A tropical grassland is a grass dominated ecosystem occurring in tropical climates, usually where seasonal rainfall, fire, herbivory, soil conditions or combinations of these factors prevent the development of continuous closed forest. Tropical grasslands range from nearly treeless systems to landscapes containing scattered trees and shrubs. Many are strongly seasonal, with a pronounced wet season during which grasses grow rapidly and a dry season during which vegetation becomes dormant or dries out. Savannas are the best known form of tropical grassland, particularly in Africa, but tropical grasslands also occur in South America, Australia and parts of Asia.
Temperature is generally high throughout the year in tropical grasslands, so seasonality is often controlled more by rainfall than by temperature. During the wet season, grasses and other plants can produce large quantities of biomass. During the dry season, water becomes limiting, grasses may turn brown and many trees reduce their activity or shed leaves. This seasonal contrast creates a strong ecological rhythm involving plant growth, herbivore migration, fire, decomposition and water availability. The length and severity of the dry season vary considerably and help determine whether an area supports open grassland, wooded savanna or tropical forest.
Fire is an important process in many tropical grasslands and savannas. Dry grasses provide highly combustible fuel, and natural lightning as well as human ignition can produce fires during dry periods. Recurrent fire can suppress the growth of woody vegetation and favour grasses and fire adapted plants. Some trees survive through thick bark, protected buds or underground structures, while grasses can regrow rapidly after burning. Fire regimes differ among regions and are influenced by rainfall, vegetation productivity, human land management and herbivore activity. Excessively frequent or intense fire can also degrade some ecosystems, so fire is not universally beneficial.
Large herbivores are particularly important in many tropical grasslands. African savannas support species such as elephants, zebras, wildebeest, giraffes, buffalo and numerous antelope species. Their grazing and browsing influence vegetation structure and nutrient cycling, while predators such as lions, cheetahs, leopards and hyenas interact with herbivore populations. Similar ecological processes occur in other tropical grasslands, although species differ by continent. Herbivore movement can be strongly seasonal because animals track rainfall and fresh vegetation. Large mammals can therefore connect distant parts of the landscape through migration and nutrient transport.
Tropical grassland soils vary widely. Some savanna soils are relatively fertile and support productive vegetation, while others are strongly weathered or nutrient limited. Seasonal wetting and drying influence nutrient cycling, soil structure and water availability. Grasses often allocate substantial biomass below ground, helping them survive drought and disturbance while contributing organic matter to soils. Tree roots can access deeper water than grasses in some environments, producing competition as well as facilitation. The balance between grasses and woody plants therefore depends on water, nutrients, fire, herbivory and plant traits.
Tropical grasslands occur in several major world regions. African savannas form the largest and most famous examples, including extensive areas in East, Southern, Central and West Africa. South America contains savanna systems such as the Brazilian Cerrado and Venezuelan and Colombian Llanos. Australia contains tropical and subtropical savannas across large parts of the north. Tropical grasslands and savannas also occur in India and other parts of South and Southeast Asia. These ecosystems share broad structural and climatic characteristics but contain distinct species and evolutionary histories.
Tropical grasslands are important for biodiversity, carbon cycling, pastoralism, agriculture, tourism and water regulation. Many iconic wildlife populations depend on them, while millions of people rely on grazing livestock or cultivated crops within seasonal tropical landscapes. Conversion to agriculture, overgrazing, invasive species, altered fire regimes and climate change can modify vegetation structure. At the same time, excluding fire or large herbivores from systems that evolved with regular disturbance can also cause ecological changes. Effective conservation therefore requires understanding the natural disturbance regime rather than simply preventing all disturbance.
Tropical grassland is a broad ecological category, while savanna is a more specific vegetation formation generally characterised by grasses with scattered trees or shrubs. Not every tropical grassland is a savanna, and not every savanna has the same climate or species composition. Tropical grasslands are also distinct from tropical rainforests because they experience stronger seasonal water limitation and generally have a more open vegetation structure. Their existence demonstrates that tropical climates do not inevitably produce dense rainforest; rainfall seasonality, soil, fire and herbivory can maintain extensive open ecosystems even under high temperatures.
