Australia has hardly any native deciduous trees. Why do we mostly have evergreens? Is it too hot for deciduous trees to have evolved, or is there another reason? Source: ABC News
There are only a few trees native to Australia that are deciduous and that makes us pretty unique.
“The fascinating thing about Australia is the remarkable absence of deciduousness in our flora,” said Dr David Bowman, a professor of environmental change biology at the University of Tasmania.
“We have some deciduous trees, but they are very, very outnumbered by evergreens.”
In most parts of the world, deciduous trees get a clear message from the environment – in the form of day length, temperature and moisture – about when to lose their leaves.
But in Australia, plants have evolved in a much more variable climate, said Dr Bowman.
Good growing conditions can happen at any time of year – and that makes it smarter to be an evergreen ready to jump into action the minute they appear he said.
“If you’re in outback Australia and a thunderstorm goes past, you can immediately suck up the rain, whereas if you are deciduous you can’t do anything because you don’t have a leaf – you can’t be opportunistic,” Dr Bowman said.
So to put it bluntly, being deciduous in a variable climate is risky.
“You could be tricked into putting out your leaves, which are very expensive to make, and then the climate doesn’t play. Then you take a hit,” Dr Bowman said.
While Australian soils are generally poor in nutrients, most soils are deep, so trees can tap into a source of groundwater far below the surface.
During dry seasons, trees can keep running in ‘maintenance mode’, waiting for the next rainy opportunity for a growth spurt.
While evergreen leaves are even more expensive to make than deciduous leaves, they last longer.
According to ‘leaf economics’, which weighs up the cost and benefit of being evergreen versus deciduous, it pays to be evergreen in Australia.
“Being deciduous is not a winning strategy,” Dr Bowman said.
Most deciduous trees in Australia, such as the red cedar (Toona ciliata), white cedar (Melia azedarach), and boab (Adansonia gregorii), live in tropical or subtropical regions, where they lose some or all of their leaves in preparation for the dry season.
This is different to deciduous trees in temperate areas, which lose their leaves in the lead up to the cold season.
Australia has just one true temperate deciduous native tree – the deciduous beech or Fagus (Nothofagus gunnii) in Tasmania, which puts on a stunning autumn display before dropping all its leaves in anticipation of cold winter weather.
