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'Thieving' willows destroy waterways
WANT more water in your local
creek? Try getting rid of the wil-
For many people, this time of year in
the Adelaide Hills prompts fond
images of deciduous trees and falling
But there is wider recognition that
the exotic willow, ash and poplar trees
that create this visual backdrop also
cause significant harm to the local
creeks and waterways that help make
the Mount Lofty Ranges one of South
Australia's most productive and biolog-
ically diverse regions.
While deciduous trees may be rela-
tively harmless in an urban setting, the
story is quite different when it comes
to rural and bushland waterways
where they often outcompete native
vegetation, clog creeks, cause erosion,
and guzzle water that could otherwise
support a healthy native environment.
Willows in particular have colonised
significant lengths of creeks in the
Mount Lofty Ranges, forming thick,
shady canopies that limit the growth of
native vegetation, resulting in bare
banks prone to erosion.
It is little wonder that some species
of willow are now considered Weeds of
Native trees and plants tend to drop
leaves constantly throughout the year
at a pace that suits the Australian
bush's unique 'digestive system'. On
the other hand, willows and other
deciduous trees drop most of their
leaves in one hit during autumn. Being
softer, they decompose rapidly in
watercourses, releasing large amounts
of nutrients that strip oxygen from the
water, causing serious problems for
aquatic fauna like fish and macro-
Willows also fail as habitat for native
fauna. They do not form wildlife-shel-
tering hollows and their bark is not as
welcoming for native insects.
Combined with the suppression of
understory, willows tend to create a
very sterile environment.
Willows can spread rapidly and
dominate entire reaches in short peri-
ods. When infesting creeks, the wil-
low's dense root mat allows it to grow
out into the watercourse channelling
water into nearby banks, causing ero-
sion and bank widening.
One of the willow's less well-known
attributes is its ability to slurp water at
a phenomenal rate. New South Wales
Government research has shown that
willows in watercourses annually con-
sume 3-4 megalitres of water per
hectare more than natural riparian veg-
This is water that could otherwise be
available for users such as our native
flora and fauna.
If the average willow-infested water-
course had a canopy width of 10
metres, every kilometre would har-
bour 1ha of willow canopy (10m x
1000m = 10,000 square metres),
draining an extra 3-4mL of water a
year from the environment.
In the Upper Onkaparinga alone,
above Mount Bold, it has been estimat-
ed there are more than 400 kilometres
of willow-infested watercourses, which
would equate to between 1200mL and
1600mL of water 'lost' from the catch-
ment each year.
If these spectacular numbers do not
already sound bad enough, then let's
think about the potential conse-
quences of this water use.
During dry times the fish, frogs and
aquatic invertebrates that live in our
waterways retreat to small permanent
pools throughout our catchments.
These refuges also play a vital role in
providing resources to other non-
aquatic species such as birds and
mammals. In some areas, it is quite
possible that willows could deny the
critical, rare summer water flows
required to keep these refuges topped
up.Anecdotally, there have been reports
of an apparent increase in water avail-
ability after removing willows. For
example, when a community group
removed willows from several hun-
dred metres of a creek passing through
an Adelaide Hills reserve, they noticed
more frequent flows.
So next time your dam runs dry, you
see a stand of water-stressed red gums,
or wonder where the frogs have gone,
think about the water thieving willows
upstream and ask yourself if they real-
ly deserve it.
• Need to know more?
Natural Resources Centre 5 Aldinga
Road, Willunga 08 8550 3400; 1 Adelaide
Lobethal Road, Lobethal 08 8389 5900;
or 8 Adelaide Road, Gawler South 08
Deciduous trees in rural and bushland waterways often outcompete native
vegetation, clog creeks, cause erosion, and guzzle water.
Deciduous trees dominate
Unfriendly to fauna
Use too much water
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