Section 1: Prickly acacia—ecology and threat - Weeds Australia

‘Some
individual
trees have
produced
more than
175 000
seeds
annually.’
8
Preferred habitat
Prickly acacia prefers tropical and subtropical
regions where it is found in woody grasslands
and savannas (W. Palmer, 2003, pers. comm.,
21 Nov). It prefers heavy, cracking clay soils
such as those found on the Mitchell grasslands
of northern Australia. However, it will also
grow on heavy coastal clays, basalt soils and
occasionally on lighter soils and sands. It
requires 250–1500 mm of rainfall annually
(Mackey 1998).
Life cycle
Seeds germinate after significant rainfall in
late spring and summer. Seedling growth can
be rapid, and trees flower and set seed within
two to three years after germination under
ideal conditions with unlimited water
(e.g. along bore drains).
Flowering generally begins in late February
and continues through to June. Pods ripen
and fall from late October through to January.
Trees growing in association with a
permanent or semi-permanent water source
such as a bore drain, dam, drainage line,
creek or river tend to produce a large number
of flowers and pods annually, while those on
the open downs produce low numbers except
in high rainfall years. Trees along a bore drain
have been recorded as producing 30 000
seeds annually (Mackey 1998), while some
individual trees have produced more than
175 000.
Although seeds may remain viable in the soil
for seven years (Bolton, Carter & Dorney
1987), most seeds germinate or are destroyed
within two years. As it can survive for 30–60
years, prickly acacia is a relatively long-lived
tree (Carter 1994).
matures
in 2-5 years
long lived
- 30-60 years;
out-competes
native plants;
forms dense
thickets;
900 trees/ha;
fire
sensitive.
MATURE
PRICKLY
ACACIA
THICKETS
flowers
Feb-Jun;
throughout the
year but peak
after rains;
seedlings
resistant to
competition once
established;
reduced by
frost and low
moisture.
GERMINATION
SEED
BANK
30
thousand
seeds per
square metre
per year;
seeds remain
viable for
7 years.
SPREAD
AND
DISPERSAL
Figure 1: The life cycle of prickly acacia
(adapted from Prickly Acacia National Strategic Plan)
animal
droppings;
fresh
water;
grazing
imported stock;
deliberate
plantings.
Dispersal
Stock, particularly cattle, are the main agents
for dispersing prickly acacia seed. Cattle pass
about 80 per cent of ingested seed in their
faeces, and about 40 per cent of these remain
viable (Mackey 1998). The faeces also provide
an environment that promotes germination
and survival (Harvey 1981). Since seed takes
about six days to pass through the digestive
tract, stock moved by road transport can
disperse viable seed over large distances.
In comparison, sheep pass few viable seeds in
their faeces but spit out about 35 per cent of