Reindeer grazing not global warming is shifting the tree line in Torneträsk

Strolling reindeer (Rangifer tarandus) in the ...

Strolling reindeer: iImage via Wikipedia

New research shows that the advance of the tree line upwards in the Swedish mountains was due to reduced reindeer grazing and not due to any global warming.

Swedish Radio P1 reports today: (freely translated)

It is not primarily a warmer climate which causes the tree line to crawl
up in many places in the Swedish mountains. A new study from the Torneträsk area shows that there are several other factors that affect tree spread rather than just higher temperatures. Climate change plays a very minor role. It is mainly grazing reindeer, insect infestation, and several other factors that affect mountain forest coverage, rather than changing temperature conditions.
“That the tree line can go up or down or remain stationary within the same climate period has not been shown before “, says Professor Terry Callaghan, one of the researchers who carried out the study.

The tree line advanced up the mountains most during the cold period at the end of the 1960s and 1970s. It was primarily because it was a time with fewer reindeer. A warmer climate may actually have an indirect effect (to reduce the advance northwards) by adding to the number of  insects and insect infestations that can damage trees.

Many climate models expect that the forest in the tundra and other Arctic areas will expand considerably northwards in the next one hundred years because of higher temperatures. But the new research suggests that these simple assumptions can be grossly inaccurate. One must reckon with how to account for the impact of insects and grazing reindeer and moose. “It now requires that much more detailed information be added into the models”, says Professor Terry Callaghan, director of the Abisko research station.

The article is published in the Journal of Biogeography

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