Today, when it becomes warmer the treeline is moving north in Siberia. That it gets warmer is manifested in longer growing season, spring comes earlier, but as in Sweden the height of summer heat does not increase, the average temperature in July does not rise. See chart of July's temperature from the Kola Peninsula. The lower part shows recruitment of pines at the tree line in a couple of periods. The average temperature in July has declined since the 1930s as well as in Sweden. In Sweden, the average temperature in July, about two degrees cooler in the late 1900s, compared with 30, 40 and 50s. But the tree line is moving north because of longer growing season and this causes problems for the nomadic peoples of northern Russia. But still the treeline has not reached areas where there was forest in the Middle Ages and during the Holocene Termal Maximum (HTM).
"However, conifers have not yet recolonized many areas where trees were present during the Medieval Warm period (ca AD 800–1300) or the Holocene Thermal Maximum (HTM; ca 10000–3000 years ago). Reconstruction of tree distributions during the HTM suggests that the future position of the treeline due to global warming may approximate its former Holocene maximum position." http://rstb.royalsocietypublishing.org/content/363/1501/2283.full
The chart below shows how the tree line has changed since 10,000 years ago from the end of the Ice Age. We see a shift southwards gradually. The reason is climate deterioration. The place is Yamal peninsula.
On the map you can see the circled Yamal peninsula which is known for a number of studies on tree rings and change of the tree line. Scientist has worked extensively with larches along the river beds. Because of this, Holocene has been divided in three parts. Below a comparison with the tree line in the third period, Late Holocene and today. After 1600 BC changes have not been as great as in previous millennia.
Further reading about this with tree lineshttp://climateaudit.org/2009/01/06/the-supra-long-finnish-chronology-2/