The tree line is the only line on a mountain map that moves. It is also the only one nobody drew.
The boundary between forest and bare mountain is what decides how a mountain looks when you stand in front of it. Below it the slope is dark and shaggy, above it pale and naked. And unlike a contour line it is not a convention but a biological relationship that changes.
Two boundaries, not one
The terms are often confused. The forest line is the line connecting the uppermost continuous stands of forest. The tree line sits higher and marks the very topmost individual trees, in the Nordic countries usually single mountain birches. The difference between them is on the order of a hundred vertical metres.
In the county monitoring programme the tree line is defined as the elevation at which trees can exist given the climate prevailing at the site. A tree normally counts as at least two metres tall.
The level varies considerably with latitude and exposure. In central Sweden the tree line sits roughly between 700 and 800 metres, while the forest line across the range as a whole moves between about 600 and 1,000 metres.
What the measurements actually show
Since 2006 the county administrations of Jämtland and Dalarna have run a joint programme following the tree line along a large number of fixed transects, that is, lines up mountain slopes where the uppermost trees are surveyed. The sites include the south side of Åreskutan, Getryggen near Storulvån, Sönfjället and Hamrafjället.
The follow-up rests on older material going back to 1915 and on Leif Kullman's studies at Umeå University from the 1970s onwards.
Over the long term the picture is clear. On Getryggen, a little west of Åre, the tree line today sits above 1,000 metres, more than 200 metres higher than a hundred years ago. Mountain birch grows where there used to be bare ground.
And the part that makes it interesting
Over the short term, nothing shows.
When the programme's first cycle was evaluated, the periods 2006-2007 and 2010-2013 were compared. The conclusion in the report was that no change in the position of the tree line could be demonstrated over that interval, despite studies across longer intervals showing a rise.
That is not a contradiction. It is a reminder of which timescale you are looking at. A tree needs decades to reach two metres at that altitude, a single hard winter can knock an entire front back, and the boundary therefore moves in jerks rather than evenly. Five years is simply too short a period in which to measure something that responds over a hundred.
Why it matters for how a mountain looks
The tree line is the reason Swedish mountains feel bare. We ski and walk above it already at altitudes where in the Alps you would still be in dense spruce forest. That gives the open, expansive character that is the whole point of Swedish mountain walking.
Technically it also matters for how terrain can be measured. A laser scan hits both tree canopies and ground, and the points have to be sorted for the ground to become visible. On bare mountain there is barely anything to sort out, which is one reason mountain terrain comes out so clean in an elevation model. More on that in the article on elevation models.
The terrain shape itself is not affected at all. The ground beneath the forest is the same whatever grows on it, which is one of the reasons we build pieces of ground rather than of landscape photographs. See for instance Helags or Åre, and read on about why Swedish mountains have the shape they do.