Some savannas pack in more trees per hectare than neighbouring rainforest
Picture a savanna and you probably imagine lonely trees dotting a sea of grass. Yet Brazil's cerrado can hold 800 to 3,300 trees per hectare, matching or beating nearby tropical forest. What makes a savanna is not sparse trees but a canopy that never closes, letting sunlight feed a continuous grass layer.
Ecologists define a savanna as grassy woodland: trees are spaced so the crowns never knit into a closed roof, and enough light reaches the ground for an unbroken carpet of grasses. They recognise four forms, from savanna woodland with a light canopy, through tree savanna and shrub savanna, to grass savanna where woody plants are almost absent. Densities vary widely; Guinean savanna averages 129 trees per hectare against 103 in riverside forest, and eastern Australian sclerophyll forests are comparable to the savannas around them.
Rain comes mostly in a single season. Savannas often sit in the transition between desert and forest, and cover about 20 percent of Earth's land. Unlike North American prairies or Eurasian steppes with their cold winters, they occupy warm to hot regions in Africa, Australia, South America and India.
The word travelled from the Taíno language of the West Indies, meaning treeless grassland, into Spanish as sabana. It first appeared in English in 1555 as zauana. Peter Martyr recorded it as the local name for a plain near the court of the cacique Carlos in what is now Panama, probably around Madugandí or the Guna Yala coast. The explorer William Dampier, in his 1697 to 1705 accounts of a voyage around the world, spelled it savannah with a final h, which became the British form, applying it to grassland with palmettos on an island in the Bay of Campeche and later to scenes in Sumatra and the Philippines.
Definitions have since tangled. Before the mid-19th century, many mixed tree-and-grass landscapes were simply called savanna. The Köppen climate system then framed tropical savanna as a climatic climax, an oversimplification that clashes with everyday vegetation usage and has sometimes left huge savannas north and south of the Congo and Amazon off savanna maps.
Source: Savanna
