Survey of threatened plant species in South East Queensland biogeographical region queensland cra/rfa steering committee survey of threatened plant species in

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3.29 plectranthus omissus P.I. Forst.

3.29.1 Summary

Plectranthus omissus is endemic to south-eastern Queensland where it is recorded from the Kilkivan and Gympie districts. The distribution of P. omissus has a range of approximately 70 km and encompasses an area of occurrence of approximately 700 km². It is recorded from 3 sites all within State Forests lands. The population at Timber Reserve (TR) 26 was estimated to comprise 30-40 plants. There is no information available on population sizes at the other sites. The species is not within any areas set aside for conservation purposes. The species is threatened by weed infestation by lantana (Lantana camara) and blue billygoat weed (Ageratum houstonianum).

3.29.2 Species description and identification

The genus Plectranthus is placed in the mint family (Lamiaceae). Plectranthus is widespread throughout the tropical regions of Africa to Asia and Australia with approximately 200 species (Mabberley 1997). In Australia it comprises approximately 40 species of herbs and herbaceous shrubs.

Plectranthus omissus was formally described in 1992 from material collected in TR 26, approximately 8 km north of Kilkivan (Forster 1992). The botanical description of Plectranthus omissus is as follows:
Subshrub to 100 cm high, foliage slightly aromatic. Stems or lateral branches erect to straggling, the lower parts woody and up to 10 mm thick; upper parts with sparse to dense, retrorse, 6-8-celled non-glandular silver trichomes up to 0.6 mm long but commonly much shorter, lacking glandular trichomes, with sparse sessile 4-celled glands. Leaves concolorous; petiole 25-36 mm long; lamina broadly ovate, 20-80 mm long, 30-60 mm wide, somewhat fleshy, serrate with 17-19 teeth on each margin; both surfaces with dense, retrorse, 6-8-celled non-glandular hairs silver hairs, lacking glandular hairs, with sparse to dense sessile 4-celled glands. Flowers in compact cymose clusters, arranged into terminal racemes 15-35 cm long. Clusters 14-16-flowered; pedicels 3-3.5 mm long. Calyx 2.6-2.8 mm long. Corolla 8.5-8.8 mm long, lilac; tube 4.8-5.0 mm long, abruptly curved at an angle of c. 110-130° c 1.4 mm from base, with scattered, divaricate, 4-6-celled non-glandular trichomes to 0.4 mm long, lacking glandular trichomes, with scattered sessile 4-celled glands; upper lobes subcircular, 2.4-2.5 mm long, 2.4-2.5 mm wide; lateral lobes ovate, 1.0-1.2 mm long, c. 0.8 mm wide; lower lobe, oblong, 4.4-4.5 mm long, 3.0-3.2 mm wide. Fruiting calyx 4.0-4.8 mm long; upper lobe lanceolate-triangular, 1.7-1.8 mm long, 1.2-1.4 mm wide; lateral lobes lanceolate-falcate, 1.3-1.5 mm long, c. 1.2 mm wide; lower lobes lanceolate-falcate, 2.4-2.5 mm long, 0.5-0.6 mm wide. Seeds hemi-spherical, 0.7-0.8 mm long, c. 0.7 mm wide, 0.4 mm thick(Forster 1992). For a more detailed description and illustration refer to Forster (1992).
Plectranthus omissus is superficially similar to P. argentatus and P. torrenticola but differs from them by a number of characters that are set out in Table 3.29.1.

3.29.3 Current conservation status

Plectranthus omissus is presently listed on the schedule of the Queensland Nature Conservation Act 1992 as “endangered wildlife”. It is also listed on Schedule 1 Part 1 (endangered) of the Commonwealth Endangered Species Protection Act 1992. The species has not been assigned a national conservation status by ANZECC (1993).
Table 3.29.1. Characters distinguishing Plectranthus omissus, Plectranthus argentatus and Plectranthus torrenticola

Character	P. omissus	P. torrenticola	P. argentatus
4-celled sessile glands	present	absent	present
flower number/cluster	14-16	10-12	6-10
corolla tube angle (°)	110-130	30	0-10
trichomes on corolla tube	present	absent	absent
corolla upper lobe size (length x width mm)	2.4-2.5 x 2.4-2.5	1.7-2.1 x 1.9-2.2	1.4-1.5 x 1.5-1.7
corolla lower lobe size (length x width mm)	4.4-4.5 x 3-3.2	4.6-5.6 x 2.7-4.5	4-4.2 x 4-4.2

3.29.4 Distribution and abundance

Plectranthus omissus was not surveyed during the present study. Details from Queensland Herbarium specimens of P. omissus are listed in Appendix 1.28. P. omissus is restricted to the Kilkivan and Gympie districts in south-eastern Queensland. There are two locality records on the database with another record currently being processed. All sites are within state forest lands (beauty spot 11, TR 26 Fty 250; SF 393 Fty 1146; SF 67 Fty 1173). The population at TR 26 was estimated to comprise 30-40 plants (Forster 1992). There is no information available on population sizes at the other sites. The distribution of P. omissus has a range of approximately 70 km and encompasses an area of occurrence of approximately 700 km².

3.29.5 Habitat

P. omissus has been recorded on steep rocky outcrops approximately 300-400 m above sea level on the margin of vine forest or sclerophyll forests.

3.29.6 Life history and ecology

There have been no studies into the biology or autecology of Plectranthus omissus. P. omissus is a herbaceous shrub with an unknown life span. It is easily propagated by cuttings. The main method of reproduction is by sexually produced seeds. P. omissus as with most other Plectranthus produces roots from branch nodes where the branches come in contact with the soil surface. It is believed that the plants are fire-sensitive (P. Forster pers. comm.).

P. nitidus has been recorded flowering in February and March. The pollinators of P. omissus are unknown. Pollinators reported for the family Lamiaceae include bees, birds, flies, wasps, butterflies and hawkmoths (Huck 1992). The most common pollinators for Plectranthus are bees and flies (Huck 1992). Fruits develop and mature soon after flowering. The fruit comprise 4 (1-seeded) nutlets which fall from the fruit after maturity. There is no information available on seed viability and germination of this species.

3.29.7 Threats

The area around the population in TR 26 is infested with lantana (Lantana camara) and blue billygoat weed (Ageratum houstonianum). These exotic species are considered to be a potential threat to the long term survival of the species.

The species is suspected to be fire-sensitive and incapable of regenerating from underground organs. The continued existence of the species in the wild after a fire would require the regeneration of the species from seed (obligate seeder). If fires were at a frequency that did not allow the production of adequate seed for future recruitment events then the population would decline and the species would be eliminated from the area. This is a potential threat at all sites. However, the habitat in which the species has been observed provides some degree of protection from frequent burning in that the rocky outcrops may buffer plants from fires.

3.29.8 Management, research and conservation measures

Very little is known about this species. Previously recorded sites should be examined to establish the population sizes and extent. Areas of likely habitat in the vicinity of the known populations should be searched for further occurrences. Observations should be made on potential threatening processes. The spread of weeds, the impact of fire and the condition of habitat on rock faces should be monitored. Monitoring and control of the spread of weeds on the rock face at TR 26 should be undertaken.

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