In The Natural Interest
by Greg Stone
As I sit to write this
month's contribution, the weather is unusually
cold and wet for mid-November, however experience
has taught us that just a few hot, windy days can
rapidly increase the threat of bushfires. With
this in mind, fire in the bush seems to be a
relevant topic to explore.
The majority of remnant
communities of natural vegetation in the Tallong
-Wingello - Penrose - Canyonleigh area occur on dry,
well-drained and infertile soils based on Hawkesbury
Sandstone, and most of the plant species growing in
these fire-prone sclerophyllous (hard-leaved)
communities have been specifically made with inbuilt
'bushfire survival' mechanisms. Most of us will have
seen a dark and desolate bush landscape remaining
after a devastating fire and wondered how anything
could survive, let alone recover to flourish again.
Yet several years later the same site is again full
of green foliage with only the blackened trunks of
the Eucalypts testifying to previous events.
How do the plants survive
and recover? In regard to fire response, plants
divide themselves into two basic groups - the
'sprouters' and the 'seeders' (for want of better
The 'sprouters' are
those plants that respond to
fire damage by sending out new vegetative growth.
Some of the most common sprouting mechanisms are
epicormic buds, lignotubers, rhizomes and
underground rootstocks and fire induced flowering.
Beneath the bark of many Eucalyptus species are
specialised 'epicormic' buds which lie dormant
until the canopy of the tree is either removed
or scorched by fire. The loss of leaves triggers
a burst of growth from these buds (epicormic shoots)
which provides an almost immediate leaf growth to
sustain life and aid the recovery of the plant.
Eucalyptus trunks clothed in fresh, green foliage
are a common sight after a bushfire.
Most Eucalypts have a specialised root system in the
form of a 'lignotuber'. This is actually a swollen
tap root which contains numerous dormant buds
protected by the soil surface. Most soils are
poor conductors of heat and provide adequate
protection to lignotubers. As with epicormic buds,
lignotuber buds respond when the upper parts of the
tree are damaged by fire. Lignotubers are most
common in 'mallees' or smaller, multi-stemmed Eucalypts.
Rhizomes and underground
Rhizomes are horizontal underground stems which
obviously enjoy the benefit of some protection from
fires burning overhead. Plants such as the common
Spiny-headed Mat Rush (Lomandra longifolia) may be
destroyed by fire, but are afterwards able to send
up new shoots from their rhizomes.
The Grass Trees (Xanthorrhoea spp.) often produce
a large flowering stalk immediately after a fire
in order continue their regenerative processes as
soon as possible.
The 'seeders' obviously survive a bushfire by means
of a variety of seed-based methods, which include
protective seed-coats, storage in the soil and
protection in woody seed capsules.
Hard seeds in the soil
Leguminous shrubs e.g. Acacia (Wattles) and the many
members of the Fabaceae (Pea flowers) family are
usually destroyed by fire but regenerate from the
huge number of seeds stored in the soil or leaf
litter. Most legumes produce seeds with a hard
coating which is impermeable to water and will
therefore not germinate until cracked open by fire.
These species are usually the first plants to
regenerate after a fire.
Woody seed capsules
Shrub species such as Banksias and Hakeas have the
capacity to regenerate from rootstocks as 'sprouters'
but also store their seed for many years in thick,
woody capsules. A hot fire will often kill the adult
plant but also assist in the release of the seeds
from the capsules. The capsules are desiccated by
the heat and in shrinking release the seeds into
an ash bed favourable to germination.
Leptospermums (Tea Trees), Melaleucas (Paperbarks)
and Callistemons (Bottlebrushes) employ a similar
mechanism, but the smaller seed capsules are not
as efficient at retaining seed, and seed-fall may
occur every few years whether a fire is present or not.
That's not all...
If time and space permitted, we could also consider
the plant species that send in 'air-drops' of
wind-borne seeds to germinate in fire-devastated areas.
Or perhaps the Burrawangs (Macrozamia spp.) which
keep a supply of food buried beneath the ground to
enable a quick post-fire response. Or the different
ways in which both rough and smooth Eucalyptus bark
protects the trunk from fire damage. Maybe later in
this fire season we will explore this subject a
C/- P. O. Wingello
Wingello N.S.W. 2579