Savannas and temperate grasslands are expansive biomes dominated by grasses, covering significant portions of the Earth. While both are characterized by vast stretches of grassy vegetation, they differ in fundamental ways. This article explores a primary characteristic that sets savannas apart from temperate grasslands.
Understanding Savannas and Temperate Grasslands
Savannas are found in tropical and subtropical regions, transition zones between forests and deserts. They span large areas across Africa, South America, Australia, and India. These environments experience distinct wet and dry seasons, supporting a mix of grasses and scattered trees. Annual rainfall ranges from 50.8 to 127 cm (20-50 inches), concentrated in a six to eight-month wet season. Temperatures remain warm to hot, between 20 to 30°C (68 to 86°F).
In contrast, temperate grasslands are treeless plains located in the mid-latitudes, experiencing a wider range of seasonal temperatures. Notable examples include the North American prairies, the South American pampas, and the Eurasian steppes. These regions have hot summers and cold winters, with temperatures dropping to -40°C (-40°F) in winter and rising above 38°C (100°F) in summer. Annual precipitation is lower than in savannas, ranging from 25.4 to 88.9 cm (10-35 inches), and can occur more evenly throughout the year or as snow in winter.
The Defining Presence of Trees
A primary difference between savannas and temperate grasslands is the presence of scattered trees in savannas, and their general absence in temperate grasslands. This distinction is directly tied to the unique climatic patterns and ecological disturbances prevalent in each biome. The term “savanna” itself implies a grassy ecosystem with a notable component of woody plants.
Savannas maintain their tree component due to sufficient rainfall during their wet seasons, allowing trees to establish. However, the prolonged dry season, which can last five to seven months, and frequent natural fires prevent the development of a dense, closed-canopy forest. During the dry period, lightning strikes commonly ignite the dry grasses, leading to widespread fires. These fires help control woody plants and promote grass growth, maintaining the savanna’s open structure. Trees in savannas possess adaptations like thick bark or the ability to resprout from roots, enabling them to survive these periodic fires.
Conversely, temperate grasslands lack trees, with woody vegetation limited to areas along rivers or in sheltered depressions where moisture is more consistent. The combination of lower overall precipitation, very cold winters, and the significant role of fire restricts tree establishment. Temperate grassland fires, often ignited by lightning, are a natural and recurring disturbance that maintains the ecosystem by suppressing woody plants and favoring grasses, which are well-adapted to regrow from underground structures after burning. Without regular fires, shrubs and trees would encroach, gradually converting grasslands into more wooded areas. The extreme temperature fluctuations, particularly the cold winters where water can be locked up as ice, also pose a significant challenge for tree survival in these regions.