The world’s great grasslands—known regionally as steppes, prairies, savannas, and pampas—are expansive ecosystems characterized by the dominance of grasses over large trees. These biomes cover nearly one-quarter of the Earth’s land surface, and their existence is dictated by climate. The specific combination of temperature and precipitation limits the growth of woody vegetation, creating the open, grassy landscapes that define the biome. This climatic control shapes the soil and the animal life supported by these vast plains.
Defining Characteristics of Grassland Climates
Grasslands are defined by a semi-arid climate, receiving more annual precipitation than a desert but significantly less than a forest. Annual rainfall typically falls within the range of 500 to 950 millimeters (20 to 35 inches). This is sufficient to support a dense cover of grasses but insufficient for the sustained growth of large trees. This moderate precipitation is paired with high rates of evaporation, which limits the water available in the soil for deeper-rooted woody plants.
These regions are also characterized by pronounced seasonality, oscillating between distinct wet/warm and dry/cold periods. Evaporation is intensified by persistent, strong winds that sweep across the flat, treeless terrain. These winds increase the rate of water loss from both the soil and the plant surfaces, exacerbating dry conditions even during periods of moderate rainfall. The combination of limited precipitation and high water loss maintains the open grassland structure.
Climate Variations Between Temperate and Tropical Grasslands
The global grassland biome is broadly divided into two major types, each with a distinct climate profile: temperate and tropical.
Temperate Grasslands
Temperate grasslands, such as the North American prairies, the Eurasian steppes, and the South American pampas, are found in the interior of continents and experience high temperature variability. These regions are known for having hot summers where temperatures can reach above 30°C (86°F), followed by extremely cold winters where temperatures can drop well below freezing, sometimes falling to -40°C (-40°F) in the northern latitudes.
The precipitation in temperate grasslands is often distributed relatively evenly throughout the year, but the total amount is low, typically ranging between 300 and 600 millimeters annually. This lack of concentrated rainfall, coupled with the wide seasonal temperature swings, results in a dormant period during the cold winter months when plant growth ceases. The extreme cold causes the grasses to die back to the roots each year, with the dense root network protecting the plant buds from the harsh winter conditions.
Tropical Grasslands (Savannas)
In contrast, tropical grasslands, commonly known as savannas, are located closer to the equator and maintain consistently warm temperatures year-round, generally fluctuating between 15°C and 35°C (59°F and 95°F). The climate of the savanna is defined not by temperature variation but by a dramatic shift in precipitation, dividing the year into a distinct wet season and a long, severe dry season. Tropical grasslands receive a higher overall amount of rain than their temperate counterparts, typically between 500 and 1,500 millimeters annually.
The annual rainfall is heavily concentrated in a short period, leaving a prolonged dry season that can last as long as eight months. This extended period of water scarcity limits the growth of trees and shrubs, although scattered, drought-adapted woody plants may be present. The consistent warmth allows grasses to grow rapidly during the wet season, but the severe, predictable dry period imposes a strong constraint on overall biomass accumulation and tree establishment.
The Dynamics of Seasonal Extremes
The inherent characteristics of grassland climates—low to moderate precipitation and high evaporation—make them highly susceptible to frequent and intense periods of drought. This dry condition prevents the successional transition of the ecosystem into a forest, thus maintaining the dominance of grasses.
Compounding the effect of low rainfall is the constant presence of strong winds across the flat terrain, which increases the rate of water vapor loss from the ground and vegetation. This high evaporative demand quickly depletes soil moisture, accelerating the onset and severity of drought cycles. The resulting accumulation of dry, dead grass material, known as fuel load, makes fire a natural and recurring dynamic.
Fire is a fundamental process that maintains the open structure of grasslands, preventing the encroachment of woody species. During the long dry seasons, whether due to a lack of rain in the tropics or a cold, dry winter in temperate regions, the continuous layer of dry grass becomes highly flammable. These fires, often ignited by lightning, rapidly consume the dry biomass, recycling nutrients and ensuring the ecosystem remains dominated by fire-adapted grasses.