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Smoke Stimulates the Germination of Many Western Australian Plants
Kingsley Dixon and Shauna Roche
Fire has played a significant role in the evolution of the Australian flora at least since the arrival of arid conditions in the mid-Tertiary (about 30 million years ago). For many taxa, response to fire has moulded plant growth and development and has been responsible for the derivation of analogous structures and life forms often in disparate taxonomic groups. In fire-prone floras, particularly those of Mediterranean zones, fire has been shown to be crucial for the recruitment from seed of a wide variety of taxa. For seeder (fire-sensitive) species and fire ephemerals, habitat burning is the single most important cue for triggering germination of the dormant soil seed bank. For many fire-responsive taxa, germination of viable seed under controlled conditions has been difficult or impossible using conventional treatments other than excised embryo culture or special pretreatments including hormonal applications.
The Role of Smoke in Germination
Following the discovery that smoke stimulated germination of the rare South African plant Audounia capita, the exploration of the benefits of smoke-mediated germination has expanded to different continents and has been applied in nurseries, for land management and in rare flora conservation.
This article gives an overview of the development of smoke-stimulated germination of native Australian species and describes recent applications of the process to the germination of horticulturally significant species.
Smoke-Stimulated Germination of Australian Species
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).
Calytrix leschenaultii is a colourful small shrub which is native to south-western Australia. Calytrix species are difficult to propagate from seed by normal methods. Select the thumbnail image or plant name for a higher resolution image (32k).
- 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).
Lechenaultias can be propagated easily by vegetative means but are difficult to grow from seed. This is Lechenaultia biloba the best known of this colourful genus. Select the thumbnail image or plant name for a higher resolution image (37k).
- The promotive effect of smoke is independent of seed size and shape and plant life form, ie. 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.
- Control (unsmoked) and smoked seed attained final germination at the same rate, eg. Conostylis species.
- First seedling emergence occurred earlier in smoked seeds.
- Control germination was limited to first week or so whereas smoked seed continued to germinate over a longer period.
- 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 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 which respond to summer/autumn smoking.
This research was funded by the Minerals and Energy Research Institute of Western Australia, Rural Industries Research and Development Corporation and Land and Water Resources Research and Development Corporation with support from Alcoa Australia, RGC Mineral Sands and wildflower growers and enthusiasts from around Australia .
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 reprinted from the August 1995 issue of the "SGAP Journal", the newsletter of the South Australian Region of the Society.
Dr Kingsley Dixon is Assistant Director and Shauna Roche is a research botanist at Kings Park and Botanic Garden, West Perth, Western Australia.
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