National recovery plan for Boronia granitica (Granite Boronia)

NSW National Parks and Wildlife Service, July 2002
ISBN 0 731 36889 4

5 Ecology

5.1 Life form and longevity

Boronia granitica is a perennial evergreen woody shrub with an open branched growth habit. The species appears to have a slow growth rate. Although longevity data are lacking, the CRA Threatened Flora Expert Workshops for the Upper North-East Region of NSW estimated B. granitica to have a longevity of about 15 years. This equates to that suggested for the closely related B. boliviensis m.s. (Morsley and Falconer in prep.).

Few data on Boronia exist concerning the time required for seedlings to mature and produce their first flowers (the primary juvenile period). B. ledifolia (closely related to B. granitica) and B. serrulata are fire sensitive species that were found to flower four or five years after fire in the Hawkesbury Sandstone vegetation (Benson 1985). B. parviflora, a resprouter of swampy heaths, is estimated to have a primary juvenile period of more than five years, although this period is thought to generally be longer in resprouters than seeders (Keith 1991). Preliminary observations suggest that B. granitica may flower in its third or fourth year (J. Westaway pers. obs.), which is consistent with the estimated primary juvenile period of 4-8 years for other restricted outcrop shrubs with similar regenerative strategies (Clarke and Fulloon 1999).

5.2 Reproductive biology

B. granitica is a dicotyledonous angiosperm that, once mature, flowers and sets seed annually. The species appears to have an extended flowering season from July to December, concentrated over spring. Fewer individuals flower in other months, although this may vary annually with local climatic conditions.

Little is known of the breeding system of B. granitica. All Boronia section Valvatae are described as self-incompatible (Weston et al. 1984). Thus, pollen transfer is required for successful fertilisation. Self-incompatibility implies an outcrossing breeding system with isolated pollen centres (populations) diverging genetically over time according to conventional speciation theory. However, the reduction in size of pollen centres, for instance due to clearing, could potentially force inbreeding. The ramifications of such changes are unknown but may speculatively include declines in genetic integrity and population viability. A detailed investigation of pollination mechanisms in B. granitica would improve understanding of the species' breeding system.

The flowers from plants in the Boronia genus emit volatile oils. The location and timing of these emissions suggest a role in attracting pollination vectors (Weston et al. 1984). There are no known limits to pollination in B. granitica, although little information is available because pollination vectors and mechanisms have not been studied. Unidentified beetles were attending flowers at one location during spring 1998 (J. Westaway pers. obs.) and beetles are thought to be involved in pollination of other Boronia species (M. Duretto pers. comm.). The role of introduced honeybees in pollination of B. granitica at sites in the Torrington SRA may also be significant given the proximity of at least 40 apiary leases to the area. More research is needed into the pollination ecology and population genetics of B. granitica.

Little is known of the fate of seed released from B. granitica but seeds of more than 30 percent of the sclerophyll shrub flora of low nutrient soils in Australia are ant dispersed (myrmecochory) (Beattie 1991). Seed of many Boronia species is likely to be ant dispersed (Berg 1975) and this is the case for the closely related B. boliviensis m.s. where seed has been found in excavated ant nests (Morsley and Falconer in prep.).

Seeds buried by ants are safe from fire and predation and are removed from competition with sympatric species for establishment microsites (Beattie 1991). The depth of burial affects dormancy breaking, germination and seedling emergence. In a study of ant-seed interaction in sclerophyll forest in Queensland, Drake (1981) found that the removal of seeds by ants could potentially deplete the available seed supply for regeneration, especially if taken to unsuitable germination sites such as wood ant nests. In most cases, however, sufficient seed for regeneration remains after predator satiation has been reached (Drake 1981).

Beattie (1991) suggested that ants transport seeds mostly within 10-20 metres of the parent plant. Seed dispersal by rainfall runoff may exceed such distances where microtopographic variation is minimal allowing unhindered flow of high rainfall events down slope. Nonetheless, seed dispersal ability in B. granitica seems to be relatively poor when compared with other vascular plants.

The small hard seeds of B. granitica accumulate in the soil until dormancy is broken by germination cues. The related B. keysii, of more mesic environments in southern Queensland accumulated a large viable seed bank in the soil over an inter-fire period extending two decades (Leigh and Briggs 1992). The soil-stored seed bank of B. granitica is thought to be relatively small and suspected to be moderately long-lived (Clarke and Fulloon 1999). More research is needed into the role of ants in dispersing the seed of B. granitica and into long term viability of seed banks in soil.