高山林线形成机理及植物相关生理生态学特性研究进展

高山林线作为极端环境条件下树木生存的界限,由于其对气候变化的敏感性,在全球变化研究中得到了广泛关注。研究高山林线形成机理以及林线地带植物相关生理生态学特性成为预测未来气候变化条件下植被动态变化的出发点。对于高山林线形成机理研究主要关注两方面问题：（1）林线地带外界环境如何限制乔木生长和分布,其内在机理如何;（2）灌木及草本相对于乔木在林线地区有哪些生存优势,从乔木到灌木及草本生活型演变的功能及意义如何。综述了当前高山林线形成机理及相关生理生态特性的国内外最新研究成果,指出尽管温度（尤其是生长季低温）在全球尺度上能解释大部分高山区域林线的分布,但树木生长和生存受限的内在机理并没有弄清楚,目前主要存在“碳受限”以及“生长受限”假说两大争论焦点。另外,理论上受温度控制的高山林线对气候变化的响应表现出不同的模式,表明全球变化对林线分布和植被生长影响的复杂性和不确定性。因此,未来的研究应该关注影响林线地区植被生长的多种生理生态学过程,比如水分及养分利用过程,以及从乔木到灌木及草本生活型演变的功能意义,从而为林线形成机理以及对气候变化的响应提供更好的解释。     

高山生物多样性对气候变化响应的研究进展

高山带是指自然气候森林边界即林线到雪线之间的无林区域。受低温限制的高山生物对气候变化具有高度的敏感性, 因此高山带被视为监测气候变化的理想试验场所。气候变暖加速了高山冰雪消融, 也加剧了高山生物多样性的波动, 因而高山生物多样性变化对于指示全球气候变化具有十分重要的意义。目前, 高山生物多样性对气候变暖响应的研究主要集中在高山物种组成和群落结构的变化、物种的分布格局和适宜生境的变化、林线交错带的位移、种间关系的变化等方面。气候变化与人为干扰等因素的叠加效应为预测未来生物多样性的变化增加了很多不确定性, 从长期来看, 气候变化效应相对于其他因素会显得越来越重要。未来的重点研究领域包括高山带生物多样性对极端气候变化的响应、全球气候变化背景下生物多样性与生态系统过程的关系、高山带地上/地下生物多样性的相互作用关系及其对气候变化的响应与适应、全球气候变化与人类活动干扰的叠加效应对高山生物多样性格局的影响等。     

林线更新关键影响因子研究进展

林线作为森林上限连接树种线的特殊生态过渡带,能够迅速地对气候变化做出反应,是理想的气候变化生态监测器。在全球变暖影响下,全球林线位置表现为一半上升,其余则保持相对稳定的趋势。林线能否上升取决于种子生产、向上扩散的能力及幼苗建成。大多学者从温度、降水、光照与干扰等方面分析林线更新及动态机制,主要认为低温限制林线上升,然而单一因素很难解释全球林线变化。国内外研究多关注于非生物因子对林线分布的作用机制,缺乏林线群落交错区基于种子扩散者等生物因子与幼苗定居,特别是林线交错带中种子扩散者对林线变化的作用。未来的研究可关注全球气候变暖下的林线幼苗幼树分布,如林线群落区分散贮食动物种子贮藏行为对林线幼苗更新的影响,为从种子扩散者与林线动态互作角度探究生物对气候变暖的响应提供科学依据。     

干扰对高山林线再形成过程的影响

高山林线是一类典型的生态交错带,因其特殊的结构和功能以及对外界环境的高度敏感性而成为全球气候变化研究的热点之一.本文简要介绍了高山林线的相关概念及其界定,从高山林线海拔位置波动、植被格局变化、生态交错带物种组成变化及其生理生态特征变化等几个方面阐述了干扰对高山林线再形成过程的不同影响,总结了高山林线物种对干扰的两种基本响应方式,即退行和入侵.认为人为干扰在一定程度上弱化了当前气候变暖对高山林线波动的影响,因而在不同地区必须紧密结合当地可能的干扰来讨论高山林线的波动,否则结果有可能因误差较大而失去应有的价值.指出该研究在高海拔地区进行植被恢复的指导意义.     

高山林线与气候变化关系研究进展

20世纪全球气候经历了异常的变化。20世纪是过去1000年中增暖最大的1个世纪,并且90年代是最暖的10年。作为两个生态系统的过渡地带,生态过渡带是监测全球变化的重要地点,而森林和苔原之间的高山林线是全球变化最为敏感的地点。从高山林线树木个体对气候变化的响应、气候变化下林线处树木的更新、林线格局变化以及高山林线与气候变化关系研究中所采用的研究方法等方面,综合论述了国内外的研究进展,最后提出了高山林线研究中需要注意的问题,并对今后的研究趋势作了展望。     

高山林线变化的更新受限机制研究进展

全球林线位置对气候变暖的响应表现为上升、无变化或下降等截然不同趋势,表明影响林线位置及动态的因子十分复杂,除了较普遍认为的低温调控机制外,还存在其它控制林线位置变化的机制。林线向上迁移开始于种子向林线以上的传播及幼苗在林线以上的定居,这些过程中的限制因子均会影响林线的位移,因此研究更新过程及其限制因子对理解高山林线对气候变化的响应具有重要的科学意义。主要从种子和幼苗两个关键阶段综述高山林线森林更新的研究进展。在种子阶段,夏季积温不足导致种子产量和活力下降,风速过低和浓密灌丛限制种子向林线以上传播,近地表的霜冻/水分胁迫和灌木释放的化感物质会阻碍种子在林线以上萌发。在幼苗阶段,除冬季低温外,生长季内较大的温度日振幅和偶然出现的冻害事件也是导致幼苗死亡的重要原因,而低温环境下的强烈光照引起的低温光抑制会显著降低生长季的光合作用;土壤低温、由土壤温度昼夜变化引起的冻举事件、夏季土壤干旱可能会导致幼苗光合作用下降和死亡率上升;积雪太浅会导致生长季早期幼苗水分供应的严重缺乏,但积雪太深会导致幼苗感染真菌的可能性增加;浓密的灌木和草本植物以及植食动物的啃食也会降低林线以上的幼苗存活率。气候变暖对林线幼苗定居的影响复杂且具有很大不确定性,需要进一步研究气候变暖导致的环境因子变化对林线更新各关键阶段的影响。未来气候变暖无疑会导致生长季起始日提前,结束日推迟,这很可能会增加生长季期间尤其是早期的低温冻害事件,对高山林线树种幼苗的存活具有重要影响。在未来研究中,需要找出定义生长季冻害事件的温度阈值,利用长期气象观测数据分析增温背景下生长季早期冻害事件特征的变化趋势,并进一步开展野外模拟增温实验以深刻理解林线树种的种子萌发和幼苗定居与生长季冻害事件的关系,加强对不同地区林线树种的繁殖策略研究,这将有助于人们进一步理解不同区域林线的形成机制并预测未来气候变化条件下林线的动态变化趋势。     

Carbon and nutrient physiology in shrubs at the upper limits: a multispecies study

高山分布上限灌木的碳与养分生理碳水化合物不足是高山林线树种生长限制假说之一,国内外以碳水化合物为基础的高山林线研究已有很多,而与高山林线相比,人们对碳水化合物在灌丛线形成中的作用知之甚少。除此之外,土壤养分亦被视为限制高山树种向上分布的重要因素之一。本研究将探究欧亚多种高山灌木不同器官中非结构性碳水化合物(NSCs)、氮(N)和磷(P)在不同季节及高低海拔上的含量变化规律。研究结果显示,除了与夏季相比冬季细根中具有较低的P含量以外,不同海拔与季节对灌木不同器官中的N和P含量均无显著影响。冬季灌木枝条中的NSCs和可溶性糖含量显著高于夏季。海拔与季节对细根中NSCs、淀粉、可溶性糖和糖与淀粉比值的影响均有显著的交互作用。在冬季,灌木细根中的可溶性糖与淀粉含量在海拔上限处要显著低于其在低海拔处;而在夏季,这些指标在高低海拔间均无显著差异。本研究结果表明,与高山林线树种相似,海拔分布上限的灌木冬季细根中较低的非结构性碳水化合物含量可能限制了灌木的向上分布。     

高山植物对其环境的生理生态适应性研究进展

高山林线植物(林木和草本)由于生长环境特殊而形成其独特的适应环境的生理生态特性.该文对近年来国内外有关高山植物、特别是林线林木在形态解剖结构、光合作用、养分利用和碳水化合物及抗氧化系统等方面对高山环境的生理生态适应性的研究进展进行综述,指出了林线树种生理生态适应性研究方面的不足,并提出了今后高山林线树种生理生态需要研究的方向,以期为研究气候变化下高山林线植物的应对策略和适应机制提供参考.     

芦芽山不同海拔白杄非结构性碳水化合物含量动态

高山林线对环境变化具有高度的敏感性, 但林线形成机制仍然没有明确的结论。为了检验高山林线形成是由碳限制还是生长限制决定, 并探讨林线树种适应高山环境的生理生态机制, 选择山西省吕梁山脉北端芦芽山, 沿3个海拔梯度测定了林线树种白杄(Picea meyeri)各组织非结构性碳水化合物(NSC)及其组分含量。结果表明: 白杄总体及各组织NSC含量均随海拔升高而增加, 林线树木不存在碳限制; 白杄NSC源、汇均随海拔升高而增加, 源-汇比在3个海拔之间没有差异, 表明源-汇平衡关系对海拔的适应性, 林线树木碳源活动没有受到限制; 各组织中可溶性糖与淀粉的比值随海拔升高呈增大趋势, 说明树木生长的环境越寒冷, 树木组织中表现出越明显的保护策略, 也可能暗示林线区域的树木更多地受到生长限制。研究结果在一定程度上支持“生长限制”假说。     

高山林线生态交错区木本植物幼苗分布特征、更新机制及其对气候变化的响应

木本植物幼苗是高山林线生态交错区的重要组成部分,其更新对气候变化背景下树线的移动至关重要.本研究通过对近几十年来全球范围内林线生态交错区的木本植物幼苗分布特征、更新机制及其对气候变化响应的研究总结得出:林线生态交错区木本植物幼苗的空间分布类型主要为渐变型和聚集型,且不同分布类型对树线动态的指示意义各异.在全球尺度上,其分布的海拔高限通常与生长季长度、均温和物种特性等有关,而在区域尺度上则多受降水影响.在幼苗更新初期,种源在很大程度上决定了种子的萌发及分布位置,之后微环境的促进作用为幼苗的定植提供庇护,提高其存活率,而在更新后期多种生物和非生物因素及其相互作用则非常关键.气候变暖促使林线生态交错区气温升高、降水充沛,有利于幼苗生长,使其向高海拔区域扩张而成为树线上移的先兆,但部分物种受遗传特性或适应策略影响,仅表现为密度增加,使树线保持相对稳定.未来应借助树轮、14C等精确定年技术,通过长期的野外定位观测和室内模拟,加强多时空尺度下林线幼苗的空间分布特征和更新机制研究,分析不同类型林线内木本植物幼苗的适应策略,预测气候变化背景下的树线动态,为山地生态系统恢复及保护提供科学依据.     

Global distribution and bioclimatic characterization of alpine biomes

Although there is a general consensus on the distribution and ecological features of terrestrial biomes, the allocation of alpine ecosystems in the global biogeographic system is still unclear. Here, we delineate a global map of alpine areas above the treeline by modelling regional treeline elevation at 30 m resolution, using global forest cover data and quantile regression. We then used global datasets to 1) assess the climatic characteristics of alpine ecosystems using principal component analysis, 2) define bioclimatic groups by an optimized cluster analysis and 3) evaluate patterns of primary productivity based on the normalized difference vegetation index. As defined here, alpine biomes cover 3.56 Mkm² or 2.64% of land outside Antarctica. Despite temperature differences across latitude, these ecosystems converge below a sharp threshold of 5.9°C and towards the colder end of the global climatic space. Below that temperature threshold, alpine ecosystems are influenced by a latitudinal gradient of mean annual temperature and they are climatically differentiated by seasonality and continentality. This gradient delineates a climatic envelope of global alpine biomes around temperate, boreal and tundra biomes as defined in Whittaker's scheme. Although alpine biomes are similarly dominated by poorly vegetated areas, world ecoregions show strong differences in the productivity of their alpine belt irrespectively of major climate zones. These results suggest that vegetation structure and function of alpine ecosystems are driven by regional and local contingencies in addition to macroclimatic factors.     

Warming‐induced upward migration of the alpine treeline in the Changbai Mountains,northeast China

Treeline responses to environmental changes describe an important phenomenon in global change research. Often conflicting results and generally too short observations are, however, still challenging our understanding of climate‐induced treeline dynamics. Here, we use a state‐of‐the‐art dendroecological approach to reconstruct long‐term changes in the position of the alpine treeline in relation to air temperature at two sides in the Changbai Mountains in northeast China. Over the past 160 years, the treeline increased by around 80 m, a process that can be divided into three phases of different rates and drives. The first phase was mainly influenced by vegetation recovery after an eruption of the Tianchi volcano in 1702. The slowly upward shift in the second phase was consistent with the slowly increasing temperature. The last phase coincided with rapid warming since 1985, and shows with 33 m per 1°C, the most intense upward shift. The spatial distribution and age structure of trees beyond the current treeline confirm the latest, warming‐induced upward shift. Our results suggest that the alpine treeline will continue to rise, and that the alpine tundra may disappear if temperatures will increase further. This study not only enhances mechanistic understanding of long‐term treeline dynamics, but also highlights the effects of rising temperatures on high‐elevation vegetation dynamics.     

Vegetation patterns at the alpine treeline ecotone: the influence of tree cover on abrupt change in species composition of alpine communities

Aims: The upper elevation limit of forest vegetation in mountain ranges (the alpine treeline ecotone) is expected to be highly sensitive to global change. Treeline shifts and/or ecotone afforestation could cause fragmentation and loss of alpine habitat, and are expected to trigger considerable alterations in alpine vegetation. We performed an analysis of vegetation structure at the treeline ecotone to evaluate whether distribution of the tree population determines the spatial pattern of vegetation (species composition and diversity) across the transition from subalpine forest to alpine vegetation. Location: Iberian eastern range of the Pyrenees. Methods: We studied 12 alpine Pinus uncinata treeline ecotones. Rectangular plots ranging from 940 to 1900 m² were placed along the forest‐alpine vegetation transition, from closed forest to the treeless alpine area. To determine community structure and species distribution in the treeline ecotone, species variation along the forest‐alpine vegetation transition was sampled using relevés of 0.5 m² set every 2 m along the length of each plot. Fuzzy C‐means clustering was performed to assess the transitional status of the relevés in terms of species composition. The relation of P. uncinata canopy cover to spatial pattern of vegetation was evaluated using continuous wavelet transform analysis. Results: Vegetation analyses revealed a large degree of uniformity of the subalpine forest between all treeline ecotone areas studied. In contrast, the vegetation mosaic found upslope displayed great variation between sites and was characterized by abrupt changes in plant community across the treeline ecotone. Plant richness and diversity significantly increased across the ecotone, but tree cover and diversity boundaries were not spatially coincident. Conclusions: Our results revealed that no intermediate communities, in terms of species composition, are present in the treeline ecotone. Ecotone vegetation reflected both bedrock type and fine‐scale heterogeneity at ground level, thereby reinforcing the importance of microenvironmental conditions for alpine community composition. Tree cover did not appear to be the principal driver of alpine community changes across the treeline ecotone. Microenvironmental heterogeneity, together with effects of past climatic and land‐use changes on ecotone vegetation, may weaken the expected correlation between species distribution and vegetation structure.     

Competitor or facilitator? The ambiguous role of alpine grassland for the early establishment of tree seedlings at treeline

Alpine treelines are expected to move upslope with a warming climate. However, so far treelines have responded inconsistently and future shifts remain difficult to predict since many factors unrelated to temperature, such as biotic interactions, affect responses at the local scale. Especially during the earliest regeneration stages, trees can be strongly influenced by alpine vegetation via both competition and facilitation. We aimed to understand the relative importance of these two types of interaction in different vegetation structures for treeline regeneration dynamics. Effects of herbaceous alpine vegetation on seedling emergence and first‐year performance were studied in a field experiment in the French Alps (2100 m a.s.l.) with five important European treeline tree species: Larix decidua, Picea abies, Pinus cembra, Pinus uncinata and Sorbus aucuparia. Total emergence and locally‐germinated seedling survival were not affected, but for seedlings planted at two months of age, negative vegetation impacts dominated for all response parameters: first‐year survival, growth and carbohydrate accumulation. However, in the winter half‐year, evergreen tree seedlings increased carbohydrate reserves under the protection of senescent herbs. Also, responses of locally‐germinated seedlings suggest facilitative vegetation effects in the first two months after emergence. Thus, the interaction switched between competition and facilitation according to ontogenetic stage and seasons. Still, the net outcome after one year was negative, but species differed in their susceptibilities. Because initial establishment is the first bottleneck determining whether treelines remain stable or move upslope, understanding establishment, including site‐, life‐stage and species‐specific processes, is essential for understanding observed treeline spatial patterns and dynamics. When developing predictive models of treeline dynamics, all these ‘local’ aspects should be incorporated in addition to more global drivers like changes in temperature.     

贡嘎山雅家埂峨眉冷杉林线种群的时空动态

通过对贡嘎山雅家埂峨眉冷杉种群林线附近6个3000 m2样地(阴阳坡各3个)中峨眉冷杉(Abies fabri Craib)种群的定位调查,分析了过去100a间该区峨眉冷杉种群的时间-空间动态。结果表明:1)雅家埂林线附近峨眉冷杉种群密度在过去100 a(主要是近50 a)有显著的升高,但树线的海拔位置并无明显的爬升;2)阴阳坡林线格局存在显著的坡向分异:阴坡林线和树线的海拔高度显著高于阳坡(分别比阳坡高152.5 m和135.8 m),阳坡林线附近峨眉冷杉早期的生长速率在大于阴坡,但后期的生长速率却低于阴坡;3)热量(温度)控制假说不能完全解释雅家埂目前的树线格局,除气候因素之外,其它因素也限制了雅家梗地区树线位置的变化。     

Asymmetric effects of daytime and nighttime warming on alpine treeline recruitment

Trees at their upper range limits are highly sensitive to climate change, and thus alpine treelines worldwide have changed their recruitment patterns in response to climate warming. However, previous studies focused only on daily mean temperature, neglecting the asymmetric influences of daytime and nighttime warming on recruitments in alpine treelines. Here, based on the compiled dataset of tree recruitment series from 172 alpine treelines across the Northern Hemisphere, we quantified and compared the different effects of daytime and nighttime warming on treeline recruitment using four indices of temperature sensitivity, and assessed the responses of treeline recruitment to warming-induced drought stress. Our analyses demonstrated that even in different environmental regions, both daytime and nighttime warming could significantly promote treeline recruitment, and however, treeline recruitment was much more sensitive to nighttime warming than to daytime warming, which could be attributable to the presence of drought stress. The increasing drought stress primarily driven by daytime warming rather than by nighttime warming would likely constrain the responses of treeline recruitment to daytime warming. Our findings provided compelling evidence that nighttime warming rather than daytime warming could play a primary role in promoting the recruitment in alpine treelines, which was related to the daytime warming-induced drought stress. Thus, daytime and nighttime warming should be considered separately to improve future projections of global change impacts across alpine ecosystems.     

Nitrogen and carbon source-sink relationships in trees at the Himalayan treelines compared with lower elevations

No single hypothesis or theory has been widely accepted for explaining the functional mechanism of global alpine/arctic treeline formation. The present study tested whether the alpine treeline is determined by (1) the needle nitrogen content associated with photosynthesis (carbon gain); (2) a sufficient source-sink ratio of carbon; or (3) a sufficient C-N ratio. Nitrogen does not limit the growth and development of trees studied at the Himalayan treelines. Levels of non-structural carbohydrates (NSC) in trees were species-specific and site-dependent; therefore, the treeline cases studied did not show consistent evidence of source/carbon limitation or sink/growth limitation in treeline trees. However, results of the combined three treelines showed that the treeline trees may suffer from a winter carbon shortage. The source capacity and the sink capacity of a tree influence its tissue NSC concentrations and the carbon balance; therefore, we suggest that the persistence and development of treeline trees in a harsh alpine environment may require a minimum level of the total NSC concentration, a sufficiently high sugar:starch ratio, and a balanced carbon source-sink relationship.     

Mass elevation effect and continentality have a stronger impact on global treelines than spatial isolation

Aim

The global relationship between treeline elevation and temperature (or latitude as a proxy) is well established. However, additional large-scale and regional abiotic influences such as mass elevation effect (MEE), continentality and isolation are superimposed onto the latitude-treeline relationship. To quantify these effects, we apply globally applicable measures and test the effects of MEE, an aspect of continental climate and isolation on treeline elevation.

Location

Global treeline elevations (n = 629).

Methods

We sampled treeline sites using earth observation. We calculated MEE as the distance to the nearest mountain chain limits. Continentality was assessed by the distance to the nearest coastline. Isolation was calculated by the nearest distance of a mountain chain to another mountain chain within a comparable elevational band.

Results

The global latitudinal pattern showed a distinct bimodal latitude-treeline elevation relationship. Treeline elevations increased substantially with increased MEE and distance to coastlines while isolation even decreased treeline elevations.

Main Conclusions

Our study shows a globally consistent effect of MEE and distance to the coastline on treeline elevation, contributing to our basic understanding of large-scale biogeographic processes governing treeline formation. MEE and continentality reduce cloudiness and increase solar radiation, resulting in higher treeline elevations. Isolation effects are not consistent and may be influenced by immigration and speciation. Understanding global treeline formation using comprehensive measures contributes to a better understanding of how environmental conditions determine vegetation boundaries at large spatial scales.