at high-altitude nearby regions is that R. przewalskii has a weaker positive growth response to summer temperature and winter minimum temperature, but a much stronger negative response to winter temperature. Due to the strong climatic signal recorded in the growth curves of R. przewalskii, this dwarf shrub should be useful for climate–growth studies in alpine regions where no forests are present.     

Annual increments of juniper dwarf shrubs above the tree line on the central Tibetan Plateau: a useful climatic proxy

Background and Aims

Dendroclimatology is playing an important role in understanding past climatic changes on the Tibetan Plateau. Forests, however, are mainly confined to the eastern Tibetan Plateau. On the central Tibetan Plateau, in contrast, shrubs and dwarf shrubs need to be studied instead of trees as a source of climate information. The objectives of this study were to check the dendrochronological potential of the dwarf shrub Wilson juniper (Juniperus pingii var. wilsonii) growing from 4740 to 4780 m a.s.l. and to identify the climatic factors controlling its radial growth.

Methods

Forty-three discs from 33 stems of Wilson juniper were sampled near the north-eastern shore of the Nam Co (Heavenly Lake). Cross-dating was performed along two directions of each stem, avoiding the compression-wood side as far as possible. A ring-width chronology was developed after a negative exponential function or a straight line of any slope had been fit to the raw measurements. Then, correlations were calculated between the standard ring-width chronology and monthly climate data recorded by a weather station around 100 km away.

Key Results

Our study has shown high dendrochronological potential of Wilson juniper, based on its longevity (one individual was 324 years old), well-defined growth rings, reliable cross-dating between individuals and distinct climatic signals reflected by the ring-width variability. Unlike dwarf shrubs in the circum-arctic tundra ecosystem which positively responded to above-average temperature in the growing season, moisture turned out to be growth limiting for Wilson juniper, particularly the loss of moisture caused by high maximum temperatures in May–June.

Conclusions

Because of the wide distribution of shrub and dwarf shrub species on the central Tibetan Plateau, an exciting prospect was opened up to extend the presently existing tree-ring networks far up into one of the largest tundra regions of the world.     

灌木年轮学研究进展

灌木往往分布在树线以上或以北的高海拔和高纬度地区以及干旱、半干旱区,是把传统上以乔木为主的树轮研究扩展至森林分布界限以外的唯一选择.尽管灌木具有以上研究潜力,迄今用于树木年代学研究的灌木种类仅有30种左右.介绍了灌木年轮研究方法,综述了过去几十年来环北极高纬度地区,干旱、半干旱区以及高海拔地区的灌木年轮研究的主要进展.主要研究进展如下:(1)发掘一些灌木的树木年代学潜力;(2)揭示限制灌木生长的主要环境因子,并尝试利用灌木年轮宽度等指标重建过去区域气候变化历史;(3)探讨全球变暖的背景下,灌木的生长或分布范围的变化;(4)通过人为控制增温来揭示变暖对灌木生理特征和生长的影响.这些研究展示了灌木在扩展传统乔木树轮研究网络方面的潜力,也是树木年代学研究中最有前景的研究方向之一.目前的灌木年轮学研究多集中于环北极苔原带.作为地球的第三极,青藏高原具有广泛的高山灌木分布,具有把青藏高原边缘区以乔木为主的树木年轮网络扩展至更高海拔和高原内部的潜力.青藏高原高山灌木的年轮学研究并没有引起足够的重视.青藏高原高山灌木的生长是如何适应极端环境条件的,全球变暖的背景下,青藏高原高山灌木的分布和生长正在发生哪些变化等,都有待深入研究.     

Assessment of tree-ring analysis of high-elevation Cedrus deodara D. Don from Western Himalaya (India) in relation to climate and glacier fluctuations

A 458-year-long regional tree-ring-width index chronology of Himalayan cedar (Cedrus deodara D. Don) prepared from three high-elevation sites of Western Himalaya has been presented. Dendroclimatological investigation indicates significant positive relationship of tree-ring index series with winter (December–February) temperature and summer precipitation and inverse relationship with summer temperature. Higher growth in the recent few decades detected in the tree-ring chronology has been noticed coinciding with the rapid retreat of the Himalayan glaciers. Suppressed and released growth patterns in tree-ring chronology have also been observed to be well related to the past glacial fluctuation records of the region. The higher tree growth in recent decades may be partially attributed to the warming trend over the region, particularly increasing the winter warmth, and thus to the regional manifestations of global warming.     

Some Biological Problems comected with the Himalaya

(1) The Himalaya underwent a period of uplift from sea-level commencing in Eocene times and reached their present approximate elevation in Pliocene times. The Quaternary Glacial Period caused intense cold on the Himalayas and Tibetan Plateau, but precipitation was insufficient to form an extensive ice-cap.
(2) Modern distribution in the Himalaya and Indian Peninsula were not much influenced by the Quaternary Glacial Period.
(3) Discontinuous distribution is evidence of previous continuous distribution, intervening extermination being brought about by some form of competition.
(4) The afforested area of the Himalaya from Kashmir to Bhutan, and up to timber-line, is a sub-region of the Oriental Region and not of the Palæarctic Region. It was colonised almost entirely from the east or the hills of western China.
(5) The Tibetan Plateau is definitely a sub-region of the Palmirctic Region, and was probably colonised from the north.
(6) The Kashmir sub-region is probably more correctly placed in the Palæarctic than the Oriental Region, and appears to have been colonised from the north or north-west.
(7) A Transition Zone above timber-line forms a true contact zone between the Palæarctic and Oriental Regions along the Himalayan Divide from Kashmir to Bhutan and beyond.     

Phylogeny of spiny frogs Nanorana (Anura: Dicroglossidae) supports a Tibetan origin of a Himalayan species group

Recent advances in the understanding of the evolution of the Asian continent challenge the long‐held belief of a faunal immigration into the Himalaya. Spiny frogs of the genus Nanorana are a characteristic faunal group of the Himalaya–Tibet orogen (HTO). We examine the phylogeny of these frogs to explore alternative biogeographic scenarios for their origin in the Greater Himalaya, namely, immigration, South Tibetan origin, strict vicariance. We sequenced 150 Nanorana samples from 62 localities for three mitochondrial (1,524 bp) and three nuclear markers (2,043 bp) and complemented the data with sequence data available from GenBank. We reconstructed a gene tree, phylogenetic networks, and ancestral areas. Based on the nuDNA, we also generated a time‐calibrated species tree. The results revealed two major clades (Nanorana and Quasipaa), which originated in the Lower Miocene from eastern China and subsequently spread into the HTO (Nanorana). Five well‐supported subclades are found within Nanorana: from the East, Central, and Northwest Himalaya, the Tibetan Plateau, and the southeastern Plateau margin. The latter subclade represents the most basal group (subgenus Chaparana), the Plateau group (Nanorana) represents the sister clade to all species of the Greater Himalaya (Paa). We found no evidence for an east–west range expansion of Paa along the Himalaya, nor clear support for a strict vicariance model. Diversification in each of the three Himalayan subclades has probably occurred in distinct areas. Specimens from the NW Himalaya are placed basally relative to the highly diverse Central Himalayan group, while the lineage from the Tibetan Plateau is placed within a more terminal clade. Our data indicate a Tibetan origin of Himalayan Nanorana and support a previous hypothesis, which implies that a significant part of the Himalayan biodiversity results from primary diversification of the species groups in South Tibet before this part of the HTO was uplifted to its recent heights.     

High-elevation tree-ring record of 263-year summer temperature for a cold-arid region in the western Himalaya,India

The unavailability of weather records from the orography dominated high Himalayas restricts our understanding in long term perspective. However, remote high-altitude regions of Himalaya silently testify the regional climate and can provide valuable insights of real climatic challenges in the absence of instrumental observatories. The tree-species over such high-altitude regions with negligible anthropogenic pressure have the potential to reveal the clear climate upheavals in long-term perspective. In the present study tree-ring samples of Himalayan birch from a high-altitude cold-arid region of Lahaul-Spiti, Himachal Pradesh were analysed and two ring-width chronologies were developed. The response function analyses showed direct relationship between the summer temperature and ring-width chronologies of Himalayan birch. Using the relationship we have reconstructed mean summer temperature (June-July) back to AD 1752 for the Lahaul-Spiti region of Himachal Pradesh. We have developed the first record of summer temperature from the Indian western Himalaya using tree-ring-width chronologies that have direct relationship with summer temperature. Further, our study in accordance with instrumental as well as other tree-ring based summer temperature records suggested that the high-altitude western Himalaya is not warming unprecedently during summers. However, slight warming pattern have been observed in the summer temperature in the later part of the reconstruction. The temperature reconstruction also reflects strong spatial correlation with gridded temperature for the western Himalaya.     

Glacial Lake Expansion in the Central Himalayas by Landsat Images, 1990–2010

Glacial lake outburst flood (GLOF) is a serious hazard in high, mountainous regions. In the Himalayas, catastrophic risks of GLOFs have increased in recent years because most Himalayan glaciers have experienced remarkable downwasting under a warming climate. However, current knowledge about the distribution and recent changes in glacial lakes within the central Himalaya mountain range is still limited. Here, we conducted a systematic investigation of the glacial lakes within the entire central Himalaya range by using an object-oriented image processing method based on the Landsat Thematic Mapper (TM) or Enhanced Thematic Mapper (ETM) images from 1990 to 2010. We extracted the lake boundaries for four time points (1990, 2000, 2005 and 2010) and used a time series inspection method combined with a consistent spatial resolution of Landsat images that consistently revealed lake expansion. Our results show that the glacial lakes expanded rapidly by 17.11% from 1990 to 2010. The pre-existing, larger glacial lakes, rather than the newly formed lakes, contributed most to the areal expansion. The greatest expansions occurred at the altitudinal zones between 4800 m and 5600 m at the north side of the main Himalayan range and between 4500 m and 5600 m at the south side, respectively. Based on the expansion rate, area and type of glacial lakes, we identified 67 rapidly expanding glacial lakes in the central Himalayan region that need to be closely monitored in the future. The warming and increasing amounts of light-absorbing constituents of snow and ice could have accelerated the melting that directly affected the glacial lake expansion. Across the main central Himalayas, glacial lakes at the north side show more remarkable expansion than those at the south side. An effective monitoring and warning system for critical glacial lakes is urgently needed.     

Inconsistent response times to precipitation and soil moisture in Picea crassifolia growth

Quantifying the effects of environmental variables on radial growth has real significance for reasonably predicting the impacts of environmental changes on tree dynamics. This study used Picea crassifolia, a widely distributed dominant evergreen coniferous tree species found on the north-eastern fringe of the Tibetan Plateau, as a case study to analyse the associations of radial growth with environmental variables during 1960–2018 using a correlation analysis and sliding correlation analysis. The responses of radial growth to different moisture conditions were further quantitatively evaluated through the generalised linear model and relative dominance analysis. The results show that the radial growth of P. crassifolia is mainly influenced by moisture conditions in the study area. Specifically, the response times of P. crassifolia radial growth to soil moisture and precipitation differ, as radial growth has a significant positive correlation with precipitation in the early growth period. Notably, radial growth has a remarkable and stable correlation with soil moisture in the autumn and winter seasons of the previous year. This study provides a theoretical foundation and scientific grounds for analysing the response of Tibetan Plateau forests to climate change and can act as a reference for future research on the response of radial growth to soil moisture in alpine regions.     

Patterns and dynamics of tree-line response to climate change in the eastern Qilian Mountains,northwestern China

Tree-line ecotones are strongly climatically limited and serve as potential monitors of climate change. We employed annual growth increment from tree-rings, and tree density and age structure data derived from two Juniperus przewalskii tree-line sites in the eastern part of the Qilian Mountains, northeastern Tibetan Plateau, to detect the responses of tree growth and population dynamics to climate change. High temperature favors tree growth and is associated with increased tree density at tree-line, and an advance in tree-line position. Significantly positive correlations were found between ring-width and mean monthly air temperatures in current and previous June, July and August. Tree recruitment began to increase rapidly at the two sites after the Little Ice Age, but then decreased starting in the 1970s. The number of trees established coincides with temperature changes. The warming trend after the Little Ice Age favors increases of tree density and an advance of tree-line. The majority of trees established during the period of 1931–1970, which coincides well with the rapid radial growth of the trees.     

Winter warming as an important co‐driver for Betula nana growth in western Greenland during the past century

Growing season conditions are widely recognized as the main driver for tundra shrub radial growth, but the effects of winter warming and snow remain an open question. Here, we present a more than 100 years long Betula nana ring‐width chronology from Disko Island in western Greenland that demonstrates a highly significant and positive growth response to both summer and winter air temperatures during the past century. The importance of winter temperatures for Betula nana growth is especially pronounced during the periods from 1910–1930 to 1990–2011 that were dominated by significant winter warming. To explain the strong winter importance on growth, we assessed the importance of different environmental factors using site‐specific measurements from 1991 to 2011 of soil temperatures, sea ice coverage, precipitation and snow depths. The results show a strong positive growth response to the amount of thawing and growing degree‐days as well as to winter and spring soil temperatures. In addition to these direct effects, a strong negative growth response to sea ice extent was identified, indicating a possible link between local sea ice conditions, local climate variations and Betula nana growth rates. Data also reveal a clear shift within the last 20 years from a period with thick snow depths (1991–1996) and a positive effect on Betula nana radial growth, to a period (1997–2011) with generally very shallow snow depths and no significant growth response towards snow. During this period, winter and spring soil temperatures have increased significantly suggesting that the most recent increase in Betula nana radial growth is primarily triggered by warmer winter and spring air temperatures causing earlier snowmelt that allows the soils to drain and warm quicker. The presented results may help to explain the recently observed ‘greening of the Arctic’ which may further accelerate in future years due to both direct and indirect effects of winter warming.     

Annual and intra-annual growth dynamics of <Emphasis Type="Italic">Myricaria elegans</Emphasis> shrubs in arid Himalaya

Key message

To disentangle complex drivers of Myricaria elegans growth in arid Himalaya, we combined tree-ring analysis with detailed dendrometer records. We found that the combination of winter frost, summer floods, and strong summer diurnal temperature fluctuations control annual and intra-annual growth dynamics. The relative importance of these drivers is, however, changing with ongoing climate change.

Abstract

High-mountain areas are among the most sensitive environments to climate change. Understanding how different organisms cope with ongoing climate change is now a major topic in the ecology of cold environments. Here, we investigate climate drivers of the annual and intra-annual growth dynamics of Myricaria elegans, a 3–6 m tall tree/shrub, in a high-elevation cold desert in Ladakh, a rapidly warming region in the NW Himalayas. As Myricaria forms narrow stands around glacier streams surrounded by the desert, we hypothesized that its growth between 3800 and 4100 m will be primarily limited by low temperatures and summer floods. We found that warmer and less snowy conditions in April and May enhance earlywood production. Latewood formation is mostly driven by the June–July temperatures (T). The positive effect of warmer summers on both annual and intra-annual growth is related to fluctuating daily T (from +30 to 0 °C). In particular, dendrometer measurements over a 2-year period showed that net daily growth increments increased when the summer night T remained above 6 °C. While high night T during generally cold desert nights promoted growth, high daytime T caused water stress and growth inhibition. The growth–temperature dependency has gradually weakened due to accelerated warming since the 1990s. In addition, positive latewood responses to high March precipitation during the colder 1960s–1980s have become negative during the warmer 1990s–2000s, reflecting an intensification of summer floods. Latewood width increased while earlywood width decreased from the 1990s, indicating a prolonged growing season and a higher risk of drought-induced embolism in earlywood vessels. Due to a multiplicity of environmental drivers including winter frost, intensified floods and strong summer diurnal T fluctuations, Myricaria growth is not controlled by a single climate parameter. Similar results are increasingly reported from other Himalayan treelines, showing that ongoing climate change will trigger complex and probably spatially variable responses in tree growth. Our study showed that these complex climatic signals can be disentangled by a combination of long-term data from tree-rings with detailed, but short-term, records from dendrometers.

     

Climate signal shift under the influence of prevailing climate warming – Evidence from Quercus liaotungensis on Dongling Mountain,Beijing, China

Increasing climate warming is inducing drought stress and resulting in forest growth decline in many places around the world. The recent climate of northern China has shown trends of both warming and drying. In this study, we obtained tree ring width chronology of Quercus liaotungensis Koidz. from Dongling Mountain, Beijing, China. We divided the temperature series of the study area into cooling (1940–1969) and warming intervals (1970–2016). The climate–tree growth response analysis showed that temperature exerted a limiting impact on the annual radial growth of Q. liaotungensis during the cooling period, whereas the influence of temperature was lower during the warming period. The moving correlation analysis showed that the influence of summer temperature decreased with the warming climate since the 1970s, and that the influence of winter and spring temperatures decreased since the 2000s. The correlation values between the chronology and precipitation decreased during the cooling period, whereas spring and early summer precipitation correlations began to increase in the 1970s and reached significance (p < 0.05) in the 1990s. Our results show that the positive influence of temperature on radial growth of Q. liaotungensis in the study area has weakened, whereas precipitation has become the dominant regulator with climate warming. These findings suggest that forest growth on Dongling Mountain will decline if climate warming continues in the future.     

Growth variation in Abies georgei var. smithii along altitudinal gradients in the Sygera Mountains, southeastern Tibetan Plateau

A network of nine Smith fir (Abies georgei var. smithii) ring-width chronologies was constructed from sites ranging in elevation from 3,550 to 4,390 m above sea level (a.s.l.) in the Sygera Mountains, southeastern Tibetan Plateau. High-elevation trees had lower growth rates than did low-elevation trees. The mean tree-ring series intercorrelation (RBAR) increased with elevation. Principal component analysis identified three elevation zones (around 3,600, 3,800, and >4,200 m a.s.l.) with distinctive tree-ring growth patterns. Five chronologies with elevation >4,200 m a.s.l. were highly correlated. Overall, the initiation of tree-ring growth in Smith fir is controlled by common climatic signals, such as July minimum temperature, across a broad altitudinal range. Precipitation was not a growth-limiting factor across stands. Regardless of differences in stand elevation, topographical aspect, and tree age, the radial growth of Smith fir trees was markedly similar in response to common climatic signals, perhaps as a result of the relatively high-elevation of these forests (above 3,550 m a.s.l.) and the abundant summer monsoon rainfall. In addition, radial tree growth along the altitudinal gradients was indicative of a recent warming trend on the southeastern Tibetan Plateau.     

地表水热要素在青藏高原草地退化中的作用

在全球气候变暖和频繁的人类活动影响下,青藏高原草地生态系统发生了生产力下降、生物多样性减少及生态功能退化等一系列现象。与传统观测技术相比,遥感技术具有大范围、快速和连续监测等优点,因此被广泛用于区域尺度的草地植被长时间序列监测。以往对青藏高原草原植被影响因子的研究多集中在气温与降水,而相比较于气温和降水,地表温度和土壤湿度直接作用于植物的根部,对植物种子的萌芽和植株的生长也都有着重要影响,所以地表温度和土壤湿度与植被生长的关系更加紧密。基于遥感技术,利用青藏高原草地区域的MODIS和AVHRR数据,选择草地植被覆盖度作为草地退化的遥感监测指标,建立了青藏高原草地退化遥感监测和评价指标体系,并对青藏高原2001-2017年的草地退化状况进行了遥感监测和评价。同时,利用遥感数据获取青藏高原区域尺度的地表温度和温度植被干旱指数数据,用于指示地表水热状况,最后基于回归方法分析了地表水热要素在青藏高原草地退化中的作用。结果表明：从2001-2017年,青藏高原植被退化程度空间差异明显,柴达木盆地和青海湖附近退化较为严重,喜马拉雅山脉北部、昆仑山脉南部、冈底斯山脉北部交汇的地区退化也较严重。在2001-2017年间,青藏高原草地未退化面积从50.60%上升到59.00%,说明青藏高原草地整体上在朝着改善的方向发展。2001-2017年内,青藏高原草地整体上大部分时间处于轻度退化状态,但是2001年和2015年这两个年份青藏高原草地退化整体上达到中等退化水平。通过回归分析发现,土壤湿度主导的对青藏高原草地的影响面积达到14.04%。地表温度主导的影响面积达到草地总面积的约36.61%。但地表温度与植被之间相互影响,且主要呈现负相关关系。其中,在温性草甸地区,当植被覆盖度较低时,地表温度正向影响植被生长。     

Dendrochronological potential of the alpine shrub Rhododendron nivale on the south-eastern Tibetan Plateau

Background and Aims

Shrubs and dwarf shrubs are wider spread on the Tibetan Plateau than trees and hence offer a unique opportunity to expand the present dendrochronological network into extreme environments beyond the survival limit of trees. Alpine shrublands on the Tibetan Plateau are characterized by rhododendron species. The dendrochronological potential of one alpine rhododendron species and its growth response to the extreme environment on the south-east Tibetan Plateau were investigated.

Methods

Twenty stem discs of the alpine snowy rhododendron (Rhododendron nivale) were collected close to the tongue of the Zuoqiupu Glacier in south-east Tibet, China. The skeleton plot technique was used for inter-comparison between samples to detect the growth pattern of each stem section. The ring-width chronology was developed by fitting a negative exponential function or a straight line of any slope. Bootstrapping correlations were calculated between the standard chronology and monthly climate data.

Key Results

The wood of snowy rhododendron is diffuse-porous with evenly distributed small-diameter vessels. It has well-defined growth rings. Most stem sections can be visually and statistically cross-dated. The resulting 75-year-long standard ring-width chronology is highly correlated with a timberline fir chronology about 200 km apart, providing a high degree of confidence in the cross-dating. The climate/growth association of alpine snowy rhododendron and of this timberline fir is similar, reflecting an impact of monthly mean minimum temperatures in November of the previous year and in July during the year of ring formation.

Conclusions

The alpine snowy rhododendron offers new research directions to investigate the environmental history of the Tibetan Plateau in those regions where up to now there was no chance of applying dendrochronology.Key words: South-east Tibetan Plateau, Rhododendron nivale, alpine shrub, growth ring, cross-dating, dendroclimatological potential, climate/growth association     

From ‘third pole’ to north pole: a Himalayan origin for the arctic fox

The ‘third pole’ of the world is a fitting metaphor for the Himalayan–Tibetan Plateau, in allusion to its vast frozen terrain, rivalling the Arctic and Antarctic, at high altitude but low latitude. Living Tibetan and arctic mammals share adaptations to freezing temperatures such as long and thick winter fur in arctic muskox and Tibetan yak, and for carnivorans, a more predatory niche. Here, we report, to our knowledge, the first evolutionary link between an Early Pliocene (3.60–5.08 Myr ago) fox, Vulpes qiuzhudingi new species, from the Himalaya (Zanda Basin) and Kunlun Mountain (Kunlun Pass Basin) and the modern arctic fox Vulpes lagopus in the polar region. A highly hypercarnivorous dentition of the new fox bears a striking resemblance to that of V. lagopus and substantially predates the previous oldest records of the arctic fox by 3–4 Myr. The low latitude, high-altitude Tibetan Plateau is separated from the nearest modern arctic fox geographical range by at least 2000 km. The apparent connection between an ancestral high-elevation species and its modern polar descendant is consistent with our ‘Out-of-Tibet’ hypothesis postulating that high-altitude Tibet was a training ground for cold-environment adaptations well before the start of the Ice Age.     

Growth–climate relationships along an elevation gradient on a southeast-facing mountain slope in the semi-arid eastern Qaidam Basin,northeastern Tibetan Plateau

Key message

Both temperature and precipitation are strong factors of radial tree growth at all elevations in the semi-arid study area, except at the upper treeline where temperature becomes the major controlling factor.

Abstract

Several recent studies across the Tibetan Plateau found consistent growth–climate relations at all elevations from the lower treelines to the upper treelines. These findings seem to challenge the general principle of dendroclimatology that precipitation serves as the controlling factor of radial tree growth at lower elevations while temperature serves as the controlling factor at higher elevations in semi-arid regions. Such conclusions also question the potential of temperature reconstruction using ring-width data in these regions if precipitation remains the dominant factor of tree growth at the upper treelines. In this study, radial growth of Qilian juniper (Sabina przewalskii Kom.) was examined along an elevation gradient between ~3820 and 4230 m in the mountains east of the Qaidam Basin, northeastern Tibetan Plateau, to determine the limiting factors of radial tree growth at different elevations. Rotated principal component analysis revealed two modes of variation patterns. The first mode presents mostly tree ring data from the lower elevation zones (3820–4100 m) and contains strong signals of precipitation variation. The second mode represents the higher elevation zones (approx. 4100–4230 m) and contains strong signals of both temperature and precipitation variations. When signals of precipitation variation are removed from the tree ring data using partial correlation, the growth–temperature relationships become more evident on the upper slope. When correlations between individual tree-ring series and climate variables were examined, we discovered that there were better chances of finding tree ring samples strongly correlated to temperature variables (r = 0.6 or higher) at the elevation zone within ~100 m of the upper treeline, but uncommon at lower elevations. We also found that topographic variables, such as slope gradient and growing-season direct solar radiation may have minor influences on the growth–climate relationships.

     

Recent decline of high altitude coniferous growth due to thermo-hydraulic constrains: evidence from the Miyaluo Forest Reserve,Western Sichuan Plateau of China

Tree growth decline has been reported in many places around the globe under the context of increasingly warming climate, and strengthening drought intensity is detected to be the primary factor for such decline, particularly in northern forest sites, as well as arid and semi-arid areas. Yet, the forest growth decline in high altitude, high mountain sites certainly merits investigation. Here, we reported faxon fir (Abies fargesii var. faxoniana) forest growth decline (slope = -0.64) at the tree line (4150 m above sea level) in Miyaluo Forest Reserve (MFR) at the Western Sichuan Plateau, southwestern China since 2000. We investigated the cause of tree growth decline by applying dendrochronological approaches. We took tree-ring samples from fir trees at the tree line and developed tree-ring width (TRW) chronology. The tree growth – climate relationship analysis showed that maximum temperature (Tmax) was the primary factor limiting the radial growth of fir trees in the investigated area. The moving correlation analysis indicated the strengthening positive influence of Tmax, spring precipitation, and cloud cover during winter and monsoon period on radial growth since 2000s. Our results have shown that both thermal and hydraulic constrains accounted for the radial growth decline of fir trees at the tree line of MFR in the western Sichuan Plateau.