Revolutionary new findings about the history of trees and climate in Scandinavia

Scientists at Umeå University in Sweden are putting forward an entirely new picture of climate change and the first immigration of trees following the last Ice Age. Research shows that 8,000-14,000 years ago the climate was considerably warmer than was previously thought. When it was at its warmest 9,000-10,000 years ago, the timberline was 500 m higher than today, and leafy trees grew in the mountains. The spruce immigrated considerably earlier that was assumed until now, and it probably came from the west, not the east. What’s more, research provides some support for the hypothesis that humankind has affected the climate in more recent times. Deciduous trees are again being spotted in mountain terrain!

The project combines in a unique way a long-term historical perspective with more contemporary events. For example, the reasons for the ever warmer climate of the last hundred years are being analyzed. “This research adds strength to the suspicion that the warmer climate may be related to the influence of human activity on the environment,” says Professor Leif Kullman, who heads this comprehensive physical geographical research project at the Department of Ecology and Environmental Science at Umeå University.

The spruce came from the west

High mountain peaks, like Åreskutan, thawed free as early as 14,000 years ago, which is 4,000-5,000 years earlier than earlier studies have indicated. After that, tree species like the mountain birch, pine, and spruce rapidly immigrated to these extremely high levels. Everything indicates that the climate was substantially warmer than today. One especially sensational finding is that the spruce appeared in the mountain chain more than 11,000 years ago. It has previously been taken for granted that the spruce came in from the east (from Russia) 3,000-4,000 years ago. The new discoveries show that the spruce was not only established earlier but also that it came from the west. Perhaps the trees ‘hibernated’ through the Ice Age on the emerged sea floor southwest of Norway.

Some 8,000-9,000 years ago, the tree flora of the mountains comprised previously unknown species for these regions, such as the Siberian larch, oak, lime, elm, hazel, and alder. The latter five are leafy trees that prefer an entirely different and warmer climate than that of today.

Changes in the timberline indicate climatic changes

The long-term trends of the climate have been reconstructed by analyzing changes in the timberline. The highest timberline and the warmest summers occurred about 9,500 years ago. At that time trees grew more than 500 m higher up the mountain slopes compared with today’s conditions. Subsequently there was incremental cooling up to the end of the 19th century. The timberline was then lower and the bare slopes greater than ever before.

Support for the greenhouse effect?
About 100 years ago there was a unique reversal of the trend. The climate suddenly became considerably warmer, and the timberline climbs 100-150 m during the following 100 years. The mountain forests were revitalized, and deciduous trees even started to appear on the mountain fringe. The project has managed to evaluate the ever warmer climate of the 20th century and the higher and higher elevation of the timberline. “Our present warm climate stands out as unique, at least in regard to the last 3,500 years, exceeding the natural variations that might be expected in the climate. We may be looking at one of the most concrete proofs that humans have started to affect the earth’s climate and ecosystem,” says Leif Kullman.

New methods yield more reliable results

The success of the project is based, apart from hard and patient field work, on the consistent application of a new methodology. The history of trees, forests, and the climate has been recreated by so-called megafossil-analysis. This means that large plant remains, like trunks, roots, and cones that have been preserved in peat and sea sediment, are sought out and dated using the C14 method. This yields a decidedly more reliable picture than previous research did, being based almost exclusively on pollen analysis. A more general aspect of the findings of the project is that earlier research into the history of vegetation based on pollen analysis can and should be reinterpreted.