Temporal dynamic of wood formation in Pinus cembra along the alpine treeline ecotone and the effect of climate variables

We determined the temporal dynamic of cambial activity and xylem development of stone pine (Pinus cembra L.) throughout the treeline ecotone. Repeated micro-sampling of the developing tree ring was carried out during the growing seasons 2006 and 2007 at the timberline (1,950 m a.s.l.), treeline (2,110 m a.s.l.) and within the krummholz belt (2,180 m a.s.l.) and the influence of climate variables on intra-annual wood formation was determined. At the beginning of both growing seasons, highest numbers of cambial and enlarging cells were observed at the treeline. Soil temperatures at time of initiation of cambial activity were c. 1.5°C higher at treeline (open canopy) compared to timberline (closed canopy), suggesting that a threshold root-zone temperature is involved in triggering onset of above ground stem growth. The rate of xylem cell production determined in two weekly intervals during June through August 2006–2007 was significantly correlated with air temperature (temperature sums expressed as degree-days and mean daily maximum temperature) at the timberline only. Lack of significant relationships between tracheid production and temperature variables at the treeline and within the krummholz belt support past dendroclimatological studies that more extreme environmental conditions (e.g., wind exposure, frost desiccation, late frost) increasingly control tree growth above timberline. Results of this study revealed that spatial and temporal (i.e., year-to-year) variability in timing and dynamic of wood formation of P. cembra is strongly influenced by local site factors within the treeline ecotone and the dynamics of seasonal temperature variation, respectively.     

Carbon sink limitation and frost tolerance control performance of the tree <Emphasis Type="Italic">Kageneckia angustifolia</Emphasis> D. Don (Rosaceae) at the treeline in central Chile

The decrease in temperature with increasing elevation may determine the altitudinal tree distribution in different ways: affecting survival through freezing temperatures, by a negative carbon balance produced by lower photosynthetic rates, or by limiting growth activity. Here we assessed the relative importance of these direct and indirect effects of altitudinal decrease in temperature in determining the treeline in central Chile (33°S) dominated by Kageneckia angustifolia. We selected two altitudes (2000 and 2200 m a.s.l.) along the treeline ecotone. At each elevation, leaf non-structural carbohydrates (NSC) and gas exchange parameters were measured on five individuals during the growing season. We also determined the cold resistance of K.␣angustifolia, by measuring temperatures that cause 50% seedling mortality (LT₅₀) and ice nucleation (IN). No differences in net photosynthesis were found between altitudes. Although no differences were detected on NSC concentration on a dry matter basis between 2000 and 2200 m, when NSC concentration was expressed on a leaf area basis, higher contents were found at the higher elevation. Thus, carbon sink limitations may occur at the K. angustifolia’s upper altitudinal limit. For seedlings derived from seeds collected at the 2200 m, LT₅₀ of cold-acclimated and non-acclimated plants were −9.5 and −7 °C, respectively. However, temperatures as low as −10 °C can frequently occur at this altitude during the end of winter. Therefore, low temperature injury of seedlings seems also be involved in the treeline formation in this species. Hence, a confluence of global (carbon sink limitation) and regional (freezing tolerance) mechanisms explains the treeline formation in the Mediterranean-type climate zone of central Chile.     

Seasonal dynamics of mobile carbon supply in Quercus aquifolioides at the upper elevational limit

Many studies have tried to explain the physiological mechanisms of the alpine treeline phenomenon, but the debate on the alpine treeline formation remains controversial due to opposite results from different studies. The present study explored the carbon-physiology of an alpine shrub species (Quercus aquifolioides) grown at its upper elevational limit compared to lower elevations, to test whether the elevational limit of alpine shrubs (<3 m in height) are determined by carbon limitation or growth limitation. We studied the seasonal variations in non-structural carbohydrate (NSC) and its pool size in Q. aquifolioides grown at 3000 m, 3500 m, and at its elevational limit of 3950 m above sea level (a.s.l.) on Zheduo Mt., SW China. The tissue NSC concentrations along the elevational gradient varied significantly with season, reflecting the season-dependent carbon balance. The NSC levels in tissues were lowest at the beginning of the growing season, indicating that plants used the winter reserve storage for re-growth in the early spring. During the growing season, plants grown at the elevational limit did not show lower NSC concentrations compared to plants at lower elevations, but during the winter season, storage tissues, especially roots, had significantly lower NSC concentrations in plants at the elevational limit compared to lower elevations. The present results suggest the significance of winter reserve in storage tissues, which may determine the winter survival and early-spring re-growth of Q. aquifolioides shrubs at high elevation, leading to the formation of the uppermost distribution limit. This result is consistent with a recent hypothesis for the alpine treeline formation.     

Positive associations between the cushion plant Azorella monantha (Apiaceae) and alpine plant species in the Chilean Patagonian Andes

Low growing, compact cushion plants are a common and often dominant life form in temperate and subpolar alpine habitats. The cushion life-form can modify wind patterns, temperature and water availability and thus cushion species could be expected to act as nurse-plants facilitating the establishment of other alpine plant species on their surfaces. It has been suggested that the nurse effect should be most pronounced under more stressful environmental conditions, as found with increasing elevation in the alpine. One of the approaches used to detect the nurses has been the study of spatial associations among species, in which extreme clumping within or beneath one species has been interpreted as evidence of nursing. We characterized microclimatic conditions (soil and air temperature) within and outside cushions of Azorella monantha at two elevations (700 m a.s.l., corresponding to an elevation just above treeline, and 900 m a.s.l., corresponding to the upper limit of the cushion belt zone) on Cerro Diente in the Patagonian alpine of southern South America (50° S) and recorded all plant species growing upon cushions of various sizes and for paired sampling areas of equivalent sizes outside cushions. At 5 cm depth, soil temperature was slightly higher under cushions than under bare ground, but only significantly so at 900 m. Air temperature at ground level was significantly higher in the cushion microhabitat at both 700 m and 900 m, with the difference being more exaggerated at the highest elevation. At 700 m, a total of 27 species were recorded growing within cushions as compared to 29 outside cushions. At 900 m the corresponding numbers were 34 and 18. At the highest elevation, significantly more species grow within cushions than for equal areas outside cushions. Here moreover, 17 (48.6%) species grew preferentially within cushions, with eight of the latter being limited to the cushion microhabitat at this elevation. However, at 700 m there was no significant difference in species richness in the two microhabitats, and only one species (3.1%) grew preferentially on cushions. Considering individual species, nine occurring at both elevations showed non-preferential recruitment on cushions at 700 m, but significantly higher frequencies on cushions at 900 m. Results suggest striking altitudinal variation in the association with Azorella monantha on Cerro Diente, ranging from a very strong at 900 m to near absence at 700 m. Milder air and soil temperatures, shelter from wind, and greater water availability within cushions as opposed to outside cushions are discussed as possible factors favoring strong plant recruitment on cushions at higher elevations in the harsh Patagonian alpine environment. This revised version was published online in August 2006 with corrections to the Cover Date.     

Responses of white spruce (Picea glauca) to experimental warming at a subarctic alpine treeline

From 2001 to 2004 we experimentally warmed 40 large, naturally established, white spruce [Picea glauca (Moench) Voss] seedlings at alpine treeline in southwest Yukon, Canada, using passive open‐top chambers (OTCs) distributed equally between opposing north and south‐facing slopes. Our goal was to test the hypothesis that an increase in temperature consistent with global climate warming would elicit a positive growth response. OTCs increased growing season air temperatures by 1.8°C and annual growing degree‐days by one‐third. In response, warmed seedlings grew significantly taller and had higher photosynthetic rates compared with control seedlings. On the south aspect, soil temperatures averaged 1.0°C warmer and the snow‐free period was nearly 1 month longer. These seedlings grew longer branches and wider annual rings than seedlings on the north aspect, but had reduced Photosystem‐II efficiency and experienced higher winter needle mortality. The presence of OTCs tended to reduce winter dieback over the course of the experiment. These results indicate that climate warming will enhance vertical growth rates of young conifers, with implications for future changes to the structure and elevation of treeline contingent upon exposure‐related differences. Our results suggest that the growth of seedlings on north‐facing slopes is limited by low soil temperature in the presence of permafrost, while growth on south‐facing slopes appears limited by winter desiccation and cold‐induced photoinhibition.     

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。     

A comparison of water and carbon dioxide exchange at a windy alpine tundra and subalpine forest site near Niwot Ridge,Colorado

Eddy covariance measurements of the surface energy balance and carbon dioxide exchange above high-elevation (3,480 m above sea level) alpine tundra located near Niwot Ridge, Colorado, were compared to simultaneous measurements made over an adjacent subalpine forest over two summers and one winter, from June 9, 2007 to July 3, 2008. The surface energy balance closure at the alpine site averaged 71 and 91%, winter and summer, respectively, due to the high wind speeds, short turbulent flux footprint, and relatively flat ridge-top location of the measurement site. Throughout the year, the alpine site was cooler with higher relative humidity, and had a higher horizontal wind speed, especially in winter, compared to the forest site. Wind direction was persistently downslope at the alpine site (summer and winter, day and night), whereas upslope winds were common at the forest site during summer daytime periods. The latent and sensible heat fluxes were consistently larger in magnitude at the forest site, with the largest differences during summer. The horizontal advective flux of CO₂ at the alpine site averaged 6% of the net ecosystem exchange (NEE) during summer nights (5% during summer daytime), and was small in relation to the high wind speeds, relatively flat site, and weak sources of CO₂ upwind of the site. The magnitudes and diurnal behavior of the alpine NEE calculated using three methods; eddy-covariance, friction velocity filter, and with advection and storage calculations, gave similar results. The period of net CO₂ uptake (negative NEE) was 100 days at the alpine site with a net uptake of 16 g C m⁻², compared to 208 days at the forest site with a net uptake of 108 g C m⁻², with initiation of net uptake coinciding with air temperatures reaching +10°C. Winter respiration loss at the alpine site was 164 g C m⁻² over 271 days, compared to 52 g C m⁻² over 175 days at the forest site, with the initiation of net loss coinciding with air temperatures reaching −10°C at each site.     

How the scrub height of dwarf pine Pinus pumila decreases at the treeline

Plant height decreases much within narrow altitudinal spans near treelines. We compared the stem age, stem inclination and shoot elongation rates of alpine dwarf pine Pinus pumila between the upper distribution limit (treeline, 2,850 m a.s.l.) and the lower distribution limit (2,500 m a.s.l.) on Mount Norikura in central Japan, to examine how the growth traits of P. pumila change with altitude. The mean stem height at the upper distribution limit (49 cm) was about a quarter of that at the lower distribution limit (187 cm). The mean ratio of stem height to length was lower at the upper distribution limit than at the lower distribution limit, indicating that P. pumila stems inclined more at the higher altitude. The mean stem age at the upper distribution limit (48 years) was less than a half of that at the lower distribution limit (109 years). Although the shoot elongation rate positively correlated with stem length at the two altitudes, the shoot elongation rate at a given stem length was lower at the upper distribution limit than at the lower distribution limit. Thus, less developed scrub at the upper distribution limit than at the lower distribution limit was due to shorter stem age, more creeping stems and lower shoot elongation rates. Generally, wind velocity is greater in higher altitudes. Probably, strong wind reduces the growth and mean stem age of P. pumila stems at the upper distribution limit. Therefore, this study concludes that the scrub height of P. pumila is controlled not only by temperature, but also by strong wind.     

A novel framework for disentangling the scale‐dependent influences of abiotic factors on alpine treeline position

Low‐temperature growth limitation largely determines alpine treeline position globally, but treeline elevation also varies locally at a range of scales in response to multiple biotic and abiotic factors. In this study, we conceptualise how variability in treeline elevation is related to abiotic factors that act as thermal modifiers, physiological stressors, or disturbance agents. We then present a novel analytical framework for quantifying how abiotic factors influence treeline elevation at different spatial scales using New Zealand Nothofagus treelines as a case study. We delineated Nothofagus treelines in a GIS, along which we extracted data for treeline elevation and eight abiotic explanatory variables at 54 000 points. Each location was classified at each of five spatial scales based on nested river catchments, ranging from large regional to small hillslope catchments. We used hierarchical linear models to partition the variation in both treeline elevation and the eight abiotic variables by spatial scale, and then quantified the relationships between these at each spatial scale in turn. Nothofagus treeline elevation varied from 800–1740 m a.s.l. across New Zealand. Abiotic factors explained 82% of the variation in treeline elevation at the largest (regional) scale and 44–52% of variation at the four finer scales. Broad‐scale variation in Nothofagus treeline elevation was strongly associated with thermal modifiers, consistent with the idea that treelines coincide with a temperature‐driven, physiological limit. However, much of the finer‐scale variation in treeline elevation was explained by a combination of thermal, physiological stress‐related, and disturbance variables operating at different spatial scales. The conceptual model and analytical methods developed here provide a general framework for understanding treeline variation at different spatial scales.     

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.     

The role of stem recharge in reducing the winter desiccation of Picea engelmannii (Pinaceae) needles at alpine timberline

The winter desiccation of needles is thought to limit tree growth and survival within alpine timberline ecotones of the southern Rocky Mountains, USA. To better understand the factors contributing to this desiccation damage, the extent to which stem water was available to needles of Picea engelmannii undergoing desiccation at timberline near Monarch Pass, Colorado, was monitored throughout the winter. Severed shoots experienced significantly greater desiccation than did intact shoots, indicating the availability of stem water to needles despite presumably frozen soil, roots, and stems. A model of water relations during winter predicted more extreme desiccation of severed shoots than observed. This suggests that one or more of the common assumptions concerning the winter water relations of timberline trees is in error. The influence of cold, dry conditions on the cuticular conductance of Picea engelmannii needles is not known and therefore not accounted for in current models of winter water relations. The assumption that cuticular conductance is not influenced by temperature or humidity is a likely source of error in such models.     

Alpine treelines as ecological indicators of global climate change: Who has studied? What has been studied?

As the “front line” of coping with global climate change, the alpine treeline has been widely investigated by ecologists for a long time. In this study, 3180 articles in the Web of Science database from 2000 to 2021 are visualized by using CiteSpace bibliometric software from the perspectives of basic literature features (volume, discipline, journal, author, institution, and country), academic layout, research hotspot, and research frontier to have a deeper understanding of the development laws and hot spots in the alpine treeline. Results show that: (1) The number of papers published in alpine treeline research field is increasing annually, and the professional degree of papers (Sp) is 0.13–0.14, it is lower than that in other fields, implying a tendency of multi-disciplinary integration . (2) The core journals are headed by Arctic and Alpine Research, and J. Julio Camarero is the most prolific author in the field. (3) From the analysis of countries and institutions, the United States and the Chinese Academy of Sciences have leading positions in this research field. (4) Over the last 20 years, alpine treeline researchers have primarily focused on three key words (climate change, environmental characteristics, interspecific interactions, etc.), research sites (National Glacier Parks, the Alps, the Sygera Mountains, etc.), and research species (Norway spruce, European spruce, Abies georgei, etc.). (5) The first phase (2000–2009) focuses on the formation and changes of the alpine treeline in the context of climate change, and the second phase (2010−2021) focuses on the driving mechanisms of climatic factors, physiological change of tree species and grasslands on the treeline ecotones, and the influence of human activities (logging, grazing, etc.) on the alpine treeline. (6) Academic publications in this discipline have a short half-life (3.85 ± 1.70 years), and the aging rate is high and changing quickly. In the coming years, more research on alpine treeline will be devoted to explain the mechanism of tree species limitation in alpine treeline under climate change, influence of treeline changes the carbon and water cycles. There is a press need to explore the driving mechanism for treeline shift from the perspectives of plant physiology and soil carbon and nitrogen cycles.     

Limited prospects for future alpine treeline advance in the Canadian Rocky Mountains

Treeline advance has occurred throughout the twentieth century in mountainous regions around the world; however, local variation and temporal lags in responses to climate warming indicate that the upper limits of some treelines are not necessarily in climatic equilibrium. These observations suggest that factors other than climate are constraining tree establishment beyond existing treelines. Using a seed addition experiment, we tested the effects of seed availability, predation and microsite limitation on the establishment of two subalpine tree species (Picea engelmannii and Abies lasiocarpa) across four treelines in the Canadian Rocky Mountains. The effect of vegetation removal on seedling growth was also determined, and microclimate conditions were monitored. Establishment limitations observed in the field were placed in context with the effects of soil properties observed in a parallel experiment. The seed addition experiment revealed reduced establishment with increasing elevation, suggesting that although establishment within the treeline ecotone is at least partially seed limited, other constraints are more important beyond the current treeline. The effects of herbivory and microsite availability significantly reduced seedling establishment but were less influential beyond the treeline. Microclimate monitoring revealed that establishment was negatively related to growing season temperatures and positively related to the duration of winter snow cover, counter to the conventional expectation that establishment is limited by low temperatures. Overall, it appears that seedling establishment beyond treeline is predominantly constrained by a combination of high soil surface temperatures during the growing season, reduced winter snowpack and unfavourable soil properties. Our study supports the assertion that seedling establishment in alpine treeline ecotones is simultaneously limited by various climatic and nonclimatic drivers. Together, these factors may limit future treeline advance in the Canadian Rocky Mountains and should be considered when assessing the potential for treeline advance in alpine systems elsewhere     

Development of acute frost drought in Rhododendron ferrugineum at the alpine timberline

Summary Acute winter frost drought in Rhododendron ferrugineum at the alpine timberline was provoked by removing the snow that had covered the shrubs and by irradiating and rewarming the twigs in situ in a climatized chamber simulating snow melt conditions. Water potential decreased to -2.3 MPa within three days. After activation of photosynthesis on the 1st day, the COr-uptake decreased during the 2nd day and approached zero at the 3rd day. With advanced desiccation, the internal COr concentration of the leaves increased indicating an impairment of chloroplast functions. The procedure of the disturbances appears to be similar to that of other acute drought events.     

Winter water relations of timberline larch (Larix leptolepis Gord.) on Mt. Fuji

 A krummholz mat of larch, Larix leptolepis, is the predominant growth form near the upper limit (near 2500 m) of the timberline ecotone on the south slope of Mt. Fuji, Japan. On the south-eastern slope, the tree line is lowered to 1600 m because of the last volcanic eruption. The extent and causes of winter desiccation were compared in timberline larch between 2500 m and 1600 m elevation over two winters. Bark abrasion due to wind-blown fine volcanic gravels caused a decrease in bark resistance to water loss and resulted in severe desiccation damage to current-year shoots of krummholz larches in the winter of 1986 – 87 at 2500 m, whereas abraded shoots at 1600 m maintained high water content during both winters. In the winter of 1985 – 86, shoots of krummholz larches at 2500 m did not experience bark abrasion and high water contents were maintained. Experimental abrasion of shoot surfaces resulted in similar results at each elevation. Thus, in timberline larch at 2500 m, abrasion by wind-blown fine volcanic gravels is the primary factor causing winter desiccation damage and krummholz formation. Based on field experiments, the estimated amount of water movement to non-abraded shoots was the same for the two elevations. At 2500 m, water movement to abraded shoots was less than to non-abraded shoots, but the reverse situation was noted at 1600 m. Water supply to abraded shoots at 2500 m was limited and insufficient to compensate for water loss. A cause of limited water supply at 2500 m may be xylem embolism. Received: 13 January 1995 / Accepted: 31 January 1996     

Climatic signals in tree ring of Picea schrenkiana along an altitudinal gradient in the central Tianshan Mountains, northwestern China

Tree-ring samples of Picea schrenkiana (Fisch. et Mey) were studied along an altitudinal gradient in the central Tianshan Mountains, and ring-width chronologies were developed for three sites at different altitudes: low-forest border (1600–1700 m a.s.l.), interior forest (2100–2200 m a.s.l.), and upper treeline (2600–2700 m a.s.l.). Annual ring-width variations were similar among the three sites but variability was greatest at the low-forest border site. The statistical characters of the chronologies showed that mean sensitivity (MS) and standard deviation (SD) decreased with increasing elevation. In other words, the response of tree growth to environmental changes decreased with increasing altitude. To understand the differing response of trees at different elevations to the environmental changes, response function analysis was used to study the relationships between tree-ring widths and mean monthly temperature and total monthly precipitation from 1961 to 2000. The results showed that precipitation was the most important factor limiting tree radial growth in the arid central Tianshan Mountains, precipitation in August of the prior growth year played an important role on tree's radial growth across the entire altitudinal gradient even at the cold, high-elevation treeline site. It is expected that with increasing altitude air temperature decreased and precipitation increased, the importance of precipitation on tree growth decreased, and the response of tree growth to environmental changes decreased, too. This conclusion may be helpful to understand and research the relationship between climatic change and tree growth in arid and semiarid area.     

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.     

Radial Growth of Qilian Juniper on the Northeast Tibetan Plateau and Potential Climate Associations

There is controversy regarding the limiting climatic factor for tree radial growth at the alpine treeline on the northeastern Tibetan Plateau. In this study, we collected 594 increment cores from 331 trees, grouped within four altitude belts spanning the range 3550 to 4020 m.a.s.l. on a single hillside. We have developed four equivalent ring-width chronologies and shown that there are no significant differences in their growth-climate responses during 1956 to 2011 or in their longer-term growth patterns during the period AD 1110–2011. The main climate influence on radial growth is shown to be precipitation variability. Missing ring analysis shows that tree radial growth at the uppermost treeline location is more sensitive to climate variation than that at other elevations, and poor tree radial growth is particularly linked to the occurrence of serious drought events. Hence water limitation, rather than temperature stress, plays the pivotal role in controlling the radial growth of Sabina przewalskii Kom. at the treeline in this region. This finding contradicts any generalisation that tree-ring chronologies from high-elevation treeline environments are mostly indicators of temperature changes.     

Growth and reproduction of the alpine grasshopper <Emphasis Type="Italic">Miramella alpina</Emphasis> feeding on CO<Subscript>2</Subscript>-enriched dwarf shrubs at treeline

The consequences for plant-insect interactions of atmospheric changes in alpine ecosystems are not well understood. Here, we tested the effects of elevated CO₂ on leaf quality in two dwarf shrub species (Vaccinium myrtillus and V. uliginosum) and the response of the alpine grasshopper (Miramella alpina) feeding on these plants in a field experiment at the alpine treeline (2,180 m a.s.l.) in Davos, Switzerland. Relative growth rates (RGR) of M. alpina nymphs were lower when they were feeding on V. myrtillus compared to V. uliginosum, and were affected by elevated CO₂ depending on plant species and nymph developmental stage. Changes in RGR correlated with CO₂-induced changes in leaf water, nitrogen, and starch concentrations. Elevated CO₂ resulted in reduced female adult weight irrespective of plant species, and prolonged development time on V. uliginosum only, but there were no significant differences in nymphal mortality. Newly molted adults of M. alpina produced lighter eggs and less secretion (serving as egg protection) under elevated CO₂. When grasshoppers had a choice among four different plant species grown either under ambient or elevated CO₂, V. myrtillus and V. uliginosum consumption increased under elevated CO₂ in females while it decreased in males compared to ambient CO₂-grown leaves. Our findings suggest that rising atmospheric CO₂ distinctly affects leaf chemistry in two important dwarf shrub species at the alpine treeline, leading to changes in feeding behavior, growth, and reproduction of the most important insect herbivore in this system. Changes in plant-grasshopper interactions might have significant long-term impacts on herbivore pressure, community dynamics and ecosystem stability in the alpine treeline ecotone.     

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.