Encroachment drives facilitation at alpine shrublines

Ongoing encroachment is driving recent alpine shrubline dynamics globally,but the role of shrub-shrub interactions in shaping shrublines and their relationships with stem density changes remain poorly understood.Here,the size and age of shrubs from 26 Salix shrubline populations along a 900-km latitudinal gradient(30°-38°N)were measured and mapped across the eastern Tibetan Plateau.Point pattern analyses were used to quantify the spatial distribution patterns of juveniles and adults,and to assess spatial associations between them.Mean intensity of univariate and bivariate spatial patterns was related to biotic and abiotic variables.Bivariate mark correlation functions with a quantitative mark(shrub height,basal stem diameter,crown width)were also employed to investigate the spatial relationships between shrub traits of juveniles and adults.Structural equation models were used to explore the relationships among conspecific interactions,patterns,shrub traits and recruitment dynamics under climate change.Most shrublines showed clustered patterns,suggesting the existence of conspecific facilitation.Clustered patterns of juveniles and conspecific interactions(potentially facilitation)tended to intensify with increasing soil moisture stress.Summer warming before 2010 triggered positive effects on population interactions and spatial patterns via increased shrub recruitment.However,summer warming after2010 triggered negative effects on interactions through reduced shrub recruitment.Therefore,shrub recruitment shifts under rapid climate change could impact spatial patterns,alter conspecific interactions and modify the direction and degree of shrublines responses to climate.These changes would have profound implications for the stability of alpine woody ecosystems.

Spatiotemporal variability in tree ring's δ^(13)C of Picea crassifolia in the Qilian Mountains:climatic significance and responses to rising CO_2

Abstract：The carbon isotopic composition （δ13C） of tree rings was used to assess changes in intrinsic water-use efficiency （Wi） to increasing atmospheric CO2 and climate change during the period of 1891–2003. Five Qinghai spruce （Picea crassifolia） stands were selected in the Qilian Mountains, growing along a precipitation gradient. All five δ13C were correlated to each other, but two sites （DDS and CLS）, which are far from the main body of the mountains, show relative weak connections to other sites. Although trees at all sites had improved their Wi in response to increasing atmospheric CO2 concentration, spruce growing in the regions far away from the main body of the mountains were less sensitive to improved Wi than those of other sites. Based on the correlation between carbon isotope discrimination （Δ） and Palmer Drought Severity Index （PDSI）, the drought history covering the period of 1891–2003 was reconstructed in the study region. The two most severe drought epochs of the late 1920s and the last decade were caused by reduced precipitation and climate warming, respectively. Our results will be useful in assessing any further spatial climate-related bioclimatic information.

火山喷发对陆地植被与碳循环影响的研究进展

1991年,Pinatubo火山喷发导致全球大气二氧化碳浓度上升态势出现停滞,这一现象揭示了地球内部系统可以通过火山活动对地表生物地球化学循环产生重大影响.近些年,火山喷发对陆地生态系统的影响及其驱动机制引起科学界的广泛讨论.本文系统梳理了火山喷发对陆地生态系统影响的研究,总结了火山活动的辐射效应和气候效应对树木生长和陆地碳循环产生影响的机制.大型火山喷发形成的平流层气溶胶一方面引起散射辐射增加促进光合作用,另一方面,还引起总辐射减弱抑制光合作用,由于散射辐射的影响相比更强,总体上火山辐射效应有利于陆地碳汇的增加.火山的气候效应体现在全球尺度的降温和区域降水的改变.降温作用在低纬地区促进植被的光合作用,在高纬地区抑制光合作用,同时在全球尺度减弱生态系统的呼吸作用,总体上也有利于增加碳汇.对火山造成降水变化影响的评估目前还存在较大不确定性.在对这些机制总结的基础上,本文评述了当前的认识盲点和研究难点,以期为未来进一步推进火山喷发影响的研究、完善地球内外系统联动机制的认识提供参考.

Linking leaf elemental traits to biomass across forest biomes in the Himalayas

Plants require a number of essential elements in different proportions for ensuring their growth and development.The elemental concentrations in leaves reflect the functions and adaptations of plants under specific environmental conditions.However,less is known about how the spectrum of leaf elements associated with resource acquisition,photosynthesis and growth regulates forest biomass along broad elevational gradients.We examined the influence of leaf element distribution and diversity on forest biomass by analyzing ten elements(C,N,P,K,Ca,Mg,Zn,Fe,Cu,and Mn)in tree communities situated every 100 meters along an extensive elevation gradient,ranging from the tropical forest(80 meters above sea level)to the alpine treeline(4200 meters above sea level)in the Kangchenjunga Landscape in eastern Nepal Himalayas.We calculated communityweighted averages(reflecting dominant traits governing biomass,i.e.,mass-ratio effect)and functional divergence(reflecting increased trait variety,i.e.,complementarity effect)for leaf elements in a total of 1,859 trees representing 116 species.An increasing mass-ratio effect and decreasing complementarity in leaf elements enhance forest biomass accumulation.A combination of elements together with elevation explains biomass(52.2%of the variance)better than individual elemental trait diversity(0.05%to 21%of the variance).Elevation modulates trait diversity among plant species in biomass accumulation.Complementarity promotes biomass at lower elevations,but reduces biomass at higher elevations,demonstrating an interaction between elevation and complementarity.The interaction between elevation and mass-ratio effect produces heterogeneous effects on biomass along the elevation gradient.Our research indicates that biomass accumulation can be disproportionately affected by elevation due to interactions among trait diversities across vegetation zones.While higher trait variation enhances the adaptation of species to environmental changes,it reduces biomass accumulation,especially at higher elevations.

High-resolution reconstruction of April-September precipitation and major extreme droughts in China over the past∼530 years

Extreme drought events have increased,causing serious losses and damage to the social economy under current warming conditions.However,short-term meteorological data limit our understanding and projection of these extremes.With the accumulation of proxy data,especially tree-ring data,large-scale precipitation field reconstruction has provided opportunities to explore underlying mechanisms further.Using point-by-point regression,we reconstructed the April-September precipitation field in China for the past~530 years on the basis of 590 proxy records,including 470 tree-ring width chronologies and 120 drought/flood indices.Our regression models explained average 50%of the variance in precipitation.In the statistical test on calibration and verification,our models passed the significance level that assured reconstruction quality.The reconstruction data performed well,showing consistency and better quality than previously reported reconstructions.The first three leading modes of variability in the reconstruction revealed the main distribution modes of precipitation over China.Wet/drought and extremely wet/drought years accounted for 12.81%/10.92%(68 years/58 years)and 1.69%/3.20%(9 years/17 years)of the past~530 years in China,respectively.Major extreme drought events can be identified explicitly in our reconstruction.The detailed features of the Chongzhen Great Drought(1637-1643),the Wanli Great Drought(1585-1590),and the Ding-Wu Great Famine(1874-1879),indicated the existence of potentially different underlying mechanisms that need further exploration.Although further improvements can be made for remote uninhabited areas and large deserts,our gridded reconstruction of April-September precipitation in China over the past~530 years can provide a solid database for studies on the attribution of climate change and the mechanism of extreme drought events.

互利作用促进了高山树线幼苗的存活

高山树线被视为记录气候变化对树木和森林影响的生态监测器。迄今为止,大多数树线研究都集中在气候变化如何驱动树线动态的变化。然而,关于生物相互作用如何调节树线变化,特别是在树木更新这一树线动态的瓶颈方面,人们知之甚少。本研究假设种间和种内促进作用决定了高山树线树苗的定居和存活。为了验证这一假设,我们在青藏高原东南部的色季拉山高山树线交错区(海拔4300–4500 m)的3个生长生境(林冠内、林冠外和草地)中共计移植了630株不同年龄(4–6年、7–9年和10–15年)的急尖长苞冷杉(Abies georgei var.smithii)幼苗,连续3年监测了移植后幼苗的微气候、高度生长、死亡率和叶片功能性状。结果发现,叶片功能性状的变化是由微气候驱动的。移植后,叶片可溶性糖和淀粉浓度以及C:P比增加,而叶面积减小。幼苗的资源利用逐渐转向更保守的策略,其背后机制主要受到了生理性状(非结构性碳水化合物和植物养分)的介导。同时,由结构方程模型发现太阳辐射、温度和水分条件在植物相互作用下,通过介导叶片形态性状的变化最终影响了幼苗的死亡率和年生长。上述研究结果表明,植物间的互利作用通过改变太阳辐射和叶片功能性状,最终在决定幼苗生长和存活上起到至关重要的作用,同时也为理解全球气候变化背景下高山树线动态变化的驱动机制提供了新的见解。

青藏高原生态系统对气候变化的响应及其反馈

近几十年来,青藏高原正经历快速的气候变化,高原生态系统因此发生了深刻变化,并对周边地区产生了深远影响.本文围绕青藏高原生态系统结构和功能对气候变化的响应与反馈这一主线,系统总结了气候变化对物候、高山树线、生物多样性、植被生产力和生态系统碳汇功能的影响,阐述了青藏高原植被变化对区域气候的反馈及对亚洲季风的远程影响的研究进展.主要结论如下:气候变暖导致植被返青期总体提前,高原树线位置上升,高寒草原植物物种丰富度和多样性下降;气候变暖总体促进了高原植被生产力、增强了生态系统碳汇功能,但受限于土壤极大的空间异质性和对深层土壤碳动态理解的匮乏,目前对高原土壤碳库及土壤碳汇功能大小的估算仍具有较大不确定性.同时,青藏高原植被变化对近地表气温产生'负反馈'作用;植被活动增强还对东亚季风产生远程影响,导致我国东部夏季降水变化呈现'华南增加-长江黄河中间区域减少'的空间分异格局.未来的研究需要在完善观测体系基础上,加强对高寒生态系统对气候变暖的适应机理及生物地球物理反馈等过程的认知,为优化生态系统管理和保障青藏高原的生态安全提供理论基础.

Critical minimum temperature limits xylogenesis and maintains treelines on the southeastern Tibetan Plateau

Physiological and ecological mechanisms that define treelines are still debated. It has been suggested that the absence of trees above the treeline is caused by low temperatures that limit growth. Thus, we hypothesized that there is a critical minimum temperature （CTmin） preventing xylogenesis at treeline. We tested this hypothesis by examining weekly xylogenesis across three and four growing seasons in two natural Smith fir （Abies georgei var. srnithii） treeline sites on the southeastern Tibetan Plateau. Despite differences in the timing of cell differentiation among years, minimum air temperature was the dominant climatic variable associated with xylem growth; the critical minimum temperature （CTmin） for the onset and end of xylogenesis occurred at 0.7 ±0.4 ℃. A process-based modelling chronology of tree-ring formation using this CTmin was consistent with actual tree-ring data. This extremely low CTmin permits Smith fir growing at treeline to complete annual xylem production and maturation and provides both support and a mechanism for treeline formation.

Strong link between large tropical volcanic eruptions and severe droughts prior to monsoon in the central Himalayas revealed by tree-ring records

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 Ni?a 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.

No benefits from warming even for subnival vegetation in the central Himalayas

普遍认为变暖有利于高山植被生态系统,并已经导致冰缘植物向高海拔的扩张.珠峰山谷金字塔气象站(海拔5050 m)1994~2018的气象记录显示,高海拔山区呈显著的暖干化趋势.然而,我们依然不清楚暖干化是否对冰缘植被更新和分布造成显著影响?本研究在喜马拉雅山中段3个山谷建立了8块(30 m×120 m)滇藏方枝柏(Juniperus indica)和高山柏(Juniperus squamata)灌木线样地(海拔介于4344~5074 m之间).样地调查揭示,自20世纪90年代之前所有样地种群更新呈增加的趋势,然而90年代以来更新持续下降.种群更新与春季、夏季温度之间呈现显著的负相关关系,暗示了暖干化加剧了水分胁迫,并限制了种群的更新.另外,近60年来所有灌木线位置稳定.近30年遥感数据也证实,研究区冰缘植被无变绿趋势,甚至出现2%-6%的植被生长减弱现象.因此,进一步研究应该探讨冰缘植被生长减弱对喜马拉雅山区水循环和生物多样性的影响.

Toward a sustainable grassland ecosystem worldwide

Globally,grasslands,covering about 40%of the Earth’s land area,are vital for supporting important ecosystem functions,services,and livelihoods of millions of humans.Currently,grassland degradation is a major threat to the maintenance of ecological services,1 food security,and sustainable development,and directly hinders the global efforts with meeting goals and targets such as the The UN Decade on Ecosystem Restoration and Sustainable Development Goals(SDGs).Remote sensing approaches have the advantages of spanning large geographical areas withmultiple spatial,spectral,and temporal resolutions.In global scale,remote sensing methods used normalized difference vegetation index to determine net primary productivity(NPP),which still is the effectivemethod to indicate grassland conditions.To master the general situation of grassland,we analyzed the global spatial-temporal variation of NPP from 2001 to 2019 at the pixel level across the globe.As presented in Figure 1A,the NPP values of global grasslands showed an obvious variation trend,which indicated a considerable distribution pattern of spatial heterogeneity.The decreasing and increasing trend in grassland NPP covered approximately 25.3%and 74.5%of the total grassland area,respectively.