FACTBOX - Key facts about Australia's bushfires

(Reuters) - Australian bushfires have killed at least 35 people and destroyed more than 100 homes in the country's worst bushfires in 26 years.

Here are key facts on Australia's annual bushfires and what is done to manage them.

- High temperatures and low rainfall make bushfires a natural hazard. Natural tree oils in native eucalypt forests fuel fireballs.

- Common causes are lightning strikes or humans, who drop cigarettes or light blazes deliberately. Australia's science agency, the CSIRO, says controlled burns, where fires are deliberately lit in high-risk areas, are key to management. But backburns can spiral out of control.

- Bushfires have killed more than 250 people in Australia in the last 40 years. In February 1983 blazes killed 75 people as they swept across Victoria and South Australia.

- Trained volunteers and professional firefighters typically team to fight fires. In severe seasons volunteers come from the United States and New Zealand. Light aircraft and helicopters are used for water-bombing.

- As well as human impacts, long-term bushfire effects include loss of bird and animal habitats, reduced soil fertility lasting decades and contamination of water catchments with ash and debris.

Sources: Reuters, Emergency Management Australia (www.ema.gov.au), Department of Sustainability and Environment (www.dse.vic.gov.au), Commonwealth Scientific and Industrial Research Organisation (www.csiro.au), Environmental Literacy Council (www.enviroliteracy.org/article.php/46.html)

(Editing by David Fox)