The sensitivity and response of northern hemisphere altitudinal and polar treelines to environmental change are increasingly discussed in terms of climate change, often forgetting that climate is only one aspect of environmental variation. As treeline heterogeneity increases from global to regional and smaller scales, assessment of treeline sensitivity at the landscape and local scales requires a more complex approach than at the global scale. The time scale (short-, medium-, long-term) also plays an important role when considering treeline sensitivity. The sensitivity of the treeline to a changing environment varies among different types of treeline. Treelines controlled mainly by orographic influences are not very susceptible to the effects of warming climates. Greatest sensitivity can be expected in anthropogenic treelines after the cessation of human activity. However, tree invasion into former forested areas above the anthropogenic forest limit is controlled by site conditions, and in particular, by microclimates and soils. Apart from changes in tree physiognomy, the spontaneous advance of young growth of forest-forming tree species into present treeless areas within the treeline ecotone and beyond the tree limit is considered to be the best indicator of treeline sensitivity to environmental change. The sensitivity of climatic treelines to climate warming varies both in the local and regional topographical conditions. Furthermore, treeline history and its after-effects also play an important role. The sensitivity of treelines to changes in given factors (e.g. winter snow pack, soil moisture, temperature, evaporation, etc.) may vary among areas with differing climatic characteristics. In general, forest will not advance in a closed front but will follow sites that became more favourable to tree establishment under the changed climatic conditions.

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For over 100 years, mountain treelines have been the subject of varied research endeavors and remain a strong area of investigation. The purpose of this paper is to examine aspects of the epistemology of mountain treeline research-that is, to investigate how knowledge on treelines has been acquired and the changes in knowledge acquisition over time, through a review of fundamental questions and approaches. The questions treeline researchers have raised and continue to raise have undoubtedly directed the current state of knowledge. A continuing, fundamental emphasis has centered on seeking the general cause of mountain treelines, thus seeking an answer to the question, ''What causes treeline?'' with a primary emphasis on searching for ecophysiological mechanisms of low-temperature limitation for tree growth and regeneration. However, treeline research today also includes a rich literature that seeks local, landscape-scale causes of treelines and reasons why treelines vary so widely in three-dimensional patterns from one location to the next, and this approach and some of its consequences are elaborated here. In recent years, both lines of research have been motivated greatly by global climate change. Given the current state of knowledge, we propose that future research directions focused on a spatial approach should specifically address cross-scale hypotheses using statistics and simulations designed for nested hierarchies; these analyses will benefit from geographic extension of treeline research.

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Wild herbivorous mammals may damage treeline vegetation an cause soil erosion at a local scale. In many high mountain areas of Europe and North America, large numbers of red deer have become a threat to the maintenance of high-elevation forests and attempts to restore the climatic treeline. In northern Fennoscandia, overgrazing by reindeer in combination with mass outbreaks of the autumnal moth are influencing treeline dynamics. Moose are also increasingly involved damaging treeline forest. In the Alps, the re-introduction of ibex is causing local damage to subalpine forests and tree establishment above the forest limit as well as aggravating soil erosion. High-elevation forests and treeline in Europe are susceptible to the deleterious impact of wild ungulate populations because of former extensive pastoral use. Rodents may damage tree seedlings and saplings by girdling, root cutting, bark stripping and burrowing. Hares damage young trees by gnawing. Large numbers of small rodents may occasionally impede tree regeneration by depleting the seed sources. Rodents do not contribute to forest expansion beyond the current treeline. Among birds, nutcrackers are highly effective in influencing tree distribution patterns and treeline dynamics. Without the nutcracker caching of stone pine seeds any upward advance of the trees in response to climatic warming would be impossible. Some bird species such as black grouse, willow grouse and ptarmigan can impair tree growth by feeding on buds, catkins and fresh terminal shoots.

The term "Krummholz" is applied to the bush-like growth-forms of Pinus mugo prostrata, Alnus viridis, and some other species which are genetically determined. The term has also long been used by English-speaking botanists to refer to the climatically stunted and distorted trees in the forest-tundra ecotone. Thus, distinct ecological and plant-geographical differences are disguised which, in a more precise differentiation of the mountain plant cover, ought to be accentuated. The author tries to explain this by observation and field studies in the European high mountains and in the Rocky Mountains and suggests restricting the German term "Krummholz" to the genetically controlled shrub-like species; the timberline species shaped by climatic influences should be called cripple-trees. Thus, the elfin wood is nothing but a cripple facies of the upper montane forest. RESUME Que veut vraiment dire le terme "Krummholz"? Observations avec reference particulaire aux Alpes et aux Montagnes Rocheuses du Colorado. En allemand, on appelle "Krummholz" les especes de bois comme Pinus mugo prostrata et Alnus viridis qui sont d'une pousse tordue par voie d'heredite. En anglais, on appelle "Krummholz" depuis quelque temps aussi les formes des arbres a la limite forestiere superieure qui sont occasionees par le climat. Avec ce transfert, des differences importantes en vue de de ecologie et de la geographie floristique sont effacees qui devront etre souligne'es pour obtenir la differenciation la plus exacte de la vegetation de haute montagne. Cela, l'auteur essaye de demontrer au moyen des observations comparatives dans les hautes montagnes europeennes et dans les montagnes rochenses. II suggere de reserver le terme "Krummholz" pour les especes de pousse tordue fixee genetiquement et d'appeler arbres deformes les formes de pousse tordue forcees par les constraints du climat. Alors, le "elfin wood" n'est qu'une forme tordue de la foret pres de sa limite superieure. ZUSAMMENFASSUNG Was bedeutet der Ausdruck "Krummholz" wirklich? Beobachtungen die sich speziell auf die Alpen und die Rocky Mountains beziehen. Als Krummholz werden im Deutschen die erblich krummwuchsigen Holzarten wie Pinus mugo prostrata und Alnus viridis bezeichnet. Im Englischen ist es seit geraumer Zeit ublich geworden, auch die klimatisch bedingten Wuchsformen im Waldgrenzbereich "Krummholz" zu nennen. Mit dieser Ubertragung werden aber wesentliche okologische und pflanzengeographische Unterschiede verwischt, die im Hinblick auf eine moglichst genaue Differenzierung der Hochgebirgsvegetation eigentlich hervorgehoben werden miufiten. Dies versucht der Autor anhand vergleichender Beobachtungen und Studien in europaischen Hochgebirgen und in den Rocky Mountains zu begriinden. Er schlagt vor, den Termninus "Krummholz" den Arten mit genetisch fixierter Krummwuichsigkeit vorzubehalten und die klimatisch induzierten Zwangswuchsformen als Kruiippelwuchsformen oder Baumkrippel zu bezeichnen.

The present study aims to contribute to a fine regional differentiation of Scots pine (Pinus sylvestris) response to climate change at its altitudinal margins in subarctic Finland north of 69˚ N (Utsjoki) and to find out whether a prompt establishment of new pines in response to climate change can be expected above the old pine tree limit in and above the mountain birch zone. In 10 sampling areas, distribution, site characteristics, and condition (damage degree, growth forms) of the new pines (pines that have become established since the mid 20th century) were checked in a zone 50 m to the left and right of our field routes. The field routes extended from the scattered birch forest to the treeless alpine zone and mountaintops and covered a total area of more than 4 km 2. In total, 213 new pines were found. Tree height was measured and age estimated by counting the whorls. The degree of damage was estimated and then attributed to four damage classes. Pine establishment was most successful during the 1970s and up until the end of the last century. Pines younger than 10 years are rare (< 3%) in the study areas, with one exception (about 8%). Pine recruitment is comparatively intense in close proximity to old pines in the birch forest while it is sporadic within the scattered birch stands at higher elevations and in the alpine tundra. More than 80% of the new pines show disturbed growth forms due to frequent winter injury, reindeer, and moose. About 66% exhibit severe damage, and 15% have already died. On windswept terrain, microsite facilitation is essential for pine establishment. Lack of local seed sources and severe site conditions at high elevations have probably delayed pine altitudinal advance. New pine generations may become effective seed sources speeding up pine advance beyond the present seed trees. In view of the high proportions of severely damaged and dead new pines, we do not expect that climatic warming will bring about a rapid advance of the pine tree limit.

Elevational and polar treelines have been studied for more than two centuries. The aim of the present article is to highlight in retrospect the scope of treeline research, scientific approaches and hypotheses on treeline causation, its spatial structures and temporal change. Systematic treeline research dates back to the end of the 19th century. The abundance of global, regional, and local studies has provided a complex picture of the great variety and heterogeneity of both altitudinal and polar treelines. Modern treeline research started in the 1930s, with experimental field and laboratory studies on the trees’ physiological response to the treeline environment. During the following decades, researchers’ interest increasingly focused on the altitudinal and polar treeline dynamics to climate warming since the Little Ice Age. Since the 1970s interest in treeline dynamics again increased and has considerably intensified from the 1990s to today. At the same time, remote sensing techniques and GIS application have essentially supported previous analyses of treeline spatial patterns and temporal variation. Simultaneously, the modelling of treeline has been rapidly increasing, often related to the current treeline shift and and its implications for biodiversity, and the ecosystem function and services of high-elevation forests. It appears, that many seemingly ‘new ideas’ already originated many decades ago and just confirm what has been known for a long time. Suggestions for further research are outlined.