Across a large area of western Siberia, shrubs are rapidly growing into trees more than six feet tall, a process that is expected to further increase temperatures in this rapidly warming part of the Arctic, according to a new study. Relying on satellite images and fieldwork, scientists from Oxford University and Finland found that in 8 to 15 percent of a 36,000-square-mile region in western Siberia, willow and alder shrubs had turned into trees over the last 30 to 40 years as temperatures have climbed. Oxford scientists said their research showed that the growth of shrubs could be an even more important factor in the greening of the tundra than the migration of trees northward from the boreal forest. The rapid growth of trees is expected to further warm the Arctic for two reasons. In the Arctic spring and autumn, shrubs are often buried under snow, but trees grow above the snow, their dark surfaces absorbing sunlight. In addition, trees create a microclimate that traps heat. “The speed and magnitude of the observed change is far greater than we expected,” said Bruce Forbes of the University of Lapland and co-author of the paper, which was published in Nature Climate Change.