How does rainfall affect the grasses that grow across North American prairies?

Rainfall is the primary factor determining the species composition and distribution of grassland vegetation across North American prairies. As precipitation levels decrease from east to west, the prairie transitions through three distinct subtypes:1

• Tallgrass Prairie: Found in the humid eastern region where rainfall reaches approximately 100 cm (40 inches), this area supports deep-rooted, sod-forming grasses like big bluestem and prairie cordgrass, alongside bunchgrasses in drier upland sites.1
• Midgrass (Mixed-grass) Prairie: Occupying the central region, this subtype supports a mix of bunchgrasses and sod-forming species, including wheatgrass, grama grass, and buffalo grass.1
• Shortgrass Plains: Located in the driest western areas where rainfall drops below 30 cm (12 inches), the vegetation is dominated by shorter, drought-tolerant species such as buffalo grass and grama grass.1

This moisture gradient also influences soil development; regions with higher rainfall support heavier grass cover and deep, organic-rich mollisols, while more arid regions feature thinner, lighter soils.2