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CONFERENCE PROCEEDINGSAustralian Bushfire Conference, Albury, July 1999 |
copyright 1999 |
Belinda J. Kenny
Department of Environmental Science, University of Technology, SydneyGrevillea species have a soil-stored seedbank which responds to the passage of fire. Recruitment is minimal in mature vegetation communities, while post-fire emergence exhibits a definite pulse pattern whereby the proportion of seedlings emerging is greatest within a few months of fire and then tapers off. This behaviour has been shown for G. caleyi, G. longifolia [Auld 1994; Auld 1995], G. buxifolia, G. speciosa [Auld and Tozer 1995], and G. barklyana [Vaughton 1998]. The level of dormancy is variable (most species appear polymorphic, that is there is a reasonable proportion of seeds that are not dormant) and the method by which fire breaks dormancy little studied.
The role of fire in seed germination was first observed as the action of heat in breaking hard seed coats. The heat required to fracture a seed coat varies with species, but is generally in the range 60-120oC, with an optimum between 80-100oC [Keeley et al. 1981; Auld and O’Connell 1991]. This range of temperatures can be expected within the soil underneath the passage of a fire [Bradstock and Auld 1995].
Heat has shown mixed success as a germination cue for Grevillea species. Auld and Tozer [1995] got no heat response from six species tested, while Morris [1999] had four of seven species react positively to heat. Edwards and Whelan [1995] found heat treatment to only be successful when it resulted in obvious cracking of the seed coat, ie as a scarification agent.
Heat, however, is not the only possible germination cue provided by a fire, there are also the combustion products such as the charcoal left behind and the smoke released. The first report of a combustion product aiding in a seed’s germination was that of Wicklow [1977] who performed both laboratory and field experiments where charred wood was found to be the agent in the fire related germination of the chaparral shrub Emmenanthe penduliflora. Charcoal has since been found to be a successful germination agent in a wide range of chaparral species [Keeley 1987].
Research into smoke responses is a relatively new field, with the majority of work so far being conducted in South Africa [eg Brown 1993a] and Western Australia [eg Dixon et al. 1995; Roche et al. 1997], where a large number of species from a wide range of families have been successfully germinated using smoke treatment. Many species in which germination had previously been unreliable or impossible to induce are now known to respond to a smoke cue.
For Grevillea species smoke treatment shows greater promise than heat, with eight species showing a positive response: G. wilsonii [Dixon et al. 1995], G. buxifolia, G. diffusa, G. juniperina, G. linearifolia, G. mucronata, G. sericea and G. speciosa [Morris 1999]. Morris [1999] also found that smoke and heat treatments combined had an additive effect on five species.
This paper investigates the influence of three possible fire cues, heat, smoke and charcoal, on the germination rate and magnitude of two Sydney Grevillea species.
Seeds were collected from Ku-ring-gai Chase National Park in northern Sydney, NSW. Two seed lots of Grevillea sericea (Sm.) R. Br. were collected, one in the winter months of 1997 (July-August), the other in the summer months from November 1997 to January 1998. Grevillea speciosa (Knight) McGillivary was collected only in the summer months of December 1997 to January 1998. Seeds were stored at room temperature until experiments were performed in August 1998.
Heat, smoke and charcoal treatments were applied in all orthogonal combinations, giving a total of 8 treatments. For both G. sericea seed lots 100 seeds were used per treatment, in four replicate batches of 25. Less seed was available of G. speciosa, hence 80 seeds (in four batches of 20) were used per treatment. After treatment, seeds were placed in 9cm petri dishes on 1 layer of Whatman No. 1 filter paper kept moist with distilled water and stored in the laboratory under ambient conditions. Fungal growth on the seed coat of Grevillea species is common, so an anti-fungal solution (0.5g/l Benlate) was applied within the first week after treatment. Dishes were checked 3 times per week for germination of the seeds. Once germination was scored (after emergence of the radicle) seeds were removed. The trial ran for 8 weeks, at the end of which seeds remaining ungerminated were assessed for health using a cut test. Viability as expressed in the results refers to seeds with a plump, white, firm embryo.
Heat treatment was applied to loose seeds in an oven pre-heated to 80oC for 10 minutes. This temperature was chosen as (a) simulating conditions in the seedbank (upper 5cm of soil profile) under the passage of fire; (b) being within the optimum temperature range for heat-shock effects on many species; (c) in a preliminary heat trial temperatures of 100oC and above were shown to be lethal for G. sericea.
Smoke was produced by burning dried litter material from a eucalypt woodland in a bee keeper’s burner. The smoke was channelled through a length of hose sufficient to cool the smoke into a chamber containing the seeds. Seeds were thus fumigated for 15 minutes.
Charcoal was produced by heating wood from Eucalyptus haemastoma in a muffle furnace until fully charred but not ashed. The charred wood was ground, then 0.05g of this material scattered over the seeds once in the petri dish. While most published experiments with charcoal have used a greater amount of charcoal than this, lower concentrations, equivalent to that used here, have been shown to be just as effective [Keeley and Pizzorno 1986; Brown 1993b].
When treatments were combined they were done in the order heat, smoke, charcoal.
Data was analysed using analysis of variance (ANOVA). When the data was not appropriate (variances not homogenous) log transformation was performed prior to ANOVA or an equivalent non-parametric test (Kruskal-Wallis) was used.
Seed lot viability is shown in Table 1. Germination results for the winter seed lot of G. sericea will not be displayed. This seed lot showed the same pattern of germination with treatment as did the summer seed lot of G. sericea, but with lower viability, germination rate and percent germination.
Table 1. Viability of seed lots.
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Species |
Mean viability (%) |
Standard deviation |
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G. sericea (winter seed lot) |
71.9 |
13.68 |
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G. sericea (summer seed lot) |
99.4 |
7.58 |
|
G. speciosa |
98.5 |
2.60 |
Smoke was the only treatment that significantly influenced the commencement of germination (measured as the time taken for the first seed within a replicate batch to germinate). Smoke reduced (p=0.000) the time taken to germinate for both species (Table 2; data pooled into smoke and non-smoke treated).
Table 2. Commencement of germination.
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Species |
Smoke treatment |
Mean time to first germination (days) |
Standard deviation |
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G. sericea |
No |
27.1 |
8.57 |
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Yes |
15.8 |
1.22 |
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G. speciosa |
No |
22.6 |
2.16 |
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Yes |
15.6 |
1.21 |
The pattern of germination over time for the different treatments is shown in Figure 1. The separation between smoke and non-smoke treatments is clearly seen in both commencement of germination and subsequent levels of germination, as well as the rate of germination (comparison of slopes). Both species show three different patterns in germination rate, in the treatment groups smoked, not smoked but heated, neither smoked nor heated. These patterns are comparable between the two species.
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| Figure 1. Cumulative germination (mean percent, n=4) over the trial period (8 weeks). (a) G. sericea, (b) G. speciosa. |
Comparison of germination level between the treatments was made after 5 weeks (Figure 2). This time was chosen as the point after which there was no further significant increase in the treatment giving maximum germination (all three treatments combined).
Three factor analysis of variance showed both smoke and heat to increase germination significantly (p=0.000). A progressive increase can be seen (Fig 2a) in the treatment groups: neither heated nor smoked, heated, smoked, smoked and heated. The influence of both heat and smoke treatments is consistently additive; the difference between control and heat (22.4%) equals the difference between smoke and heat + smoke (24.9%), the difference between control and smoke (45.4%) equals that between heat and heat + smoke (47.9%). The combination of the two is also additive, such that the total of heat only and smoke only added (84.2%) equals the combination treatment (89.8%). Charcoal treatment gave no significant influence.
Three factor ANOVA showed a significant effect of smoke (p=0.000) and heat (p=0.001) as well as an interaction between these two factors (p=0.013). While germination was increased above control by both heat (18.7% increase) and smoke (51.2%), there was little increase between smoke and heat + smoke (6.8%) (Fig 2b). There was no statistically significant influence of charcoal.
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| Figure 2. Treatment effect on germination level (mean percent ± SE, n=4) after five weeks. (a) G. sericea, (b) G. speciosa. Treatments: 1=control, 2=charcoal, 3=heat, 4=heat+charcoal, 5=smoke, 6=smoke+charcoal, 7=smoke+heat, 8=smoke+heat+charcoal |
Grevillea species exhibit a post-fire pulse of seedling emergence [Auld and Tozer 1995] indicating that the soil-stored seeds have a dormancy mechanism which is broken by some element of the fire’s passage. This study has shown that both the heat and smoke produced are important germination cues for these species.
For both species smoke was shown to be the more influential cue. Applied individually the increase in germination above control level was equivalent for both species (heat 22.4% and 18.7%; smoke 45.4% and 51.2%). The main difference between the species was the high level of non-dormancy (control germination of 34.5%) in G. speciosa, so that a neat addition when heat and smoke were combined was not seen here (given that smoke alone led to near maximum germination).
That charcoal had no significant effect is odd given that many species respond to a similar degree when charcoal and smoke treatments are compared [Brown 1993b; Keeley and Bond 1997]. Perhaps the method of charcoal application used here was not appropriate (although the methods of Keeley and Pizzorno [1986] were approximately followed; application in solution [Keeley and Nitzberg 1984] was not attempted, nor was concentration investigated).
While both heat and smoke are known individually to improve the germination of many species, few species have been reported to be triggered by both. An additive relationship between heat and smoke on germination has been shown for Epacris stuartii [Keith 1997] and for four Grevillea species [Morris 1999], and between heat and charcoal for Eriodictyon crassifolium [Keeley 1987]. That the two treatments in combination appear to be purely additive implies that an individual seed does not require both, it will be triggered by one of them. However, as a population both cues are required for maximum seed bank response.
That either one of these cues alone will act on a fair proportion of the seeds gives the species a safety mechanism, such that if the temperature penetrating the soil profile is inadequate smoke will still allow significant germination, and vice versa. This could be of great importance in low intensity fires (eg prescribed fuel reduction burn) which are sufficient to kill fire sensitive adult plants but often provide insufficient soil heating to most of the soil seed bank [Auld and O’Connell 1991; Bradstock and Auld 1995]. This scenario could lead to local extinction depending on the longevity of the seed stored in the soil in relation to the time to the next fire. However, with smoke alone providing a germination trigger for a significant proportion of the seed bank, such extinction potential is averted.
The distinct pattern shown in the timing of germination under different treatments allows the emergence of seedlings to be spread over several weeks allowing for a better survival rate given the chance occurrence of rainfall. If for instance non-smoke affected seeds take longer to imbibe water they will require a greater or more prolonged rain event to allow their germination.
While the active ingredient in smoke [van Staden et al. 1995a] and charcoal [Keeley and Pizzorno 1986] remains elusive and the method of action is not positively identified, there are a few hypotheses. It is assumed that the active ingredient is acting on an embryo dormancy mechanism by initiating metabolic activity [Baldwin et al. 1994], increasing hormone sensitivity [van Staden et al. 1995b], enhancing hormone activity [Thomas and van Staden 1995], or inactivation of a germination inhibitor [Keeley and Nitzberg 1984]. Since such a wide range of plant species have reacted to smoke or charcoal treatment, however, it must be assumed that the method of dormancy breaking varies somewhat.
Recent work [Keeley and Fotheringham] has shown the seed coat may be involved. Amongst a group of 25 smoke stimulated chaparral species, most germinated well following mechanical scarification, and some after chemical scarification. These seeds, when untreated, will readily imbibe water but remain dormant. When water uptake was examined with dyes, while the testa readily absorbed water it was stopped by a subdermal barrier before reaching the endosperm. Smoke treatment, however, allowed penetration through to the embryo. Egerton-Warburton [1998] has examined cuticle changes to one of these species, Emmenanthe penduliflora. After smoke treatment, changes on the seed’s water-permeable external surface indicated that intense chemical scarification had occurred. Smoke also increased the density and width of permeate channels within the semi-permeable internal (sub-testa) cuticle. This evidence may indicate the presence of inhibiting agents within the endosperm which are too large too diffuse through the water-permeable cuticle. The effects of smoke treatment, as well as scarification, may allow the outward passage of these.
Grevillea seeds have similar characteristics to these species. Scarification is a successful germination treatment [Edwards and Whelan 1995; Morris 1999], and dormant seeds readily imbibe water [Morris 1999]. The effectiveness of the heat treatment here could be explained as a scarifying influence. Edwards and Whelan [1995] observed that heat treatment of G. barklyana was only successful when it resulted in obvious cracking of the seed coat. Failure to respond to heat [Auld and Tozer 1995; Morris 1999] may be a cause of either insufficient heat to give scarification, or too much heat damaging the embryo. If smoke is acting on the Grevillea seeds in a scarifying manner it is more effective for this purpose than is heat (Fig 2). An additional smoke influence on the sub-testa cuticle as seen in E. penduliflora [Egerton-Warburton 1998] would explain this increased effectiveness, as well as the much quicker germination of smoke treated seeds (Fig 1; Table 2). The question of whether smoke scarifies Grevillea seeds may be followed up in further experiments.
These results have importance form both ecological and horticultural perspectives. Temperate region Grevillea species (including those studied here) are generally propagated by cuttings [Olde and Mariott 1994], a method which obviously results in no genetic diversity. Knowing that the use of both smoke and heat treatment can give close to 100% germination, propagation can be more confidently attempted by seed. An easier method of smoke application in horticultural conditions might be the use of smoked water, which should give roughly equivalent results [Brown 1993a].
From the perspective of bushland conservation, knowledge of which cues are of importance in the germination of various species can aid in management decisions. For example, in choosing alternatives to prescribed burning for areas containing rare plants. Smoke treatment has proven to be a successful tool in mine site rehabilitation [Roche et al. 1997] and could be of use in remnant urban bushland where burning is often impractical.
Whether these cues have the same influence on germination of these species under field conditions is to be examined in further experiments, hopefully in comparison with an experimental fire.
Published by School of Environmental & Information Sciences Charles Sturt University