Smoke
treatment, and germination of Australian Natives
Dirk Wallace,
Yackandandah, Victoria.
In fire-prone floras, particularly those of
Mediterranean climate zones, fire has been shown to be crucial for the germination
of seed from a wide variety of taxa. Habitat burning is an important cue, for
fire-sensitive species, in triggering germination. For many of these taxa,
germination under controlled conditions has been difficult (to near
impossible!) using conventional treatments, other than excised embryo culture
or special pretreatments including hormonal applications.
Following the discovery that smoke stimulated
germination of the rare South African plant Audounia capita, much
research has been carried out around the world for the conservation of such
rare species, and for the many uses in land management and nursery production.
Research
by Dixon et al. (1995) has shown that smoke is a key principle in breaking seed
dormancy in a wide variety of native Australian species. Though this study has
concentrated on Western Australian plants, general principles have emerged
regarding the benefits of smoke for germination:
·
Smoke can promote earlier and more uniform germination
under controlled greenhouse and laboratory conditions.
·
Smoke enables germination in species previously thought
difficult or impossible to germinate by conventional means. Examples include
species in the genera: Geleznowia and Eriostemon (Rutaceae); Hibbertia
(Dilleniaceae); Stirlingia and Conospermum, Grevillea and Hakea (Proteaceae);
Verticordia and Calytrix (Myrtaceae); Pimelea (Thymeleaceae); Blancoa
(Haemodoraceae); Stylidium (Stylidiaceae).
·
Smoke substantially promotes germination in species with
otherwise low levels of germination. Examples include Anigozanthos and
Conostylis (Haemodoraceae); Dianella, Thysanotus and Burchardia (Liliaceae);
Patersonia (Iridaceae); Lechenaultia (Goodeniaceae); Gyrostemon and
Codonocarpus (Gyrostemonaceae); Stackhousia (Stackhousiaceae); Hybanthus
(Violaceae).
·
The promotive effect of smoke is independent of seed size
and shape and plant life form, i.e. whether annual, perennial, herbaceous,
seeder (fire sensitive) or resprouter (fire tolerant).
·
Aerosol smoke, smoke dissolved in water and solids
(activated clays, sand particles) that have been smoked have all been effective
in promoting seed germination.
·
High doses of smoked water can inhibit germination of many
species.
·
The germination of seed of paper daisies (Rhodanthe,
Schoenia) is suppressed by smoking.
·
Germination over time in response to smoke varies amongst
taxa.
a.
Control (unsmoked) and smoked seed attained final
germination at the same rate, eg. Conostylis species.
b.
First seedling emergence occurred earlier in smoked seeds.
c.
Control germination was limited to first week or so
whereas smoked seed continued to germinate over a longer period.
d.
Difference between control and smoked treatment became
apparent only after several weeks.
·
Seed that does not respond to smoke treatment includes
that of species in Persoonia and drupaceous Epacridaceae (those species with
large, woody fruits, compared to small-seeded species which do respond
positively to smoke). These groups have been extensively investigated to
determine possible barriers to smoke entering the seed but all attempts to
scarify the seed mechanically or with acid have not been effective in improving
germination, Persoonia has been found in other studies to respond to
gibberellic acid treatments, suggesting that factors involved in seed dormancy
in this species may require other dormancy-breaking mechanisms for germination
to proceed.
Methods of Applying Smoke
Sown
seed trays or whole seed are placed on an open-mesh, two-tiered frame in a
sealed, plastic tent approximately 2m x 2m and 1.4m high. Smoke is generated by
slow, controlled combustion, in a 200-litre drum, of a mixture of fresh and dry
leaf and twig material from a range of plants. Prunings of native species are
usually used so as to emulate the natural smokes likely to occur after a
wildfire in bushland habitats. Melaleuca has been found to readily produce
smoke. The drum is fitted with an inlet through which air is pumped at the rate
of 60 -100 litres per minute, and an outlet is connected to a 1.5 metre long,
pipe. A 2-metre length of flexible, stainless steel exhaust piping
approximately 50 mm in diameter is then connected to the plastic enclosure.
This ensures that the smoke is injected towards the roof of the tent and is
therefore spread throughout the tent.
After smoking
for 60 minutes, trays are transferred to the glasshouse and watered carefully
for the first 6-10 days to ensure that the soluble promoter in smoke comes in
contact with the seeds but is not washed through the mix before reacting with
the seed. Watering is then continued as for normal germination.
Seeds
can also be direct-smoked. In this instance, seed is laid out in a single layer
in trays. The trays are smoked for 60 minutes in the fumigation tent (as
described above) and the air-dried seed is then sown or stored dry until
required. Unlike smoke applied to soil containing sown seeds, smoked seeds can
be watered, as would be normal practice.
Smoked
Water
Smoked
water can be useful for direct priming or pre-germination of seeds prior to
sowing. Application of smoke via water has the advantage of not requiring the
use of the smoke tent and the convenience of priming seeds at will.
Smoke-water-primed seeds may germinate better than those in smoked seedling
trays. This process allows the handling of large quantities of seed such as is
required for land restoration or automated seed-sowing devices.
Smoked
water is produced by drawing smoke produced from the combustion drum operating
as for aerosol smoke, through a 20 litre container of water. Smoke bubbling is
done for approximately 60 minutes and the resultant solution is frozen until
required.
Seed is
soaked for 12 hours in a 10%, 9:1 water: smoke-water solution. The seed is then
sown, or dried for sowing as required. Seeds treated with smoked water can be watered
normally after smoking. Although this method as been shown to be useful for a
number of native species, caution is recommended, as seed of some species can
degenerate if soaked in water for prolonged periods. Also, pre-germination as a
horticultural practise for seed of Australian native plants requires some
experimentation to ensure that the process is applicable. In some cases,
pre-germination can lead to decline in seed quality and viability. It is
therefore recommended that species to be treated in this way should be tested
far tolerance to imbibing and drying treatments. (With seeds that are prone to
degeneration during soaking, it is useful to bubble air - as from a fish tank
aerator - through the water during soaking).
Habitat
Germination Studies
Smoke-fumigation
treatments can be applied directly to habitat sites. For a range of species,
germination will happen in 6 to 8 weeks after treatment.
Smoke is
generated as above and applied to sites where excess leaf litter and larger
plants have been removed to prevent 'shadowing' of the soil from smoke. Tents
of 5 m x 1 m x 40 cm high are erected over the sites and smoke is pumped in for
60 minutes. Best results are achieved if smoking is done in summer to early
autumn so that wash-down of the smoke factor coincides with the onset of the
first rains (for temperate regions of Australia). Smoking undertaken at other
times of the year appears to yield less germination for taxa that respond to
summer/autumn smoking.
Further
Reading
Dixon
K.W, Roche S and Pate J.S (1995). The promotive effect of smoke derived from
burnt native vegetation on seed germination of Western Australian plants.
Oecologia 101: 185-192
This article is derived from
information by Dr. Kingsley Dixon and Shauna Roche.
Dr
Kingsley Dixon is Assistant Director and Shauna Roche is a research botanist at
Kings Park and Botanic Garden, West Perth, Western Australia.