Oligochaetochilus excelsus, an impressive name for the winning photo, taken by John Fennell. It was originally formally described and named Pterostylis excelsa by Mark Clement in 1986 from a specimen cultivated from a tuber taken from Eyre Peninsula. This was subsequently published in the fourth edition of Black’s Flora of South Australia. In the early 2000s, the genus Pterostylis was split into several different genera of which Oligochaetochilus was one.
Although not everyone accepted this change, it is helpful for understanding the characteristics of this group. They are found only in Australia and are primarily a semi-arid inland group, sometimes the only orchids to occur in a locality.
Bob Bates has documented some of the drought tolerant and drought avoidance features found in this group as follows.
Large moisture-storing tubers, shallowly buried to take advantage of light rain.
Waxy leaves pressed flat to the soil thus avoiding water loss.
Fast growth, some species are able to flower on just two good rainfalls.
Leaves senesce (die off) before or soon after flowering begins; moisture and food from the leaves is then stored in the tuber and scape. Leaves are able to absorb moisture from dew.
Leaves also contain an anti-freeze and can tolerate black frosts.
Just one month after rain a replacement storage tuber is produced as large as the old.
Plants favour damper microhabitats, growing at the base of larger rocks and in seepage zones or on the cooler damper south side of bushes.
Tubers of desert species do not usually germinate in drought years.
Leaf senesces (dies off) before hot weather meaning plants avoid moisture loss during flowering.
Plants cease respiration in long dry periods and resume after rain even up to three months later, staying green even in bone dry soil.
Think of deserts and the image is that of a bleak barren landscape with little to see but this is not so. The conditions are harsh but there is a myriad, though not an abundance, of hardy fauna and flora if one but looks closely.
But concerning orchids – No orchids have been found in true deserts….. They also appear to be absent from the arid mountains of the far north-west, or at least no-one has ever found orchids there.
Orchids need moisture and so they do not grow on unstable soils such as dry sand-hills, gibber plains or the many saline areas of the far north but on the desert fringes there are micro-climates where the moisture, humidity and soil structure is just right (to quote Goldilocks) for orchids. This micro-climate is created by [s]hrubland [which] is … [an] … important dryland orchid habitat. Besides providing shade and shelter for the orchids, shrubs like the many species of wattles, Acacia and hop-bush Dodonea drop fine leaves which help to hold the soil together and slowly break down into humus rich with nutrient and water storing capability. These shrublands usually form in soils too dry or shallow for trees. Orchids of course have no need for deep soils as they are shallow rooted.
Of the five desert botanical regions, the Eastern region contains the most number of species with over a dozen species.
Orchids of the Eastern Region – this region is from the east of the Flinders Ranges to the New South Wales border and includes the Olary Spur and Lake Frome.
Arachnorchis toxochila – Dry Land Spider Orchid or Bow Lip Spider Orchid.
Corunastylis tepperi – Mallee Midge Orchid
Diplodium robustum – Common green shell-orchid.
Hymenochilus pagophilus – Mountain Shell-orchid
Microtis eremaea – Desert onion orchid
Microtis frutetorum – Common woodland onion orchid.
Oligochaetochilus bisetus species complex, Rusty rufous-hoods
Oligochaetochilus sp. Blue-bush Plain – Blue Bush rufous-hood (O. bisetus complex)
The Gairdner-Torrens region includes, besides the salt lakes it is named after, the Gawler Ranges and the southern part of the Great Victoria Desert. Though not as many species as the Eastern region, it contains some different species including a Sun Orchid.
The final two regions Lake Eyre and North-Western contain the vast expanses of desert of the far north of South Australia. Definitely not a place to find orchids yet one specimen has been collected from each of these two regions.
Oligochaetochilus sp. Everard Range (L. Scott 173), Mimili Orchid (possibly O. woollsii complex) from North-Western Region.
Oligochaetochilus sp. Gammon Range (O excelusus complex) from the Lake Eyre region.
It is unusual to find orchids in the desert because they only grow when there have been good winter rains which isn’t very often. But nevertheless, here in South Australia we have over 20 possible species – an astonishingly high number for such a harsh area!
Bates R J ed, South Australia’s Native Orchids, 2011 Native Orchid Society of South Australia
Map adapted from Flora of South Australia, Fourth Edition, 1986