A biome is a large-scale community of plants and animals determined by the climate and geography of a region. The grassland biome, known regionally as prairies, steppes, or savannas, is characterized by a landscape dominated by grasses and herbaceous plants rather than large trees. Found on every continent except Antarctica, this biome exists where specific climatic factors prevent the establishment of dense forests. The amount and seasonality of precipitation are the most important factors shaping these vast, open environments.
Defining Characteristics of Grassland Climate
The climate that defines and maintains grasslands sits in a middle ground between the arid conditions of deserts and the moisture-rich environments of forests. This biome is characterized by a wide annual temperature range, which can span from as low as -20°C to over 30°C, depending on the latitude. These temperature fluctuations create a challenging environment, but they are not the primary factor limiting the growth of woody species.
The most restrictive climatic factor is precipitation, which typically ranges from 250 to 950 millimeters annually. This amount of rainfall is sufficient to prevent the land from becoming a desert, allowing a continuous cover of grass to thrive. However, this precipitation level remains insufficient to support the dense, large-scale growth of trees, which require more consistent and higher moisture levels for survival. The grasses, with their fibrous root systems, are better adapted to utilize this intermediate level of moisture than the deep taproots of forest trees.
Seasonal Dynamics and Climatic Extremes
The annual climate cycle in grasslands is marked by distinct seasonal shifts that divide the year into a growing season and a dormant season. This cycle creates intense environmental pressures that further discourage the establishment of tree seedlings. Grasslands frequently experience periods of seasonal drought, which can dramatically limit water availability for all vegetation.
The open nature of the terrain means that high winds sweep across the plains year-round, significantly increasing the rate of evapotranspiration. This process actively removes moisture from the air and the soil surface, exacerbating water stress on plants. Woody plants are particularly vulnerable to this increased water demand because they have a greater structural biomass to maintain, making them more susceptible to hydraulic failure in dry conditions than grasses.
Climatic extremes, such as severe heat waves, blizzards, and periodic droughts, characterize these regions. The combination of dry grass and strong winds makes fire a natural, recurring feature of the grassland climate. These periodic fires clear out woody shrubs and tree seedlings, effectively maintaining the dominance of fire-adapted grasses whose growth points and root systems are protected underground.
Climatic Differences Between Major Grassland Types
Globally, the grassland biome is broadly categorized into two types: temperate and tropical. Temperate grasslands, also known as prairies or steppes, are found in mid-latitude continental interiors and are defined by extreme temperature variability. These regions often experience very hot summers, where temperatures can soar above 38°C, and very cold winters, with temperatures sometimes plummeting below -40°C.
Precipitation in temperate grasslands is typically concentrated in the late spring and summer months, aligning with the peak growing season for the grasses. The cold winter temperatures, often accompanied by snow, force the vegetation into a deep dormancy period.
Tropical grasslands, or savannas, are located closer to the equator and maintain a consistently warm climate throughout the year, with average monthly temperatures ranging from 10°C to 30°C. The climate here is defined not by temperature extremes but by a highly pronounced cycle of wet and dry seasons. Savannas receive their precipitation in a concentrated rainy season, often followed by an extended, severe dry season that can last anywhere from two to eleven months. The length and intensity of this dry period are the primary climatic control on the environment, limiting tree cover to scattered, drought-tolerant trees.