some places amounting to monocultures. To provide more space for spruce plantations various peatlands were drained in the 19th and 20th century. Some spruce plantations have suffered severe damage as the seeds used came from lowland specimens that were not adapted to mountain conditions. Silver fir grow naturally in the Sudetes being more widespread in past times, before clearance since the Late Middle Ages and subsequent industrial pollution reduced the stands. Many arctic-alpine and alpine vascular plants have a disjunct distribution being notably absent from the central Sudetes despite suitable habitats. Possibly this is the result a warm period during the Holocene (last 10,000 years) which wiped out cold-adapted vascular plants in the medium-sized mountains of the central Sudetes where there was no higher ground that could serve as refugia. Besides altitude the distribution of some alpine plants is influenced by soil. This is the case of Aster alpinus that grows preferentially on calcareous ground. Other alpine plants such as Cardamine amara, Epilobium anagallidifolium, Luzula sudetica and Solidago virgaurea occur beyond their altitudinal zonation in very humid areas. Peatlands are common in the mountains occurring on high plateaus or in valley bottoms. Fens occur at slopes. Timber line The higher mountains of the Sudetes lie above the timber line which is made up of Norway spruce. Spruces in wind-exposed areas display features such as flag tree disposition of branches, tilted stems and elongated stem cross sections. Forest-free areas above the timber line have increased historically by deforestation yet lowering of the timber line by human activity is minimal. Areas above the timber line appear discontinuously as "islands" in the Sudetes. At Krkonoše the timber line lies at c. 1230 m a.s.l. while to the southeast in the Hrubý Jeseník mountains it lie at c. 1310 m a.s.l. Part of the Hrubý Jeseník mountains have been above the timber line for no less than 5000 years. Mountains rise considerably above the timber line, at most 400 m, a characteristic that sets the Sudetes apart of other Mittelgebirge of Central Europe. Geology Geological research has been hampered by the multinational geography of the Sudetes with and the limitation of studies to state boundaries. Bedrock The igneous and metamorphic rocks of the Sudetes originated during the Variscan orogeny and its aftermath. The Sudetes are the northeasternmost accessible part of Variscan orogen as in the North European Plain the orogen is buried beneath sediments.