Summary1 Recent elevational range-margin performance of tree and shrub species was studied at a site in the Swedish Scandes. The methods included comparisons of historical and present-day range-margin records (m a.s.l.) in conjunction with age-determination of newly established saplings. 2 Since the early 1950s, the range-margins of Betula pubescens ssp. tortuosa (mountain birch), Picea abies ( Norway spruce), Pinus sylvestris (Scots pine), Sorbus aucuparia (rowan) and Salix spp. (willows) have advanced by 120-375 m to colonize moderate snow-bed communities. The non-native Acer platanoides (Norway maple) has become established just below the birch forest-limit. In concert with tree-limit rises by 100-150 m in the same region, the present results suggest a shift in reproduction and a significant break in the late-Holocene vegetation history. 3 Ring-counting (in 2000) of a subsample of the recovered saplings revealed that, with one exception, they were aged between 7 and 12 years, i.e. they germinated after 1987. Since 1988 there has been strong and consistent winter warming, with some very warm summers, and this may ultimately have forced the vegetational changes. 4 Reduced summer snow-retention has favoured seedling establishment and juvenile growth, and mild winters, with reduced risk of frost-desiccation, have enhanced survivorship and height increment. 5 Certain seed-regenerating tree and shrub species have tracked recent climate change quite rapidly and more sensitively than vegetatively propagating field-layer species. Such species-specific responses may give rise to novel high-elevation vegetational patterns in a hypothetically warmer future world.

Summary 1.Elevational tree line change in the southern Swedish Scandes was quantified for the period 1915-2007 and for two sub-periods 1915-1975 and 1975-2007. The study focused on Betula pubescens ssp. czerepanovii , Picea abies and Pinus sylvestris at a large number of sites distributed over an 8000-km 2 area. The basic approach included revisitations of fixed sites (elevational belt transects) and measurements of tree line positions (m a.s.l.) during these three periods. 2. Over the past century, tree lines of all species rose at 95% of the studied localities, with means of 70-90 m. All three species displayed maximum upshifts by about 200 m, which manifests a near-perfect equilibrium with instrumentally recorded air temperature change. This magnitude of response was realized only in particular topographic situations, foremost wind-sheltered and steep concave slopes. Other sites, with more wind-exposed topoclimatic conditions, experienced lesser magnitudes of upshifts. Thus, spatial elevational tree line responses to climate change are markedly heterogeneous and site-dependent. Modelling of the future evolution of the forest-alpine tundra transition has to consider this fact. Even in a hypothetical case of substantial climate warming, tree lines are unlikely to advance on a broad front and a large proportion of the alpine tundra will remain treeless. 3. During the period 1975-2007, the tree lines of Picea and Pinus (in particular) advanced more rapidly than that of Betula towards the alpine region. These species-specific responses could signal a potential trajectory for the evolution of the ecotone in a warmer future. Thereby a situation with some resemblance with the relatively warm and dry early Holocene would emerge. 4. Substantial tree line upshifts over the past two to three decades coincide with air and soil warming during all seasons. This implies that both summer and winter temperatures have to be included in models of climate-driven tree line performance. 5. Synthesis . Maximum tree line rise by 200 m represents a unique trend break in the long-term Holocene tree line regression, which has been driven by average climate cooling for nearly 10 000 years. Tree line positions are well-restored to their pre-Little Ice Age positions. Recent tree line ascent is a truly anomalous event in Holocene vegetation history and possibly unprecedented for seven millennia.

Summary1 Demographic trends of Pinus sylvestris L. (Scots pine) tree line populations are reported for a 32-year monitoring period . Functional and projective aspects of tree line performance were analysed by relating temporal variability and change of vital population parameters, such as natality/mortality, vigour, injuries, height growth and seed viability to contemporary variations in air and soil temperatures. 2 The size of the entire sampled population increased by 50% during the 32-year observation period and thereby pine has become a more prominent element on the landscape. This reverses a natural multicentennial or even millennial trend of tree line decline and recession. 3 Contrasting population trends were recorded for the subperiods 1973-87 and 1988-2005, viz. decline and increase, respectively. Mean summer temperatures (JJA) did not change perceivably over and between these intervals, although some exceptionally warm summers from 1997 onwards have contributed to population expansion by increased seed viability and seedling emergence. Winter temperatures (DJF) decreased significantly over the first subperiod and were consistently higher during the second, which has significantly lowered the mortality rates. 4 A functional link to winter temperature conditions was particularly stressed by the aetiology of individual plant vigour, injuries and final mortality. Classical symptoms of winter desiccation correlated significantly with low winter temperatures. This negative impact occurred with a high frequency during the decline phase and virtually ceased during the expansion phase from 1988 onwards, when winter air and root zone temperatures were raised to a consistently higher level. 5 Winter and summer temperatures in the air and soil, as well as positive feedback mechanisms and nonlinear responses, must be taken into account in the search for global or regional mechanical explanations for the tree line phenomenon. This insight helps to generate realistic tree line models for a high-CO 2 world, when winter warming is usually predicted to be particularly large.

Aim This paper seeks to elucidate the first post‐glacial arrival of tree species to high elevations in the Scandes. This enables testing of general theories concerning glacial refugia, immigration routes and palaeoclimate. Location The study site, 1360 m a.s.l., was close to the summit of Mt Åreskutan in the alpine region of the southern Swedish Scandes, 400–500 m above modern tree‐limits. Methods Tree megafossils (trunks, roots, cones) were retrieved (and radiocarbon‐dated) from the ground surface in the forefields of receding `perennial' snow‐patches. This approach allows elevational range‐margin reconstructions to be made with an accuracy not possible with any other method. Results and conclusions Megafossils were recovered substantially higher and earlier than previously recorded or inferred for tree growth in this part of Europe. The species were Betula pubescens Ehrh. ssp. tortuosa (Ledeb.) Nyman, Picea abies (L.) Karst. and Pinus sylvestris (L.). The oldest dates obtained are c. 14,000, 11,000 and 11,700 BP for these species, respectively. For the first time, explicit evidence of tree growth in the Late‐Glacial (including the Younger Dryas stadial) is demonstrated for central/northern Scandinavia. The swift appearance of boreal tree species at high northern latitudes and altitudes on the western fringe of Scandinavia and near the most extended margin of the Weichselian ice sheet hypothetically suggests that the first immigration of the trees considered here was from the west. Glacial refugia for these species (other plants and animals as well) on the exposed continental shelf areas west and southwest of Norway is hypothesized. By extension, the results fit into a more general pattern, suggesting the presence of boreal and temperate trees quite close to the full glacial (Weichselian) ice margins in central Europe. Thus, the Quaternary forest and landscape history of Europe seems more complex than previously believed.

In the context of projected future human‐caused climate warming, the present study reports and analyses the performance of subalpine/alpine plants, vegetation and phytogeographical patterns during the past century of about 1 °C temperature rise. Historical baseline data of altitudinal limits of woody and non‐woody plants in the southern Scandes of Sweden are compared with recent assessments of these limits at the same locations. The methodological approach also includes repeat photography, individual age determinations and analyses of permanent plots. At all levels, from trees to tiny herbs, and from high to low altitudes, the results converge to indicate a causal association between temperature rise and biotic evolution. The importance of snow cover phenology is particularly evident. Treeline advance since the early‐20th century varies between 75 and 130 m, depending on species and site. Tendencies and potentials for further upshift in a near future are evident from the appearance of young saplings of all tree species, growing 400–700 m atop of the treeline. Subalpine/alpine plant species have shifted upslope by average 200 m. In addition, present‐day repetitions of floristic inventories on two alpine mountain summits reveal increases of plant species richness by 58 and 67%, respectively, since the early‐1950s. Obviously, many plants adjust their altitudinal ranges to new climatic regimes much faster than generally assumed. Nevertheless, plants have migrated upslope with widely different rates. This produces non‐analogous alpine plant communities, i.e. peculiar mixtures of alpine and silvine species. The alpine region is shrinking (higher treeline), and the character of the remaining alpine vegetation landscape is changing. For example, extensive alpine grasslands are replacing snow bed plant communities.