Cyclones, fire, drought and flood - weeds love 'em!
Monday, 03 March 2008
By Weeds CRC
bushfire
Natural disasters, such as bushfires, give weeds a boost.

Drought, flood and fire may be the ruin of rural life, and cyclones
the curse of coastal land and other low-lying communities, but if
you're a weed you love `em, says Dr Rachel McFadyen, CEO of the Weeds CRC.

Commenting on the recent floods in northern NSW and central
Queensland, Dr McFadyen says that while our agricultural systems, and
our native plants and animals, take a hammering from these extreme
weather events, they give weeds and other biological invaders the
perfect leg up.

Alarmingly, says Dr McFadyen, extreme weather events are likely to
become more common as climate change progresses. But for many weeds,
that's great news.

`In the tropics, cyclones often break open the canopy of a forest and
let in new light to the normally dim forest floor – that's just
perfect for weeds.'

`For thousands of years the local native plants have supplied the only
seed available. This meant, given time, that the forest always
returned to near-original condition.'

But now, says Dr McFadyen, thanks to people and vehicles, there are
often seeds of new foreign species lying around in small numbers ready
to out-compete the local plants in the grab for space and light. Birds
and animals may also bring in these new seeds from towns and farmland.

`Suddenly', says Dr McFadyen, `the invaders get the break they need'.

Furthermore, some weeds are quite capable of keeping the canopy open
(e.g. African tulip tree in the far north, madeira vine and cat's claw
creeper in SE Qld, privet in NSW) by smothering the regrowth of native
trees. The new conditions with high light levels suit the weeds, and
they consolidate their position.

`With each storm or cyclone event the invaders ratchet up their
position', says Dr McFadyen, `and the forest is prevented from
repairing itself'.

Fire risk is also predicted to increase greatly under climate change
scenarios, and favours weeds by `clearing the decks' of competitors,
says Dr McFadyen. And while fires may burn up many tons of weed seed,
and favour the germination of some fire-adapted native species, it
also provides the perfect seed bed for weed seeds.

`If you're a weed with seed that is blown around by the wind, or have
fruit that birds spread, for example, then a bushfire can be good
news', says Dr McFadyen.

`Chances are that your seeds will be the first to arrive. Your
seedlings will be first up when the rain comes, and will have set seed
themselves long before any native plants recolonise the burnt ground.'

Some weed species, such as the 4m-tall African gamba grass now rapidly
spreading in northern Australia, go even further by fuelling fires so
hot that even normally fire-resistant eucalypts are killed. Native
eucalypt woodlands, with their many animals and birds, are changed
into a pure stand of gamba grass with much lower biodiversity.

`These new grasslands might be good for elephants', says Dr McFadyen,
`but are pretty useless for most Australian animals and birds.'

Drought also tips the scales in favour of weeds. If climate change
does result in more frequent and more severe droughts, then weeds will
again be the first to benefit.

`If the drought kills off all the competitors, whether these are crops
or pasture grasses, then when the rains finally come the weeds will do
what they do best – invade bare ground and grab all the moisture!',
says Dr McFadyen.

`Those invaders able to cope with drier conditions, and able to
quickly move their seed around, (eg parthenium weed, or serrated
tussock) will be the weeds we'll be battling tomorrow', she says.

Finally, floods can help weeds get a toehold in conditions they like.
Climate scientists are predicting more floods – as we are already
seeing in the north - as rainfall conditions become more variable.
Once again, this will suit the weeds nicely.

According to Dr McFadyen, Mimosa pigra in the NT didn't really get out
of hand until major floods in the 1970s spread the seed from a small
initial patch out over the wide flood plain of the Adelaide River. The
same happened with Athel pine in the river channels of central
Australia, and even prickly acacia in north Queensland.

Any weed that is spread effectively by water is a candidate for
trouble in areas where floods become more common, says Dr McFadyen.
Examples include willows, giant reed, alligator weed, cabomba and lippia.

'Whether it's cyclones, fire, drought or flood, weeds are set to
become the big winners from climate change', she says. `Extreme
vigilance and early action by landholders, agencies and individuals
are the keys to keeping these species under control.'

`Changing climate is new territory for farmers and other land
managers, and they will need all the information and understanding
that science can provide if they are to stay ahead in the weed war',
says Dr McFadyen.

It is critical that public and private stakeholders join forces and
invest wisely in building a new knowledge base, she says. `This is the
only way we will equip ourselves to meet the new weed challenge now
emerging from climate change.'

http://www.sciencealert.com.au/features/20080303-16983.html