National recovery plan for Boronia granitica (Granite Boronia)

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

6 Disturbance, threats and reasons for decline

Due to its intrinsic habitat specificity, Boronia granitica may have always had a restricted distribution, confined to heathy vegetation associated with large expanses of granite outcropping on the mid-elevation, dry western side of the New England Batholith. However, populations of B. granitica may have declined since European settlement of the area as a result of grazing by stock and feral goats, inappropriate fire regimes, mineral exploration, mining, quarrying and bushrock collection. Widespread land clearance and disturbance may have also impacted on the distribution of the species although this assumption is based principally on circumstantial evidence. A summary of impacts on B. granitica populations in various areas is given in Table 5.

Table 5: Relative impact of major current or potential threats to main population areas of Boronia granitica
Threat Parlour Mountain Howell area Kings Plains Severn River / The Barbs & adj. private land Torrington area
Browsing of plants, especially by feral goats
high
high
moderate
moderate
moderate*
Inappropriate fire regimes
moderate
high
moderate
moderate
moderate*
Soil disturbance from mining and exploration activities
low
moderate
low
low
moderate
Loss of habitat to agricultural activities or access tracks
moderate
moderate
low
moderate
low

Note:
* down from 'high' as a result of active NPWS management.

6.1 Grazing

It was noted as early as 1932 that plants in the Elsmore area identified as B. granitica (but now requiring confirmation) were 'eaten by stock in the drought' (McKie herbarium label) and that this may have contributed to the disappearance of the species from this area. At Torrington SRA, stock grazing is permitted under licence but it is unclear whether any populations of B. granitica have been adversely affected. The draft Plan of Management for the SRA recommends elimination of grazing from the reserve by 2005.

The rocky habitat of B. granitica possibly confers a degree of protection from disturbance by domestic stock, but not from goats which can gain access to the entire landscape (Quinn et al. 1995). Feral goats are now abundant in the Howell area and pose an increasing threat to the vegetation community (Hunter and Clarke 1998). A drastic reduction of the B. granitica population to critically low numbers has occurred at Parlour Mountain where two of the three plants that persist at the site have been eaten by goats (Hunter and Bruhl 1997). Feral goats and cattle also threaten vegetation on other outcrops in this area (Hunter and Clarke 1998). Evidence of feral goat activity was frequently recorded on the rocky outcrop heaths and shrubby stringybark woodlands supporting B. granitica in the Torrington area (Hunter and Clarke 1998) and Clarke et al. (1998) has stated that this threatens populations of some plant species.

Populations of B. granitica in the Howell area have declined since the type specimen was collected there nearly a century ago. B. granitica plants were observed to be heavily browsed by macropods in this area in the 1960s (Quinn et al. 1995). Macropod grazing could be considered a natural occurrence, although the populations of certain larger macropods may be artificially high due to agriculture. These reports suggest B. granitica is palatable despite bearing small, hairy leaves with oil glands. However, macropod grazing may also be related to shifts in food preferences during times of low resource availability such as drought.

6.2 Fire

The heathy vegetation that forms the habitat of B. granitica is characterised by a dominance of scleromorphic shrubs in conditions of high light penetration, low soil moisture retention, and slow litter decomposition. These factors and the volatile oils characteristic of the Eucalyptus overstorey and Boronia itself all contribute to a fire prone environment, as is typical for much of the Australian autochthonous vegetation (Specht 1981).

Many shrubs of heathy vegetation communities germinate prolifically after fire (Benson 1985, Keith 1996), including some Boronia species (M. Duretto pers. comm.). Others resprout from rootstock, such as the undescribed Boronia sp. aff. microphylla at Torrington and B. parviflora of upland swamps (Keith 1991), or are facultative resprouters such as B. falcifolia and B. safrolifera (Benwell 1998) of coastal heaths and the rare B. galbraithiae of dry sclerophyll forest in Victoria (Albrecht and Walsh 1993).

Post-fire vegetative resprouts of B. granitica have not been observed in the field. Thirty B. granitica individuals were burnt during fire research at Torrington and none resprouted (Clarke and Fulloon 1999). Lignotubers or similar root structures are absent and regeneration relies on the successful germination of seed and survival of seedlings, that is, the species is an obligate seeder.

Dormancy in soil stored seed of many obligate seeding plants is broken by heat (Keith 1996, Clarke and Fulloon 1999). Some Rutaceae have germination promoted by exposure to smoke from burnt native vegetation and an enhanced germination response to heat has been demonstrated for the persistent soil seed bank of some Boronia species (Keith 1996). A pH-sensitive enzyme is suspected of being the seed dormancy factor in Boronia and the enzyme is neutralised and denatured by basic post-fire leachates (Armstrong 1989). In recently conducted field experiments fire has been demonstrated to trigger or stimulate seed germination of B. granitica with a post-fire result of 1-10 seedlings present per square metre (Clarke and Fulloon 1999). The precise response to fire of the soil stored seed is likely to vary with components of the fire regime (intensity, season), fire residence time (duration) and local site conditions (eg. soil moisture availability).

It has been suggested that the low fire frequency at Howell may have adversely affected B. granitica populations in that area (Quinn et al. 1995) based on significant changes to the flora including the loss or reduction of several other species in the long fire-free period (Hunter 1998). Obligate seeders generally require periodic fire for substantial regeneration but some species, including the related B. keysii of southern Queensland (Leigh and Briggs 1992), achieve some regeneration in the absence of fire. Young plants of B. granitica have been observed in heathland that appeared not to have carried a fire for several years (J. Westaway pers. obs.), suggesting that fire may not always be a prerequisite for germination. Recruitment cues other than fire (eg. soil disturbance) have been suggested in the establishment of young cohorts of the closely related B. boliviensis m.s. in long unburnt heath (Morsley and Falconer in prep.). Seasonal drought may also underlie the waxing and waning of B. granitica plants occurring in low moisture-retaining habitats but these effects have not been documented.

For species to persist in a fire prone environment new recruits must not only flower between successive fires, but also replenish propagule reserves. In a study of heathy woodland regeneration in Victoria, Wark et al. (1987) found that although most shrubs commenced flowering within two years of fire, few produced seed, suggesting that early post-fire flowering does not necessarily result in seedlings. Obligate seeding species are likely to be severely disadvantaged and possibly eliminated by a series of fires at intervals less than the time taken for the new cohort to produce sufficient seed (Benson 1985).

Conversely, if inter-fire periods extend beyond the longevity of plants and soil-stored seed bank, then adult plants will age and the accumulated seed become non-viable. During long fire-free periods local population extinctions may occur. Species most vulnerable to this are perennials which are killed by fire (Benson 1985, Gill and Bradstock 1995), irrespective of whether propagules for replenishment are canopy or soil stored.

Keith (1996) listed a number of fire-driven mechanisms of plant population decline and extinction that may be relevant to B. granitica:

  1. depletion of standing plants and seeds through senescence and competition with community dominants;
  2. high seedling mortality due to resource deprivation, competition, predation and disease;
  3. low recruitment rate relative to mortality due to germination of few seeds caused by poor soil heating; and
  4. low seed availability due to failure to satiate predators.

The majority of fires recorded in B. granitica habitat are large wildfires, control burns or arson events, not small-scale lightning events that rocky outcrops are prone to. The myriad of small-scale lightning induced fires that fail to spread due to low fuel loads are mostly unrecorded let alone mapped. The patchy (mosaic) distribution of fires from lightning strikes may be an important ecological process providing B. granitica with adequate regeneration opportunities.

Field observations suggest that when burnt by intense fires and even scorch burning, standing plants of B. granitica are usually killed outright (Gill and Bradstock 1992). In their study of fire and rare plants at Torrington SRA, Clarke and Fulloon (1999) classified B. granitica as 'highly sensitive' to inappropriate fire regimes because it is an obligate seeder with a soil-stored seed bank. They recommended that plant populations of 'high sensitivity' to fire frequency (such as B. granitica) not be burnt more frequently than 8-10 years, nor have fire excluded for more than 20 years. Alternatively, Hunter (1999) has recommended even longer (30-100 year) fire exclusion periods for certain vegetation communities (including communities containing B. granitica) within Kings Plains NP.

High frequency fire is currently listed as a key threatening process on Schedule 3 of the TSC Act (NSW Scientific Committee 2000a). In the determination for this key threatening process, B. granitica was identified as a species adversely affected by high frequency fire, which disrupts lifecycle processes.

In summary, the most current and thoroughly researched studies conclude that B. granitica is adversely affected by frequent fire and that much longer fire exclusion periods than those that occur at present should be part of future fire management strategies. More research is needed into the response of B. granitica to fire, especially in relation to lightning strikes, and optimal fire frequency.

6.3 Mining, quarrying and bushrock collection

Activities associated with mineral exploration, mining, granite quarrying and bushrock collection can be highly localised, and represent significant threats to B. granitica populations. These activities often require removal or damage of native vegetation and, in the case of open-cut mining, massive soil disturbance.

It is unknown whether specific sites supporting B. granitica have been mined in the past and the species is not known to occur on specific lands subject to current mining leases. However, the Howell area has experienced considerable local soil disturbance from mining activities whilst mineral exploration and mining are permitted in Torrington SRA under various mining lease options. The species is also known to occur on lands subject to current exploration licences. The perpetuation of these lease and exploration options could present a threat to B. granitica populations in the Torrington area (Hunter and Clarke 1998) unless appropriate environmental mitigation amendments are included. The environmental assessment requirements for the Department of Mineral Resources (DMR) and the NSW National Parks and Wildlife Service (NPWS) are currently under review in relation to these leases and licences. It has been agreed that future activities will be undertaken in a manner that avoids or minimises adverse impacts on the species and that the known occurrence of the species will be supplied to future lease and licence applicants.

Bushrock removal is currently listed as a key threatening process on Schedule 3 of the TSC Act (NSW Scientific Committee 1999). In the determination for this key threatening process B. granitica was identified as a species adversely affected by bushrock removal, which destroys and disturbs habitat.

The effect of soil disturbance on the seed bank of B. granitica is unknown but the likelihood of germination seems low without heat stimulation. Young individuals of B. boliviensis m.s. occur under a powerline easement on Bolivia Hill suggesting that, for this sister species, a recruitment cue other than fire, such as scarification of the seeds, may also be involved in reproduction. The impact of limited soil disturbance on germination of B. granitica requires further research.

6.4 Land clearance

Clearing of native vegetation has been listed by the NSW Scientific Committee as a key threatening process on Schedule 3 of the Threatened Species Conservation Act 1995. Clearing of native vegetation for agriculture and other purposes has been widespread on the New England Tablelands and has often included the less productive areas surrounding granite outcrops. Although possibly affected less than plants in more fertile areas, B. granitica and the associated flora of granite outcrops are, nevertheless, restricted in distribution and vulnerable to further clearing.

Vegetation removal for activities adjacent to known populations may also pose a risk. An example is the clearing for tracks on private property adjacent to Severn River NR where an ecologically significant stand of B. granitica occurs on deep red soils (J. Hunter pers. comm.).

Herbicide application on farmlands adjacent to rocky outcrops in the Tenterfield area has encroached significantly on native vegetation communities and may also be adversely affecting B. granitica populations (P. Croft pers. comm.).

6.5 Low population numbers

Reduction in the population size and distribution of B. granitica due to the above threats can result in stochastic (chance) events becoming increasing significant in the local extinction of the species. For example, the role of prevailing winds can affect the likelihood of successful cross-pollination between plants.