The Central European forest zone embraces the entire Pidhirye in the Ukraine, the southern part of the Rostoche, and the western spurs of Volhynia and Podolia. It is distinguished by a greater variety of tree species. Here, upon damp, loamy hills, entire forests of beech are found, on the Carpathian foothills the pine, and singly or in small groups, the larch, the yew, the maple, etc. In the Northern European zone all these trees disappear, due to the increasing continentality of the climate. The predominating species of tree here is the pine, which forms large woods everywhere on sandy soil, then the birch, which always accompanies the pine, the fir on sandy soil, the oak and white beech on loamy soil. There is an admixture of a considerable number of alders on swampy ground, aspens, lindens, elms, maples, ash and wild apple, pear and cherry trees. Hazel bushes, willow (salix caprea), mountain ash, raspberry and blackberry bushes comprise the thick underbrush in these mixed forests, and contribute a great deal to the beauty of the woods, together with grass and herb vegetation, especially in numerous clearings. In truer evergreen forests the underbrush usually is very poor. There are a great many swamp forests in the Ukrainian forest region. In the Carpathian foothills they are called lasi, and are quite common there, but in the Polissye they are most widely developed. There they are usually composed of pines, with which, however, the swampy ground does not agree very well. The alders and willows, however, grow all the better. [102] The second important formation of the forest region are the luhi. They usually stretch thru the wide, flat river valleys of the region. These are luxurious meadows with a beautiful growth of grass and herbs set with single trees and clusters of trees. In dry places the oak usually grows, in damp places the alder. The third typical plant formation is that of the swamps. They are widely developed in the forest region of the Ukraine, especially in the flat river valleys of the Rostoche and Volhynia. Polissye is the greatest swamp country in Europe. Regular moors, made up of peat mosses, alternate even in the Polissye region with meadow moors, in which swamp grass and herb vegetation predominates. The forest region has played a significant part in the history of the Ukraine. When the Turkish nomad tribes, using the steppe district of the Ukraine as a convenient military road, destroyed the work of Ukrainian civilization in the steppe region, the Ukrainian people retreated into the forests and swamps of the north and west, advancing toward the southeast again, at the proper moment, to reinhabit the ravaged and desolated lands. This circle of events repeated itself frequently in the history of the Ukraine.