Some 9,550 years ago, a Norway spruce tree (Picea abies) sprouted in Sweden. The tree, now called Old Tjikko, stands 5 meters (16 ft) tall and has survived by cloning itself — the visible tree is relatively young, but its root system has been around for almost 100 centuries. But that’s not even the only interesting thing about it. Old Tjikko is a true pioneer, one of the first trees to colonize Sweden after the ice sheet withdrew from Scandinavia some 10,000 years ago, at the end of the ice age.

Norway spruce had been dominant in much of Scandinavia before the ice age. But when the ice sheet moved in, the trees had to withdraw. Once the ice started withdrawing, you’d expect them to come right back – but they didn’t.

Researchers track the migration of trees in Scandinavia (and many other parts of the world) by studying ancient pollen in lake sediments and peat deposits. In Sweden, a new study now shows that spruce started to truly take root in Sweden only in the past thousand years. This is consistent with previous studies which suggested that spruce was slow in recolonizing and re-asserting dominance in Scandinavian forests.

“Although spruce was one of the first trees to become re-established, it long failed to colonize the region to any great extent. This is surprising since pioneer plant species are usually at an advantage in this respect,” says Kevin Nota, PhD student at Uppsala University and the first author of the study.

The researchers analyzed ancient spruce DNA from 15 sites, combining them with DNA analysis of ancient clonal and contemporary spruce forest trees from central Sweden. The findings suggest that spruce came to Scandinavia early on, but then struggled to expand.

“The Swedish spruce seems to have lived through several earlier attempts to take over the forests of Scandinavia, but only its last expansion was successful,” says Laura Parducci, a researcher at Uppsala University and the Sapienza University of Rome and the principal investigator of the study.

Why exactly it took such a long time for spruce to reestablish is still pretty much a mystery. The new study suggests that the first spruce trees were related to clonal spruces that came from Sweden’s mountainous areas, but it’s unclear if this affected the spread of the trees in other areas. “We lack a good explanatory model for spruce migration,” the researchers write in the study.

Nevertheless, the study shows that analysis of ancient tree DNA found in sediments is an important tools to reconstruct ancient environments following the last Ice Age’s deglaciation. Studies like this could help us understand how different species survived and adapted when faced with climatic changes — which in turn could help us better understand how these species will adapt in the face of man-made climate change.

The study “Norway spruce postglacial recolonization of Fennoscandia” was published in Nature Communications.