Reindeer and Other Herbivors Determine the Tree Line – Not Warmth

It is mainly grazing reindeer, insect attacks, and several other factors that affect the spread of the mountainous forest, more than the changed temperature situation.

“Tree line can go up, down or stay in the same position even during the same climatic period. That has not being showed before,” says Terry Callaghan, director of the Abisko Scientific Research Station.

Researchers were able to see that precisely reindeer grazing affects more than the temperature, since the tree line advanced furthest upward during the cold period that started in the late 1960s and continued through the 1970s, it was a time of fewer reindeer.

A warmer climate has more of an indirect effect through, for example, there being more insects that can damage trees.

Many climate models expect the forest in the tundra and the upper Arctic will expand heavily northward in the next hundred years because of higher temperatures. But the new research indicates that the assumptions may be grossly inaccurate. The effect of grazing reindeer and moose must be reckoned with.

“We can not just expect the tree line to move northwards, we have to look in more detail,” Terry Callaghan says.