Savanna

• 1. Introduction

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  Savanna sounds like open grassland dotted with a few lonely trees, yet parts of South America pack in more trees than the forests beside them. In the cerrado, savanna holds between 800 and 3300 trees per hectare. Neighbouring forests there range from 800 to 2000. Even so, this is grassy country. How can a crowd of trees still count as grassland? What keeps the leaves from knitting together overhead? And why do so many of these landscapes seem to owe their shape to human hands? This is the story of a landscape that is stranger than its postcard image suggests.

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• 2. A Word From the Caribbean

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  In 1555, an English description of the islands of the kings of Spain spelled the word as Zauana. In the writing conventions of the time, that equals zavana. It descends from the Spanish sabana, which Spanish borrowed from Taíno, a language of the West Indies. There it meant a treeless grassland.

  Peter Martyr reported the word as the local name for the plain around Comagre. Comagre was the court of a chief named Carlos, in what is now Panama. The accounts are inexact about where that plain lay. Most guesses settle on Madugandí, on points along the nearby Guna Yala coast opposite Ustupo, or on Point Mosquitos. Those places are now modern cropland or jungle.

  The English spelling with a final h came from the explorer William Dampier. His volumes of A New Voyage Round the World, published between 1697 and 1705, used "savannah" throughout, and it became the British form. On the island of Trist, in the Bay of Campeche in the Gulf of Mexico, he wrote: "The West part is dry and sandy, bearing a sort of long Grass, growing in Tufts very thin. This is a sort of Savannah, with some large Palmeto Trees growing in it." He used the same spelling for similar terrain in Sumatra and the Philippines, in accounts of a later voyage to New Holland, which is northern Australia. Notice the palm trees in Dampier's first savannah, in a word that began life meaning treeless.

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• 3. Trees Without a Canopy

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  Guinean savanna holds 129 trees per hectare, while the riparian forest beside it holds 103. Eastern Australian sclerophyll forests average about 100 per hectare, which is much like the savannas of the same region. Counting trees, then, cannot tell the two apart. What matters is arrangement.

  In a savanna the trees stand far enough apart that their crowns never join into a roof. Light reaches the ground and feeds an unbroken layer of herbs, mostly grasses. Many savannas even space their trees more evenly than forests do, which overturns the picture of scattered loners.

  Four forms exist. In savanna woodland, trees and shrubs make a light canopy. Tree savanna has scattered trees and shrubs. Shrub savanna has spread-out shrubs. Grass savanna has hardly any trees or shrubs at all.

  Water sets the rhythm of the year. Most of the rain falls in a single season. Savannas often sit between forest and desert or grassland, and the step from desert to forest is the most common one. Their warm to hot climates, found in Africa, Australia, South America and India, separate them from the prairies of North America and the steppes of Eurasia, which endure cold winters. Together these lands cover roughly a fifth of the Earth's land area.

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• 4. Lines on the Map

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  Before the middle of the 19th century, writers used the word savanna for many grassy landscapes and mixed communities of trees, shrubs and grasses. Then the idea of a tropical savanna climate took hold. The Köppen climate classification system was shaped by how temperature and rainfall affect tree growth, and its oversimplified assumptions treated savanna as a "climatic climax" formation. Everyday usage now clashes with that tidy concept. As a result, extensive savannas north and south of the Congo and Amazon Rivers have sometimes been left off mapped savanna categories.

  In North America, the word has been swapped freely with barrens, prairie, glade, grassland and oak opening. Authors disagree on how many trees a savanna can have. Some set the lower limit of tree cover at 5-10% of an area, and upper limits range from 25% to 80%. The line between woodland and savanna is so vague that the two can be merged into one biome, since both have open-canopied trees with crowns that do not usually interlink, mostly forming 25-60% cover. In Belize, as elsewhere in the Americas, savanna vegetation looks similar from Mexico to South America and the Caribbean.

  Two things appear in every savanna: rainfall that varies from year to year, and wildfires in the dry season. Annual rainfall runs from about 500 to 1270 millimetres, with most of it in six or eight months, followed by drought. In climatic geomorphology, the study of landforms shaped by climate, many savannas are noted on pediplains and inselbergs. Rivers there seem to cut down little and instead erode by migrating sideways. Flooding and sheet wash have been proposed as the main forces wearing down savanna plains.

  Over many large tropical areas, climate alone cannot predict whether forest, savanna or grassland will dominate. Historical events play a key role, and in some places more than one stable biome is possible. Fire is the historical force most often singled out, and it matters greatly in what follows.

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• 5. Regions and Residents

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  The Serengeti is the savanna most people have heard of, famous for its wildlife. It belongs to the tropical and subtropical group, which also includes the Brazilian cerrado, known for its exotic and varied flora. Other members include the Kimberley, the Central Zambezian miombo woodlands, Cape York Peninsula and the Terai-Duar grasslands.

  Temperate savannas are mid-latitude lands with wetter summers and drier winters. They cover much of the plains of southeastern Australia, northern India, Southern Africa, southeastern Argentina and Uruguay. The Argentine Espinal, the Pampas and the Cumberland Plain Woodland are examples. Mediterranean savannas have mild, rainy winters and hot, dry summers. California's oak savannas fall here, as does the temperate grassland of South Australia, where eucalyptuses grow.

  Some savannas are underwater for part of the year or all of it. The Everglades, the Mesopotamian Marshes, the Pantanal and the Sudd are flooded savannas. Montane savannas are rarer, found in a few high mountain spots. The Bogotá savanna sits at an average altitude of 2550 metres on the Altiplano Cundiboyacense, in the Eastern Ranges of the Andes. The savannas of the Angolan Scarp are lower, rising to 1000 metres.

  The plants differ from continent to continent. Tropical American savannas hold broadleaved trees such as Curatella, Byrsonima and Bowdichia, over grasses such as Leersia and Paspalum. Argentina's savannas often feature Prosopis, a relative of the bean. East African savannas are home to acacias, baobabs, Combretum, Borassus and Euphorbia. Drier ones there add spiny shrubs and grasses such as Andropogon, Hyparrhenia and Themeda, while wetter ones bring Brachystegia trees and elephant grass. West African trees include Anogeissus, Combretum and Strychnos. Indian savannas are mostly cleared, but the reserved ones keep acacias, Mimosa and Zizyphus over Sehima and Dichanthium grasses. Australia's savanna is rich in evergreen, hard-leaved vegetation, including eucalyptus, Acacia, Bauhinia and Pandanus, with grasses such as Heteropogon and kangaroo grass.

  The animals follow the plants. African savannas hold giraffes, elephants, buffalo, zebras, gnus, hippopotamuses, rhinoceroses and antelope, all depending on grass or the foliage of trees. In Australia, the kangaroo and wallaby family dominates. Cattle, horses, camels, donkeys and Asian water buffalo also roam there, but people brought them in.

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• 6. Landscapes Made by Fire

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  Native Americans created the pre-Columbian woodlands of North America by burning them periodically, so that fire-resistant plants came to dominate. The same pattern appears worldwide. Fire-stick farming appears to explain the wide spread of savanna in tropical Australia and New Guinea. India's savannas result from human use of fire. The maquis shrub savannas of the Mediterranean region were also created and maintained by burning.

  Intentional burns usually stay in the herbaceous layer and do little lasting harm to mature trees. They also head off catastrophic wildfires that would do far more damage. But these fires kill or hold back tree seedlings, which stops a continuous canopy from forming and shading out the grass.

  Before European settlement in Australia, aboriginal land use, including fire, shaped the vegetation and may have maintained and modified savanna flora. Many authors suggest that this burning made the landscape structurally more open. It certainly created a habitat mosaic that probably raised biodiversity and altered the structure of woodlands and the ranges of many woodland species. When traditional burning is removed or changed, many savannas are being replaced by forest and shrub thickets with little herbaceous layer.

  Newcomers have also quieted the flames. Introduced grazers eat the herbage that would otherwise fuel fires, leaving fewer and cooler fires. Exotic pasture legumes add another twist. They keep high nutrient levels all year, so graziers no longer burn to coax a flush of green growth, and fire can harm legume populations, which makes people reluctant to burn at all.

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• 7. Hooves and Axes

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  Sheep, goats and cattle have changed much of the world's savanna. Closed forests such as rainforests are rarely grazed, because their dense structure keeps grass from growing. The open structure of savanna lets a herbaceous layer thrive, so it is commonly used for livestock. The results range from shifts in pasture composition to woody plant encroachment.

  Grazing affects woody plants in two main ways. Grasses compete with them for water in the topsoil, so removing grass can boost tree growth. Removing fuel also lowers the intensity and frequency of fires that might hold woody species in check. Animals can also browse palatable woody species directly, and unpalatable woody plants have increased under grazing. Weeds spread too, because the plants that would compete with them are thinned. Cattle and horses carry the seeds of weeds such as prickly acacia (Acacia nilotica) and stylo (Stylosanthes species).

  Hooves damage soil through compaction and break-up, and stripped plant cover invites erosion. The worst effects of overstocking come on soils of low fertility and in areas with rainfall below 500 millimetres, where nutrients concentrate at the surface and any movement of soil can cause severe degradation. Impacts can be softened. Studies of elephants in savannas found that their overall impact shrinks where there is rainfall and fences.

  In Queensland in the 2000s, about 4800 square kilometres of savanna were being cleared every year, mainly to improve pasture. Large areas of Australian and South American savannas have lost their trees, and clearing continues. In the Northern Territory of Australia, land clearing and fracking are threats. The motive is economic. Without trees competing for water, pasture production can rise two to fourfold, and better feed means more livestock. Dense tree and shrub cover also harbors predators, which raises stock losses, and woody plants hinder mustering of sheep and cattle.

  The tools changed over time. Early pastoralists used felling and girdling, which means removing a ring of bark and sapwood. In the 1950s, arboricides suitable for stem injection were developed, and war-surplus heavy machinery became available. Machines pushed timber over, or pulled it using a chain and ball strung between two machines. New pasture grasses and legumes helped drive a resurgence in clearing. The 1980s brought soil-applied arboricides, notably tebuthiuron, which work without cutting and injecting each tree.

  Pulling trees in many ways mimics fire, and most Queensland savannas are adapted to regrow after fire. Clearing slashes basal area and canopy cover, but many woody plants survive as seedlings too small to be hit or as plants that re-sprout from lignotubers and broken stumps. Even when every tree over 5 metres is uprooted from a eucalypt community, half or more of the original population of woody plants often remains.

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• 8. Weeds and Warming

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  Fewer than three percent of savanna ecosystems can be classed as highly intact. Introduced plants are one reason. Serious woody weeds now include rubbervine (Cryptostegia grandiflora), mesquite (Prosopis species), lantana (Lantana camara and Lantana montevidensis) and prickly pear (Opuntia species). Herbaceous arrivals, deliberate or accidental, include Rhodes grass and other Chloris species, buffel grass (Cenchrus ciliaris), giant rat's tail grass (Sporobolus pyramidalis), parthenium (Parthenium hysterophorus) and stylos.

  These newcomers can transform the structure and composition of savannas, and in many places they already have. They change the fire regime, add to grazing pressure, compete with native vegetation and move into previously vacant ecological niches.

  Climate change adds another question. Human-induced warming from the greenhouse effect may alter the structure and function of savannas. Some authors suggest savannas and grasslands could become even more prone to woody plant encroachment. Yet one recent case described a savanna expanding its range at the expense of forest in response to climate variation. Savannas so often sit at ecotones, the boundaries between ecosystems, that rapid and dramatic shifts in vegetation are possible there.

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