A Vegetation Survey at Gibraltar Rock, DUBBO

On Tuesday, 25 April 2000, I conducted a vegetation survey, by way of a 50m x 20m quadrat, in the south-eastern corner of the Gibraltar Rock Trig Reserve. A total of 94 species were found in and around the quadrat, with 680 within the 0.1 ha area. The site selected was in White Box ? Grey Box ? Black Cypress woodland, which is atypical of the reserve which, is largely covered by Currawang closed shrubland. Members of the Dubbo Field Naturalist Club identified an additional 20 species during a visit to the reserve in 1998.

The total survey area covers an area of approximately 45 ha and is based on the trig station on Gibraltar Rock, at approximately 371m altitude. The main lithology is rhyolite. The reserve had been completely burnt in December 1984, with fire scars and dead trees near the trig station. In addition the trunks of standing fire-killed cypress were well represented in the quadrat. Several large Cypress, both Black and White had survived the fire. The more open canopy in conjunction with the heavy recent rains had promoted a dense groundcover within the site. Summer flowering species were still identifiable and a number of winter-growing species were emerging at the time of the survey (Dichopogon, Hypoxis, Thysanotus, Pterostylis and Drosera).

Exotic species formed a small proportion of the total flora but did include such common weeds as Saffron Thistle, Barley Grass/ Rye Grass, Spear Thistle, Lepidium africanum, Paterson?s Curse and Flatweed.

The adjacent Currawang closed shrubland was investigated briefly and found to have a much lower species diversity. The common ground cover species included Barley Grass/ Rye Grass, Flatweed, Medics and Rock Fern.

A number of unusual species were seen near the site, particularly Muehlenbeckia rhytica and Pterostylis sp aff revoluta (Inland Form). The former occurred as scattered shrubs at the base of the rock outcrop particularly on the southern side (D. Shelly pers comm, 2000). These have conspicuously broad fleshy leaves, almost square in outline and the leaf base includes a conspicuous sheath of the adjacent stem. The large plant near the quadrat was in flower and found to be a male plant. Given that this species is dioecious (ie separate male and female plants) and apparently insect pollinated this population may be placed at risk of local extinction if plant numbers are reduced or all plants of one sex are eliminated.

A cluster of flowering greenhood orchids was found in a thin rock shelf on the southern face of Gibraltar Rock. Approximately 8 flowers were open and they were best keyed out to Pterostylis sp aff revoluta (Inland Form) following Bishop (1996), however it is noted that the flowers had more brown at the flower tip than the plant shown in the photo.

An analysis of TSCAB indicates that a number of threatened plant species have potential to occur in the reserve including Diuris shaeffiana, Goodenia macbarronii, Homoranthus darwinioides and Indigofera efoliata.

Further field work in the reserve is recommended, particularly covering other parts of the reserve, such as the northern and western slopes. Formal quadrats in other vegetation types, particularly the Currawang closed shrubland is recommended to enable formal comparison with the vegetation quadrat done to date.

Bishop, T. (1996) Field Guide to the orchids of New South Wales and Victoria. UNSW Press.

Robert Gibson  (1/5/2000)

Natural History of the Dubbo Area