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Helmut Bauer - One of the best experts on this subject based on the ideXlab platform.
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xylem temperatures during winter in conifers at the alpine Timberline
Agricultural and Forest Meteorology, 2006Co-Authors: Stefan Mayr, Gerhard Wieser, Helmut BauerAbstract:Trees at the alpine Timberline are exposed to extreme temperatures during winter. These temperature conditions affect the hydraulic properties of the xylem because frozen sections block water flux and freeze‐thaw events can induce embolism. In this study, winter temperature regimes in the xylem of Picea abies and Pinus cembra growing at the Central Alps Timberline were analysed. During mid winter nights minimum xylem temperatures were down to � 20.5 8C. In late winter, daytime xylem temperatures up to 22.8 K above air temperature and extreme diurnal temperature amplitudes of up to 30 K were observed. The number of freeze‐thaw events was highest (115 events from September to May) in sun exposed parts at the top of the trees, and decreased towards the trunk base as well as from the trunks surface to its centre and from distal to proximal twig sections. Freezing and thawing rates were higher at the top of the tree (up to 5.4 and 7.0 K h � 1 ) than at the trunk base (up to 4.4 and 4.5 K h � 1 ). Our study demonstrates that sun exposed parts of Timberline trees exhibit most extreme temperature minima and maxima, highest rates of temperature change, as well as thehighest numberoffreeze‐thaw events. These conditions are favourable to induce embolism.In contrast, the xylem of less exposed crown parts was found to be frozen and consequently separated from trunk water resources during prolonged periods. The observed complexity of temperature regimes may be relevant for tree life at high altitudes as temperature has been suggested to be a limiting factor at the Timberline. # 2006 Elsevier B.V. All rights reserved.
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Winter at the alpine Timberline. Why does embolism occur in norway spruce but not in stone pine
Plant Physiology, 2003Co-Authors: Stefan Mayr, Franziska Schwienbacher, Helmut BauerAbstract:Conifers growing at the alpine Timberline are exposed to frost drought and freeze-thaw cycles during winter—stress factors known to induce embolism in tree xylem. The two dominant species of the European Central Alps Timberline were studied: Norway spruce (Picea abies [L.] Karst) and stone pine (Pinus cembra), which usually reaches higher altitudes. We hypothesized to find embolism only at the Timberline and to observe less embolism in stone pine than in Norway spruce due to avoidance mechanisms. Seasonal courses of embolism and water potential were studied at 1,700 and 2,100 m during two winter seasons and correlated to vulnerability (to drought-induced embolism), leaf conductance, and micrometeorological data. Embolism was observed only at the Timberline and only in Norway spruce (up to 49.2% loss of conductivity). Conductivity losses corresponded to low water potentials (down to −3.5 MPa) but also to the number of freeze-thaw events indicating both stress factors to contribute to embolism induction. Decreasing embolism rates—probably due to refilling—were observed already in winter. Stone pine did not exhibit an adapted vulnerability (50% loss of conductivity at −3.5 MPa) but avoided critical potentials (minimum −2.3 MPa): Cuticulare conductance was 3.5-fold lower than in Norway spruce, and angles between needles and axes were found to decrease in dehydrating branches. The extent of conductivity losses in Norway spruce and the spectrum of avoidance and recovery mechanisms in both species indicates winter embolism to be relevant for tree line formation.
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winter drought induced embolism in norway spruce picea abies at the alpine Timberline
Physiologia Plantarum, 2002Co-Authors: Stefan Mayr, Marion Wolfschwenger, Helmut BauerAbstract:At the Timberline in the Central Alps, climatic conditions during winter frequently cause excessive drought stress (frost drought, 'Frosttrocknis'), which we hypothesized to induce cavitation in trees. We investigated the extent of winter-embolism in Norway spruce (Picea abies (L.) Karst.) growing near the Timberline and analysed adaptations in vulnerability and anatomy. We found conductivity losses of up to 100% at the highest elevation (2020 m) correlated with low water potentials down to - 4.0 MPa. Vulnerability thresholds (50% loss in conductivity) decreased from - 3.39 MPa at 800 m to - 3.88 MPa at 1600 m corresponding to a decrease in tracheid cross-sectional area as well as pit and pit pore diameters. These thresholds were lower than potentials measured in embolized twigs near the Timberline at the sampling dates probably due to lower potentials and/or a role of freeze-thaw events earlier in winter. Data indicated refilling processes, which may be of particular relevance for trees at the Timberline, since adaptations in drought-induced vulnerability failed to prevent winter-embolism.
Michal Hejcman - One of the best experts on this subject based on the ideXlab platform.
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natural layering foliation fertility and plant species composition of a fagus sylvatica stand above the alpine Timberline in the giant krkonose mts czech republic
European Journal of Forest Research, 2012Co-Authors: Stanislav Vacek, Michal HejcmanAbstract:Layering, long-term development of foliation, beech nut fertility and plant species composition have never before been studied in Fagus sylvatica stands above the alpine Timberline in Central Europe. F. sylvatica forms and is present above the Timberline (1,260 m a. s. l.) on only one south-facing slope along a 1-km-long stretch of the Giant Mts., which represents the northernmost Timberline featuring F. sylvatica worldwide. We investigated the long-term development of this unique stand in which layering of F. sylvatica was discovered (LP—1,310 m a. s. l.), comparing it with a control plot (CP—1,190 m a. s. l.). Research plots were established in 1980, with monitoring of foliation, masting, beech nut fertility and plant species composition performed over the following 30 years. In the LP plot, F. sylvatica was able to reproduce only clonally by layering. Development of adventitious roots on branches reaching the ground took many years. In the LP plot, F. sylvatica was able to survive heavy air pollution during the 1980s and since that time substantial improvement of its health status has been recorded. Generative reproduction of F. sylvatica in the LP plot was extremely rare, as production of fertile nuts was recorded only once in 2007 and seedlings have never been recorded at all. An improvement in F. sylvatica foliation in the LP plot was probably the reason for a decrease in species richness and cover of bryophytes. The herb layer, dominated by Calamagrostis villosa, was highly stable in the LP plot over the 30-year period, contrasting significantly with the marked changes in species composition observed in the understory of the CP plot where massive regeneration of F. sylvatica occurred. Layering, stability of herb layer and rare masting can be expected in F. sylvatica stands above the alpine Timberline.
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natural layering foliation fertility and plant species composition of a fagus sylvatica stand above the alpine Timberline in the giant krkonose mts czech republic
European Journal of Forest Research, 2012Co-Authors: Stanislav Vacek, Michal HejcmanAbstract:Layering, long-term development of foliation, beech nut fertility and plant species composition have never before been studied in Fagus sylvatica stands above the alpine Timberline in Central Europe. F. sylvatica forms and is present above the Timberline (1,260 m a. s. l.) on only one south-facing slope along a 1-km-long stretch of the Giant Mts., which represents the northernmost Timberline featuring F. sylvatica worldwide. We investigated the long-term development of this unique stand in which layering of F. sylvatica was discovered (LP—1,310 m a. s. l.), comparing it with a control plot (CP—1,190 m a. s. l.). Research plots were established in 1980, with monitoring of foliation, masting, beech nut fertility and plant species composition performed over the following 30 years. In the LP plot, F. sylvatica was able to reproduce only clonally by layering. Development of adventitious roots on branches reaching the ground took many years. In the LP plot, F. sylvatica was able to survive heavy air pollution during the 1980s and since that time substantial improvement of its health status has been recorded. Generative reproduction of F. sylvatica in the LP plot was extremely rare, as production of fertile nuts was recorded only once in 2007 and seedlings have never been recorded at all. An improvement in F. sylvatica foliation in the LP plot was probably the reason for a decrease in species richness and cover of bryophytes. The herb layer, dominated by Calamagrostis villosa, was highly stable in the LP plot over the 30-year period, contrasting significantly with the marked changes in species composition observed in the understory of the CP plot where massive regeneration of F. sylvatica occurred. Layering, stability of herb layer and rare masting can be expected in F. sylvatica stands above the alpine Timberline.
Stefan Mayr - One of the best experts on this subject based on the ideXlab platform.
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Die hard: Timberline conifers survive annual winter embolism
New Phytologist, 2019Co-Authors: Eric Badel, Barbara Beikircher, Feng Feng, Stefan MayrAbstract:During winter, Timberline trees are exposed to drought and frost, factors known to induce embolism. Studies indicated that conifers cope with winter embolism by xylem refilling. We analysed the loss of hydraulic conductivity (LC) in Picea abies branch xylem over 10 years, and correlated winter embolism to climate parameters. LC was investigated by direct X-ray micro-computer tomography (micro-CT) observations and potential cavitation fatigue by Cavitron measurements. Trees showed up to 100% winter embolism, whereby LC was highest, when climate variables indicated frost drought and likely freeze-thaw stress further increased LC. During summer, LC never exceeded 16%, due to hydraulic recovery. Micro-CT revealed homogenous embolism during winter and that recovery was based on xylem refilling. Summer samples exhibited lower LC in outermost compared to older tree rings, although no cavitation fatigue was detected. Long-term data and micro-CT observations demonstrate that Timberline trees can survive annual cycles of pronounced winter-embolism followed by xylem refilling. Only a small portion of the xylem conductivity cannot be restored during the first year, while remaining conduits are refilled without fatigue during consecutive years. We identify important research topics to better understand the complex induction and repair of embolism at the Timberline and its relevance to general plant hydraulics.
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Freezing Stress in Tree Xylem
2016Co-Authors: Stefan Mayr, Thierry AmeglioAbstract:Freezing in plant xylem is a complex process affecting living and dead components. This book chapter gives a brief overview of methods for analyzing freezing dynamics and tissue damage and focuses on the effects of freezing stress in the xylem symplast and apoplast. Survival strategies, such as supercooling, extracellular freezing, or avoidance of critical bubble formation/expansion in conduits are discussed, and insights from experimental and field studies available in the literature summarized. The final part deals with trees at the Alpine Timberline, which are exposed to intense freezing as well as extreme drought stress every winter. Timberline trees are thus an interesting model system to study combined effects of drought and freezing stress in tree xylem and respective avoidance, tolerance, and repair strategies of plants.
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frost drought in conifers at the alpine Timberline xylem dysfunction and adaptations
Ecology, 2006Co-Authors: Stefan Mayr, Franziska Schwienbacher, Uwe G Hacke, P Schmid, Andreas GruberAbstract:Drought stress can cause xylem embolism in trees when the water potential (psi) in the xylem falls below specific vulnerability thresholds. At the alpine Timberline, frost drought is known to cause excessive winter embolism unless xylem vulnerability or transpiration is sufficiently reduced to avoid critical psi. We compared annual courses of psi and embolism in Picea abies, Pinus cembra, Pinus mugo, Larix decidua, and Juniperus communis growing at the Timberline vs. low altitude. In addition, vulnerability properties and related anatomical parameters as well as wood density (D(t)) and wall reinforcement (wall thickness related to conduit diameter) were studied. This allowed an estimate of stress intensities as well as a detection of adaptations that reduce embolism formation. At the alpine Timberline, psi was lowest during winter with corresponding embolism rates of up to 100% in three of the conifers studied. Only Pinus cembra and Larix decidua avoided winter embolism due to moderate psi. Minor embolism was observed at low altitude where the water potentials of all species remained within a narrow range throughout the year. Within species, differences in psi50 (psi at 50% loss of conductivity) at high vs. low altitude were less than 1 MPa. In Picea abies and Pinus cembra, psi50 was more negative at the Timberline while, in the other conifer species, psi50 was more negative at low altitude. Juniperus communis exhibited the lowest (-6.4 +/- 0.04 MPa; mean +/- SE) and Pinus mugo the highest psi50 (-3.34 +/- 0.03 MPa). In some cases, D(t) and tracheid wall reinforcement were higher than in previously established relationships of these parameters with psi50, possibly because of mechanical demands associated with the specific growing conditions. Conifers growing at the alpine Timberline were exposed to higher drought stress intensities than individuals at low altitude. Frost drought during winter caused high embolism rates which were probably amplified by freeze-thaw stress. Although frost drought had a large effect on plant water transport, adaptations in hydraulic safety and related anatomical parameters were observed in only a few of the conifer species studied.
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xylem temperatures during winter in conifers at the alpine Timberline
Agricultural and Forest Meteorology, 2006Co-Authors: Stefan Mayr, Gerhard Wieser, Helmut BauerAbstract:Trees at the alpine Timberline are exposed to extreme temperatures during winter. These temperature conditions affect the hydraulic properties of the xylem because frozen sections block water flux and freeze‐thaw events can induce embolism. In this study, winter temperature regimes in the xylem of Picea abies and Pinus cembra growing at the Central Alps Timberline were analysed. During mid winter nights minimum xylem temperatures were down to � 20.5 8C. In late winter, daytime xylem temperatures up to 22.8 K above air temperature and extreme diurnal temperature amplitudes of up to 30 K were observed. The number of freeze‐thaw events was highest (115 events from September to May) in sun exposed parts at the top of the trees, and decreased towards the trunk base as well as from the trunks surface to its centre and from distal to proximal twig sections. Freezing and thawing rates were higher at the top of the tree (up to 5.4 and 7.0 K h � 1 ) than at the trunk base (up to 4.4 and 4.5 K h � 1 ). Our study demonstrates that sun exposed parts of Timberline trees exhibit most extreme temperature minima and maxima, highest rates of temperature change, as well as thehighest numberoffreeze‐thaw events. These conditions are favourable to induce embolism.In contrast, the xylem of less exposed crown parts was found to be frozen and consequently separated from trunk water resources during prolonged periods. The observed complexity of temperature regimes may be relevant for tree life at high altitudes as temperature has been suggested to be a limiting factor at the Timberline. # 2006 Elsevier B.V. All rights reserved.
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hydraulic efficiency and safety of leader shoots and twigs in norway spruce growing at the alpine Timberline
Journal of Experimental Botany, 2003Co-Authors: Stefan Mayr, Barbara Rothart, Birgit DamonAbstract:Xylem within trees varies in its hydraulic efficiency and safety. Trees at the alpine Timberline were expected to exhibit a hydraulic architecture protecting the leader shoot from winter embolism. Hydraulic and related anatomical parameters were compared as well as seasonal courses of winter embolism in leader shoots and twigs of Norway spruce trees growing at 2000 m. Leader shoots had a 1.4-fold higher specific hydraulic conductivity (ks) as well as a 4.9-fold higher leaf specific conductivity (kl) than side twigs. Vulnerability to drought-induced embolism was lower in leader shoots with a 50% loss of conductivity occurring at a water potential (Y50) 0.7 MPa lower than in twigs. Higher ks and kl were related to 1.2-fold wider tracheid diameters in leader shoots. Lower vulnerability corresponded to smaller pit dimensions but not to wood density. High ks and kl reflect the hydraulic dominance of the leader shoot, which is important for its water supply during summer. Low vulnerability protects the leader shoot from embolism during the winter season. In field measurements at the Timberline during the winter of 2001/2002, conductivity losses of up to 56% were observed only in twigs while leader shoots showed little or no embolism. Results demonstrate that leader shoot xylem is both hydraulically efficient and safe.
Eryuan Liang - One of the best experts on this subject based on the ideXlab platform.
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does increasing intrinsic water use efficiency iwue stimulate tree growth at natural alpine Timberline on the southeastern tibetan plateau
Global and Planetary Change, 2017Co-Authors: Eryuan Liang, Ru Huang, Haifeng Zhu, Xiaohong Liu, Jussi Griesinger, Achim BrauningAbstract:Abstract Little is known about whether increasing iWUE (intrinsic water use efficiency) can stimulate tree growth in the temperature-limited natural Timberlines. Here, we presented the basal area increment (BAI) and iWUE chronologies of Smith fir ( Abies georgei var. smithii ) from 1900 to 2006 at a high-elevation (ca. 4400 m a.s.l.) Timberline in the humid Sygera Mountains, southeastern Tibetan Plateau (TP). The commonality analysis model was applied to investigate the relationships among BAI, temperatures, atmospheric CO 2 concentration ( C a ) and iWUE during 1961–2006, taking into account of both pure and joint effects. As illustrated by the commonality analysis model, the pure effect of C a (39.15%) had more stronger influence on iWUE than that of the Tmean (annul mean temperature, 0.12%), but the joint effect between C a and Tmean (49.79%) on iWUE was stronger than any pure effect for the raw data with an increasing trend. For the first-difference data with year-to-year variations, the pure effect of C a (7.72%) on iWUE was stronger than that of Tmean (0.59%) and the joint effect between them (0.59%). All above imply the C a is the dominant factor for iWUE both for the 46-year trend and interannual variations. In addition, as showed by the commonality analysis model, the pure effect of iWUE (17.57%) played a much more important role on BAI than that of temperatures (smt, mean temperature during June, July, August of current year, 5.92%; amt, mean temperature during September, October, November of previous year, 3.04%), while joint effects of iWUE and temperatures contributed more (27.96%; 13.90%; 16.47%) to the BAI than their pure effects for the raw data with an increasing linear trend. For the first-difference data with interannual variations, the pure effect of smt (12.45%) had much more effect on BAI than that of iWUE (5.49%), at the same time the joint iWUE and temperatures contributed less (3.56%; 1.9%; 1.31%) to the BAI than their pure effects. These results suggest that an increasing iWUE could enhance 46-year increasing tree growth trend at humid and high-elevation Timberlines, supporting the CO 2 fertilization hypothesis, while temperatures dominate the interannual variations of tree growth.
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is the growth of birch at the upper Timberline in the himalayas limited by moisture or by temperature
Ecology, 2014Co-Authors: Eryuan Liang, Binod Dawadi, Neil Pederson, Dieter EcksteinAbstract:Birch (Betula) trees and forests are found across much of the temperate and boreal zones of the Northern Hemisphere. Yet, despite being an ecologically significant genus, it is not well studied compared to other genera like Pinus, Picea, Larix, Juniperus, Quercus, or Fagus. In the Himalayas, Himalayan birch (Betula utilis) is a widespread broadleaf Timberline species that survives in mountain rain shadows via access to water from snowmelt. Because precipitation in the Nepalese Himalayas decreases with increasing elevation, we hypothesized that the growth of birch at the upper Timberlines between 3900 and 4150 m above sea level is primarily limited by moisture availability rather than by low temperature. To examine this assumption, a total of 292 increment cores from 211 birch trees at nine Timberline sites were taken for dendroecological analysis. The synchronous occurrence of narrow rings and the high interseries correlations within and among sites evidenced a reliable cross-dating and a common climatic ...
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has an extending growing season any effect on the radial growth of smith fir at the Timberline on the southeastern tibetan plateau
Trees-structure and Function, 2013Co-Authors: Bo Liu, Binod Dawadi, Dieter Eckstein, Liping Zhu, Eryuan LiangAbstract:The growing season of high-elevation forests will presumably lengthen in response to warming. However, little is known about long-term effects of an extended growing season, particularly on the Tibetan Plateau. Based on a strong correlation between the daily mean temperatures at an automatic weather station at Timberline (4,390 m a.s.l.) in the Sygera Mts., recorded since 2007, and at the meteorological station at Nyingchi (3,000 m a.s.l.), recorded since 1960, we modeled the variation in daily mean temperature at the Timberline back to 1960. The onset and end of the growing season at the Timberline were determined by the first and the last day within a year when the mean daily air temperature equals or exceeds, respectively falls below, +5 °C for at least 5 days. From 1960 to 2010, the estimated length of the growing season at the Timberline has significantly extended by 21.2 days, resulting mainly from a significant delay of its end (by 14.6 days) rather than from an earlier onset (by 6.6 days). Nevertheless, the variation of the length of the growing season did not exhibit any significant effect on the radial growth of Smith fir at the Timberlines. Thus, tree-ring width is still a reliable proxy for summer temperature.
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a short note on linkage of climatic records between a river valley and the upper Timberline in the sygera mountains southeastern tibetan plateau
Grid and Pervasive Computing, 2011Co-Authors: Eryuan Liang, Bo Liu, Liping Zhu, Zhiyong YinAbstract:Abstract Tree-ring data from Timberlines have been widely used to reconstruct past temperature variability. High-quality reconstructions depend on successful calibrations in which tree-ring records are compared with instrumental observations of climatic factors to establish quantitative relationships between tree growth and climate. Climatic data used in the calibrations are mostly from nearby meteorological stations, located generally in the valleys near human settlements of mountainous areas, regardless of whether climatic records at low elevations are representative for the upper Timberline. In order to better understand the characteristics of the alpine environment at the upper Timberline of blackseed juniper (Juniperus saltuaria) in the eastern side of the Sygera Mountains, southeastern Tibetan Plateau, an automatic weather station was established. We found that the variation in the daily/5-day/10-day/monthly mean temperatures and sums of precipitation on the valley bottom (3000 m a.s.l.) as well as the daily/seasonal sums of precipitation from the TRMM (Tropical Rainfall Measuring Mission) are highly correlated with the measurements at the upper Timberline (4400 m a.s.l.). Thus, the variations of the valley bottom temperature and precipitation records are confident indicators of the conditions at the upper Timberline on the southeastern Tibetan Plateau and hence can be used for the calibration in the dendroclimatic reconstructions based on Timberline tree-ring data. For 5-day mean temperature pooled by individual months, the R2 values of the regression models > 0.60 between the valley bottom and Timberline in February–August and October, and 0.60 in March–July and October–December, and
Stanislav Vacek - One of the best experts on this subject based on the ideXlab platform.
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natural layering foliation fertility and plant species composition of a fagus sylvatica stand above the alpine Timberline in the giant krkonose mts czech republic
European Journal of Forest Research, 2012Co-Authors: Stanislav Vacek, Michal HejcmanAbstract:Layering, long-term development of foliation, beech nut fertility and plant species composition have never before been studied in Fagus sylvatica stands above the alpine Timberline in Central Europe. F. sylvatica forms and is present above the Timberline (1,260 m a. s. l.) on only one south-facing slope along a 1-km-long stretch of the Giant Mts., which represents the northernmost Timberline featuring F. sylvatica worldwide. We investigated the long-term development of this unique stand in which layering of F. sylvatica was discovered (LP—1,310 m a. s. l.), comparing it with a control plot (CP—1,190 m a. s. l.). Research plots were established in 1980, with monitoring of foliation, masting, beech nut fertility and plant species composition performed over the following 30 years. In the LP plot, F. sylvatica was able to reproduce only clonally by layering. Development of adventitious roots on branches reaching the ground took many years. In the LP plot, F. sylvatica was able to survive heavy air pollution during the 1980s and since that time substantial improvement of its health status has been recorded. Generative reproduction of F. sylvatica in the LP plot was extremely rare, as production of fertile nuts was recorded only once in 2007 and seedlings have never been recorded at all. An improvement in F. sylvatica foliation in the LP plot was probably the reason for a decrease in species richness and cover of bryophytes. The herb layer, dominated by Calamagrostis villosa, was highly stable in the LP plot over the 30-year period, contrasting significantly with the marked changes in species composition observed in the understory of the CP plot where massive regeneration of F. sylvatica occurred. Layering, stability of herb layer and rare masting can be expected in F. sylvatica stands above the alpine Timberline.
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natural layering foliation fertility and plant species composition of a fagus sylvatica stand above the alpine Timberline in the giant krkonose mts czech republic
European Journal of Forest Research, 2012Co-Authors: Stanislav Vacek, Michal HejcmanAbstract:Layering, long-term development of foliation, beech nut fertility and plant species composition have never before been studied in Fagus sylvatica stands above the alpine Timberline in Central Europe. F. sylvatica forms and is present above the Timberline (1,260 m a. s. l.) on only one south-facing slope along a 1-km-long stretch of the Giant Mts., which represents the northernmost Timberline featuring F. sylvatica worldwide. We investigated the long-term development of this unique stand in which layering of F. sylvatica was discovered (LP—1,310 m a. s. l.), comparing it with a control plot (CP—1,190 m a. s. l.). Research plots were established in 1980, with monitoring of foliation, masting, beech nut fertility and plant species composition performed over the following 30 years. In the LP plot, F. sylvatica was able to reproduce only clonally by layering. Development of adventitious roots on branches reaching the ground took many years. In the LP plot, F. sylvatica was able to survive heavy air pollution during the 1980s and since that time substantial improvement of its health status has been recorded. Generative reproduction of F. sylvatica in the LP plot was extremely rare, as production of fertile nuts was recorded only once in 2007 and seedlings have never been recorded at all. An improvement in F. sylvatica foliation in the LP plot was probably the reason for a decrease in species richness and cover of bryophytes. The herb layer, dominated by Calamagrostis villosa, was highly stable in the LP plot over the 30-year period, contrasting significantly with the marked changes in species composition observed in the understory of the CP plot where massive regeneration of F. sylvatica occurred. Layering, stability of herb layer and rare masting can be expected in F. sylvatica stands above the alpine Timberline.