use while most of the forests are planted with spruce and pine. Since 1990, Thuringia's forests have been managed aiming for a more natural and tough vegetation more resilient to climate change as well as diseases and vermin. In comparison to the forest, agriculture is still quite conventional and dominated by large structures and monocultures. Problems here are caused especially by increasingly prolonged dry periods during the summer months. Environmental damage in Thuringia has been reduced to a large extent after 1990. The condition of forests, rivers and air was improved by modernizing factories, houses (decline of coal heating) and cars, and contaminated areas such as the former Uranium surface mines around Ronneburg have been remediated. Today's environmental problems are the salination of the Werra river, caused by discharges of K+S salt mines around Unterbreizbach and overfertilisation in agriculture, damaging the soil and small rivers. Environment and nature protection has been of growing importance and attention since 1990. Large areas, especially within the forested mountains, are protected as natural reserves, including Thuringia's first national park within the Hainich mountains, founded in 1997, the Rhön Biosphere Reserve, the Thuringian Forest Nature Park and the South Harz Nature Park. Demographics Demographic history During the Middle Ages, Thuringia was at the border between Germanic and Slavic territories, marked by the Saale river. The main Slavic tribe in what is now Thuringia were the Sorbs proper, who unified all tribes in what is now southern half of Eastern Germany. The Ostsiedlung movement led to the assimilation of Slavic people between the 11th and the 13th century under German rule. The population growth increased during the 18th century and stayed high until World War I, before it slowed within the 20th century and changed to a decline since 1990. Since the beginning of Urbanisation around 1840, the Thuringian cities have higher growth rates resp. smaller rates of decline than rural areas (many villages lost half of their population since 1950, whereas the biggest cities (Erfurt and Jena) kept growing). Current population The current population is 2,120,237 (December 31, 2020) with an annual rate of decrease of about 0.5%, which varies widely between the local regions. In 2012, 905,000 Thuringians lived in a municipality with more than 20,000 inhabitants, this is an urbanization rate of 42% which continues to rise. In July 2013, there were 41,000 non-Germans by citizenship living in Thuringia (1.9% of the population