 It forms the boundary between the steppe zone to the south and the forest-steppe zone to the north.It is also one of the oldest reserves in the world, being protected by law since the 18th century.The forests of the mountains surrounding Ulaanbaatar are composed of evergreen pines, deciduous larches and birches, while the riverine forest of the Tuul River is composed of broad-leaved, deciduous poplars, elms and willows.