The extent to which trees will grow defines the ‘timberline’, a zone of transition from boreal forest to treeless tundra in the polar regions, or from montane forest to the alpine belt on mountains (1). It corresponds to a temperature limit in summer, or rather, a limit in heating degree days (i.e. the length of growing season), reached either by elevation on mountains or by high latitude. Radiation conditions are quite different in the two cases.
The zone is confused by a gradation of tree types, e.g. the mountain birch grows higher (in elevation) than the Norway spruce in Scandinavia, yet the Norway spruce is found at higher latitudes in Finnish Lapland.
The position of the timberline depends chiefly on temperature during the growing season. But there are other factors too, such as temperature variability. A single frost in an otherwise sufficiently warm month can damage annual shoots and reduce growth of timberline trees. It also causes desiccation, especially in spring when there is less snow offering protection. Timberlines are also affected by forest fires, and grazing by sheep, deer, or moose, for instance. In addition there are plant diseases, insect damage, acid rain and air pollution. The result is that the timberline fluctuates.
Pollen and megafossil evidence indicate deglaciation and the growth of birch around 9400 BP in Finland, spreading north until the Climatic Optimum around 5500 BP. Then the trees retreated. In the Tatra mountains in Poland, there was an extension of pine trees from the lowlands to the present timberline level of about 1550 m, once the glaciers melted from the valleys during the early Holocene, followed by a retreat from about 4500 BP. There was a retreat again during the Little Ice Age, and there has been an advance during the 20th century, as one would expect from global warming.
The long life of many trees suggests that the timberline may be slower to retreat than to advance. Also, forests create and maintain, to some extent, a microclimate that is more suitable to tree growth than outside the forest. Therefore some forest patches may be 'fossil' evidence of different climate conditions a few hundred years ago.
(1) Heikkinen, O., B. Obrebska-Starkel and S. Tuhkanen 1995. Introduction: the timberline - a changing battlefront. Zeszty Naukowe Uniwersytetu Jagiellonskiego 1156, Prace Geograficzne - Zeszy, 98, 7 - 16.