A savanna or savannah is a mixed woodland grassland ecosystem characterised by the trees being sufficiently widely spaced so that the canopy does not close.
The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer consisting primarily of grasses. Savannas maintain an open canopy despite a high tree density. However, in many savannas, tree densities are higher and trees are more regularly spaced than in forests.
Savannas are also characterised by seasonal water availability, with the majority of rainfall confined to one season; they are associated with several types of biomesand are frequently in a transitional zone between forest and desert or grassland. The word originally entered English in  as the Latin Zauana equivalent in the orthography of the times to zavana see history of V. Peter Martyr reported it as the local name for the plain around Comagrethe court of the cacique Carlos in present-day Panama.
Many grassy landscapes and mixed communities of trees, shrubs, and grasses were described as savanna before the middle of the 19th century, when the concept of a tropical savanna climate became established. The common usage meaning to describe vegetation now conflicts with a simplified yet widespread climatic concept meaning.
The divergence has Savannah spread up close caused areas such as extensive savannas north and south of the Congo Savannah spread up close Amazon Rivers to be excluded from mapped savanna categories. Sometimes midwestern savanna were described as "grassland with trees".
Two factors common to all savanna environments are rainfall variations from year to year, and dry season wildfires. Over many large tropical areas, the dominant biome forest, savanna or grassland can not be predicted only by the climate, as historical events plays also a key role, for example, fire Savannah spread up close.
Savannas are subject to regular wildfires and the ecosystem appears to be the result of human use of fire.
For example, Native Americans created the Pre-Columbian savannas of North America by periodically burning where fire-resistant plants were the dominant species. Aboriginal burning appears to have been responsible for the widespread occurrence of savanna in tropical Australia and New Guinea and savannas in India are a result of human fire use.
These Savannah spread up close are usually confined to the herbaceous layer and do little long term damage to mature trees. However, these fires either kill or suppress tree seedlings, thus preventing the establishment of a continuous tree canopy which would prevent further grass growth.
Prior to European settlement aboriginal land use practices, including fire, influenced vegetation  and may have maintained and modified savanna flora. Aboriginal burning certainly created a habitat mosaic that probably increased biodiversity and changed the structure of woodlands and geographic range of numerous woodland species.
The consumption of herbage by introduced grazers in savanna woodlands has led to a reduction in the amount of fuel available for burning and resulted in fewer and cooler fires. The closed forest types such as broadleaf forests and rainforests are usually not grazed owing to the closed structure precluding grass growth, and hence offering little opportunity for grazing. The removal of grass by Savannah spread up close affects the woody plant component of woodland systems in two major ways.
Grasses compete with woody plants for water in the topsoil and removal by grazing reduces this competitive effect, potentially boosting tree growth. There is evidence that unpalatable woody plants have increased under grazing in savannas.
Introduced grazing animals can also affect soil condition through physical compaction and break-up of the soil caused by the hooves of animals and through the erosion effects caused by the removal of protective plant cover. Such effects are most likely to occur on land subjected to repeated Savannah spread up close heavy grazing. Alteration in soil structure and nutrient levels affects the establishment, growth and survival of plant species and in turn can lead to a change in woodland structure and composition.
Large areas of Australian and South American Savannah spread up close have been cleared of trees, and this clearing is continuing today. For example, until recentlyha of savanna were cleared annually in Australia alone primarily to improve pasture production. Clearing is Savannah spread up close out by the grazing industry in an attempt to increase the quality and quantity of feed available for stock and to improve the management of livestock.
The removal of trees from savanna land removes the competition for water from the grasses present, and can lead to a two to fourfold increase in pasture production, as well as improving the quality of the feed available. A number of techniques have been employed to clear or kill woody plants in savannas.
Early pastoralists used felling and girdlingthe removal of a ring of bark and sapwoodas a means of clearing land.
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War-surplus heavy machinery was made available, and these were used for either pushing timber, or for pulling using a chain and ball strung between two machines. These two new methods of timber control, along with the introduction and widespread adoption of several new pasture grasses and legumes promoted a resurgence in tree clearing.