Introduction
What is a treeline?
It certainly is not a line in the common sense. In the context of this course the treeline is defined as the high elevation, climate driven limit of tree growth. "Tree" is defined here as an upright woody plant with a dominant main stem reaching a height of at least 3 m. Since there are always a few isolated individuals ("outposts") in peculiar microhabitats above the treeline, a practical convention is to consider groups of trees only. The tree species line, the treeline and the timberline are coupled boundaries covering the whole transition from closed forest to treeless alpine terrain, often termed the treeline ecotone. The term "subalpine" would be a logical surrogate for the term "treeline ecotone", however, "subalpine" has been defined in so many different ways that the term will be avoided here.
Natural climatic treelines occur worldwide, but at different altitudes depending on latitude, as the classical profile of the world illustrates:
