What Are Grasslands?

Grasslands are terrestrial ecosystems in which grasses and other herbaceous plants dominate the vegetation and trees are absent or widely scattered, a category that includes the temperate prairie of North America, the steppe of Eurasia, the pampas of South America, and the tropical savanna of Africa, Australia, and India. They occupy roughly two-fifths of the ice-free land surface and occur where rainfall is too low or too seasonal to support closed forest but sufficient to prevent desert. Their persistence depends less on climate alone than on recurrent disturbance: fire, grazing by large herbivores, drought, and seasonal freezing all suppress woody seedlings that would otherwise convert the system to shrubland or forest.

The defining structural feature of a grassland is the allocation of biomass belowground. Perennial grasses invest heavily in dense fibrous root systems and underground storage organs, which lets them resprout after fire or grazing removes the aboveground shoots. That root investment also builds the deep, organic-rich soils that made temperate grasslands the world's principal grain-producing regions, and it is why so little native prairie remains.

Types and Distribution

Grasslands are conventionally divided along a temperature and rainfall gradient. Temperate grasslands experience cold winters and a pronounced growing season, and within North America they grade from tallgrass prairie in the wetter east through mixed-grass to shortgrass steppe in the rain shadow of the Rocky Mountains. Tropical and subtropical savannas remain warm year-round and are structured by a wet and dry season cycle, with a continuous grass layer beneath a discontinuous tree canopy whose density is set by fire frequency and herbivore pressure. Montane grasslands, coastal prairie, and flooded grasslands form smaller regional types. A U.S. Geological Survey synthesis of grassland management priorities treats the North Central grassland region, spanning seven states from Montana to Kansas, as a single unit precisely because these gradients cross state and land ownership boundaries.

Carbon, Soil, and Ecosystem Function

Grasslands hold a large share of global terrestrial carbon, and unusually for a major biome, nearly all of it sits belowground as root biomass and soil organic matter rather than in standing vegetation. This makes their carbon stock relatively resistant to fire, which consumes aboveground material without releasing the soil pool, but highly vulnerable to tillage, which exposes protected organic matter to rapid oxidation. Grasslands also regulate water infiltration and groundwater recharge, stabilize soils against wind and water erosion, and support pollinator and grassland bird communities that have declined sharply with habitat conversion. Where these functions are managed for livestock production, they are analyzed as rangeland ecosystem services, a framing that puts forage yield alongside carbon storage and water regulation.

Remote Sensing and Monitoring

Because grasslands are spatially extensive, structurally simple, and highly variable between seasons and years, they are well suited to satellite monitoring. Vegetation indices computed from red and near-infrared reflectance track green biomass and phenology, and time series of these indices reveal drought response, grazing pressure, and the timing of green-up. Approaches to grassland health assessment from remote sensing combine such indices with measures of cover, productivity, and species composition to produce condition indicators at landscape scale. Hyperspectral sensors extend the method to biochemical properties, and reviews of nutrient estimation in tropical grasslands describe retrieval of foliar nitrogen and phosphorus that indicate forage quality. Lidar adds canopy height and structure, and eddy covariance flux towers supply the ground measurements of carbon and water exchange used to calibrate the satellite record.

Applications

Grassland science informs work in a range of fields, including:

• Rangeland and livestock grazing management
• Soil carbon accounting and carbon offset verification
• Drought monitoring and early warning systems
• Biodiversity conservation and prairie restoration planning
• Wildfire fuel load estimation and fire risk modeling
• Agricultural land use planning and conversion assessment
• Climate model parameterization of land surface exchange