The Interaction of Heat and Smoke in the Release of Seed Dormancy in Seven Species from Southwestern Western Australia

Exposure to high temperatures (40–60 °C) over severaldays to months (simulating summer soil conditions) or to extremelyhigh temperatures (80–120 °C) for periods rangingfrom minutes to several days (simulating the heat of an extremefire) promoted germination in five (including one legume species)of the seven species tested from southwestern Australia. Heat,a collective term for high temperature (HT) and extremely hightemperature (EHT), has previously been used to promote germinationin species with impermeable seed coats (many of which are legumes).If heat is found to promote germination in species with permeableseed coats, it could be of use in the release of dormancy forcommercial development of native plants. Here we report thatelevated storage temperatures (50 °C for 45 and 90 d or60 °C for 45 d) and EHT (100 °C for 3 h and 120 °Cfor 30 min) alone, and in addition to smoke, directly promotegermination of the refractory species Actinotus leucocephalus,Anigozanthos manglesii, Gompholobium knightianum, Loxocaryastriatus and Stylidium affine. Germination of Sowerbaea laxifloraseed was not improved with heat alone but was improved by thecombination of heat and smoke. Seed of Schoenus unispiculatusdid not germinate in response to any of the treatments. Interactionsbetween the effect of temperature, the duration of heat exposureor smoke were detected in all species. Whilst most species demonstratedan interaction between heat and time, not all species were affectedby smoke alone. This was illustrated by an interaction betweensmoke and HT found only in Actinotus leucocephalus, Anigozanthosmanglesii and Stylidium affine or smoke and EHT in A. manglesiiandS. affine . Smoke and exposure time to heat influenced thegermination of Actinotus leucocephalus,Anigozanthos manglesiiand L. striatus in response to HT and only S. affine appearedto demonstrate an interaction between smoke and EHT. The variationsfound in the interactions between treatments suggest that germinationevents occur over a range of conditions both spatially and temporallywhich may facilitate seedling survival and limit competitionbetween emergents. Copyright 2001 Annals of Botany Company Smoke, heat, fire, seedbank, seed dormancy, seed germination, Western Australia     

The promotive effect of smoke derived from burnt native vegetation on seed germination of Western Australian plants

Exposure of dormant seed to cold smoke derived from burnt native vegetation had a positive influence on germination in one or more seed provenances in 45 out of 94 species of native Western Australian plants that are normally hard to germinate. When tested under controlled conditions some species showed earlier germination in smoke treatments than controls; in others smoke-treated seeds continued to germinate for several weeks after controls had achieved full germination. In the remainder, treated and control seeds germinated to similar time schedules. A group of 23 species which responded positively had previously been recorded as extremely difficult or impossible to germinate using conventional techniques. These included members of the genera Geleznowia (Rutaceae), Hibbertia (Dilleniaceae), Stirlingia (Proteaceae), Verticordia (Myrtaceae), Actinostrobus (Cupressaceae) and Pimelea (Thymelaeaceae). Both large- and small-seeded species were encountered amongst the positively responding taxa, which encompassed representatives of 15 families and 26 genera of dicotyledons, 5 families and 8 genera of monocotyledons and the gymnosperm Actinostrobus acuminatus. Sowing seeds on smoke-fumigated filter papers or watering with aqueous eluates of smoke elicited similar degrees of stimulation of germination, as did exposure to gaseous smoke in a readily germinating species Anigozanthos manglesii (Haemodoraceae) and the normally intractable species Lysinema ciliatum (Epacridaceae). Exposing recently burnt and unburnt natural bushland sites to smoke, smoked water or smoked dry sand elicited a significant germination response in 15 species. Over one third of the species sampled in the burnt site exhibited germination additional to that caused by the fire. Data are discussed in relation to previous germination studies on Australian and other taxa.     

Seed Coat Dormancy in Two Species of Grevillea(Proteaceae)

The role played by the seed coat in seed dormancy of Grevillealinearifolia(Cav.) Druce and G. wilsonii(A. Cunn.) was testedby a series of manipulations in which the seed coat was dissectedand removed, dissected and returned to the decoated seed, ordissected, removed and given a heat shock, and returned to thedecoated seed. Germination of intact seeds of both species wasalso examined after exposure to heat shock, smoke, or heat shockand smoke combined. Water permeability of the seed coat wasinvestigated by examining imbibition. For intact seeds, virtuallyno germination occurred under any treatment (G. wilsonii), orgermination was increased by exposure to either heat or smoke(G. linearifolia). Removal of the seed coat led to germinationof all decoated seeds for G. linearifolia, or a proportion ofdecoated seeds for G. wilsonii. Inclusion of smoked water inthe incubation medium led to a higher proportion of decoatedseeds germinating for G. wilsonii. Returning the seed coat,either with or without heat shock to the seed coat, did notsignificantly affect germination in either species. Seed coatswere permeable to water in both species. For the two Grevilleaspecies, there were different dormancy mechanisms that werecontrolled by the seed coat (G. linearifolia) or by both theseed coat and embryo (G. wilsonii). Copyright 2000 Annals ofBotany Company Grevillea linearifolia, Grevillea wilsonii, dormancy, seed coat dormancy, seed coat permeability, smoke, heat shock, germination     

Response surfaces for the combined effects of heat shock and smoke on germination of 16 species forming soil seed banks in south‐east Australia

Abstract There is limited understanding of how fire‐related cues such as heat shock and smoke can combine to affect the germination response of seeds from fire‐prone vegetation because combinations of multiple levels of both cues have rarely been investigated. Germination response surfaces were determined for the combination of heat shock and smoke by applying factorial combinations of temperature (up to 100°C) and aerosol smoke (0–20 min) to 16 species that form soil seed banks in the Sydney region of south‐eastern Australia. Duplicate populations of three species were also examined to assess the constancy of a species response surface. Of the 19 populations examined, 16 showed a germination response to both the fire cues, which combined interactively in 14 populations, and independently in two. No population responded only to a single cue; however, seeds of 11 populations responded to heat in the absence of smoke, and nine responded to smoke in the absence of heat. Heat applied in the absence of smoke negatively affected germination in seven populations, either progressively as temperature increased, or above a set temperature. Negative germination responses over part of the temperature range were fully reversed at higher temperatures for unsmoked seeds of four populations (curvilinear heat response). Smoke effects were most frequently positive over all or part of the range of durations used, and when combined with heat frequently fully or partially reversed negative heat effects. Three populations required the obligatory combination of smoke and heat. A novel response to the cues was observed for three species, with smoke reversing negative heat effects at 75°C, being supplanted by a positive heat response of unsmoked seed at 100°C. The response surface for duplicate populations of two of the three species examined was variable. Heat shock and smoke frequently combined to affect germination, in both positive and negative ways. Consequently, to gain an accurate assessment of the response of seeds to fires, an experimental design that samples within the potential response zones of germination cues is essential.     

Seed longevity and fire: germination responses of an exotic perennial herb in NW Patagonian grasslands (Argentina)

Fire affects grassland composition by selectively influencing recruitment. Some exotic species can increase their abundance as a consequence of fire-stimulated seed germination, but response may depend on seed age. Rumex acetosella L. (Polygonaceae, sheep's sorrel) is a cosmopolitan herb that has invaded NW Patagonia's grasslands. This species forms persistent soil seed banks and increases after disturbances, particularly fire. We studied how fire and seed longevity influence R. acetosella germination. In 2008, we conducted laboratory experiments where we exposed different-aged seeds (up to 19 years old) to heat, smoke, charcoal, ash and control treatments. Total percentage germination and mean germination time depended on both seed age and fire treatment. Germination of younger seeds decreased with increasing temperature. There was no general pattern in germination responses of different-aged seeds to smoke, charcoal and ash. While smoke improved the germination of fresh seeds, charcoal decreased germination. Germination of untreated seeds was negatively correlated with seed age, and mean germination time increased with seed age. In most treatments, fresh seeds had lower germination than 1-5-year-old seeds, indicating an after-ripening requirement. Smoke stimulates R. acetosella germination, causing successful recruitment during post-fire conditions. Fresh seeds are particularly responsive to fire factors, possibly because they have not experienced physical degradation and are more receptive to environmental stimuli. Knowing the colonisation potential from the soil seed bank of this species during post-fire conditions will allow us to predict their impact on native communities.     

Combined effects of heat shock, smoke and darkness on germination of Epacris stuartii Stapf., an endangered fire-prone Australian shrub

Epacris stuartii (Epacridaceae) is an endangered heathland shrub in which seedling recruitment occurs almost exclusively after fire. Seed viability and multiple seed dormancy mechanisms were examined to explore why levels of seedling recruitment were low after some fires, despite high rates of pre-fire seed production. The individual and combined effects of heat shock, smoke derivatives and darkness on germination were tested in the laboratory using an orthogonal logit-linear model, a form of analysis ideally suited to hypotheses concerning multiple germination cues. Seed viability (56%) was found not to be limiting. After 89 days of incubation, germination was significantly enhanced by heat shock, smoke treatment and continuous darkness but there were no significant interactions. These effects were equal in magnitude and additive such that maximum germination (42% of viable seeds) was stimulated when all three treatments were combined, significantly less germination occurred in response to any two treatments combined (22–23%), any single treatment stimulated germination at levels (10–11%) significantly less than two treatments and the lowest levels of germination occurred among untreated seeds (4%). Relative to the untreated control, germination was stimulated by smoke derivatives in high concentrations but not in low concentrations. The effect of darkness diminished with time so that after 270 days of incubation darkness had no significant effect on germination, while heat and smoke still enhanced germination significantly, additively and interchangeably. More prolonged germination of seeds exposed to light on the soil surface than buried seed may spread the risk of desiccation of emerging seedlings over multiple rainfall events. The role of heat and smoke derivates suggests that fire management is a crucial tool for influencing seedling recruitment and hence the survival of E. stuartii at its only known location. Received: 3 November 1996 / Accepted: 15 June 1997     

Interactive effects of heat shock and smoke on germination of nine species forming soil seed banks within the Sydney region

Abstract The germination response of seeds from fire‐prone vegetation to fire‐related cues such as heat shock and smoke has usually been studied by applying the cues singly. The few studies that have applied the cues in combination have shown that interactions between the cues are possible. Here, the response of seeds from a number of species to combined heat shock and smoke is reported. Heat shock (25, 50, 75 and 100°C) and aerosol smoke (0, 5, 10 and 20 min) were applied factorially to nine species that form soil seed banks in the Sydney region of south‐eastern Australia. These species were from Epacridaceae (four species), Myrtaceae (four species) and Cyperaceae (one species) and ranged from fire‐sensitive obligate seeders to fire‐tolerant facultative resprouters. Germination of Dracophyllum secundum R. Br and Sprengelia monticola (A. Cunn. ex DC.) Druce was low and did not respond to the germination cues. The positive response of Gahnia sieberiana Kunth and Kunzea ambigua (Sm.) Druce to heat shock and smoke was independent and additive. The positive response of Kunzea capitata Rchb. to the interaction between heat shock and smoke was synergistic, and the response of Baeckea diosmifolia Rudge and Baeckea imbricata (Gaertn.) Druce was unitive, with germination increase only occurring following combined heat and smoke application. Epacris coriacea A. Cunn. ex DC. and Epacris obtusifolia Sm. had low levels of dormancy and hence it was not possible to find a fire response. Gahnia sieberiana and K. capitata responded differently to the combination of heat shock and smoke than has previously been reported. Germination of species from habitats that are infrequently burnt was not affected by heat shock or smoke. Low‐intensity fire or patches within fire may be important for seedling recruitment as the 50°C heat shock stimulated germination in four of the five species that responded to the heat cue, and germination of Baeckea imbricata declined within the 100°C heat shock treatment. Germination of one species, Baeckea imbricata, was only stimulated by a specific combination of cues, indicating that regeneration niches may be narrow for some species and that the application of a range of heat and smoke doses is required to find such responses. Of the species positively responding to heat shock and smoke, a requirement for both cues was prevalent, therefore the response to these cues in isolation cannot be relied upon to give a true indication of the fire response of a species.     

Fire as a stimulant of shrub recruitment in northwestern Patagonian (Argentina) grasslands

One strategy of plant survival during post-fire succession is to persist and regenerate by recruiting new individuals from a fire-resistant seed bank. The heat, smoke, and charcoal released during plant combustion may act (individually or in combination) as a cue for post-fire seed germination. Fabiana imbricata is a shrub that forms persistent seed banks in the northwestern Patagonian grasslands and shows a high recruitment from seeds during post-fire succession. Mathematical models showed that this species is advancing over the grasslands in response to fires. To corroborate these findings, we studied the role of fire on F. imbricata seed germination. In order to achieve this, a factorial experiment was designed in laboratory conditions to study the effect of heat, charcoal, smoke, scarification, and their interactions on F. imbricata seed germination. Seeds treated with the higher temperatures required a longer period of time to germinate, thus, significantly affecting the mean germination time. Total germination percentages in F. imbricata were significantly enhanced by smoke and scarification, but the interaction of heat, smoke, and scarification was more important than the effect of each fire factor alone. The positive response to fire cues exhibited by F. imbricata indicates that this species would have an adaptive advantage to colonize these grasslands if fire frequency increased, as predicted for this environment. Hence, fire will contribute to the grassland encroachment by this species and, therefore, to the loss of biodiversity and productivity of northwestern Patagonian grasslands.     

Does Fire Trigger Seed Germination in the Neotropical Savannas? Experimental Tests with Six Cerrado Species

The Cerrado (Brazilian savanna) is a biodiversity hotspot with a history of fire that goes back as far as 10 million years. Fire has influenced the evolution of several aspects of the vegetation, including reproduction and life cycles. This study tested how fire by‐products such as heat and smoke affect the germination of six species common to two Cerrado open physiognomies: wet grasslands and the campo sujo (grassland with scattered shrubs and dwarf trees). We subjected seeds collected in northern Brazil to heat shock and smoke treatments, both separately and combined, using different temperatures, exposure times, and smoke concentrations in aqueous solutions. High temperatures and smoke did not break seed dormancy nor stimulate germination of the Cerrado study species. However, seeds were not killed by high temperatures, indicating that they are fire‐tolerant. Our findings differed from those of other fire‐prone ecosystems (mostly of Mediterranean vegetation), where fire stimulates germination. Moreover, we provide important information regarding germination strategies of non‐woody Cerrado plants, showing the importance of considering the tolerance of seeds to high temperatures when evaluating fire‐related traits in fire‐prone ecosystems.     

Impacts of fire cues on germination of Brassica napus L. seeds with high and low secondary dormancy

• Agricultural burning is used in farm management operations; however, information about the impact of fire cues on the release and/or induction of secondary dormancy in crop seeds is scarce.
• Seeds from two oilseed rape cultivars were induced for high (HD) or low (LD) secondary dormancy using polyethyleneglycol (PEG) pre‐treatment, and their germination after exposure to various fire cues was compared to control PEG pre‐treated and non‐dormant seeds.
• Non‐dormant seed germination was unaffected by various fire cues. Low doses of aerosol smoke released secondary dormancy in HD seeds, while higher doses increased dormancy of LD seeds. Dilute smoke water also released HD seed secondary dormancy, but concentrated smke water enhanced dormancy in both LD and HD seeds. The concentrated aqueous extracts from charred oilseed rape straw only promoted germination of HD seeds, while dilution inhibited LD seed germination. Heat shock (80 °C, 5 min) released secondary dormancy in HD seeds; however, higher temperatures and/or increased exposure time was associated with seed death. GC‐MS analyses of smoke water revealed two butenolides and an array of monoaromatic hydroxybenzene compounds with potential germination inhibitor or promoter activity.
• The extent of secondary dormancy induction in seeds affects their subsequent responses to fire cues. Both aerosol smoke and smoke water have both germination promoter and inhibitor activity. Lacking any butenolides, aqueous extracts of charred straw contain a potential germination stimulating steroid, i.e. ergosterol. The significance of fire‐derived cues on behaviour of oilseed rape seeds in the soil seed bank is discussed.

     

Heat increases germination of water-permeable seeds of obligate-seeding <Emphasis Type="Italic">Darwinia</Emphasis> species (Myrtaceae)

We examined the response of seeds to heat in four geographically restricted and one widespread species of shrubby Darwinia from the fire-prone region of southeastern Australia. These shrubs are killed by fire and rely on seed germination after a fire to maintain populations. We replicated the germination trials across several sites and several fruiting seasons for most species. Seeds had a high level of viability and were largely dispersed in a dormant state, except in D. glaucophylla, where seed dormancy varied significantly across fruiting seasons. The indehiscent fruit of all species readily imbibes moisture when wet and seeds are not considered to be ‘hard-seeded’. All species had increased seed germination in response to a limited range of heating temperatures (generally 80–100°C). Higher temperatures killed increasing proportions of seeds. This pattern was broadly consistent across species, population and seasons, although the proportion of seeds whose germination was promoted by heat varied from high (D. diminuta, D. fascicularis, D. glaucophylla) to moderate (D. biflora, D. procera). Our work highlights the importance of heat as a mechanism for influencing germination in species that are not hard-seeded. Consequently, soil temperatures during a fire should strongly influence post-fire germination levels in Darwinia. The roles of other cues that promote germination, i.e. smoke, seasonal temperatures and their interactions with heat, remain to be investigated.     

Seed moisture content affects afterripening and smoke responsiveness in three sympatric Australian native species from fire‐prone environments

Germination of freshly collected seeds of three sympatric herbaceous species native to fire‐prone environments in south‐western Australia was significantly improved through the application of novel combinations of dry heat, gibberellic acid, smoke water and dry afterripening. For fresh seeds, combinations of dry heat, gibberellic acid and/or smoke water resulted in >80% germination in Austrostipa elegantissima (Poaceae) and Stylidium affine (Stylidaceae) seeds and >60% germination in Conostylis candicans (Haemodoraceae) seeds, compared with <10% germination of control seeds. For fresh seeds, two broad germination patterns were observed in response to smoke water: nil – low germination for both control and smoke water‐treated seeds (A. elegantissima and S. affine); and a significant smoke response (35%) compared with control seeds (1%) (C. candicans). During afterripening, high germination for A. elegantissima seeds was achieved following 3 months storage of seeds at equilibrium relative humidities of 23–75%, but seeds stored at 5–13% equilibrium relative humidities took 6–36 months to achieve similar levels of germination. Germination of C. candicans seeds also increased after 3 months storage, to >60% at each equilibrium relative humidity and further increases over time were slight. For S. affine seeds >60% germination was achieved only after 36 months storage at 50% equilibrium relative humidity. Seeds from all three species were smoke‐responsive at some point, but the interaction/effects of afterripening on the smoke response varied significantly between species. This study highlights an apparent effect of seed dormancy status on response to smoke and a surprisingly high level of ecological variation in pre‐germination requirements (cues) for these co‐occurring species that may relate to variation(s) in microsite selection forces operating on the soil seed bank of the different species.     

Fire as a threatening factor for endemic plants of the Canary Islands

The endemic flora of the Canary Islands is remarkable but highly vulnerable to environmental changes. Several factors, such as fire, could be threatening. The purpose of this study was to analyze various treatments in endemic threatened species which could have been growing in the understory of the Canary pine forest. We attempted to improve germination through dormancy breaking, and analyze the influence of fire on these plants. Seven physical and chemical treatments were tested: control, mechanical scarification, boiling water, dry heat (50 and 120?°C), smoke and smoked water. Mechanical scarification showed a positive response in germination percentage in two species characterized by hard seed coat, but out of all of the species tested, only one demonstrated a positive response to any of the treatments related to fire. At the temperatures registered on wildfires, neither the seeds situated on the aerial structure nor seeds in seed bank seem to be able to survive and germinate. The increasing incidence of fires through anthropogenic causes could contribute to the current threat status of these species and poorness of the Canary pine forest understory.     

Comparative seed ecology of the endangered shrub, Pimelea spicata and a threatening weed, Bridal Creeper: Smoke, heat and other fire-related germination cues

Summary Pimelea spicata R. Br. is a nationally listed endangered Australian shrub threatened with extinction by habitat fragmentation and environmental weed invasion. Bridal Creeper (Asparagus asparagoides L. W. Wight) is the primary weed threat to the largest remaining populations of P. spicata in the Cumberland Plain. Fire, as part of an integrated pest management program, offers the potential to stimulate P. spicata populations while controlling Bridal Creeper. It is important, therefore, to understand how the components of fire affect the germination and growth of both species. Using laboratory experiments we investigated the effects of smoke, heat, ash and/or light on the germination of P. spicata and Bridal Creeper. We found a significant promotive effect of smoke and indication of an inhibitory heat shock (90°C for 10 min) effect on the germination of P. spicata seeds. The response of Bridal Creeper seeds to the same factors was complex; while the results of one experiment suggested an inhibitory effect of smoke and a promotive effect of heat, subsequent trials were contradictory, implying that Bridal Creeper, like many weeds, is able to germinate under a wide range of environmental conditions. Other experiments investigated the optimal germination temperature and innate dormancy of P. spicata in the absence of fire‐related germination cues. Of the incubation temperatures investigated, the optimal diurnally fluctuating regime for P. spicata germinations was 10°C and 20°C in the night and day, respectively. The innate dormancy of freshly produced seeds disappeared after 3 months. In contrast to Bridal Creeper, we found a persistent germinable seed bank of about 97 P. spicata seeds/m² located in the top 5 cm of the soil profile. While fire alone is unlikely to kill Bridal Creeper plants, fire may help to manage local infestations of the weed by limiting germination and providing opportunity for herbicide treatment of regrowth.     

Heat and smoke pre‐treatment of seeds to improve restoration of an endangered Mediterranean climate vegetation type

Invasive alien plants impact ecosystems, which often necessitates their removal. Where indigenous species recovery fails following removal alone, an active intervention involving reintroduction of seed of native species may be needed. This study investigated the potential for a combination of the fire cues of smoke and heat as a pre‐treatment of seeds in breaking dormancy and facilitating increased germination. Species were selected to represent different functional types within Cape Flats Sand Fynbos; a fire‐prone, critically endangered vegetation type in South Africa. Seeds were exposed to either a heat pulse (temperatures between 60 and 300°C for durations of between 30 s and 20 min) or dry after‐ripening (1 or 2 months at milder temperatures of 45°C or less). Thereafter, seeds were soaked in smoke solution for 18 h and subsequently placed on agar at 10/20°C for germination. Most species fell into one of two main groups: Seed germination in the first group was greatest following a lower temperature (60°C) heat pulse, an extended period of mild temperature (20/40°C or 45°C) exposure, or no pre‐treatment with heat. Seed germination in the second group was promoted after brief exposure to higher (100°C) temperatures. No germination occurred in any species following heat treatments of 150°C or higher. Species which responded better to higher temperatures were mainly those possessing physical dormancy, but seed morphology did not correlate with germination success. This study showed that heat stimulation of seeds is more widespread in fynbos plant families than previously known and will enable the development of better seed pre‐treatment protocols before large‐scale sowing as an active restoration treatment after alien plant clearing.     

Fire‐related cues break seed dormancy of six legumes of tropical eucalypt savannas in north‐eastern Australia

Abstract This paper describes an assessment of the effect of exposure to fire‐related cues (heat shock, smoke and nitrate) and the interactions between the cues on seed dormancy release of tropical savanna legumes in north‐eastern Australia. Ten legume species were tested, comprising both native and exotic species. The ten species responded variously to the treatments. Brief exposure to temperatures between 80 and 100°C was found to break the seed dormancy of the native ephemeral herbs Chamaecrista mimosoides, Crotalaria calycina, Crotalaria montana, Indigofera hirsuta and Tephrosia juncea, as well as the exotic ephemeral herb Crotalaria lanceolata. Exposure to 80°C combined with treatment with a nitrate solution produced an additive effect on the germination of Chamaecrista mimosoides and Crotalaria lanceolata. However, the four species with the heaviest seeds, two exotic ephemeral herbs (Chamaecrista absus and Crotalaria pallida) and two native perennials (Galactia tenuiflora and Glycine tomentella) displayed no significant increase in germination with exposure to fire‐related cues. Exposure to 120°C for 5 min produced seed mortality in all species tested. Two of the largest seeded species, Crotalaria pallida and Galactia tenuiflora, displayed the lowest tolerance to heat shock, with seed mortality after exposure to 100°C for 5 min. These data indicate that fire can promote the germination of some tropical savanna legumes. As a proportion of seeds of each species displayed no innate dormancy, some germination may occur in the absence of fire, especially of exotic species.     

Disentangling the role of heat and smoke as germination cues in Mediterranean Basin flora

Background and Aims

The role of fire as a germination cue for Mediterranean Basin (MB) plants is still unclear. The current idea is that heat stimulates germination mainly in Cistaceae and Fabaceae and that smoke has a limited role as a post-fire germination cue, in comparison with other Mediterranean-type ecosystems (MTEs), suggesting that fire-stimulated germination is less relevant in the MB than in other MTEs. However, recent studies showed that the assembly of Mediterranean plant communities is strongly driven by post-fire germination, suggesting an important role for fire as a germination cue. We hypothesize that both heat and smoke have important effects on the different post-fire recruitment processes of MB species (e.g. level and rate of germination and initial seedling growth).

Methods

To ascertain the role of heat and smoke in the post-fire germination response of MB woody plants, a germination experiment was performed with seven heat and two smoke treatments on 30 MB woody species from seven different families, including species with water-permeable seeds and species with water-impermeable seeds.

Key Results

Heat stimulated the germination (probability and rate) of 21 species and smoke in eight species, out of the 30 species studied. In addition, six species showed enhanced initial seedling growth after the smoke treatments.

Conclusions

The results suggest that both heat and smoke are important germination cues in a wide range of MB woody species and that fire-cued germination in woody plants of the MB may be as important as in other MTEs.     

Seed size and seed germination in the Mediterranean fire-prone shrub <Emphasis Type="Italic">Cistus </Emphasis><Emphasis Type="Italic">ladanifer</Emphasis>

The germination response of different sized seeds from individuals of a Mediterranean fire-prone shrub (Cistus ladanifer) was investigated in relation to pre-germination heating. A control (no heating), a low temperature during a short exposure time (50°C during 5 min), a high temperature during a short exposure time (100°C during 5 min) and a high temperature during a long exposure time (100°C during 15 min) were applied to seeds from different individual plants with different mean seed weight. These pre-germination treatments resemble natural germination scenarios for the studied species, absence of fire, low intensity pasture fire, typical Mediterranean shrub fire, and severe fire with high fuel load. Mean seed weight only showed a marginally significant positive correlation with the proportion of germinated seeds whatever the pre-germination treatment. These results suggest that seed dormancy is unrelated to seed size and that under the experimental conditions used in this study, the effect of seed size on seed germination is low. Nevertheless, larger seeds could be favoured in natural conditions, especially under the high competition scenario which arise after wildfires. Control seeds showed a negative correlation between seed size and germination velocity suggesting that lighter seeds could take advantage from early germination in recruitment events in the absence of wildfires. Nevertheless, even the lower pre-germination heating treatment turns this correlation in not significant, suggesting a strong selection pressure (unrelated to seed size) for early germination after fire events. In our study, different sized seeds of C. ladanifer seem to perform better under different germination scenarios suggesting that seed size variation could be maintained by the alternation of recruitments without wildfires and recruitments after wildfire events.     

Germination response of common annual and perennial forbs to heat shock and smoke treatments in the Chaco Serrano,central Argentina

Fire is a key ecological factor affecting plant dynamics. In the last few decades, fire occurrence in the Chaco region has increased noticeably, challenging the adaptive capacity of plants to regenerate after a fire. Broad‐leaved forb species have been much less studied than woody and graminoids, although they are an important component of fire dynamics. Here we analysed the germination response to heat shock of 70 and 110°C, smoke and their combination in 10 broad‐leaved herbaceous species frequently occurring in the Chaco Serrano of Córdoba province, central Argentina, including five annual (Bidens subalternans, Conyza bonariensis, Schkuhria pinnata, Tagetes minuta and Zinnia peruviana) and five perennial species (Borreria eryngioides, Sida rhombifolia, Solidago chilensis, Taraxacum officinale and Verbena litoralis). We also compared the response of annual versus perennial species. Six species had highest germination when treated with heat and smoke combined, whereas two had lowest germination under this treatment, indicating synergistic and antagonistic interaction of these factors respectively. Most of the species tolerated heat shock (i.e. germination was similar to that in control treatment), whereas others had higher germination in response to heat shock, especially under the moderate 70°C treatment. Germination was higher than control (i.e. no heat and no smoke) after smoke treatment in four species. Perennial species showed higher average germination than annuals in both heat treatments and in the control. Annual species had higher average germination for all treatments involving smoke. The high variability observed at the species level, and the limited number of species studied calls for precaution in interpreting and extrapolating results. Nevertheless, our study shows a general positive response of both perennial and annual species to fire cues, suggesting an advantage of these species for colonizing post‐fire environments, and being favoured under scenarios of increasingly frequent low‐to‐medium intensity fires.     

SUCCESSIVE PROCESSES INVOLVED IN THE GERMINATION RESPONSE OF NASTURTIUM SEEDS

1. The seeds ofNasturtium palustreDC. do not germinate, eitherin the light or darkness, at various constant temperatures,but require for their full germination a certain period of alow temperature (5°) applied immediately after light irradiation.These results indicate the existance of at least two processes,a light-dependent process and a low temperature-requiring process,in the initiation of germination ofNasturtiumseeds. Experimentalevidence indicated further that the light exposure causes twodifferent processes in the seed germination. 2. When a dark period at 23° was inserted between the lightirradiation and the low temperature treatment the germinationwas suppressed. The inhibitory effect of the inserted dark periodat 23° was eliminated by a short irradiation during thedarkness (light-break). 3. Prolonged exposure ofNasturtium seeds to any concentrationof gibberellin brought about no germination when exposure wasgiven in complete darkness. The germination was promoted onlywhen light irradiation was applied to the seeds. A short applicationof gibberellin at a fairly high concentration was, however,remarkably effective for the germination even in the darkness,and the germination was inhibited as the gibberellin applicationwas lengthened. It was considered that gibberellin could substitutefor the combined effect of light irradiation and low temperaturetreatment to induce the germination of Nasturtium seeds, andthat gibberellin was inhibitive toward the reactions followingthe above treatments which induced the germination (Received October 31, 1996; )