Gradual decline in genetic diversity in Swiss stone pine populations (Pinus cembra) across Switzerland suggests postglacial re-colonization into the Alps from a common eastern glacial refugium

Molecular genetic markers may reveal informative patterns of population processes such as historical migration, which may substantiate inference on postglacial re-colonization inferred, e.g., from fossil records. Palynological records of Swiss stone pine (Pinus cembra) suggest that the species has re-colonized the central Alps from a southeastern Alpine refugium after the last glacial maximum. Such a migration pathway likely resulted in a gradual decrease in genetic diversity with increasing distance to the glacial refugium, owing to founder events at the leading range edge. The present distribution of P. cembra in Switzerland consists of two rather distinct ranges, namely the inner-alpine parts of the Grisons and Valais, respectively, and additional disjunct occurrences in the northern and southern periphery of the Alps as well as between the two main ranges. We screened chloroplast microsatellite loci on 39 Swiss P. cembra populations and show that the genetic structure detected was congruent with a common ancestry from a single glacial refugium, likely located at the (south-)eastern periphery of the Alps. In contrast, our data rejected the alternative hypothesis of a distinct genetic separation of the two main ranges of Swiss stone pine in Switzerland. We further show that low genetic diversity within and high differentiation among peripheral populations in the northern Alps as well as the genetic differentiation between core and peripheral populations reflect genetic drift as a consequence of colonization history and limited gene flow by pollen and seed.

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Zusammenfassung  Molekulargenetische Marker enthalten wertvolle Information über Populationsprozesse wie historische Wanderungen, wodurch Annahmen zur postglazialen Wiederbesiedelung, beispielsweise abgeleitet von Fossilfunden, unterstützt werden k?nnen. Palynologische Funde von Arve (Pinus cembra) lassen vermuten, dass diese Art nach dem letzten glazialen Maximum von einem Refugium in den süd?stlichen Alpen wieder in die zentralen Alpen eingewandert ist. Ein solcher Rückwanderungsweg dürfte aufgrund von Gründereffekten an der Wanderungsfront eine graduelle Verringerung der genetischen Variation mit zunehmender Distanz zum Glazialrefugium bewirkt haben. Die heutige Verbreitung von P. cembra in der Schweiz weist zwei deutlich getrennte Gebiete auf, n?mlich inneralpine Bereiche der Kantone Graubünden und Wallis, mit jeweils isolierten Vorkommen in den n?rdlichen und südlichen Randalpen. Wir untersuchten Chloroplasten-Mikrosatelliten in 39 Schweizer Populationen von P. cembra und zeigen, dass die gefundene genetische Struktur übereinstimmt mit der Annahme einer gemeinsamen Abstammung aus einem einzigen Glazialrefugium, welches vermutlich am (süd-)?stlichen Rand der Alpen lag. Im Gegensatz dazu widerlegen unsere Daten die alternative Hypothese einer deutlichen genetischen Trennung der zwei Hauptvorkommen der Arve in der Schweiz. Im Weiteren zeigen unsere Resultate eine geringe genetische Variation innerhalb und einen hohen Differenzierungsgrad zwischen n?rdlichen Randalpenvorkommen, sowie eine genetische Differenzierung zwischen zentralen und peripheren Populationen. Dies weist auf genetische Drift hin, welche die Besiedlungsgeschichte und einen beschr?nkten Genfluss durch Pollen und Samen widerspiegelt.

     

Among‐tree variability and feedback effects result in different growth responses to climate change at the upper treeline in the Swiss Alps

Upper treeline ecotones are important life form boundaries and particularly sensitive to a warming climate. Changes in growth conditions at these ecotones have wide‐ranging implications for the provision of ecosystem services in densely populated mountain regions like the European Alps. We quantify climate effects on short‐ and long‐term tree growth responses, focusing on among‐tree variability and potential feedback effects. Although among‐tree variability is thought to be substantial, it has not been considered systematically yet in studies on growth–climate relationships. We compiled tree‐ring data including almost 600 trees of major treeline species (Larix decidua, Picea abies, Pinus cembra, and Pinus mugo) from three climate regions of the Swiss Alps. We further acquired tree size distribution data using unmanned aerial vehicles. To account for among‐tree variability, we employed information‐theoretic model selections based on linear mixed‐effects models (LMMs) with flexible choice of monthly temperature effects on growth. We isolated long‐term trends in ring‐width indices (RWI) in interaction with elevation. The LMMs revealed substantial amounts of previously unquantified among‐tree variability, indicating different strategies of single trees regarding when and to what extent to invest assimilates into growth. Furthermore, the LMMs indicated strongly positive temperature effects on growth during short summer periods across all species, and significant contributions of fall (L. decidua) and current year's spring (L. decidua, P. abies). In the longer term, all species showed consistently positive RWI trends at highest elevations, but different patterns with decreasing elevation. L. decidua exhibited even negative RWI trends compared to the highest treeline sites, whereas P. abies, P. cembra, and P. mugo showed steeper or flatter trends with decreasing elevation. This does not only reflect effects of ameliorated climate conditions on tree growth over time, but also reveals first signs of long‐suspected negative and positive feedback of climate change on stand dynamics at treeline.     

The Early Holocene treeline in the southern French Alps: new evidence from travertine formations

Aims The present paper concerns the analysis of macro‐remains (plant imprints) from high altitude travertine deposits dating back to the early Holocene (9800 BP ). Our results allow us to discuss treeline location and ecology, and to supplement previous data based on pollen from natural sediments and charcoal from natural soil. Location The travertine under study is located in the Queyras massif, in the southern French Alps, along the Italian border. The site is situated on a south‐facing slope. Methods The travertine deposit has been dated by ²³⁰Th/²³⁴U. Taxonomic identification of plant imprints was achieved by comparison of the morphology of fossil specimens with extant material. Results Imprints of Pinus uncinata (cones and needles), broad‐leaved trees (Betula cf. pubescens, Populus tremula and Salix spp.), and unidentified trunks have been found. The plant assemblage records the existence at a high altitude (2200 m) of shade‐intolerant vegetation at around 9800 BP. Our data indicate that the treeline limit was at least 100 m higher than previously thought. The morphological features of the travertine, the plant assemblage and trunk diameters indicate that during the travertinization process, vegetation around the site was probably dense with tall upright trees. Main conclusions Our data show that Pinus uncinata and broad‐leaved trees developed at 2200 m, while regional pollen analyses locate the treeline at lower altitudes. Different research methods appear to provide different results. Around 9800 BP tree regeneration and growth were made possible up to at least 2200 m a.s.l., probably as a result of warmer and wetter summers. Warmer conditions are inferred from the predicted increase in incoming solar radiation based on calculated orbital variations, and wetter conditions are inferred from the build‐up of the travertine.     

Growth trends and dynamics in sub‐alpine forest stands in the Varaita Valley (Piedmont,Italy) and their relationships with human activities and global change

Abstract. A study of the forest lines, tree lines and the structures of the sub‐alpine forest was performed in Vallone Vallanta and in Alevé forest in the Varaita Valley (Cottian Alps, Piedmont, Italy). Forest‐ and tree lines were analysed over 1728 ha while forest structures were studied on six 3000‐m² plots located at the tree line (2), at the forest line (2) and inside the sub‐alpine forest (2). Dendro‐ecological analysis of living plants and stumps showed that Larix decidua was more abundant in the past than today and that Pinus cembra has expanded, both upwards and within sub‐alpine forests. Age structure analysis revealed that the current sub‐alpine forest stands were established 200–220 yr ago, probably following a clearcut. At the forest lines the tree density decreases, and some trees are more than 500 yr old, whereas at the tree lines most of the trees (almost exclusively Pinus cembra) are younger than 100 yr. Growth dynamics were investigated both by observing Basal Area Increment (BAI) in the old and dominant trees, and by comparing the BAIs of classes of trees with a given cambial age range in different time periods. The results showed that the growth rates of mature Pinus cembra and Larix decidua had increased. These increments are more substantial for Pinus than for Larix. The growth rate of young trees (< 100 yr) of both species has decreased over recent decades. This could be due to competition caused by increased tree densities that have resulted from a decrease in grazing.     

Changes in landscape structure in the northwestern Alps over the last 7000 years: lessons from soil charcoal

Abstract. Current land‐use a bandonment and the current rise in temperature in the Alps both suggest that tree limits may change. When it is assumed that the climate of the early mid‐Holocene between 8000 and 5000 yr before present is analogous to that of the predicted climate of the late 21st century, palaeo‐ecological studies of the early Holocene may provide data for the prediction of the vegetation pattern in a century from now. It appears that mid‐Holocene charcoal assemblages can be used to reconstruct the spatial patterns of the vegetation before, or during, the practice of slash‐and‐burn. Correspondence analysis (CA) of charcoal assemblages shows that an important ecological gradient is determined by elevation. However CA also shows that charcoal assemblages in profiles between 1700 and 2100 m a.s.l. are roughly stratified: the more recent assemblages from the topmost centimetres of soil are intermediate between the lowermost assemblages and assemblages from higher elevations. This suggests that the woody communities at the highest elevation were located at lower elevations at a later date. The taxonomic diversity of the soil charcoal assemblages has been compared to that of present‐day phytosociological relevés after transformation to charcoal‐equivalent data. This comparison revealed that the vegetation pattern along the altitudinal gradient in the mid‐Holocene was different from that at present. The assemblages indicate that some communities disappeared, that Picea is a late‐Holocene invading species, and that there is no strict modern analogue for the vegetation structure prior to that of 3000 yr ago. The past structure of the woody vegetation was also different from that of today. Although past vegetation is not a good analogue for predicting future vegetation patterns, it still has potential as an indicator for the potential presence of tree species where there is none today. If we assume a temperature rise, and take into account current trends of landscape use abandonment, then we can expect strong vegetation dynamics at the upper tree line in the future: Abies alba may expand to occupy elevations of ca. 1800–2000 m in mixed communities with Picea abies, Pinus sylvestris and hardwood species, and Pinus cembra may expand up to 2500–2700 m a.s.l.     

Isolation and characterization of polymorphic nuclear microsatellite loci in Pinus cembra L

We developed eight polymorphic nuclear microsatellite markers for the Swiss stone pine (Pinus cembra L.), of which seven may be amplified in a multiplex polymerase chain reaction. Allelic polymorphism across all loci and 40 individuals representing two populations in the Swiss Alps was high (mean = 7.6 alleles). No significant linkage disequlibrium was displayed between pairs of loci. Significant deviation from Hardy–Weinberg equilibrium was revealed at three loci in one population. Cross–amplification was achieved in two related species within the genus (P. sibirica and P. pumila). Thus, the markers may be useful for population genetic studies in these three pine species. They will be applied in ongoing projects on genetic diversity and patterns of gene flow in P. cembra.     

Lateglacial and Holocene vegetation history in the Insubrian Southern Alps—New indications from a small-scale site

Fundamental uncertainties exist in the study region about the former lowland vegetation at local scales. All existing palaeoecological results are derived from sediments of medium- to large-sized sites (8–5000 ha), which are thought to record mainly regional vegetation in their pollen content. Therefore the very small mire at Balladrum (0.05 ha) was analysed for pollen, plant-macrofossils, and charcoal and the results compared with those of previous studies in the same region. Common regional signals were detected, but also new insights for the tree species Pinus cembra (L.), Abies alba (Mill.) and Castanea sativa (Mill.). Our palaeobotanical data reveal the local dominance of the timberline species P. cembra during the Lateglacial (16500–14250 cal b.p.) at low-altitudes. For A. alba an early presence in the area is suggested by pollen data, corroborating previous high-altitudinal studies indicating the presence of glacial refugia in the region. Occasional findings of C. sativa pollen throughout the Holocene may indicate the local but very rare presence of this species in the Insubrian Southern Alps, in contrast to the conventional opinion that C. sativa was introduced during the Roman Period. Altogether the results confirm the need of multiproxy palaeobotanical records from basins of variable size to assess the past composition of vegetation more accurately. Communicated by F. Bittmann     

Tree-rings,genetics and the environment: Complex interactions at the rear edge of species distribution range

Under climate change, modifications on plants’ growth are expected to be the strongest at species margins. Therein, tree acclimation could play a key role as migration is predicted to be too slow to track shifts of bioclimatic envelops. A requirement is, however, that intra-population genetic diversity be high enough for allowing such adaptation of tree populations to climate change. In this study, we tested for the existence of relationships between genetic diversity, site environmental conditions, and the response of annual tree growth to climate of Pinus cembra at its southern limit in the Alps. Site-specific climatic and environmental factors predominantly determined the response of trees along the precipitation gradient. The growth-climate interactions were chiefly linked to mean annual precipitation and temperature, slope and tree-size, and less to genetic diversity. We show that genetic background of Pinus cembra has exclusively indirect modulating power with limited effects on tree-ring formation, and within the southern limit in the Alps, genetic variability is not necessarily well expressed in the patterns of annual tree growth. Our results may imply little adaptive capacity of these populations to future changes in the water balance.     

Variation in the chloroplast DNA of Swiss stone pine (Pinus cembra L.) reflects contrasting post-glacial history of populations from the Carpathians and the Alps

Aim To characterize the genetic structure and diversity of Pinus cembra L. populations native to two disjunct geographical areas, the Alps and the Carpathians, and to evaluate the rate of genetic differentiation among populations. Location The Swiss Alps and the Carpathians. Methods We screened 28 populations at three paternally inherited chloroplast simple sequence repeats (cpSSRs) for length variation in their mononucleotide repeats. Statistical analysis assessed haplotypic variation and fixation indices. Hierarchical analysis of molecular variance (AMOVA), Mantel test, spatial analysis of molecular variance (SAMOVA) and barrier analyses were applied to evaluate the geographical partitioning of genetic diversity across the species’ range. Results Haplotypic diversity was generally high throughout the natural range of P. cembra, with the mean value substantially higher in the Carpathians (H = 0.53) than in the Alps (H = 0.35). The isolated Carpathian populations showed the highest haplotype diversity among the populations originating from the High Tatras (Velka Studena Dolina) and South Carpathians (Retezat Mountains). AMOVA revealed that only 3% of the total genetic variation derived from genetic differentiation between the two mountain ranges. Differentiation among Carpathian populations was higher (F_ST = 0.19) than among Alpine populations (F_ST = 0.04). Low, but significant, correlation was found between the geographical and genetic distances among pairs of populations (r = 0.286, P < 0.001). SAMOVA results revealed no evident geographical structure of populations. barrier analysis showed the strongest differentiation in the eastern part of the species’ range, i.e. in the Carpathians. Main conclusions The populations of P. cembra within the two parts of the species’ range still share many cpDNA haplotypes, suggesting a common gene pool conserved from a previously large, continuous distribution range. Carpathian populations have maintained high haplotypic variation, even higher than Alpine populations, despite their small population sizes and spatial isolation. Based on our results, we emphasize the importance of the Carpathian populations of Swiss stone pine for conservation. These populations comprise private haplotypes and they may represent a particular legacy of the species’ evolutionary history.     

Climate related causes of distinct radial growth reductions in Pinus cembra during the last 200 yr

Distinct radial growth reductions in Cembran pine (Pinus cembra L.) were studied at the timberline on Mt. Patscherkofel (2246 m a.s.l., Tyrol, Austria), which is situated in the inner-Alpine dry region of the Central Austrian Alps. Six timberline stands with different aspects were sampled and ring-width chronologies developed based on dendroecological techniques. Growth-climate relationships between residual chronologies and climate variables were explored using Pearson product-moment correlation coefficients. P. cembra growth at the timberline appears to be limited by cool summer (June-August) and previous autumn (September-October) temperatures and low precipitation in late winter (March). Timberline stands show concurrent growth depressions lasting ≥ 5 yr during the periods 1815–1823, 1851–1858 and 1913–1920, indicating growth depressions were steadily decreasing. Although evaluation of climate data revealed that these growth depressions can mainly be explained by occurrence of cold growing seasons and therefore the influence of recent climate warming on tree growth is plausible, an effect of tree ageing on climate sensitivity may also be involved. On the other hand, climate extremes do not inevitably induce growth responses as would be expected from growth-climate relationships. This is explained by the occurrence of synergistic and/or compensating effects of growth limiting climate variables and preconditioning of tree growth in previous years. Comparison of growth reductions with two published P. cembra timberline chronologies from inner-Alpine dry locations in the Eastern Alps revealed that investigated stands show the highest climate sensitivity during the last 200 yr. This difference in growth response to climate variability is most probably related to the special climate situation at Mt. Patscherkofel, which is exceptionally windy throughout the year and frequently exposed to warm dry winds (Föhn).     

Impact of global warming on the tree species composition of boreal forests in Finland and effects on emissions of isoprenoids

This study aims to identify how climate change may influence total emissions of monoterpene and isoprene from boreal forest canopies. The whole of Finland is assumed to experience an annual mean temperature (T) increase of 4 °C and a precipitation increase of 10% by the year 2100. This will increase forest resources throughout the country. At the same time, the proportions of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) in southern Finland (60°≤ latitude < 65°N) will be reduced from the current 40–50% to less than 10–20%, with increased dominance of birches (Betula pendula and Betula pubescens). In northern Finland (65°≤ latitude < 70°N), the proportions of Norway spruce and Scots pine will be balanced at a level of about 40% as the result of an increase in Norway spruce from the current 21% to 37% and a concurrent reduction in Scots pine from 63% to 40%. The proportion of birches is predicted to increase from the current 17% to 23%, but these will become the dominant species only on the most fertile sites. Total mean emissions of monoterpene by Scots pine will be reduced by 80% in southern Finland, but will increase by 62% in the north. Emissions from Norway spruce canopies will increase by 4% in the south but by 428% in the north, while those from birch canopies will increase by about 300% and 113%, respectively. Overall emissions of monoterpene over the whole country amount to about 950 kg km^?2 y^?1 under current temperature conditions and will increase by 17% to 1100 kg km^?2 y^?1 with elevated temperature and precipitation, mainly because of an increase at northern latitudes. Under current conditions, emissions of isoprene follow the spatial distribution of spruce canopies (the only isoprene‐emitting tree species that forms forests in Finland) with four times higher emissions in the south than in the north. The elevated temperature and the changes in the areal distribution of Norway spruce will result in increases in isoprene emissions of about 37% in southern Finland and 435% in northern Finland. Annual mean isoprene emissions from Norway spruce canopies over the whole country will increase by about 60% up to the year 2100.     

Nonannual tree rings in a climate‐sensitive Prioria copaifera chronology in the Atrato River,Colombia

In temperate climates, tree growth dormancy usually ensures the annual nature of tree rings, but in tropical environments, determination of annual periodicity can be more complex. The purposes of the work are as follows: (1) to generate a reliable tree‐ring width chronology for Prioria copaifera Griseb. (Leguminoceae), a tropical tree species dwelling in the Atrato River floodplains, Colombia; (2) to assess the climate signal recorded by the tree‐ring records; and (3) to validate the annual periodicity of the tree rings using independent methods. We used standard dendrochronological procedures to generate the P. copaifera tree‐ring chronology. We used Pearson correlations to evaluate the relationship of the chronology with the meteorological records, climate regional indices, and gridded precipitation/sea surface temperature products. We also evaluated 24 high‐precision ¹⁴C measurements spread over a range of preselected tree rings, with assigned calendar years by dendrochronological techniques, before and after the bomb spike in order to validate the annual nature of the tree rings. The tree‐ring width chronology was statistically reliable, and it correlated significantly with local records of annual and October–December (OND) streamflow and precipitation across the upper river watershed (positive), and OND temperature (negative). It was also significantly related to the Oceanic Niño Index, Pacific Decadal Oscillation, and the Southern Oscillation Index, as well as sea surface temperatures over the Caribbean and the Pacific region. However, ¹⁴C high‐precision measurements over the tree rings demonstrated offsets of up to 40 years that indicate that P. copaifera can produce more than one ring in certain years. Results derived from the strongest climate–growth relationship during the most recent years of the record suggest that the climatic signal reported may be due to the presence of annual rings in some of those trees in recent years. Our study alerts about the risk of applying dendrochronology in species with challenging anatomical features defining tree rings, commonly found in the tropics, without an independent validation of annual periodicity of tree rings. High‐precision ¹⁴C measurements in multiple trees are a useful method to validate the identification of annual tree rings.     

The role of biotic interactions in altering tree seedling responses to an extreme climatic event

Questions: We addressed two poorly understood aspects of plant response to climate change: the impact of extreme climatic events and the mediating role of biotic interactions, through a study of heatwave effects on tree seedling survival rates and ability of the tree canopy to alter seedling responses. Location: Mountain belt of the northern French Alps (Maurienne Valley). Methods: The survival rates of two seedling cohorts from four tree species (Abies alba, Acer pseudoplatanus, Fraxinus excelsior and Picea abies) were measured during both the 2003 European heatwave and an average summer (2004) in deciduous broadleaf mountain forests. Seedlings were transplanted into two soil moisture conditions, and in experimental gaps or under the tree canopy. Results: The heatwave strongly decreased tree seedling survival rates, while there was an important species‐specific mediating role of biotic interactions. In the wettest conditions, the tree canopy strongly increased survival of Abies, buffering the negative impact of the heatwave. In contrast, in the driest conditions, the tree canopy decreased survival of Picea and Acer, amplifying the negative impact of the heatwave. We found evidence of increasing soil water stress in the understorey of the driest community, but further studies including vapour pressure deficit measurements are needed to elucidate the driving mechanism of facilitation. Conclusions: The high species specificity of the mediating role of biotic interactions and its variation along stress gradients leads to questions on our ability to predict large‐scale responses of species to climate changes.     

Characterization of tree macroremains production in a recently burned conifer forest in northern Québec,Canada

The production of plant macroremains was studied in a conifer forest twomonths after it burned in 1996 in northern Québec. The proportions ofvarious types of charred and uncharred pieces (needles, cones andwood) produced by black spruce (Piceamariana) and jack pine (Pinusbanksiana) were determined by sampling around individual trees.Both species produced equivalent masses of charred material, but pieces of woodcharcoal from jack pine are generally larger that those of black spruce. Theproportion of charred versus uncharred needles is the bestindicator of the species dominance in the forest. Although the fall of uncharredneedles is delayed from the time of a fire, they contribute to more than half ofall remains produced. Jack pine cones remain on the tree for a long time after afire, while charred cones of black spruce are dehiscent (cones come off thebranches easily). Trees are poor wood charcoal producers compared toundergrowth shrubs. As a result, the macroremains assemblage associated with afire event is made up of large amount of uncharred material from trees and alarge proportion of charred pieces produced by undergrowth vegetation. Modernassemblages of plant macroremains indicate that in order to reconstruct pastvegetation associated with fire disturbance, it is important to distinguishbetween the various types of remains, because wood charcoal is mainly producedby material that is already dead.     

Responses to recent climatewarming of Pinus sylvestris and Pinus cembra within their montane transition zone in the Swiss Alps

Abstract. Altitudinal and latitudinal distribution limits of trees are mainly controlled by temperature. Therefore climate warming is expected to induce upslope or poleward migrations. In the Swiss Central Alps, summers in the period 1982-1991 were on average 0.8 °C warmer than those of the period 30 yr before. We investigated whether populations of conifers at the montane Pinus sylvestris-Pinus cembra ecocline exhibit demographic trends in response to that warming. We found no evidence for this. Young seedlings of Pinus sylvestris, the species which is expected to expand its range upward in a warmer climate, were virtually absent from all sites, whereas large fractions of Pinus cembra populations were observed in the seedling and juvenile categories even below the present lower distribution limit of adult trees. This suggests that there are no major altitudinal shifts in response to the recent sequence of warmer summers. Germination and seedling survival trials with Pinus sylvestris suggest that temperature per se would not exclude this species even from establishing at the current treeline in the Swiss Central Alps. Similar results were found at the polar treeline. Phytotron tests of seedling survival showed much less drought resistance in Pinus sylvestris than in Pinus cembra which is in contrast to their phytogeographic distributions. Thus, the montane pine ecocline in the Swiss Central Alps seems to be stabilized by species interactions and may not be directly responsive to moderate climatic change, which needs to be taken into account in predictive attempts.     

Glacial refugia and postglacial expansion of the alpine–prealpine plant species Polygala chamaebuxus

The shrubby milkwort (Polygala chamaebuxus L.) is widely distributed in the Alps, but occurs also in the lower mountain ranges of Central Europe such as the Franconian Jura or the Bohemian uplands. Populations in these regions may either originate from glacial survival or from postglacial recolonization. In this study, we analyzed 30 populations of P. chamaebuxus from the whole distribution range using AFLP (Amplified Fragment Length Polymorphism) analysis to identify glacial refugia and to illuminate the origin of P. chamaebuxus in the lower mountain ranges of Central Europe. Genetic variation and the number of rare fragments within populations were highest in populations from the central part of the distribution range, especially in the Southern Alps (from the Tessin Alps and the Prealps of Lugano to the Triglav Massiv) and in the middle part of the northern Alps. These regions may have served, in accordance with previous studies, as long‐term refugia for the glacial survival of the species. The geographic pattern of genetic variation, as revealed by analysis of molecular variance, Bayesian cluster analysis and a PopGraph genetic network was, however, only weak. Instead of postglacial recolonization from only few long‐term refugia, which would have resulted in deeper genetic splits within the data set, broad waves of postglacial expansion from several short‐term isolated populations in the center to the actual periphery of the distribution range seem to be the scenario explaining the observed pattern of genetic variation most likely. The populations from the lower mountain ranges in Central Europe were more closely related to the populations from the southwestern and northern than from the nearby eastern Alps. Although glacial survival in the Bohemian uplands cannot fully be excluded, P. chamaebuxus seems to have immigrated postglacially from the southwestern or central‐northern parts of the Alps into these regions during the expansion of the pine forests in the early Holocene.     

Les pelouses à Festuca paniculata du Tessin (Suisse) dans un contexte Alpin

Vittoz P., Selldorf P., Eggenberg S. and Maire S. 2005. Festuca paniculata meadows in Ticino (Switzerland) and their Alpine environment. Bot. Helv. 115: 33–48.Festuca paniculata (L.) Schinz & Thellung locally dominates montane and subalpine meadows of the Alps and other mountains of southern Europe. Vegetation relevés were carried out in Switzerland and northern Italy to study the site conditions under which Festuca paniculata meadows occur in this part of the Alps, their species composition and phytosociological status, and their relationship to Festuca paniculata meadows described previously from the French Alps (Centaureo-Festucetum spadiceae) and Austrian Alps (Hypochaerido uniflorae-Festucetum paniculatae). The Swiss meadows were found to have a similar ecology to those in France and Austria. They occur mostly between 1600 and 2100 m a.s.l on steep slopes with southern aspect, generally on crystalline rocks, but sometimes on calcareous rocks if soils have been decalcified. The species composition of the Swiss meadows is closer to the Austrian than to the French communities, and we attribute them to the association Hypochaerido uniflorae-Festucetum paniculatae with the new subassociation polygaletosum chamaebuxi. Climate is probably the main factor separating vegetation units in the Alps: the Centaureo-Festucetum spadiceae occurs where summers are dry, whereas the Hypochaerido uniflorae-Festucetum paniculatae occurs where rainfall is not a limiting factor in summer.Manuscrit accepté le 10 février 2005     

A century of tree line changes in sub‐Arctic Sweden shows local and regional variability and only a minor influence of 20th century climate warming

Aim Models project that climate warming will cause the tree line to move to higher elevations in alpine areas and more northerly latitudes in Arctic environments. We aimed to document changes or stability of the tree line in a sub‐Arctic model area at different temporal and spatial scales, and particularly to clarify the ambiguity that currently exists about tree line dynamics and their causes. Location The study was conducted in the Torneträsk area in northern Sweden where climate warmed by 2.5 °C between 1913 and 2006. Mountain birch (Betula pubescens ssp. czerepanovii) sets the alpine tree line. Methods We used repeat photography, dendrochronological analysis, field observations along elevational transects and historical documents to study tree line dynamics. Results Since 1912, only four out of eight tree line sites had advanced: on average the tree line had shifted 24 m upslope (+0.2 m year^?1 assuming linear shifts). Maximum tree line advance was +145 m (+1.5 m year^?1 in elevation and +2.7 m year^?1 in actual distance), whereas maximum retreat was 120 m downslope. Counter‐intuitively, tree line advance was most pronounced during the cooler late 1960s and 1970s. Tree establishment and tree line advance were significantly correlated with periods of low reindeer (Rangifer tarandus) population numbers. A decreased anthropozoogenic impact since the early 20th century was found to be the main factor shaping the current tree line ecotone and its dynamics. In addition, episodic disturbances by moth outbreaks and geomorphological processes resulted in descent and long‐term stability of the tree line position, respectively. Main conclusions In contrast to what is generally stated in the literature, this study shows that in a period of climate warming, disturbance may not only determine when tree line advance will occur but if tree line advance will occur at all. In the case of non‐climatic climax tree lines, such as those in our study area, both climate‐driven model projections of future tree line positions and the use of the tree line position for bioclimatic monitoring should be used with caution.     

Chloroplast DNA Transgresses Species Boundaries and Evolves at Variable Rates in the California Closed-Cone Pines (Pinus radiata, P. muricata, and P. attenuata)

We studied phylogenetic relationships among populations and species in the California closed-cone pines (Pinus radiata D. Don, P. attenuata Lemm., and P. muricata D. Don) via chloroplast DNA restriction site analysis. Data on genetic polymorphism within and among 19 populations in the three species were collected using9 to 20 restriction enzymes and 38 to 384 trees. Because only five clades and extremely low intraclade diversity were found, additional phylogenetic data were collected using a single representative per clade and two outgroup species, P. oocarpa Schiede and P. jeffreyi Loud. In total, 25 restriction enzymes were employed and approximately 2.7 kb surveyed (2.3% of genome). The five clades recognized were Monterey pine, knob-cone pine, and the southern, intermediate, and northern races of bishop pine. On the basis of bootstrapping, both Wagner and Dollo parsimony analyses strongly separated the northern and intermediate races of bishop pine from the southern race; knobcone pine from Monterey and bishop pines; and the closed-cone pines from the two outgroups. Approximate divergence times were estimated for the lineages leading to knob-cone pine and to the intermediate and northern populations of bishop pine. The position of Monterey pine relative to bishop pine within their monophyletic clade was unresolved. Surprisingly, Montery pine and the southern race of bishop pine were much more similar to one another than was the southern race of bishop pine to its conspecific intermediate and northern races. Both the Monterey and southern bishop pine lineages also evolved severalfold more slowly than did the knobcone pine and intermediate-northern bishop pine lineages. These results differ significantly from a recent allozyme study, corroborating previous observations that chloroplast genome phylogeny can depart substantially from that of nuclear genes.     

Spatial analysis of structural and tree‐ring related parameters in a timberline forest in the Italian Alps

Abstract. We investigated the variability in spatial pattern of some structural, dendrochronological and dendroclimatological features of a mixed Larix decidua‐Pinus cembra forest at the timberline in the eastern Italian Alps at fine geographical and temporal scales. Forest structure variables such as stem diameter, tree height, age and tree‐ring related parameters (yearly growth index, mean sensitivity, first order autocorrelation and some dendroclimatic variables) have been compared at various scale levels. We observed that most of the variables show positive autocorrelated structures due to both forest dynamics and fine‐scale driving forces, probably related to microrelief. Spatial structure of yearly indexed radial growth appears sensitive to extreme climatic events. Secondary succession after past disturbances drives the forest towards a structure governed by a gap regeneration dynamics that seems to ensure the different requirements of the two main tree species present. Small spatial scale studies of forest structures, especially if integrated to dendro‐ecological data, seem an efficient tool to assess the disturbance regime and species sensitivity to environmental change.