National recovery plan for the Downy Wattle (Acacia pubescens)
NSW National Parks and Wildlife Service
Environment Australia, February 2003
ISBN 0 7313 6504 6
7. Biology and Ecology
Acacia pubescens is a bushy or weeping shrub, 1-4 m high (Robinson 1991, Benson & McDougall 1996). The plants may be single-stemmed or multi-stemmed, forming dense patches from suckering. The longevity of individuals is reported to be 50 years (Benson & McDougall 1996), though this may be an underestimate, as individuals of clonal species have been known to survive for much longer periods (M. Matthes, NPWS pers. comm.).
Acacia pubescens is a clonal species. Clonal plants exhibit two levels of organisation: the genetic individual (the 'genet') and the module produced by vegetative growth (the 'ramet') (Sydes & Peakall 1996).
A. pubescens appears to sucker at most sites (S. Burke, NPWS pers. obs.). This has resulted in some dense patches of the species, several metres wide. Preliminary genetic work (Moore et al., 1999) has shown that in most cases these dense patches (with in some cases hundreds of stems) are in fact one individual. It is important for land managers to recognise that a census based on counts of 'individuals' may therefore overestimate the number of genetically distinct individuals.
- 7.3.1 Breeding system
- 7.3.2 Flowering and pollination
- 7.3.3 Fruit and seed production
- 7.3.4 Seed viability and germination factors
- 7.3.5 Recruitment and population structure
Acacia pubescens has bisexual flowers (ie. each flowers has both male and female components). The breeding system of Acacia species can vary from highly self incompatible (Bernhardt et al. 1984; Kenrick & Knox 1989 in Auld 1996) to a mixture of out-crossing and self-compatibility (Morrison & Myerscough 1989 in Auld 1996). It is unknown as to what extent the species is self-pollinating or outcrossing. Some level of outbreeding is suggested by the knowledge that hybridisation occurs with closely related Acacias, such as A. baileyana. It is particularly important to understand the breeding system of plants that reproduce vegetatively. There are two possible situations which would have an effect on the survival of the species:
- if the species is self-incompatible, populations with few genets may face lowered seed set because of mate scarcity (Sipes & Wolf 1997). Mate scarcity may be a particular problem for this species because such a large proportion of sites appear to have small numbers of individuals; and
- if the species is self-compatible, pollen transfers within clones will increase the level of inbreeding which may reduce the amount of genetic variability within the species (Peakall & Beattie 1991). Populations with low genetic variability may be at increased risk of extinction because of inbreeding depression and reduced sexual reproduction (Sipes & Wolf 1997).
Flowering has been recorded to occur from August to October, with a peak in September (Carolin & Tindale 1994, Benson & McDougall 1996). Plants first start flowering when they are approximately 3-5 years old (Wrigley pers. comm. in Maryott-Brown & Wilks 1993). Pollination of Acacia flowers occurs by insects (mostly by beetles, wasps and bees) (Tame 1992) and birds (Auld 1996).
Pods mature in October to December, with a peak in November (Benson & McDougall 1996). Plants produce the first seed crop when approximately 3-5 years old (Wrigley pers. comm. in Maryott-Brown & Wilks 1993). The percentage of seed fall is unknown, but may be low. Seed on the plants appears to suffer from heavy predation, with the result that few seeds drop and are available for germination (D. Thomas, consultant pers. comm.). Seed that drops is often attacked by an insect which bores a small hole into the pod, which dries out the seed (S. Douglas, consultant pers. comm.).
Pod production is known to be low for the species. Pods were only observed at 30% of the sites visited as part of this Recovery Plan, and at these sites, only a small percentage of individuals (NPWS pers. obs.). In contrast, suckering plants at Mount Annan Botanic Garden produce fruit (P. Cuneo, RBG pers. comm.). It may be that plants which set seed heavily are different genotypically to those that are poor setters of seed (R. Johnson, RBG, pers. comm.). The generally low levels of pod production may also be caused by a lack of pollinators, low pollen viability, inbreeding or some other factor. An article from 1914 remarks that the species is known to be a 'exceptionally shy bearer' of fruit (Anon 1914), thus the low fruit setting may be natural for the species and may not be due to stresses that the species is currently subject to. It may be that the species naturally spends less resources on seed production than vegetative growth.
Fruit production of individual plants has also been observed to vary from year to year, possibly due to environmental conditions, such as rainfall (D. Little, consultant pers. comm.). The variation in seed crop of individuals may also be due to the increasing age of the plants. For example, for A. suaveolens, the seedbank reaches a maximum some ten years after a fire, after which it declines in response to seed decay in the soil, adult mortality and a decline in adult fecundity (Auld & Myerscough 1986). The triggers for fruit and seed production, and the causes of the low numbers of fruit produced, require further investigation. Management of the species must include consideration of the long-term persistence of the species at sites, not just survival of current individuals. Information on factors influencing fruit and seed production will be essential for long-term persistence of the species.
There is no published information about seed viability of the species. Acacia species generally have high seed dormancy and long-lived persistent soil seedbanks (Auld 1996). Propagation work on the seed suggests this is also the case for A. pubescens, as treatment of seeds by scarification and hot water results in high levels of germination (D. Bishop, RBG pers. comm.). Mount Annan Botanic Garden has also recorded high seed viability ten years after collection (P. Cuneo, RBG pers. comm.). Germination of Acacia seed is also known to be linked to fire (Auld 1996), though this has not been investigated for this species. Investigation of the seed viability and the rates of germination will be vital information for the management of populations.
In the Fabaceae, primary seed dispersal is short (generally 0-2 m). Acacia seeds are also known to be secondarily dispersed by ants, birds and possibly water (Auld 1986a). In woodlands (where A. pubescens occurs), it is thought that dispersal is likely to be by ants and is likely to be limited to a distance of a few metres (Auld 1996). Through their harvesting of seed, ants often bury the seed and thus ensure germination only after penetrating rain (Tame 1992).
The low seedling recruitment that is observed in A. pubescens is not unusual. Seedling recruitment in clonal plants is usually infrequent and irregular (Eriksson 1993). Despite this, it has been found that only a low rate of seedling input into established populations is needed to maintain genetic variability (Eriksson 1993).
As stated above, seed production of the species is low, and of the seed that is produced, a large number suffers from predation. Although there is no published data on the subject, it is assumed from this information that recruitment is more commonly from vegetative reproduction rather than from seedlings. From a study of the genetic variation at 10 sites of A. pubescens, clones were detected at all sites (Moore et al. 1999). Also, regeneration after disturbances such as slashing, appears to occur from suckers rather than seed, since newly emerging shoots are usually clustered together (S. Burke pers. obs.).
The suckering mechanism of A. pubescens allows the species to tolerate some levels of disturbance, though this level hasn't been quantified. The trigger for suckering to occur is unknown, but it is thought to occur as a result of disturbance, such as slashing or fire (G. Errington, RBG pers. comm.) or herbivory. However, there are suckering plants at Mount Annan Botanic Garden that have not been subjected to disturbance (P. Cuneo, RBG pers. comm.). It will be important to understand the trigger for suckering and the level of tolerance to disturbance, so that better in situ management decisions can be made.
Four aspects of fires are important for germination of seeds and for vegetative recruitment - frequency, intensity, duration and seasonality (Auld 1986b). The optimal fire characteristics are not available for this species, but other Acacia species have been studied.
- fire frequency:
Comprehensive studies of optimal fire frequencies for Western Sydney vegetation have not been carried out. Some research that has been done on fire frequency in Cumberland Plain Woodland has shown that high frequency regimes (1-2 years) lead to the gradual decline and removal of shrub species (Thomas 1994). It is assumed that such high fire frequency regimes would also affect seedling recruitment of A. pubescens. Sufficient time would be needed between fires for seedlings to flower and replenish the soil seedbanks, as well as for the resprouting juveniles to become fire-resistant. In contrast, fire is required to break dormancy of Acacia seeds (Auld 1996). Thomas (1994) suggests that a minimum fire-free period of 5-7 years would be appropriate for legumes.
There is no published information about the effects of fire frequency on the vegetative recruitment of A. pubescens. It is assumed that plants would sucker after fire. Since the species tends more often to reproduce vegetatively than from seedling recruitment, it will be important for the management of the species to investigate this response to fire frequency. It may be the case that high frequency fire regimes eventually lead to the decline of genets. In contrast, fire may be needed at certain intervals to trigger vegetative reproduction or seed germination.
- fire intensity and duration:
The stems are killed by fire but resprouting occurs from suckering roots (Benson & McDougall 1996). The impact of fire temperature and fire duration are not known. The intensity of the fire will affect the degree of recruitment that occurs. Under intense fires, there is likely to be more resprouting plants killed and longer periods needed before the next fire for recovery, compared with mild fires. However, fires also need to be intense enough to break the dormancy of seeds in the soil (Auld 1996). For A. suaveolens, optimal fire temperatures for germination of seed are between 60°C and 80°C for any duration, or up to 100°C for duration's less than one hour. Exposure to temperatures less than 60°C leaves seed dormant and viable, whereas seed death occurs with increasing exposure to temperatures greater than 80°C (Auld 1986b). These figures hold implications for the amount of fuel load that will be appropriate for germination, as well as for other burning conditions. Smoke has also been found to be a factor that promotes germination (Roche et al. 1997). Experimental heating and possibly smoke trials could be carried out on this species to establish optimal fire conditions.
- fire seasonality:
Little work has been done on the effect of the season of fires on the species composition of Western Sydney vegetation. It is assumed that late summer and autumn fires would be preferable, since the seedlings at these times should encounter favourable moisture conditions for growth (Auld 1996). Fires at this time would also promote the germination of a fresh seed crop, which is released from October to December.