NSW National Parks and Wildlife Service, September 2002
ISBN 0 731 36905 X
5 Distribution and habitat
- 5.1 Historical and current distribution
- 5.2 Tenure
- 5.3 Habitat
- 5.4 Geology and Soils
- 5.5 Aspect and slope
- 5.6 Climate
Pterostylis gibbosa was first collected by Robert Brown on a roadside "between Parramatta and Green Hills" in September 1803. George Caley, collector for Joseph Banks, also found specimens at "Cowpastures" between "Prospect and South Creek" in October 1803 (Jones 1997). Although Caley was the first to prepare a description of P. gibbosa, it was never published, and the species was formally described by Brown in 1810 in "Prodramus Florae Novae Hollandiae et Insulae Van Diemen".
There is no published information on the numbers and distribution of P. gibbosa between its first discovery in 1803 and its re-discovery in 1967. This is probably because a large number of now separate species of Pterostylis (including P. gibbosa) were considered variations of Pterostylis rufa during this period (Quality Environmental Management 1994).
Currently, P. gibbosa is known from five locations: three sites in the Illawarra, one site in the Hunter Valley at Milbrodale, and one site near Nowra in the Shoalhaven (Figure 2).
The above historic accounts are the last recordings of P. gibbosa on the Cumberland Plain. Searches by the NPWS and the Australasian Native Orchid Society (ANOS) between 1996 and 1998 have failed to find P. gibbosa individuals on the Cumberland Plain (see 7.2.2).
In 1967, the species was collected by Brian Whitehead (McAlpine 1982) on privately owned, timbered grazing land south of Dapto at a site generally referred to as 'Yallah Bush'. In 1981 the Yallah Bush site was sold and foreshadowed for future urban development. To prevent destruction of the orchids the Heritage Council of NSW placed an Interim Conservation Order on the property. An inspection by the then Wollongong and District Native Orchid Society (now called the Illawarra ANOS) in 1987 recorded at least 100 plants at the Yallah Bush site (G Bradburn, ANOS pers. comm.). The Interim Conservation Order lapsed in 1988. It is not known if P. gibbosa still occurs at this site as parts of the area have been destroyed. No searches have been made of this site since 1991.
In 1983, P. gibbosa was found in bushland at Shellharbour Council's Croom Road Sports Complex at Albion Park. The 1993 census indicated that this site contained 575 individuals with the bulk of the population occurring in one clump of about 460 individuals (Quality Environmental Management 1994). In December 1999, the population was estimated to contain over 1000 individuals with the bulk still occurring in one clump (I. Taylor pers. comm.).
In 1984, botanical surveys for a proposed coal transport corridor indicated that additional suitable habitat for P. gibbosa occurred in open woodland in the Yallah area (Muston & Robinson 1984). This was confirmed in 1987 when Graeme Bradburn of the Illawarra ANOS found two groupings of P. gibbosa on private property at Yallah (Bradburn 1988).
In 1988, two groups of P. gibbosa with an estimated total of approximately 740 individuals (including 312 in Area 1 and 428 in Area 2) were found in woodland patches to the north of the Yallah substation on property owned by TransGrid (formerly Pacific Power) (Muston & Bradburn 1988).
An annual census of P. gibbosa abundance at the Transgrid site has been undertaken since 1991. The census follows the design set up by Whelan and Kohler (1991) where randomly placed and selected one metre quadrats were positioned within the two fenced areas of the site. It should be noted that the census is not a population count but provides an estimate of fluctuation in the number of individuals present at the site over time.
The 1993 census for the ANCA plan counted 413 plants in quadrats (180 in Area 1 and 228 in Area 2) (Bradburn & Tunstall 1993). In 1997, there were 169 P. gibbosa plants in the quadrats (Areas 1 and 2) only at Yallah (Bradburn and Tunstall 1997). In 1999, the number of orchids in each of the quadrats (Areas 1 and 2) was 405 providing a total of 810 plants in those two areas (G Bradburn, ANOS pers. comm.).
In 1969 Brian Whitehead found a flowering specimen of P. gibbosa among plants of Pterostylis mutica in the Browns Creek catchment near Nowra (Whitehead 1970). This part of the catchment has now been partially urbanised. Some areas of native vegetation remain in the catchment but these have been altered by grazing, clearing and burning.
In June 1997 between 300-600 P. gibbosa individuals were found in the former Currambene State Forest (now Worrigee Nature Reserve), about 1.5km south-east of Browns Creek. A survey was carried out in September 1998 to establish the distribution of P. gibbosa in the State Forest. To date, no attempts have been made to determine the exact size of this population.
In 1989, orchids thought to be P. gibbosa were discovered on private property in the Milbrodale area of the Hunter Valley (Quality Environmental Management 1994). In 1996, these were confirmed as being P. gibbosa by David Jones of the Australian National Herbarium. In 1999, the Milbrodale population was estimated as comprising at least 1000 plants but it may contain up to 2000 or more (I. Taylor pers. comm.).
A count of the population has not been undertaken although monitoring of tagged plants has been carried out in 1997, 1998 and 1999. Surveys in the vicinity of the Milbrodale population did not reveal any other populations. Further surveys in the Hunter region are recommended by this Plan.
Pterostylis gibbosa occurs on private property and land owned by Shellharbour Council, TransGrid and the NPWS. A list of tenure, zoning and management responsibility for each location is detailed in Appendix 1.
The habitat preferences of Pterostylis gibbosa are difficult to identify due to variation in the species composition and vegetation types between the sites where P. gibbosa is known to occur.
Nothing is known of the habitat of the original specimens of P. gibbosa collected on the Cumberland Plain west of Sydney, apart from the comment in George Caley's diary that the orchid was "an inhabitant of barren land" and was "much concealed by the grass". However the soil types and rainfall of the Illawarra is similar to the pattern on the Cumberland Plain (Quality Environmental Management 1994).
Yallah - Albion Park
Pterostylis gibbosa at Yallah and Albion Park occurs in woodlands with a native grass understorey. The isolated patches of woodland in which P. gibbosa is found in the Illawarra have, until the last few decades, been grazed by domestic stock. The woodland patches presently at these sites are not considered to be representative of the original vegetation. The remnants represent species which are tolerant to disturbance and include Eucalyptus teriticornis (Forest Red Gum), Melaleuca decora (White Feather Honey myrtle), Acacia falcata, Leucopogon juniperinus (Bearded Heath) and Themeda australis (Kangaroo Grass).
At Nowra, the plant community is an open forest. The presence of Eucalyptus maculata (Spotted Gum) is a distinguishing feature of the forest in the area. Structurally and floristically this area is distinct from other P. gibbosa sites in the Illawarra and the Hunter with species such as Eucalyptus paniculata (Grey Ironbark) and Exocarpus cupressiformis (Cherry Ballart). The leaf litter at this site is quite abundant and in some years it is very deep.
In the Hunter Valley P. gibbosa occurs in open woodland dominated by Eucalyptus crebra (Narrow-leaved Ironbark), Eucalyptus moluccana (Grey Box) with a sub-dominant of Callitris endlicheri (Black Cypress Pine). The understorey is thick with Dodonaea cuneata with varying amounts of leaf litter. Some of the P. gibbosa plants were found on bare soil underneath stands of Callitris endlicheri.
A list of major species associated with P. gibbosa at the various sites is provided in Appendix 2.
The populations at Albion Park, Yallah and Nowra all occur on soils derived from the Berry formation, a Permian sediment composed of undifferentiated siltstone, shale and sandstone. Yallah and Albion Park are between 10-20 metres above sea level while the Nowra populations are between 20-30 metres.
The soils have a seasonally hardsetting, acid (pH 5 - 5.5) surface horizon, 10 - 20 cm thick with a loam to clay loam texture. This overlies a mottled, acid (pH 5 - 5.5) subsoil with a heavy clay texture. These soils are poorly drained, although in the areas where P. gibbosa occurs they are only likely to be waterlogged for short periods following heavy rain.
The population at Milbrodale in the Hunter Valley is found at an altitude of 150-160 metres on soils derived from the Narrabeen Group, a Triassic sediment composed of sandstone, conglomerate, red and green claystone and shale. The topsoil is a sandy loam with an orangey-yellow clay subsoil. Ironstone gravel is common in the profile.
The soils are all leached, acidic and have a clay subsoil that impedes drainage.
All sites are flat or only gently sloping. In the Illawarra and at Nowra, the populations occur on the coastal plain to the east of the escarpment. In the Hunter Valley, the population occurs on a north-west facing terrace about 50 metres above the valley floor. The terrace at this point is between 100 - 500 metres wide before the land rises another 270 metres to the ridgetop.
The mean total annual rainfall using available data between 1970 and 1999 for Wollongong University, which is the closest site to the Yallah populations, is 1401.1 mm. January receives an average of 143.3 mm, June receives 120.2 mm and September receives a mean of 73.6 mm.
The mean total annual rainfall using available data between 1892 and 1999 for Albion Park is 1102.1 mm. January receives an average of 107.9 mm, June receives 59.1 mm and September receives a mean of 62.0 mm.
The mean total annual rainfall using available data between 1969 and 1999 for Milbrodale is 772.6 mm. January receives an average of 102.5 mm, June receives 41.1 mm and September receives a mean of 46.6 mm.
The mean total annual rainfall using available data between 1942 and 1999 for Nowra which is the closest measuring site for the Worrigee Nature Reserve population is 1090.7 mm. January receives an average of 91.4 mm, June receives 107.8 mm and September receives a mean of 65.7 mm.
The seasonal pattern of rainfall is broadly similar at all sites. Rainfall is highest in summer and autumn falling to a low in late winter and increasing throughout the spring.