Dormancy and the Fire-Centric Focus: Germination of Three Leucopogon Species (Ericaceae) from South-Eastern Australia
Total Page:16
File Type:pdf, Size:1020Kb
Annals of Botany 98: 421–430, 2006 doi:10.1093/aob/mcl118, available online at www.aob.oxfordjournals.org Dormancy and the Fire-centric Focus: Germination of Three Leucopogon Species (Ericaceae) from South-eastern Australia MARK K. J. OOI1,2,*, TONY D. AULD2 and ROBERT J. WHELAN1 1Institute for Conservation Biology, School of Biological Sciences, University of Wollongong, NSW, 2522, Australia and 2Biodiversity Conservation Science Section, Policy and Science Division, Department of Environment and Conservation (NSW), PO Box 1967, Hurstville, NSW 2220, Australia Downloaded from https://academic.oup.com/aob/article/98/2/421/315339 by guest on 29 September 2021 Received: 21 October 2005 Returned for revision: 6 March 2006 Accepted: 10 April 2006 Published electronically: 30 May 2006 Background and Aims Germination studies of species from fire-prone habitats are often focused on the role that fire plays in breaking dormancy. However, for some plant groups in these habitats, such as the genus Leucopogon (Ericaceae), dormancy of fresh seeds is not broken by fire cues. In the field, these same species display a flush of seedling emergence post-fire. Dormancy and germination mechanisms therefore appear complex and mostly unknown. This study aimed to identify these mechanisms by establishing dormancy class and testing the effects of a set of typical germination cues, including those directly related to fire and entirely independent of fire. Methods To classify dormancy, we assessed seed permeability and embryo morphology, and conducted germina- tion experiments at seasonal temperatures in incubators. To test the effects of fire cues on germination, factorial combinations of smoke, heat and dark treatments were applied. Ageing treatments, using burial and seasonal incubation, were also tested. Germination phenology was established. Key Results Seeds were dormant at release and had underdeveloped embryos. Primary dormancy of the study species was classified as morphophysiological. Seasonal temperature changes overcame primary dormancy and controlled timing of germination. Fire cues did not break primary dormancy, but there was a trend for smoke to enhance germination once this dormancy was overcome. Conclusions Despite the fact that fire is a predominant disturbance and that many species display a flush of emergence post-fire, seasonal temperatures broke the primary physiological dormancy of the study species. It is important to distinguish between fire being responsible for breaking dormancy and solely having a role in enhancing levels of post-fire germination for seeds in which dormancy has been overcome by other factors. Biogeographical evidence suggests that morphological and physiological factors, and therefore seasonal temperatures, are likely to be important in controlling the dormancy and patterns of post-fire germination of many species in fire-prone regions. Key words: Morphophysiological dormancy, fire, embryo morphology, dormancy classification, germination, seasonal temperature, south-eastern Australia, Leucopogon exolasius, Leucopogon setiger, Leucopogon esquamatus, Ericaceae, Epacridaceae. INTRODUCTION around the world. This is partly due to the large number of species that display obvious and rapid germination Dormancy in plant propagules can restrict germination responses to fire characteristics, such as heat and smoke and seedling emergence to times when there are suitable (Auld and O’Connell, 1991; Keeley, 1991; Brown, 1993; environmental conditions (Baskin and Baskin, 1998; Dixon et al., 1995; Whelan, 1995; Bond and van Wilgen, Bell, 1999). The germination of many species from fire- 1996). As a result, the majority of seed germination prone habitats is related, either directly or indirectly, to studies have concentrated on the extent to which fire. Germination cues that are directly related to fire dormancy is broken by fire rather than exploring all the include heat shock (Keeley, 1987; Auld and O’Connell, mechanisms that might be responsible for breaking 1991; Bell et al., 1993), smoke or smoke products dormancy (Keeley, 1995). Seed dormancy is defined as (de Lange and Boucher, 1990; Dixon et al., 1995; Keeley a characteristic of the seed rather than of its environment and Fotheringham, 1997) and combinations of these (Vleeshouwers et al., 1995). Overcoming primary (Keith, 1997; Kenny, 2000; Thomas et al., 2003). Indirect dormancy is therefore distinct from stimulating germina- cues include light and ambient temperature conditions tion from a seed bank (Thompson et al., 2003). Factors (Bond and van Wilgen, 1996) that are altered as a result of required to break primary dormancy may be completely the canopy and leaf litter being burned and removed. unrelated to fire; however, fire may provide subsequent These cues can lead to a pulse of seedling emergence in germination cues and suitable environmental conditions the post-fire environment, where there are increased (Bell, 1999). resources available as well as reduced competition (Gill, There is growing evidence that many species in fire- 1981; Keeley, 1991; Whelan, 1995; Bell, 1999). prone environments do not have primary dormancy broken There is still limited understanding of the range of by fire. Germination can be enhanced after fire in dormancy mechanisms operating in fire-prone regions some species, but in others it can be diminished or * For correspondence. Email mark.ooi@environment.nsw.gov.au unchanged (Keeley, 1987; Davis et al., 1989). Laboratory Ó The Author 2006. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org 422 Ooi et al. — Dormancy and the Fire-centric Focus studies support this, because a significant number of required. The first set of processes are required to break species show no response to heat or smoke treatments primary dormancy, with subsequent processes then (Dixon et al., 1995; Roche et al., 1997; Keeley and promoting increased levels of germination post-fire. Fotheringham, 1998; Clarke et al., 2000; van Staden et al., In this study, three Leucopogon (Ericaceae) species 2000). Deciphering which cues are controlling primary from south-eastern Australia were used to investigate dormancy in these species may be more difficult than with dormancy mechanisms and timing of germination, and species that respond directly to fire cues. the potential role that burial, seasonal temperature regimes, Seed dormancy classification, using a system such as dark and fire cues play in controlling these mechanisms. that outlined by Baskin and Baskin (2004), distinguishes Leucopogon is the largest genus within the sub-family between physical, morphological and/or physiological Styphelioideae, (formerly recognized as Epacridaceae) mechanisms as controls on dormancy and can help to with approx. 230 species, and is part of the largest tribe, identify the most likely factors involved in overcoming Styphelieae. Dormancy mechanisms for this genus (and Downloaded from https://academic.oup.com/aob/article/98/2/421/315339 by guest on 29 September 2021 dormancy. Stratification at seasonal temperatures is the tribe) appear complex, and seeds have been described as main known requirement for overcoming both morpholo- difficult to germinate (Dixon et al., 1995). Unlike many gical and physiological factors, whereas heat shock or other species within the same plant communities, scarification can overcome physical dormancy. To classify Leucopogon displays a seasonal emergence pattern from seed dormancy, information on permeability of seeds, a soil-stored seed bank, with peak densities recorded in embryo morphology and germination response at different autumn and winter (Ooi et al., 2004a). There is no distinct seasonal temperatures, among other things, is required rainfall season, but seedlings emerge at the same time of (Baskin and Baskin, 2003). Classification can help to gain year, irrespective of rainfall or the timing of a fire event. an understanding of a species’ dormancy by enabling Nevertheless, there is increased germination post-fire. comparisons with other species within the same dormancy Subsequently, the post-fire flush of emergence can be class. delayed in relation to many other co-occurring species Several studies have suggested that morphological (Ooi et al., 2004a). and/or physiological factors play a role in controlling The specific aims are to answer the following questions. dormancy of species from fire-prone regions (Schatral (a) Are seeds of the study species dormant at the time of et al., 1997; Keeley and Fotheringham, 1998; Bell, 1999; release from the parent plant? (b) What classification of Tieu and Egerton-Warburton, 2000; Allan et al., 2004). seed dormancy do the three Leucopogon species have? However, the role of fire as a cue for breaking primary (c) Is dormancy broken during burial? What factors dormancy involving morphological and physiological promote loss of dormancy? (d) Do fire cues break primary mechanisms is yet to be clarified. dormancy? What are the relative effects of fire cues on In Australia’s fire-prone habitats, the predominance of both fresh seeds and buried seeds? (e) What drives the fire as a disturbance appears to focus most study effort on seasonal emergence pattern displayed by Leucopogon the role that fire plays in breaking dormancy, and less on species in the field? other mechanisms. Many common species from temperate understorey vegetation, including several members of the Ericaceae, Rutaceae and Dilleniaceae, have unknown MATERIALS AND METHODS dormancy