<P> Over many large tropical areas, the dominant biome (forest, savanna or grassland) cannot be predicted only by the climate, as historical events plays also a key role, for example, fire activity . In some areas, indeed, it is possible the occurrence of multiple stable biomes . </P> <P> 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 . Pine barrens in scattered locations from New Jersey to coastal New England are remnants of these savannas . 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 . The maquis shrub savannas of the Mediterranean region were likewise created and maintained by anthropogenic fire . </P> <P> These fires 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 . It has been suggested by many authors that aboriginal burning created a structurally more open savanna landscape . Aboriginal burning certainly created a habitat mosaic that probably increased biodiversity and changed the structure of woodlands and geographic range of numerous woodland species . It has been suggested by many authors that with the removal or alteration of traditional burning regimes many savannas are being replaced by forest and shrub thickets with little herbaceous layer . </P> <P> 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 introduction of exotic pasture legumes has also led to a reduction in the need to burn to produce a flush of green growth because legumes retain high nutrient levels throughout the year, and because fires can have a negative impact on legume populations which causes a reluctance to burn . </P>

Which of the following characteristics is an important adaptation in the plants of the savanna