Variation of mobile carbon reserves in trees at the alpine treeline ecotone is under environmental control

? In low temperature-adapted plants, including treeline trees, light-saturated photosynthesis is considerably less sensitive to temperature than growth. As a consequence, all plants tested so far show increased nonstructural carbohydrate (NSC) tissue concentrations when exposed to low temperatures. Reduced carbon supply is thus an unlikely cause for low temperature range limits of plants. For altitudinal treeline trees there is, however, a possibility that high NSC genotypes have been selected. ? Here, we explored this possibility using afforestations with single-provenance conifers along elevational gradients in the Southern Chilean Andes and the Swiss Alps. ? Tree growth was measured at each of four approximately equidistant elevations at and below the treeline. Additionally, at the same elevations, needle, branch and stem sapwood tissues were collected to determine NSC concentrations. ? Overall, growth decreased and NSC concentrations increased with elevation. Along with previous empirical and experimental studies, the findings of this study provide no indication of NSC reduction at the treeline; NSC increased in most species (each represented by one common population) towards their upper climatic limit. The disparity between carbon acquisition and structural carbon investment at low temperature (accumulation of NSC) thus does occur even among genotypes not adapted to treeline environments.     

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。     

Temporal variations of mobile carbohydrates in Abies fargesii at the upper tree limits

Low temperatures are associated high‐altitude treelines, but the functional mechanism of treeline formation remains controversial. The relative contributions of carbon limitation (source activity) and growth limitation (sink activity) require more tests across taxa and regions. We examined temporal variations of mobile carbon supply in different tissues of Abies fargesii across treeline ecotones on north‐ and south‐facing slopes of the Qinling Mountains, China. Non‐structural carbohydrate (NSC) concentrations in tissues along the altitudinal gradient on both slopes changed significantly in the early and late growing season, but not in the mid‐growing season, indicating the season‐dependent carbon supply status. Late in the growing season on both slopes, trees at the upper limits had the highest NSC concentrations and total soluble sugars and lowest starch concentrations compared to trees at the lower elevations. NSC concentrations tended to increase in needles and branches throughout the growing season with increasing elevation on both slopes, but declined in roots and stems. NSC concentrations across sampling dates also indicated increases in needles and branches, and decreases in roots and stem with increasing elevation. Overall altitudinal trends of NSC in A. fargesii revealed no depletion of mobile carbon reserves at upper elevation limits, suggesting limitation of sink activity dominates tree life across treeline ecotones in both north‐ and south‐facing slopes. Carbon reserves in storage tissues (especially roots) in the late growing season might also play an important role in winter survival and early growth in spring at upper elevations on both slopes, which define the uppermost limit of A. fargesii.     

Variation in carbohydrate source–sink relations of forest and treeline white spruce in southern, interior and northern Alaska

Two opposing hypotheses have been presented to explain reduced tree growth at the treeline, compared with growth in lower elevation or lower latitude forests: the carbon source and sink limitation hypotheses. The former states that treeline trees have an unfavorable carbon balance and cannot support growth of the magnitude observed at lower elevations or latitudes, while the latter argues that treeline trees have an adequate carbon supply, but that cold temperatures directly limit growth. In this study, we examined the relative importance of source and sink limitation in forest and treeline white spruce (Picea glauca) in three mountain ranges from southern to northern Alaska. We related seasonal changes in needle nonstructural carbohydrate (NSC) content with branch extension growth, an approach we argue is more powerful than using needle NSC concentration. Branch extension growth in the southernmost Chugach Mountains was much greater than in the White Mountains and the Brooks Range. Trees in the Chugach Mountains showed a greater seasonal decline in needle NSC content than trees in the other mountain ranges, and the seasonal change in NSC was correlated with site-level branch growth across mountain ranges. There was no evidence of a consistent difference in branch growth between the forest and treeline sites, which differ in elevation by approximately 100 m. Our results point to a continuum between source and sink limitation of growth, with high-elevation trees in northern and interior Alaska showing greater evidence of sink limitation, and those in southern Alaska showing greater potential for source limitation.     

Carbohydrate reserves in the facilitator cushion plant <Emphasis Type="Italic">Laretia acaulis</Emphasis> suggest carbon limitation at high elevation and no negative effects of beneficiary plants

The elevational range of the alpine cushion plant Laretia acaulis (Apiaceae) comprises a cold upper extreme and a dry lower extreme. For this species, we predict reduced growth and increased non-structural carbohydrate (NSC) concentrations (i.e. carbon sink limitation) at both elevational extremes. In a facilitative interaction, these cushions harbor other plant species (beneficiaries). Such interactions appear to reduce reproduction in other cushion species, but not in L. acaulis. However, vegetative effects may be more important in this long-lived species and may be stronger under marginal conditions. We studied growth and NSC concentrations in leaves and stems of L. acaulis collected from cushions along its full elevational range in the Andes of Central Chile. NSC concentrations were lowest and cushions were smaller and much less abundant at the highest elevation. At the lowest elevation, NSC concentrations and cushion sizes were similar to those of intermediate elevations but cushions were somewhat less abundant. NSC concentrations and growth did not change with beneficiary cover at any elevation. Lower NSC concentrations at the upper extreme contradict the sink-limitation hypothesis and may indicate that a lack of warmth is not limiting growth at high-elevation. At the lower extreme, carbon gain and growth do not appear more limiting than at intermediate elevations. The lower population density at both extremes suggests that the regeneration niche exerts important limitations to this species’ distribution. The lack of an effect of beneficiaries on reproduction and vegetative performance suggests that the interaction between L. acaulis and its beneficiaries is probably commensalistic.     

Carbon dynamics in the deciduous broadleaf tree Erman's birch (Betula ermanii) at the subalpine treeline on Changbai Mountain,Northeast China

Premise of the Study

The growth limitation hypothesis (GLH) and carbon limitation hypothesis (CLH) are two dominant explanations for treeline formation. The GLH proposes that low temperature drives the treeline through constraining C sinks more than C sources, and it predicts that non‐structural carbohydrate (NSC) levels are static or increase with elevation. Although the GLH has received strong support globally for evergreen treelines, there is still no consensus for deciduous treelines, which experience great asynchrony between supply and demand throughout the year.

Methods

We investigated growth and the growing‐season C dynamics in a common deciduous species, Erman's birch (Betula ermanii), along an elevational gradient from the closed forest to the treeline on Changbai Mountain, Northeast China. Samples were collected from developing organs (leaves and twigs) and main storage organs (stems and roots) for NSC analysis.

Key Results

Tree growth decreased with increasing elevation, and NSC concentrations differed significantly among elevations, organs, and sampling times. In particular, NSC levels varied slightly during the growing season in leaves, peaked in the middle of the growing season in twigs and stems, and increased continuously throughout the growing season in roots. NSCs also tended to increase or vary slightly in developing organs but decreased significantly in mature organs with increasing elevation.

Conclusions

The decrease in NSCs with elevation in main storage organs indicates support for the CLH, while the increasing or static trends in new developing organs indicate support for the GLH. Our results suggest that the growth limitation theory may be less applicable to deciduous species' growth than to that of evergreen species.     

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.     

Unique responses of respiration,growth, and non-structural carbohydrate storage in sink tissue of conifer seedlings to an elevation gradient at timberline

The role of carbon balance, and particularly carbon sinks, to forest boundaries and climate responses is a major question in forest ecophysiology. At timberline, low-temperature limitations on carbon-sink processes of stem and especially root tissue have been implicated in hypotheses of the upper range limits to tree distributions. Studies on carbon sinks in root and stem tissue of trees at timberline typically report variation in only one carbon sink, such as either growth, respiration, or non-structural carbohydrates (NSCs). However, these carbon sinks may differ in their response to elevation. We asked how three carbon-sink processes in root and stem tissue (i.e. all tissue below the crown of needles) change in conifer seedlings growing from the lower (2450 m) to the upper (3000 m) edges of the timberline ecotone throughout their first growth season. We repeatedly measured respiration (mg⁻¹ and individual⁻¹), growth (relative growth rates [RGR] and biomass), and NSCs in root and stem tissue of Abies lasiocarpa and Pseudotsuga menziesii.RGR of root and stem tissue were less at the upper compared to lower elevation, but only for the first few weeks of the growing season. Total biomass of root and stem tissue was generally less at the upper site, apparently due to low early season RGR, but ultimately did not significantly differ between sites by the end of the growing season. Unlike growth, respiration rates (mg⁻¹) did not differ between elevations during any period of the growing season. Nevertheless, total respiration of CO₂ from root and stem tissue (individual⁻¹) was 22% less at the upper site, which was attributable to less biomass. NSCs of root and stem tissue, specifically starch, were overall greater at the upper site, particularly for A. lasiocarpa at the end of the season, which did not parallel spatiotemporal trends in growth or respiration. The differences in seasonal trends and the effects of elevation on carbon sinks indicate a degree of independence or uncoupling of growth, respiration, and NSCs of root and stem tissue, which is not commonly appreciated in hypotheses about physiological limitations for trees at timberline.     

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.     

Further records for Skye

As one of the most conspicuous boundaries between different types of ecosystem, the alpine timberline has attracted the interest of researchers in Innsbruck for more than 85 years. It is evident that the tree life form here reaches its absolute limit and is constrained by the harsh environment. However, the nature of constraints is less obvious and requires detailed ecophysiological analyses as exemplified in this review, which concentrates on the Central Tyrolean Alps. After focusing on the timberline environment, effects of elevation on the water relations and the CO₂ gas exchange of timberline-associated conifer species will be outlined towards conclusions on tree growth and treeline fluctuations. Presently, temperature is suggested to be the key environmental factor in determining the transition from forests to alpine shrub and grassland. As many physiological aspects are influenced directly or indirectly by the temperature regime at the alpine timberline, tree life at the timberline exists close to a number of physiological limits which interact to determine the position of the alpine timberline and modulate the upper boundary of tree life. Nevertheless, understanding of the altitude of the treeline must also consider seedling establishment, especially when evaluating treeline advances and global change effects in a changing environment.     

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.     

End of season carbon supply status of woody species near the treeline in western China

As trees and shrubs approach the high elevation tree limit, it is often assumed that they fall short in photosynthate (source limitation). Alternatively, low temperature may restrict carbon investment (growth, sink limitation). The content of mobile non-structural carbohydrates (NSC) in tissues is considered a measure of the carbon source–sink balance. To test the source vs. sink limitation hypothesis, we compared late-season NSC concentrations of various woody taxa across altitudinal gradients from the subalpine forest to the treeline at the eastern edge of the Tibetan Plateau. Since we were interested in the generality of trends, we present “community” trends across four taxa, namely Quercus aquifolioides, Abies faxoniana, Rhododendron fabri subsp. prattii and Sorbus rufopilosa. NSC concentrations increased significantly with altitude in branch wood, current-year and last-year leaves, while there were no significant trends in stem sapwood and root xylem. The sugar to starch ratio was roughly 1:1 in branches and evergreen leaves, while stems and roots showed a higher starch fraction. Analyses of total nitrogen in leaves and wood tissues indicated no change in the trees’ nitrogen supply with elevation. The overall altitudinal trends of NSC in this group of woody plant species revealed no depletion of carbon reserves near the tree limit, suggesting that sink limitation predominates woody plant life across this treeline ecotone community.     

Similar variation in carbon storage between deciduous and evergreen treeline species across elevational gradients

Background and Aims

The most plausible explanation for treeline formation so far is provided by the growth limitation hypothesis (GLH), which proposes that carbon sinks are more restricted by low temperatures than by carbon sources. Evidence supporting the GLH has been strong in evergreen, but less and weaker in deciduous treeline species. Here a test is made of the GLH in deciduous–evergreen mixed species forests across elevational gradients, with the hypothesis that deciduous treeline species show a different carbon storage trend from that shown by evergreen species across elevations.

Methods

Tree growth and concentrations of non-structural carbohydrates (NSCs) in foliage, branch sapwood and stem sapwood tissues were measured at four elevations in six deciduous–evergreen treeline ecotones (including treeline) in the southern Andes of Chile (40°S, Nothofagus pumilio and Nothofagus betuloides; 46°S, Nothofagus pumilio and Pinus sylvestris) and in the Swiss Alps (46°N, Larix decidua and Pinus cembra).

Key Results

Tree growth (basal area increment) decreased with elevation for all species. Regardless of foliar habit, NSCs did not deplete across elevations, indicating no shortage of carbon storage in any of the investigated tissues. Rather, NSCs increased significantly with elevation in leaves (P < 0·001) and branch sapwood (P = 0·012) tissues. Deciduous species showed significantly higher NSCs than evergreens for all tissues; on average, the former had 11 % (leaves), 158 % (branch) and 103 % (sapwood) significantly (P < 0·001) higher NSCs than the latter. Finally, deciduous species had higher NSC (particularly starch) increases with elevation than evergreens for stem sapwood, but the opposite was true for leaves and branch sapwood.

Conclusions

Considering the observed decrease in tree growth and increase in NSCs with elevation, it is concluded that both deciduous and evergreen treeline species are sink limited when faced with decreasing temperatures. Despite the overall higher requirements of deciduous tree species for carbon storage, no indication was found of carbon limitation in deciduous species in the alpine treeline ecotone.     

长白山不同海拔岳桦非结构碳水化合物含量的变化

 通常认为, 随着林木不断接近其海拔分布极限, 光合作用产量不断下降, 导致碳水化合物供应不足(碳供应限制), 或者低温限制了碳投资(生长限制)。植物组织内非结构性碳水化合物(Nonstructural carbohydrates, NSC)的含量反映了植物碳供应与碳吸收的平衡。为了检验“碳供应限制”和“生长抑制”假说, 我们对长白山海拔1 700~ 2 050 m的自然生境下生长的岳桦(Betula ermanii)的叶片和枝条组织的NSC含量进行了比较。结果表明: 岳桦叶片的NSC含量随海拔升高变化不显著, 枝条的NSC含量随海拔升高显著增加; 叶片和枝条中淀粉含量与可溶性总糖含量的比值均随海拔的升高而减小; 林线附近的岳桦林木不存在碳水化合物供应不足的问题, 这在一定程度上表明生长限制导致长白山岳桦林线的形成。     

贡嘎山地区不同海拔冷杉比叶质量和非结构性碳水化合物含量变化

以贡嘎山地区磨西山谷东北坡和康定山谷西南坡为样地,研究了高山林线和低海拔冷杉休眠期和生长旺盛期针叶比叶质量和组织非结构性碳水化合物含量.结果表明:生长在温暖湿润的磨西山谷东北坡的冷杉针叶比叶质量和组织非结构性碳水化合物含量均高于炎热干燥的康定山谷西南坡;高山林线冷杉比叶质量大于低海拔冷杉;高山林线冷杉组织非结构性碳水化合物含量总体高于低海拔冷杉,且生长期比休眠期更为显著.研究结果不支持"碳供应不足导致高山林线形成"的假说.     

Variation in ecophysiological properties among conifers at an ecotonal boundary: comparison of establishing seedlings and established adults at timberline

Question: Environmental limitations on carbon acquisition and use can impact successful establishment and restrict a species range, such as for trees at timberline. How do ecophysiological properties associated with carbon uptake and allocation change along an elevation gradient for adult compared to seedling conifers in a timberline ecotone? Location: Teton Range in the Rocky Mountains, Wyoming, USA Methods: Photochemical efficiency (F_v/F_m), specific leaf area (SLA) and foliar nonstructural carbohydrates (NSC) were compared along an elevation gradient (2200‐3050 m) among two age classes (seedling and adult) and species (Abies lasiocarpa and Pseudotsuga menziesii) at timberline during mid‐summer. Results: F _v/F_m values were relatively high in both seedlings and adults across the elevation gradient, with the exception of a low F_v/F_m for seedlings in the site having the lowest soil temperatures. SLA was surprisingly constant within each age class and species across the timberline ecotone. Foliar NSCs did not increase or decrease consistently with elevation in either age class. Nonetheless, NSCs were highly variable among sites, but only in seedlings and not in adults. Conclusions: Elevation effects on these indicators of the efficiency of interception and use of sunlight in the timberline ecotone were minimal during the optimum period of the growing season. However, establishing seedlings had a tendency to exhibit greater responses to the timberline environment, particularly in their allocation of photosynthate to NSC, which may be a constraint to tree establishment at high elevations.     

Gradual expansion of moisture sensitive Abies spectabilis forest in the Trans-Himalayan zone of central Nepal associated with climate change

Population structure and tree recruitment dynamics in the natural treeline ecotone of high mountains are strong indicators of vegetation responses to climate. Here, we examined recruitment dynamics of Abies spectabilis across the treeline ecotone (3439–3638 m asl) of Chimang Lekh of Annapurna Conservation Area in the Trans-Himalayan zone of central Nepal. Dendrochronological techniques were used to establish stand age structure by ring counts of adults, and by terminal bud scar count for seedlings and saplings. The results showed abundant seedling recruitment, higher regenerative inertia and colonization with a consistent range shift of the A. spectabilis treeline. The upward expansion of this sub-alpine treeline was found to be driven by a strong dependence of seedling recruitment and radial growth on snowmelt and precipitation as temperatures rise. The radial growth of A. spectabilis at the alpine timberline ecotone (ATE) and closed timberline forest (CTF) showed sensitivity to spring season (March–May) climate. Tree ring indices of CTF showed a strong positive correlation with spring and annual precipitation, and a significant negative correlation with spring and annual temperature, however, moisture sensitivity was less strong at ATE than CTF.     

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.     

Global patterns of mobile carbon stores in trees at the high‐elevation tree line

Aim Across all latitudes, high‐elevation tree lines represent a drastic change in the dominant plant life‐form, from upright trees to low‐stature alpine plants. Although associated with low temperatures, the physiological mechanisms controlling this boundary are still not clear. The growth‐limitation hypothesis assumes a direct low‐temperature restriction of tissue formation at otherwise sufficient photoassimilation. In order to test this hypothesis, we present a global synthesis of previously published and new data on tree carbon supply status at high‐elevation tree lines. Location Global; 13 regions between 68° N and 45° S. Methods Late‐season concentrations of non‐structural carbohydrates (NSC) in foliage and branch wood were measured at three elevations across the tree line ecotones, from upper tall forests (timber line) to the edges of aborescent tree growth (tree line). Year‐round records of ?10 cm soil temperatures were taken at the tree line. Results Despite large differences in elevation and season length, the mean growing season temperature at the tree line was similar (approximately 6.6 °C) between all sites. NSC concentrations were not depleted at any of the elevation gradients between timber line and tree line, indicating no shortage of C at the uppermost tree stands. Tested across all sites, NSC concentrations in fact significantly increased with elevation. On average, NSC increased by 18% in leaves and 26% in branch wood from the lowest to the highest stands, primarily due to higher starch concentrations rather than free sugars. Hence, these responses do not reflect osmotic adjustments to lower temperatures at the end of the growing season. Main conclusions This global data set contributes to a mechanistic understanding of tree line formation based on biological principles across climatic zones and tree genera. No evidence of C shortage was found at the high‐elevation tree line in either seasonal and non‐seasonal regions. The increasing trend of NSC concentrations with elevation is in line with the growth‐limitation hypothesis.     

Altitudinal variations in seedling and sapling density and age structure of timberline tree species in the Sergyemla Mountains, southeast Tibet

The growth situation of seedlings of treeline species can be used as an indicator for the upper or lower shift of treeline under global climate change, since any advance of a treeline would require seedling survival above the current treeline elevation. We investigated seedling and sapling density of two dominant conifers Abies georgei var. smithii and Sabina saltuaria at lower, middle and upper altitude of two contrasting slopes in Sergyemla Mountain, southeast Tibet. Seedling and sapling age structure of A. georgei var. smithii was analyzed further. Seedling density of both species, as well as sapling density of S. saltuaria, showed a declining trend as elevation increased. Sapling density of A. georgei var. smithii was somewhat higher at middle altitude than other positions. The frequency of old-aged A. georgei var. smithii seedlings/saplings (age >10 year) was the lowest at the highest altitude and tended to increase with the declining elevation. The results indicate that the higher density of seedling and sapling in high-altitude might be attributed to better soil moisture availability and solar insolation than in the mid- and low-altitudes during the growth season. Yet confined by other environmental factors, A. georgei var. smithii seedlings in high-altitude suffer from lower survival rate and lower possibility to grow tall compared with those in mid- and low-altitudes.