Semi-Arid Steppe and Transitional Ecosystems

Between true desert and the more humid grassland, savanna, or forest biomes lies the semi-arid steppe, classified under the Köppen system as "BSh" (hot steppe) or "BSk" (cold steppe) and receiving enough seasonal precipitation, typically 250 to 500 millimeters annually, to support continuous grass cover even though it remains too dry for reliable rain-fed tree growth. The Eurasian Steppe forms the largest such belt on Earth, stretching roughly 8,000 kilometers from the Hungarian Puszta in the west, across Ukraine and southern Russia, through Kazakhstan and Mongolia, to Manchuria in the east; this vast, largely treeless corridor of grassland historically functioned as a highway for nomadic pastoralist peoples, most famously enabling the rapid 13th-century expansion of the Mongol Empire under Genghis Khan, whose mounted armies depended on the steppe's grass to sustain enormous horse herds across thousands of kilometers of otherwise forbidding terrain.