A grassland biome is an expansive, open area predominantly covered by various types of grasses, with only a limited presence of trees or shrubs. While temperature and soil composition influence these ecosystems, the amount and timing of precipitation are the primary climatic forces shaping their existence and the types of plant life that can thrive.
Precipitation Levels in Major Grassland Types
Grassland biomes globally receive annual precipitation ranging from approximately 250 to 950 millimeters (10 to 35 inches). The height of the grasses reflects the amount of rainfall received, with taller grasses growing in wetter areas.
Temperate grasslands, such as the North American Prairies and Eurasian Steppes, receive between 254 to 889 millimeters (10 to 35 inches) of rain annually. Prairies receive 508 to 889 millimeters (20 to 35 inches), with much moisture arriving in late spring and early summer. Steppes, which are drier, receive 254 to 508 millimeters (10 to 20 inches) per year.
Tropical grasslands, known as savannas, are found closer to the equator, between 5 and 20 degrees north and south. These warmer regions receive between 508 to 1500 millimeters (20 to 59 inches) of rain annually. African savannas, for example, receive 1000 to 1500 millimeters (40 to 60 inches) each year.
The Importance of Seasonal Rain Patterns
Beyond the total annual amount, the timing and distribution of rainfall profoundly influence grassland ecosystems. Tropical savannas are defined by pronounced wet and dry seasons, rather than consistent year-round precipitation. The wet season, occurring during the summer months, triggers rapid and vigorous grass growth across the landscape.
As the wet season transitions into the dry season, many plants enter a state of dormancy to conserve water. This distinct seasonality directly impacts animal behaviors, such as the large-scale migrations seen in African savannas. Herds of animals like wildebeest and zebra follow these seasonal rains, moving across vast distances in search of fresh grazing lands and water sources.
How Precipitation Prevents Forest and Desert Formation
The amount of precipitation grasslands receive positions them uniquely between forests and deserts, creating a landscape dominated by grasses. The rainfall is insufficient to support dense forest canopies, which require more than 2,000 millimeters (79 inches) of rain annually. Tree seedlings struggle to survive drier periods or cannot compete with the extensive root systems of grasses for limited water resources.
Conversely, grasslands receive enough moisture to prevent them from becoming barren deserts, which receive less than 300 millimeters (12 inches) of precipitation per year. The consistent, moderate rainfall allows for a continuous cover of grasses to thrive. If precipitation were distributed evenly throughout the year, many savanna areas would likely transform into tropical forests.
Impacts of Changing Precipitation on Grasslands
Changes in precipitation patterns can significantly alter grassland ecosystems. Prolonged periods of lower-than-average rainfall, known as droughts, weaken grasses and increase the risk of soil erosion. Recent studies indicate that a single year of extreme drought can decrease grassland vegetation productivity by as much as 40 to 60 percent. This reduction in plant growth impacts the ecosystem’s ability to sequester carbon and reduces available forage for livestock.
A sustained increase in rainfall can change the balance of a grassland, allowing woody shrubs and trees to establish themselves. This encroachment can convert grasslands into shrublands or woodlands, changing the ecosystem’s structure and biodiversity. Such fluctuations in precipitation, including more frequent and intense drought events, are increasingly linked to broader climate change trends, posing challenges for grassland conservation and management.