Smoke Infusion for Seed Germination
in Fire-adapted Species

 

Daniela Shebitz, Anne Andreu, Marlo Mytty, Doug Schmitt, and Mike Cooksey

 

Introduction

 

Numerous species that inhabit fire-dependent ecosystems have evolved reproductive strategies to adapt to factors associated with fire (Van Staden et al. 2000).  These adaptations are particularly evident in seeds that respond to the physical (i.e. temperature and light) and/or chemical (smoke, gas, nutrients) germination cues associated with fire.  In fact, many species have evolved barriers to seed germination that are overcome only by fire-related cues (Keeley 1998). 

 

Seeds of many species germinate in response to physical signals associated with fire, such as fracturing or desiccation of the seed coat by heat (Jeffrey et al. 1998).  Heat may also stimulate the embryo directly (Blommaert 1972).  For a substantial number of species with fire-triggered germination, however, chemicals from combustion induce germination, not the heat (Keeley 1998).  

 

In western North America and South Africa, numerous species have been stimulated by exposure to chemicals in charred wood.  While it is unclear whether the chemicals in charred wood are the same as those responsible for smoke-induced germination, the chemicals in smoke have also been found to stimulate germination of seeds.  Plants whose seeds have been stimulated by smoke belong to a variety of environments ranging from South American fynbos shrub to savannas, the Great basin, Australian heath shrubland and California chaparral (Keeley 1998). 

 

In this paper, we will first present the effects that smoke has on seed germination, discuss why this relationship can be incorporated into habitat restoration, and provide information on species and ecosystems that can potentially benefit from smoke technology.  We will then discuss various methods of incorporating smoke technology into restoration, explaining in detail our method of choice.

 

General effects of smoke on germination

 

Smoke is clearly one of the products generated as a consequence of fire.  There is no evident indication of the mechanisms by which smoke affects germination. It is known, however, that the chemical signals of smoke not only influence seeds during fires and in the immediate post-fire environment, but the signals last for considerable periods after the fire, and perhaps most importantly, can travel to communities long distances away from the fire (Van Staden et al. 2000).   

 

Due to the fact that smoke particles can adhere to plant surfaces, persist in the soil, and be adsorbed to soil particles, smoke particles have major effects on scarified seeds in the soil (Van Staden et al. 2000).   Egerton-Warburton (1998) demonstrated that this ability of smoke to adhere to soil and plant surfaces plays a role in the germination process by changing the morphology of the seed and causing an intense chemical scarification of the seed surface.   

 

Roche et al. (1997) found that some species respond only to smoke application to the soil seed bank, and not to the application of smoke to freshly collected seed.  The authors suggest that some seeds need to enter the soil seed bank before they are receptive to the germination-promoting effects of smoke. 

 

In some species, such as Erica sessiliflora, smoke treatment on seeds can substitute for a light requirement.  Such a response, which was also observed for light-sensitive Grand Rapids lettuce seeds (Drewes et al. 1995), makes seedling recruitment more probable if smoke dissolved in water penetrates into the soil.  This characteristic ensures that even in the dark, there will be some germination of light-sensitive seeds in the absence of major soil disturbance (Van Staden et al. 2000). 

 

Until recently, the role of chemical cues in seed germination received little attention (Van Staden et al. 2000).  In addition to heat, vegetation fires release chemical cues such as ethylene and ammonia.  While both of these gases are known to stimulate germination, it has been shown that ethylene is not the active compound in smoke solutions that stimulates germination. (Jager et al. 1996).  Numerous studies have attempted to determine the chemical components responsible for charred wood and smoke-stimulated germination, but have not successfully identified the active components (Keeley 1998).

 

Smoke technology in habitat restoration

 

With increased urbanization, fire as a restoration tool is not always feasible.  Smoke technology provides the ability to incorporate the chemicals associated with fire in to a restoration when fire is not possible.  There are numerous benefits of incorporating smoke into a habitat restoration project.  Below are some examples that were presented by Van Staden et al. (2000):

 

  1. Testing seed viability – Smoke and smoke extracts can test the viability of seeds from soil seed banks in areas that have become invaded by exotic plant species.  Germinating the seeds using smoke will assist in determining whether a viable reserve of native species seeds that benefit from fire still exists in the area. 

 

  1. Giving existing native species an “edge” - Physically removing exotic species and then smoking the soil using smoke tents or applying aqueous smoke solutions may assist in restoring native plants.

 

  1. Stimulate germination of native seeds in the existing soil seed bank – If there are native species in an area that has been characterized by frequent fires in the past, you can stimulate the germination of seeds by smoking the soil (see 2).

 

  1. Pretreating broadcast seed with aerosol smoke to increase the number of germinants – Compared to unsmoked seed, pretreatment of broadcast seeds with aerosol smoke has been found to result in significant increases in the total number of germinants and responding species.
  2. Germinating seeds that are otherwise difficult or impossible to germinate – Many wildflower species in the families Asteraceae, Bruniaceae, Ericaceae, Thymelaeaceae, and Restionaceae that responded to smoke were previously difficult or impossible to germinate in a nursery.

 

  1. Removing the need for further dormancy-breaking treatments before sowing – Numerous plants such as vegetable crops have a potential to be primed with aqueous smoke extracts and then stored for later use.

 

  1. Flexibility in scheduling – Seeds may be smoke-treated immediately before sowing, or they may be treated prior to sowing and then dried and stored.

 

  1. Protection for seeds – Roche et al (1997) suggest that high levels of smoke by protect seeds against predation and microbial attack.

 

Species and ecosystems that can benefit from smoke technology

 

Many species, especially those from fire adapted ecosystems, respond to germination cues from heat, smoke, or a combination of the two. Since research in this area is fairly new, isolation of the specific mechanisms by which germination is stimulated are often not known. Species that have been found to respond to heat do so because of “heat shock”.  Species that have been studied are “hard-seeded”, with a prominent waxy cuticle and dense palisade layer of sclerids that enforces dormancy by making the seed coat impermeable to water. Brief heat shock between 80° and 120° C is sufficient to cause the seed to imbibe water by loosening cells or possibly denaturing germination inhibitors. For some species, heat shock alone may work, but some heat-induced species also require light and/or cold stratification. Heat-shock germination is widespread in the following families: Fabaceae, Rhamnaceae (includes Ceanothus), Convolvulaceae, Malvaceae, Cistaceae, and Sterculiaceae and is found in many ecosystems. (Keeley 1998).

 

Other fire-evolved species have been found to respond not to heat, but to chemicals from combustion - either from smoke or in charred wood. Compared to heat shock, little is known regarding which chemicals stimulate germination and how.  Charred wood has been shown to stimulate germination in the species Emmenanthe penduliflora, a California chaparral annual as well as many other western North American species. Smoke has been shown to stimulate germination in the 22 California species listed in Table 1. (Keeley 1998)

 

Table 1.  Chaparral species demonstrating highly statistically significant smoke-induced germination. Seeds of all annual species were collected in southern California from first-year burns and others from 2-3 year old burns. Seeds were 6-18 months old at the time of experiments. In all of these species, charred wood also induced germination, but heat had no effect.

Family

Species

Growth Form

Asteraceae

Chaenactis artemisiifolia

Annual

Boraginaceae

Cryptantha clevelandi

Annual

 

C. micrantha

Annual

Brassicaceae

Caulanthus heterophyllus

Annual

Caryophyllaceae

Silene multinervia

Annual

Hydrophyllaceae

Emmenanthe penduliflora

Annual

 

Eucrypta chrysanthemifolia

Annual

 

Phacelia grandiflora

Annual

 

P. minor

Annual

Lamiaceae

Salvia apiana

Shrub

 

S. columbariae

Annual

 

S. leucophylla

Shrub

 

S. mellifera

Shrub

Loasaceae

Mentzelia micrantha

Annual

Onagraceae

Camissonia californica

Annual

Papaveraceae

Romneya coulteri

Suffrutescent*

Polemoniaceae

Allophyllum glutinosum

Annual

Scrophulariaceae

Antirrhinum coulterianum

Annual

 

A. kelloggii

Annual

 

A. nuttallianum

Annual

 

Mimulus clevelandii

Suffrutescent*

 

Penstemon centranthifolius

Suffrutescent*

 

 

 

* Suffrutescent  = herbaceous with woody caudex.

Source: Keeley 1998.

 

Experiments performed by Keeley (1998) found that the length of exposure to smoke was very important in some species - a 3 minute difference in exposure resulted in the death of some seeds.  Some closely fire-linked species didn’t germinate under heat or smoke treatments alone.  In some cases burial for one year or cold stratification are required in addition to smoke exposure.  All of these factors can have an effect on germination and should be considered when using smoke or chemicals in smoke to induce germination.

 

Structurally, there are characteristics shared by smoke-stimulated species found by Keeley (1998). For most species, the outer cuticle was weakly developed and the exterior of the testa highly sculptured, in contrast to the smooth character of Ceanothus and many heat-stimulated seeds. For an in-depth discussion of the difference between seeds that are heat and smoke-stimulated, see Keeley (1998).

 

Blank and Young (1998) showed the following sagebrush-steppe species in the western United States to have increased germination from smoke or smoke compounds: Achnatherum occidentalis (Sierra Nevada needlegrass), Achnatherum hymenoides (Indian ricegrass), and Purshia tridentata (antelope bitterbrush).  Emergence of Achnatherum thurberianum (Thurber’s needlegrass) and Hesperostipa comata (needle-and-thread) increased due to heat. They also found that in some species, including Festuca idahoensis (Idaho fescue), exposure to smoke increases leaf production and/or root mass. (Blank and Young 1998)

 

In addition to those listed in the introduction, other ecosystems for which smoke (or chemicals from smoke) has been successful in inducing germination of native plants include: Mediterranean-type ecosystems, western Australia ecosystems, for which Roche et al. (1997) reported increased germination on 75 new species, and specifically dry sclerophyll spotted gum (Corymbia maculata) forest communities in New South Wales, Australia. (Read et. al 2000).

 

Techniques for smoke pre-treatments of seeds for use in restoration

 

There are two basic methods for exposing seeds to smoke or the chemicals derived from smoke that are thought to promote germination in many seeds.  The first is to expose seeds directly to smoke and the other is to indirectly expose seeds to the particulates of smoke through the use of “smoke water” or smoke distillates in a dry form. Multiple ways to approach both of these seed exposure techniques exist.

 

Direct seed exposure to smoke:

·        Place clean, dry seeds on permeable trays, mesh racks or petri dishes and place them in a poly tent.  Burn native vegetation in a metal drum adjacent to the tent and use a fan or compressed air to blow cooled smoke created by the fire into the poly tent through a long plastic pipe.  Using a long pipe allows the smoke to cool before entering the tent.  The optimum exposure time is variable for different seeds but 1 hour is common. (Tieu et al. 2001)

·        Sow seeds in nursery flats in a soil-less potting mix.  Place the flats in a poly tent and follow the same directions as above.  Exposure time is generally 1-3 hours.  After smoke exposure, spray the flats with a fine mist of water to settle smoke particles on the soil surface (Read et al. 2000).

·        Spread soil seed bank samples on a thin layer of soil-less potting mix and follow the same directions as above.  Exposure time is generally 1-3 hours. After smoke exposure, spray the area with a fine mist of water to settle smoke particles on the soil surface (Read et al. 2000).

·        Force smoke into low poly tents to treat soil seed bank in situ. 

·        Force smoke into a poly tent pitched over vegetation with ripe seeds that have not dispersed.  This is probably most effective for smoke responsive shrub species (Greening Australia.).

When smoking seeds sown in nursery flats or in situ, be careful not to over water in the first weeks or until germination occurs.  Over watering will wash away smoke particulates, reducing effectiveness of the treatment. (Greening Australia)

 

Indirect exposure to chemicals in smoke:

·        Soak seeds in smoke water (directions for making below).  A fish tank aerator can be used to minimize seed rotting.

·        Germinate seeds on smoked filter paper.

 

·        Use smoked filter paper in the presoaking of seeds.  Soak the filter paper in water to allow smoke particulates to diffuse into water, then soak in the water while aerating.

·        Use commercially available dry smoke products in the potting soil or native soil in which smoke responsive seeds will be planted. 

 

Figure 1. Various methods for smoke application.

 

Smoke can be applied directly to seeds or soils by:

Build a poly tent:  Using PVC or metal poles build a tent frame and cover with poly sheeting. 

 

Build a smoke generator:

 

1.      Make holes in the bottom and on the side near the top of a large metal drum and attach piping to the side hole.  Position the piping so that it enters the bottom of the poly tent.  Place green and dry native woody and leafy vegetation into the drum and light it.  Place a lid over the drum and blow air through the bottom hole with a fan or compressed air.  This air will feed the fire and force smoky air out of the side hole, through the pipe and into the tent.

 

2.      Using a beekeepers smoker, burn chipped or shredded native vegetation as described above and blow the smoke into a small tent or other small chamber (such as a chromatography chamber) (Morris 2000).

 

Place seeds on permeable trays, petri dishes or sown seeds in flats inside the tent for 30 minutes to 1 hour (Tieu et al. 1999).

 

Smoke water can be created by:

1.      Using the same drum technique described above, attach the piping to another drum containing water.  Force the smoky air produced in the first drum through the water in the second drum by using a small fan or compressed air.

 

2.      Using a small grill, burn charcoal on half of the base of the grill (as normal) and on the upper grill surface place a pan of water on the other side and native vegetation (woody and leafy) on the side above the charcoal.  Cover the grill.  As the coals burn the native vegetation the smoke that is created will be infused in the water in the pan.  Be careful not to allow the water to boil away.  The water created in these ways can be cooled and used immediately or frozen until needed. 

 

3.      Use commercially available smoke infused products:

Liquid smoke

Smoke infused paper discs

Dry smoke infused material to add to planting medium:  such as Regen 2000

 

In mine site rehabilitation in Australia, it is common to smoke seeds prior to sowing them, or to use smoke water to treat overburden before applying it to strip mine sites (Greening Australia 2003).

 

Testing Seed Responsiveness to Smoke Treatments:

Before investing much effort in creating smoke treatment facilities for restoration projects, it would be wise to test the responsiveness of seeds to be used in the restoration to smoke treatments. 

 

Using sterile petri dishes lined with sterile Whatcom filter papers, add distilled water or smoke water to the dishes.  If using distilled water, use pre-smoke treated seeds to test the effectiveness of the treatment on germination.  If using smoke water, use untreated seeds to test the effectiveness of smoke water treatment on germination.  Use control petri dishes using untreated seeds and distilled water.  Place the dishes in a growth chamber or temperature/light controlled greenhouse and examine regularly for evidence of germination (extension of radicle from the seeds). (Brown 1992) 

 

 

 

Recommended Technique for Incorporating Smoke in Seed Germination

 

We have developed an apparatus which allows for the germination of seeds through the use of:

 

1.      Smoke (cooled)

2.      Smoke water

3.      Heat and smoke

 

Figure 2.  Our apparatus for germinating seeds by smoke, smoke-water or heat.

 

The required materials for creating and operating this apparatus are:

 

  1. poly tent with vents
  2. frame for tent
  3. 50 gallon drums (2)
  4. shovel to dig
  5. shelving (with 5 shelves)
  6. seed flats
  7. screens for seeds
  8. seeding mix
  9. trays for water
  10. voltage regulator
  11. air inlet fan
  12. cooling pipe
  13. tubing
  14. charcoal
  15. fire source (matches)
  16. native vegetation to burn for smoke
  17. electrical wiring

 

Heat germination

 

The apparatus that we have developed is illustrated in Figure 2.  For those species that require heat to break the seed coat, a smoke generating pit is located under-ground, under the smoke tent.  Charcoal will be lit in the pit, and when the flames die down, add native vegetation on the grill grate and then place the seed trays in the tent. Within the smoke tent are shelves which, when heat is incorporated, will hold trays with seeds sown in seeding mix.  Alternatively, if you plan to sow the seeds immediately following the heat/smoke germination, the seeds can be on screens, and not be planted.  For this use of the tent, it is recommended that the bottom two shelves are not used, to avoid over-heating.  A thermometer is located outside the tent that can read the temperature inside, and vents are located throughout the tent to manipulate the heat within, and to control the amount of smoke within the tent.  We recommend keeping the seeds in the tent for up to one hour if heat is incorporated.

 

Smoke and smoke-water germination

 

For those seeds are not benefited by heat, the below-ground pit is not used.  Instead, smoke is created in the above-ground generator and is cooled through a pipe that connects the generator to the smoke tent.  A voltage regulator controls the air inlet fan speed that is connected to the smoke generator.  Within the smoke tent, the shelves can hold the sown seeds on trays, seeds on screens, or can hold pans of water that can be smoke-infused, depending on the germination method of choice.  As with the heat germination, the temperature and the smoke within the tent can be manipulated through the vents.  We do not recommend exposing seeds to the smoke in the tent for more than an hour.  If you are using water, the trays of water should remain in the smoke tent for two hours.

 

Following smoke exposure

 

For each of the treatments, the steps to take following smoke-exposure are listed below:

  1. Heat with smoke – If seeds are in seeding mix, place in greenhouse until germination.

 

  1. Cooled smoke with seeds in seeding mix – place in greenhouse until germination.

 

  1. Cooled smoke with seeds on screens – You can sow seeds on desired seedbed immediately following smoke treatment or store until needed.

 

  1. Smoke-infused water – Remove trays of smoke-infused water and pour water into glass jars.  Add seeds to the jar of water. Put an electric air circulator/filter into the water and circulate water for 24 hours.  After 24 hours, you may either dry and store the seeds until they are needed or sow the seeds directly, being sure to water daily for 6-10 days (see figure 1 for an illustrated description).

 

 

Conclusion

 

With increased urbanization, fire as a restoration tool is not always feasible.  Smoke technology provides the ability to incorporate the chemicals associated with fire in to a restoration when fire is not possible.  Numerous species inhabiting fire-dependent ecosystems have evolved reproductive strategies to adapt to factors associated with fire. The use of smoke has been shown to increase the germination rate among some of these species.  Seeds may be saturated with smoke either in or out of soil, heated in smoke, or soaked in smoke-infused water. Because seeds have different requirements the use of one or more of these techniques may be necessary to stimulate germination.

 

A simple smoke infusion system can be used to induce germination in dormant seeds.  We designed a versatile apparatus to accommodate all the methods of infusion mentioned above.  With the use of this type of technology, native plants with fire-related germination requirements may be more readily used in restoration.

 

 

Literature Cited

 

Blank, R. R. and J. A. Young. 1998. Heated substrate and smoke: influence on seed emergence and plant growth. Journal of Range Management 51: 577-583.

 

Blommaert, K.L.J. 1972.  Buchu seed germination.  Journal of South African Botany 38: 237-239.

 

Brown, N. A. 1993. Promotion of germination of fynbos seeds by plant-derived smoke. New Phytology 123: 575-583.

 

Egerton-Warburton L.M. 1998. A smoke-induced alternation of the sub-testa cuticle in seeds of the post-fire recruiter Emmenanthe penduliflora Benth (Hydrophyllaceae).  Journal of Experimental Botany 49: 1317-1327.

 

Coffey, M. 2003. Greening Australia.  Accessed on 2 June 2003.  www.greening australia.org.au/GA/NAT/TipsAndTools/Library/Smokeseed.htm.

 

Jager, A.K.; A. Strydom; J. Van Staden. 1996. The effect of ethylene, octanoic acid and a plant-derived smoke extract on the germination of light-sensitive lettuce seeds.  Plant Growth Regulation 19: 197-201.

 

Jeffrey, D.J.; P.M. Holmes; A.G. Rebelo. 1988. Effects of dry heat on seed germination in selected indigenous and alien legume species in South Africa.  South African Journal of Botany 54: 28-34.

 

Keeley, J.E. 1998. Smoke-induced seed germination of California chaparral. 

Ecology. October.

 

Morris, E. C. 2000. Germination response of seven east Australian Grevillea species (Proteaceae) to smoke, heat exposure and scarification. Australian Journal of Botany 48: 179-189.

 

Read, T. R., S. M. Bellairs, D. R. Mulligan and D. Lamb. 2000. Smoke and heat effects on soil seed bank germination for the re-establishment of a native forest community in New South Wales. Austral Ecology 254: 48-57.

 

Regen 2000. 2003. Accessed on 2 June 2003. http://www.tecnica.com.au/regen%20home.html

 

Roche, S., K.W. Dixon, and J.S. Pate. 1997. Seed ageing and smoke: partner cues in the amelioration of seed dormancy in selected Australian native species. Australian Journal of Botany 45: 783-815.

 

Roche, S.; J. Koch; K.W. Dixon. 1997.  Smoke-enhanced seed germination for mine ehability in the south-west of Western Australia.  Restoration Ecology 5: 191-203.

 

Tieu, A., K.A. Dixon, K. Sivasithamparam. 1999. Germination of four species of native western Australian plants using plant-derived smoke. Australian Journal of Botany 47: 207-219.

 

Van Staden, J.; N.A.C. Brown; A.K. Jager; and T.A. Johnson.  2000.  Smoke as a germination cue.  Plant Species Biology 15: 167-178.